8. AGRICULTURAL MANAGEMENT

8.1 Introduction
8.2 Crop Choice
8.3 Water Management at Farm Level
8.4 Tillage and Cultivation Methods
8.5 Correcting Acidity by Liming
8.6 Fertilizer Use
8.7 Crop Protection

8.1 Introduction

Before cropping commences the farmer has to make important decisions which profoundly affect the subsequent operational management. First he has to choose the crop; the operational requirements generally follow from this. Such operational requirements at farm level include:

i. Water management in relation to crop moisture requirements.
ii. Tillage and land preparation.
iii. Liming or acidity control.
iv. Fertilizer use.
v. Crop protection.

Many standard management practices used on mineral soils are also applicable to organic soils so they are not discussed in detail. Emphasis is placed on all aspects of agricultural management specific to conditions prevailing in organic soils.

8.2 Crop Choice

8.2.1 Choice of crop under natural drainage conditions
8.2.2 Choice of crops under improved drainage with water-table at less than 40 cm depth
8.2.3 Choice of crops assuming deep drainage

Crop choice is dependent upon many factors of which suitability of soil is but one. For most farmers profitability is the overruling factor, but in the case of reclaimed peatswamps there are a number of factors which influence or limit the freedom of choice and which are beyond the control of the farmer.

In the case of large peatswamps reclaimed by a Government, or government agency, the potential land use has been decided early in the scheme. Large-scale reclamation schemes need careful overall planning and the control of drainage cannot be left to the individual farmer. However, within the limits set by the controlling body it is possible to have some influence on water control at the farm level by such means as stop boards, but generally the margins are small. It is, therefore, the depth to which the groundwater is maintained in the reclaimed area that largely controls and limits the choice of crop.

In cases where an individual farmer endeavours to reclaim small stretches of swamp, he has more room for taking his own decisions. However, if his land is part of a much larger swamp he has the moral, if not legal, obligation not to disrupt farming activities on fields lying downstream. Such conditions are frequently found in long narrow interior valleys especially when under private ownership. In such cases it would be sensible to look for more participants in a concerted reclamation effort which would ensure a balanced development of the complete swamp. It would be unwise to develop one part of the same swamp for deep rooting crops demanding a drainage depth of say 90 cm, if on a neighbouring farm attempts are made to keep the water level at 40 cm for pasture development. Crop choice is also influenced by independent climatic factors and socio-economic considerations such as marketability, which are liable to fluctuations.

Peat soils are remarkably versatile in their suitability for crop growth. They have few inherent qualities which limit growth, although they require intensive and often costly improvement to natural conditions to make cropping profitable. Profitability is again largely dictated by the local economy.

Peat is a good stoneless rooting medium, it has large moisture retention capacity and hence transplanted crops establish themselves much faster than on mineral soils. Cultivations are easier than on mineral soil, even under exceptionally wet conditions. There are, however, also serious limitations to cropping:

i. Waterlogged conditions, requiring drainage.
ii. Very low chemical fertility, requiring large applications of fertilizers.
iii. High acidity, requiring liming.
iv. Low trafficability, preventing intensive mechanized farming.

Most countries have already developed or should develop their own peat suitability ratings, adapted to local conditions, taking into account the limitations mentioned and based on local economic considerations (Chapter 6). A very general guideline, based on experience in South East Asia, and which can be adopted elsewhere with similar conditions, is to limit farming to peats which are less than 2 m thick. Greater thicknesses, particularly those with a low level of management, usually have insurmountable problems. This depth requirement can be adjusted to fit local situations and where the input levels of water control, fertilization and crop protection are high deeper peats can be contemplated.

Although factors mentioned play an important role in crop choice, they are specific to a locality and should be studied and evaluated for each site.

Despite this complexity of factors, it is possible to discuss crop selection by looking at the various land use options possible under three systems of water-table management; natural conditions, somewhat improved drainage but with shallow water-tables (less than 40 cm depth), and with deep drainage (water-tables generally greater than 60 cm depth).

8.2.1 Choice of crop under natural drainage conditions

There is an increasing interest in developing an adapted form of agriculture which leaves the peatswamps largely in their natural state. Crop selection is directed to finding suitable swamp-adapted plant species of economic value. This type of farming would to a large extent solve the problem of subsidence (Shih et al. 1982). The main crops suited to such conditions are:

Sago

Sago (Metroxylon sagus, a smooth variety, and Metroxylon rumphii, the thorny variety) is grown in the natural peatswamps of Sarawak and elsewhere in the Malay-Indonesian archipelago under wild and semi-wild conditions. There is evidence that the crop could give reasonable returns, and organized production on a large scale could be lucrative. The produced starch can be used, as a food, by industry and as a raw material for the production of methanol. As a result of better vegetative dry matter production per day and a longer period of closed canopy, the sago palm is superior in potential to cassava and rice (Ahmed and Sim 1976).

Raffia

The raffia palm (Raphia spp.) and papyrus (Cyperus papyrus), indigenous species in African peatswamps, can be grown commercially provided that local markets are available and the range of its industrial use can be widened.

Rice

Rice is probably the most swamp-adapted foodcrop. Wild rice (Zizania aquatica and Zizania palustris), of which a number of varieties are commercially grown in North America, even as far south as Florida, appears to offer good possibilities under the right climatic conditions in natural peatswamps. This rice, famed as a gourmet accompaniment for wild duck and game, also has a ready market as a dietary food because it is a good source of thiamine, riboflavin and nicotinic acid (Morton et al. 1980). Wild rice needs some regulation of water-tables for optimum growing conditions although it thrives in waterlogged conditions. Aeration of the water is however essential and regular flooding with oxygenated water is beneficial. The optimum pH is between 7.5 and 8.0 which excludes most tropical peats unless they occur near a brackish water source in the fringe areas between mangroves and freshwater swamp forest. For a number of reasons the domesticated rice (Oryza sativa) requires very good water-table control and is therefore not suitable for peatswamps. It is discussed under the section dealing with conditions of shallow drainage.

Wetland taro

Another starch foodcrop, wetland taro (Colocasia esculenta, var. aquatilis) has many wetland cultivars. It is grown in and around the Pacific as well as in India and Africa in similar locations to wetland rice. Larger plantings require field levelling, puddling by ploughing, disking, harrowing and grading. It is also necessary to have constant water circulation to avoid footrot. Since many tropical peatswamps, particularly in their centres, have deoxygenized water, favourable conditions only occur near their edges where peat merges into mineral deposits. Nutritionally taro is a better choice of crop than rice, as the amino acid content is as high as in potatoes and the crop should be grown more widely in wetlands.

Miscellaneous crops

Water celery (Oenanthe javanica), water spinach (Ipomoea aquatica), and the Chinese water chestnut (Eleocharis tuberosus) when grown commercially, require some expertise and the vegetable crops are limited by location and marketability. Water chestnuts, like rice, are grown in fields covered with 5-10 cm depth of water. Tubers can be eaten in salads and soups, the juice has antibiotic uses and the sedge can be made into matting although it is not very durable.

8.2.2 Choice of crops under improved drainage with water-table at less than 40 cm depth

Pasture

By far the greatest area of reclaimed peatswamps in temperate regions is used for pasture and these only require the water-table to be lowered to about 40 cm depth. In the Netherlands a water-table depth of about 40 cm has been maintained for centuries which has minimized the rate of subsidence. The largest area of peat in the Florida Everglades is used for pasture, particularly St Augustinegrass (Stenotaphrum secundatum). Apart from requiring shallow drainage, grassland use does not necessitate intensive mechanization which is characteristic of large scale vegetable growing. Pastures can be used for direct grazing but the high water-tables lead to low surface bearing strength and pastures are unsuited to frequent and heavy traffic. In the tropics pastures on deep oligotrophic peats require high levels of fertilizer application and therefore the nature of the peat should be carefully examined and likely improvements considered in the light of local economic perspectives, before large scale pasture development is contemplated. Conditions will most likely only be favourable in a few selected localities. The growing of fodder crops such as Napier grass is probably locally more attractive than grazing. The careful selection of grass species also counteracts the effects of poaching and in Florida, St Augustinegrass is used effectively in this capacity.

Rice

Rice (Oryza sativa) has received much attention as being the crop most adapted to swamp conditions. The cultivation of rice on true tropical peats of oligotrophic nature has a number of drawbacks of which the most important are sterility, causing empty panicles, and the crop’s dependence on good water control. Although the latter is only essential for obtaining high yields, the former is often the cause of complete failure. The cause of sterility is not exactly known although copper deficiency probably plays a major role and dressings of copper can alleviate the problem. Driessen and Suhardjo (1976) suggest that copper deficiency retards the inactivation of the toxic phenols and causes male sterility. However, Japanese work by the Central Agricultural Experimental Station and reported upon by Miyake (1982) indicates that the disorder is caused by harmful substances produced by delayed and incomplete decomposition of organic materials. He postulates that disorder can be cured by drainage improvement. Both findings may be complementary, in that it is possible that retarded decomposition causes the formation of copper-fixing organic compounds, and thus, by stimulating the decomposition rate by drainage, the same effect would be obtained as a copper dressing. In both cases good management is required.

Rice fields in peatswamps where water levels are not adequately controlled may be frequently inundated by undrained rainwater and floodwaters become too high; such periods may be followed by periods of water shortage causing drought stresses in the crop. Rice growing on peat, though having some potential, therefore requires good water management involving both drainage and irrigation, and consequently level fields. The generally low or very low fertility of the majority of tropical peats, added to the problems of water control and sterility makes rice growing unprofitable in most countries, unless good management can be given and well-tried adapted cultivars are used.

Vegetables

In many countries organic soils are highly regarded for vegetable growing because of their excellent physical properties as a medium for plant growth. The choice of which vegetables to grow is dictated by many factors some of which (climate and locality) cannot be influenced by management. The minimum depth of water-table required for maximum yield and quality also limits the crop choice. Depth of water-table is often standard over large areas. Table 24 and Appendix 3 give indications of vegetable crops which can be grown with shallow drainage (water-table at less than 60 cm depth). The ultimate choice however often depends on the demand and price.

Some tropical and sub-tropical crops such as chili, soya bean and tobacco should also be mentioned here because their method of cultivation is comparable with that of vegetables. They can also be grown successfully on peat soils with shallow drainage.

Where peatswamps are in areas remote from large population centres, vegetable growing is not a commercial proposition particularly for rapidly perishable leafy vegetables. Non-perishable crops which can be stored and/or dried can often be grown profitably some distance from a market.

Horticultural crops

In temperate regions, particularly in the Netherlands, horticultural cropping is favoured on peat soils. Here the expansion of the horticultural industry was largely based on the excellent qualities of the local peat soils. Sandy mineral soil, from nearby riverbanks or dunes, is often mixed with the surface soil further improving the inherently good physical characteristics of the peat (Hidding 1982). In Japan mineral soil dressing is also practised for wet rice cultivation (Miyake 1982). Transplanting horticultural crops is easy and successful in peat because of the good root balls formed by the plants. Horticulture can be profitable in the tropics on oligotrophic peats, drained to 50 cm depth, assuming a good local market and an adequate level of management.

Crop	Florida	Indiana (Depth in cm)	Minnesota
Beans	45-60	-	45
Beets (red)	-	71	-
Cabbage	45-60	66	60
Carrots	-	66	-
Celery	45	60	45
Corn	45-75	75	60
Lettuce	75-90	-	75
Mint	-	75	-
Onions	45-60	75	90
Potatoes	45-60	66	60
Parsley	35-40	-	-
Radish	35-40	-	45
Pasture-sod	30-50	45	-
Sugar cane	60-75	-	-

8.2.3 Choice of crops assuming deep drainage

Deep drainage is defined as greater than 60 cm depth but some crops require a depth greater than 90 cm (Table 24 and Appendix 3).

Perennial crops

The growing of perennial crops, particularly plantation crops on tropical peats, has been under study in Malaysia for some time (Kanapathy 1978; Tie and Kueh 1979). One of the most difficult problems to counteract is the poor root anchorage provided by the soft peat, especially for crops such as coconut palm (Cocos nucifera), oil palm (Elaeis guineensis) and rubber (Hevea brasiliensis) (Plates 10 and 11) which become top-heavy when mature. Peat subsidence, a consequence of intensive drainage, uproots the shallow-rooting trees causing them to lean progressively and eventually topple. This problem can be partly alleviated by using dwarf varieties. Attempts are currently underway to breed a dwarf oil palm for this purpose (Dolmat et al. 1982).

It is only where the peat is shallow (less than 2 m thick) and contains an appreciable amount of mineral matter that such crops are feasible. Thus deep oligotrophic peats are unsuited because of low fertility and poor anchorage though small subsistence farmers may have marginal success when world prices are favourable.

Fruit trees and horticulture

The yield potential of some promising perennials and previously mentioned annuals grown on deep oligotrophic peat in South East Asia is shown in Table 25. This potential is only realized under very good management and with high inputs of fertilizers. The economic feasibility is therefore low.

Large-scale banana (Musa spp.) growing in the Ivory Coast with drainage at 80-100 cm depth on acid oligotrophic coastal peat, comparable with the lowland peats of South East Asia, indicate that yields in the first years are in the range of 25-30 t/ha but could eventually reach a level of 35-40 t/ha. However, initial investments are higher than on other soils and banana cultivation on peats is difficult and demands good management (Lassoudière 1976). In general this is true for most horticultural crops grown on tropical peats. Sarawak sources (Tie and Kueh 1979) indicate a high potential for mulberry (Morus alba) which, however, would require the initiation of a local silk industry.

In general many fruit crops, though performing well, are plagued by bird damage, insect pests, nematode and disease problems. Therefore, these crops also need a high level of crop protection for success.

Food crops

Root crops require well drained conditions to prevent tuber rot and large inputs of fertilizer. Under subsistence farming peat burning is traditionally practised to ensure adequate potassium levels but because this exacerbates wastage it should not be encouraged. Cassava (Manihot esculenta) or tapioca, yielding up to 50 t/ha with good management, is an important food crop on well drained deep oligotrophic peat in the tropics.

Miscellaneous field crops

A number of perennial or semi-perennial crops are difficult to place in the categories mentioned above. Of these pineapple (Ananas comosus) has a very good potential, because it both flourishes in the acid conditions prevalent in peat soils, and it is relatively low growing and not susceptible to being uprooted at maturity. The Malaysian pineapple industry is based predominantly on deep oligotrophic peats.

Sugar cane (Saccharum officinarum) is grown successfully on the Everglade peats in Florida and on deep coastal peats in southern states of Brazil, the latter for methanol production. Good management and assured markets are prerequisites to the economic feasibility of sugar cane growing.

Summary

While drained peats form an excellent growing medium for many crops it is only through the development and employment of specific technology for each crop that the inherent potential can be realized. In many countries the capital investment necessary for improvement is too high and management levels too low to justify reclamation.

Common name	Botanical name	Sarawak	West Selangor
		Yield (t/ha)
Pineapple	Ananas comosus	40.0 fresh fruit	40.0 fresh fruit
Tapioca (cassava)	Manihot esculenta	50.0 fresh fruit	49.0 fresh tuber
Tobacco	Nicotiana tabacum	0.7 dry leaf	1.0 dry leaf
Groundnut	Arachis hypogaea	1.0 dry seed	3.5 fresh nuts
Soya bean	Glycine max	1.5 dry seed	-
Cowpea	Vigna unguiculata	2.1 dry seed	-
Bambara groundnut	Vigna subterranea	1.5 dry seed	-
Sorghum	Sorghum bicolor	1.5 dry seed	2.5 dry seed
Sweet potato	Ipomoea batatas	14.0 fresh tuber	24.0 fresh tuber
Castor oil	Ricinus communis	2.5 dry seed	-
Ginger	Zingiber officinale	15.0 fresh rhizome	15.0 fresh rhizome
Okra	Hibiscus esculentus	6.0 fruit fruit	15.0 fresh fruit
Oil palm	Elaeis guineensis	19.0 fresh fruit bunch	-
Sago	Metroxylon sagus	6.0 dry starch	-
Coffee	Coffea liberica	1.7 fresh berries	-
Annatto	Bixa orellana	2.0 dry seed	-
Mulberry	Moms alba	13.0 fresh leaf	7.5 dry leaf

8.3 Water Management at Farm Level

8.3.1 Systems of open drains
8.3.2 Subsurface drains
8.3.3 Irrigation

The provision and maintenance of adequate water control systems in peatswamp reclamation require two distinct types of management; one at the individual farm level and another for the peatswamp as a whole. The latter is responsible for major constructions such as dams, dike’s, levees, canals and main ditches, whereas the farmers are usually responsible for the construction and maintenance of farm ditches and/or field drains.

The aim of water control systems is both to provide adequate drainage for optimum crop yield and to maintain the water-table at an optimum depth to prolong the life of the organic soil. Initially stagnant surplus water must be drained away, thereafter the water-table should be lowered to and maintained at a depth where crop growth becomes possible. However, the peat should never be allowed to dry out to such an extent that irreversible drying will set in. In tropical areas, where the climate is characterized by a pronounced dry season, drainage facilities should be supplemented by irrigation, so that in periods of drought water levels can be maintained at the desired height by infiltration. An adequate source of irrigation water must therefore be available.

The reclamation of small swamps within a farm or group of farms is usually carried out by the farmer or farmers concerned. Large swamps are usually reclaimed by public bodies. This Bulletin describes the reclamation management required at farm level and does not describe at length the engineering requirements for large reclamation schemes. The problems of co-ordinating and integrating drainage requirements of many individual farms must not be underestimated. Further, with regard to swamp drainage on an individual basis, the effects on groundwater levels and risk of flooding on neighbouring farms, should be borne in mind.

8.3.1 Systems of open drains

The size of open drains is related to the amount of rainfall which must be removed and the height of the groundwater-table to be maintained. Climate, in particular rainfall, and the lateral and vertical hydraulic conductivity of the peat are the most important criteria in drainage design.

Generally, the nearer the water-table is held to the optimum depth for the individual crop, the better the yield. Tie and Kueh (1979) quote recommendations for drains at 90-150 cm depth at 100-200 m intervals together with field drains at 50-80 cm depth, placed at right angles to the main drains at 15-30 m spacing. Tay (1969) describes a similar system used by the Drainage and Irrigation Department of Malaysia for peat less than 1.5 m thick (Fig. 26). For thicker peats a system of ring and feeder drains is recommended (Fig. 27). Such systems do not allow irrigation in periods of drought. In Indonesia a system is used which allows both drainage and irrigation by utilizing the tidal differences in water levels of the main canals (Fig. 28). The system works as follows: during high tides water enters the canal system backing up and raising the fresh water in the system so that parts of the land can be irrigated by submersion. During low tides the water levels in the system fall, including the main tertiary and field drains. Owing to the relatively long length of the system, not all of the drainage water is discharged to the river. To catch this, water tanks are constructed at strategic points to be filled during the next tide. This water is conveyed to the system when the next fall in water level reaches its minimum at the bottom of the tertiary drains. In practice, serious siltation occurs in the tanks, but where the silt load in rivers is low the system is feasible. However, it is only through the provision of structures such as tidal gates, sluices and pumps that good use can be made of a combined irrigation and drainage system in coastal lowlands. As a means of water control at field level stop-boards are widely used in tertiary drains. The height of the water-table can then be easily adjusted within limits to suit the need of the individual crop and growing period.

Figure 26. Drainage system for relatively shallow (<1.5 m) peat (source Tay 1969)

The land is divided by main drains (1.5 m deep, 1.2 m wide) into rectangular blocks of 200 x 600 m. Within each block, secondary (1.2 m deep, 0.9 m wide) and field (0.9 m deep, 0.6 m wide) drains are constructed.

8.3.2 Subsurface drains

In Florida an increasing use is made of corrugated plastic tubing to replace field drains. This is easier to install than clay tiles, although it may be more expensive. Drain pipes should be 10-30 m apart depending on the permeability of the peat, the rainfall and the cropping system. Observations of water levels at about 2 m intervals between the pipes give information on the efficiency of the subsurface drains. These dipwells can be made out of 1.5 m long by 10 cm diameter downspouts with a stabilizing collar placed 30 cm from the top (Lucas 1982).

The disadvantages of tile drainage are their vulnerability to silting up with either organic or iron oxide compounds, the cost and their possible disruption by tree roots. Mole drainage is an alternative method but its effectiveness depends on the nature of the peat. In finely-divided materials mole channels soon close up and become ineffective. An advantage of underdrainage is that the tiles can be used to irrigate in times of drought by reversing the direction of water flow in the main open ditches or canals, but its success depends on soil permeability and the smoothness of the field surfaces.

Figure 27. Drainage system for deep peat (> 1.5 m) (source Tay 1969)

A large circular drain is constructed round the land with feeder drains proceeding inwards to the centre.

Figure 28. Combined drainage and irrigation system using tidal differences (source ESCAP 1978)

8.3.3 Irrigation

The amount of water available to plants at critical periods in their growth is crucial to obtaining good yields. For optimum growing conditions it is therefore necessary to monitor moisture conditions in the peat soil. Neither excess nor insufficient water should be present in the rooting zone. This is particularly so for high-quality vegetable crops that demand large investment of capital and labour for optimum production. The peat surface should never be allowed to dry out and frequent watering may be necessary in dry periods. Where this cannot be maintained manually, overhead sprinkler devices, drip irrigation or surface flooding is necessary. Subsurface irrigation, as explained above, is generally favoured because of low costs and there are no problems with the quality of water sources (low alkalinity or salinity hazards). Overhead irrigation is necessary wherever fields are not level. A great many systems, catering for many varied local conditions, are in use, each with merits and handicaps. The factors to be considered when choosing equipment are labour costs, ease of handling, damage to crops, water distribution patterns and field puddling. Water used for irrigation should preferably have a conductivity of less than or 750 mmho/cm (650 ppm of salt).

8.4 Tillage and Cultivation Methods

Good tillage on peat avoids breaking down the peat particles to dust. Too much tillage leads to wind erosion of the fine particles and rapid dessication in dry spells.

Organic soils are inherently loose structured and require little power to cultivate, although variations occur because of the relative proportions of mineral particles and wood. Too many wood fragments prevents good ploughing and large fragments have often to be removed by hand. Deep ploughing is often carried out in temperate countries, particularly in the Netherlands, to mix the peat with underlying mineral soil, frequently sand. This mixing of sand and peat creates an excellent medium for plant growth and also increases the bearing capacity. If the mineral subsoil is clayey it is more difficult to obtain a good mixture, but clay admixtures have greater adsorption power for fertilizers. In Japan the system of topsoil dressing of deep peat soils with mineral materials is well established for padi cultivation.

The physical characteristics of peat often require adjustments to conventional ploughing equipment. Peat can be too loose for ploughing so that it is pushed by the plough rather than inverted.

In the tropics, high wood content and hummocky peat surfaces are often the main obstacles in preparing a good seedbed. Mechanization is difficult because of the lack of levelling equipment and manual tillage is therefore often practiced. Perennial crops or those requiring root stock propagation are therefore favoured by the tropical subsistence farmer rather than crops produced from seed.

In temperate regions, farming on peat soils is highly mechanized, while in developing countries conditions are generally unsuited to mechanized farming. Large-scale mechanized farming is often not cost-effective on the majority of tropical peats and much of the mechanized equipment is not discussed here.

It is common practice to plant on ridges or raised beds, particularly in the case of vegetables. Damage from waterlogging after heavy rains coupled with inefficient drainage can be avoided in this way. The danger of desiccation is however enhanced and in certain climates the soil surface has to be irrigated frequently (Plate 1).

To overcome the problem of uprooting with top-heavy perennial tree crops or palms, the crops in Malaysia are planted by the double hole, or hole-in-hole method. A hole of 1 m² and 30 cm deep is dug in the freshly reclaimed and drained peat. Within this large hole an oil palm seedling is planted in a normal size planting hole of 45 cm diameter and 35 cm deep. As the surface of the peat subsides because of shrinkage and compaction, the base of the young palm becomes level with the peat surface. By this method the impact of the first rapid and intensive subsidence of about 40 cm in two years can be cushioned. Subsequent subsidence is rather slow and of less significance.

There has been very little research on cultivation techniques on tropical peats, probably because most peats appear to be economically unsuited for large-scale cultivation. The exception is for pineapples but apart from this there has been little stimulation to research.

8.5 Correcting Acidity by Liming

8.5.1 Lime requirements
8.5.2 Materials used

The acid or very acid condition of many tropical peat soils does not suit most commercial crops. Liming is therefore a prerequisite for most agricultural enterprises. The relationship between acidity and base exchange characteristics in peats has been described in Section 4.2.

Many crops require a pH of over 4.5 for optimum growth. A few, such as pineapple and sago, like the low pH of peat soils. The amount of lime required depends on the natural acidity of the peat and the specific requirement of the crop. It is impossible to indicate here specific lime requirements for the great variety of crops which can be grown on peat soils. However, some general points can be made.

8.5.1 Lime requirements

Many soil testing laboratories use buffer solutions to help estimate lime requirements. Some of these solutions are not suitable for organic soils as they were developed for mineral soil with different exchange characteristics. Mehlich (1942) developed a suitable test using a pH 6.6 buffer containing triethanolamine, acetic acid, ammonium chloride and sodium glycerophosphate. Liming was recommended if soil in the buffer was less than pH 5.5 (Lucas 1982). It should be added that lime recommendation also depends on the crop grown.

Natural pH values and optimum CaCO₃ content of peat for agriculture is confusing because the type of peat is rarely given when results are discussed. The amount of lime required per unit change of pH varies for different soils depending on exchange characteristics. The proportion of mineral matter and the amount of exchangeable Al are both important. Generally a pH rise of 0.1 can be achieved by thoroughly mixing 0.7 t/ha of limestone to a depth of 15 cm. Lassoudière (1976) reports that an application of 5 t/ha of lime raised the pH one unit in coastal peat of the Ivory Coast. Similarly in Sarawak an application of 5 t/ha raised the pH from 5.7 to 6.6. However, the depth to which the pH increase is effective is also important as indicated in Table 26, based on liming studies with blanket bogs in Ireland which are chemically similar to oligotrophic peats in the tropics.

Table 26 EFFECT OF LIME AND FERTILIZER ON THE NUTRIENT STATUS OF THE SOIL (source O’Toole 1968)

¹ Ca, P and K as ppm in wet peat

Several factors modify the critical pH for good plant growth, including the crop sensitivity to active calcium content. In general, organic soils with low Fe and Al contents can have an optimum pH value as low as 4.5 for certain crops, whereas peat soils containing appreciable amounts of Fe and Al have an optimum pH value approaching 5.0 for the same crops.

Liming to neutral state is expensive and unnecessary. It may affect the availability of trace elements and over-liming may influence denitrification, producing toxic levels of nitrate-nitrogen. O’Toole (1968) showed that where an adequate supply of nitrogen fertilizer is applied to pasture the pH can be maintained at lower levels than where no applications are given. The influence of liming on the dry matter output of a mixed grass-clover sward is shown in Figure 29. Comparison with data in Table 26 illustrates that the best results are obtained when 2.5 t/ha of calcium carbonate is applied which raises the pH to 4.8.

Figure 29. Effect of calcium carbonate on dry matter output from a surface seeded grass-clover sward (source O’Toole 1968)

Liming an acid peat to a pH of over 5.2 appreciably depresses the phosphate recovery and large quantities of calcium, and in the case of dolomite application also magnesium, may interfere with the absorption of potassium by the plants. It is sometimes alleged that liming increases the rate of decomposition of peat but the results of research give variable results and are inconclusive. The type of peat probably plays an important role (section 7.3; subsidence).

Tie and Kueh (1979) give general recommendations for liming deep oligotrophic lowland peats of South East Asia based on the work of several researchers in Malaysia. They indicate that pineapple and sago need no liming, as does tapioca (cassava) if the initial pH is above 4.0 but at pH 3.5 and below, 5.0-7.5 t/ha of ground dolomite is recommended. Most other crops, like sweet potato, maize, groundnut, soya bean, sorghum, coffee and napier grass require between 5 and 10 t/ha of ground dolomite. This order of application is very costly, particularly as to maintain the pH at the required level necessitates an annual application of about 1.25 t/ha.

In many cases, therefore, liming is a prerequisite for profitable farming. The optimum pH values and rates of application to achieve this vary considerably from crop to crop and between different types of peat. When assessing adequate levels of liming local experimentation is important as optimum pH levels are partly dependent on local economic factors.

8.5.2 Materials used

There are two main materials used for liming; limestone, which is relatively pure CaCO₃ with less than 1 percent MgO, and dolomite, a CaCO₃ and MgCO₃ mixture containing over 15 percent MgO. The pure materials act faster to raise the pH than the dolomitic ones though the latter supply magnesium which is deficient in many peats. Proximity to source and transport costs of these bulky materials often determine the local choice of materials. Occasionally marl, which usually has admixtures of mineral material, and coral lime are used locally. In all cases to be effective, the limestone and related materials need to be finely ground to pass through a 100 mesh sieve.

Lime when applied to peat soils is relatively immobile and a thorough mixing to the required depth is therefore important. This is laborious to achieve manually and even with mechanized means several diskings will be necessary. In the tropics, the low level of mechanization leads to inefficient liming at the field scale. Where deep mixing is necessary, split applications can be given, one half ploughed under, the other top dressed after ploughing. Split applications also allow the use of both pure and dolomitic limestone.

8.6 Fertilizer Use

8.6.1 Introduction
8.6.2 Burning
8.6.3 Basic principles
8.6.4 Nitrogen requirements
8.6.5 Phosphorus requirements
8.6.6 Potassium requirements
8.6.7 Calcium and magnesium requirements
8.6.8 Micro-nutrients or trace element requirements
8.6.9 Conclusions

8.6.1 Introduction

One of the most important factors that has prevented large scale use of peat soils in the tropics is the very low chemical fertility. Oligotrophic peats, which areally are the most important in the tropics, are inherently poor in all plant nutrients. Eutrophic or mesotrophic peats, which are locally important, contain more nutrients than oligotrophic peats but they also need manuring or artificial fertilizers for commercial farming. This section concentrates on oligotrophic peats since their nutrient requirements have been studied in the tropics, particularly in Malaysia and Indonesia.

Studies of the chemical fertility and nutrient deficiencies of peats have received more attention than either the more important physical changes of peat soils upon drainage, and the water management of drained peats. Nutrient deficiencies are easier to remedy than some of the detrimental physical changes caused by reclamation. The interest in the nutrient requirements of peats probably reflects the parallel interest in the case of mineral soils where it is often the most important constraint to improvement.

8.6.2 Burning

Reclaimers and settlers of peatswamps realized the importance of fertilization of peat soils from the beginning. They initially fertilized by burning the peat, and this practice survives among traditional farmers employed in shifting cultivation. Kanapathy (1976 and 1977) has shown that burning is beneficial in increasing the pH value from 3.5 to occasionally over 5 which is desirable for food crops such as maize. Burning also adds potassium to the soil and changes unavailable phosphorus, stored in the organic compounds of the peat, into available forms.

Other added benefits of burning include the increase in rate of decomposition caused by the rise in pH, producing an increase in nitrogen stored in the peat. Polak and Supraptohardjo (1951) demonstrated that heating peat to between 105 and 128°C produced a flux of ammonia, to which maize showed a marked response. Finally, burning releases copper compounds, usually fixed in peat, causing these soils to be frequently deficient in copper for most crops.

The beneficial effects of peat burning are, however, short-lived and after only two years another burn is needed to support cropping. Regular burning leads to a rapid lowering of the peat surface, causing problems of waterlogging and often ultimately to abandoning agricultural activities. Where sustained agriculture is the aim, burning should be stopped, though at present it is often the only means available for the poor traditional farmer to maintain his subsistence agriculture. The regular use of fertilizer is the only way to sustain agriculture on these soils but ultimately however the peat will disappear (Chapter 7).

8.6.3 Basic principles

Each crop has individual nutrient requirements and it is therefore difficult to discuss specific details. Instead an attempt is made to provide general fertilizer guidelines and principles for peat soils. Rates of fertilizer-use depend on both the cropping and type of peat present. Where there is no local research work available, the peat type needs to be identified and the specific requirements of the crop reviewed. Then it is possible to apply the results of research work done elsewhere to the local circumstances.

Most information on fertilizer-use on tropical peats has either been carried out in the Everglades, Florida (eutrophic and mesotrophic peats), or on the oligotrophic coastal peats of Malaysia and Indonesia. Basic research on oligotrophic peats in temperate climates, particularly that on nitrogen, can also be applied to tropical peats.

Admixtures of mineral material, which can be up to 50 percent of organic soils (Histosols) can have profound effects on the behaviour of peat soils. Extrapolation of research findings, particularly on fertility status, should never be attempted if the research results are not accompanied by an accurate analysis of the soils in question.

8.6.4 Nitrogen requirements

The nitrogen status of peat soils is discussed in section 4.2, where it is shown that the total nitrogen content ranges widely because of differences in both the nature of peats and their decomposition rate. Generally, total N contents are high when compared with mineral soil. However, the amount of N available to the plant is important. While available N is indirectly determined by factors influencing the total N content other factors such as temperature, moisture, aeration and acidity play a role. The latter three affect the activity of soil organisms responsible for the breakdown of the organic compounds. The effect on soil N of liming Irish blanket bog soils decreased after a short time (O’Toole 1968). Chew et al. (1976a and b), in a study of the effect on nitrogen of liming oligotrophic peats under napier grass in Malaya, discovered an almost identical decrease in response to liming in the uptake of soil N. The availability of N is also affected in time by liming. This is because the easily decomposed nitrogenous components of the peats are mineralized first and relatively quickly after liming, leaving the less easily decomposed forms of organic N (Hardon and Polak 1941). Liming has the primary effect of raising the pH and any other effect on nitrogen liberation may be short-lived.

The carbon/nitrogen (C/N) ratio of peat is important in assessing the available N content. In tropical organic materials C/N values lower than 16 are commonly regarded as indicative of soils where nitrogen stress will form a constraint to crop growth. However, temperature also plays a role. In temperate or colder regions available N varies with the season because of changing microbial activity.

Available N content is also considerably influenced by drainage depth. In the Netherlands a drainage depth of 50 cm requires N-fertilization for good pasture growth whereas this is not necessary with deeper drainage. Likewise trials in the USA also indicated that no response to N fertilizer was found with deep drainage as opposed to a 10-67 percent yield increase, dependent on crop type, experienced when shallow drainage is practised (Table 27).

It should be noted that nitrification proceeds rapidly under high temperatures and nitrite poisoning may affect forage crops. These, when consumed by ruminants, may in turn be responsible for nitrite poisoning and death of the animals.

Table 27 AVERAGE YIELD RESPONSE TO NITROGEN DRESSINGS ACCORDING TO DEPTH OF WATER-TABLE (source Lucas 1982)

Crop	Years in test	Water-table at 40 cm	% Yield increase Water-table at 60 cm	Water-table at 80 cm
Corn	7	43	6	2
Potatoes	4	67	9	4
Onions	5	23	3	2
Peppermint	5	10	6	3

General recommendations for the application of the nitrogen on peat soils to fit all conditions are difficult to give. Rates in the USA vary from 0-200 kg/ha. Generally more emphasis is given to phosphorus and potassium fertilization.

From experimental work in Malaysia and Indonesia on many crops, it is agreed that nitrogen is required in quantity by all crops except legumes. A range of rates are specified but Tie and Kueh (1979) give the following rates of nitrogen application for oligotrophic lowland peats of South East Asia:

Vegetables including long beans, French beans, green pepper and chilli - 280 to 560 kg/ha, cucumber - 560 kg/ha.

Soya bean, groundnut and cowpea - 45 to 78 kg/ha.

Maize - 180 kg/ha (on shallow peat with 20 percent mineral matter).

Tapioca - 200 kg/ha.

Tobacco - 140 kg/ha.

Pineapple - 280 to 420 kg/ha (depending on variety).

Cattle manure or slurry is traditionally used as a fertilizer on pastures in the Netherlands. In Japan the application of farmyard manure or compost appears beneficial (Miyake 1982). In general, rice on peat soils receives rates of 40 kg/ha of nitrogen as it is assumed that the fertilizer acts as a starter in the initial growth stage and that large amounts of ammonium nitrogen will be released from the peat under submerged conditions (Miyake 1982).

In conclusion, the application of nitrogen fertilizers to crops grown on peat is dependent on a great many variable factors. For each situation and type of peat rates should be carefully assessed by trial. However, it is important to note that, without adequate nitrogen the response to other supplied elements will be small. This is particularly the case with oligotrophic peats.

8.6.5 Phosphorus requirements

The total phosphorus (P) content of tropical peat is generally low, but as is the case with nitrogen, we are more concerned with its availability. In oligotrophic peats in the tropics, available phosphorus content is generally larger than in upland soils. This is mainly caused by the low phosphate fixation experienced in these peats due to very low levels of Al and Fe. Most phosphorus is present in the organic form and upon mineralization this becomes readily available. Peats which have been drained and farmed for some time may increasingly develop phosphate-fixing powers upon decomposition, because of a relative accumulation of Al and Fe compounds in the mineral admixture. For this reason added phosphate may become partly fixed.

Available phosphorus values in peat soils are difficult to determine. This is an analytical problem inherent to the characteristics of organic soils. It is important to realize that each type of peat may require its own specific interpretation dependent on the extraction method used. The experience with oligotrophic peats in South East Asia indicates that phosphate requirements for most crops are not high. There can be considerable difference between deep and shallow peats. Perennials planted on peats less than 1 m thick may find their phosphorus source in the mineral subsoil. Type and depth of peat are therefore important variables.

General recommendations are again difficult to make because of the large variability in conditions and crop demand, for example, pineapple requires less than one tenth of the amount of phosphorus than it requires of nitrogen that is 14-28 kg/ha. Tapioca one quarter of the demand for nitrogen, thus 50 kg/ha, whereas vegetables require more than half the nitrogen requirement. Table 28 gives the general phosphorus recommendations used for organic soils in the USA.

Table 28 PHOSPHORUS FERTILIZER RECOMMENDATIONS FOR A RANGE OF CROPS BASED ON AVAILABLE SOIL PHOSPHORUS (source Lucas 1968)

ppm of available soil phosphorus 1				Phosphorus recommended (kg/ha)
			5	112
		5	10	90
		10	20	67
	5	20	30	45
5	10	30	40	34
10	20	40	50	22
20	30	50+	60+	17
30+	40+	-	-	11
Blueberry	Alfalfa	Cabbage	Broccoli
Buckwheat	Asparagus	Carrot	Cauliflower
Clover	Barley	Cucumber	Celery
Grass	Bean	Endive	Onion
Oat	Corn	Lettuce	Tomato
Rye	Mint	Parsnip
Soya bean	Pea	Potato
Pasture	Radish	Pumpkin
	Sudan grass	Spinach
	Turnip	Sugar beet
	Wheat	Table beet

¹ Extracted with 0.018-N-acetic acid, using one part air dried soil (by weight) with 10 parts of extracting solution

8.6.6 Potassium requirements

Most peat soils, particularly the oligotrophic types, are deficient in potassium (K). While much of the K found in peat soils is readily available, once it is used up, K deficiency becomes severe. Traditional subsistence farmers try to supply the required potassium for food cropping by burning. As is the case with nitrogen, insufficient drainage affects potassium uptake. Shallow drainage aggravates the deficiency, and responses to K fertilizer are good. Experience in temperate climates with most crops grown on peat soils indicates that potassium is the most important nutrient for crop production.

There are a number of important properties of potassium in relation to organic soils:

i. K fixation, which is noticeable in many mineral soils, is absent.

ii. Peat soils, although having a high cation exchange capacity, do not readily adsorb exchangeable K.

iii. A large proportion of the total available K is always present in the soil solution and is therefore strongly mobile and prone to leaching. Losses therefore, can be substantial particularly under waterlogged conditions.

Table 29 POTASSIUM FERTILIZER RECOMMENDATIONS FOR A RANGE OF CROPS BASED ON AVAILABLE SOIL POTASSIUM (source Lucas 1968)

ppm of available soil potassium 1					Potassium recommended (kg/ha)
				80	560
				200	448
			80	320	359
		80	160	440	269
	80	200	300	580	179
50	200	300	410	690	112
150	285	390	490	780	67
250	345	450	550	840	34
300	400	500	600	900	0
Barley	Bean	Alfalfa	Broccoli	Celery
Blueberry	Clover	Asparagus	Cauliflower
Grass	Corn	Cabbage	Onion
Oat	Mint	Carrot	Potato
Rye	Pea	Cucumber	Sugar beet
Pasture	Soya bean	Lettuce	Table beet
Wheat	Sudan grass	Parsnip	Tomato
	Sweet corn	Radish
	Turnip	Spinach

¹ Extracted with 1-N-neutral ammonium acetate (1 part soil to 20 parts extract)

Because of the high potassium content in both the ash from fruit bunches of oil palm and from plant debris of crops such as sugar cane, it is good practice to return these materials to the land as a source of potassium fertilizer. Such materials also raise the pH. The liberal use of dolomitic limestone for correcting acidity increases the need for potassium to counteract the effect of high levels of magnesium. In such cases purer forms of limestone should, if possible, be used for liming.

8.6.7 Calcium and magnesium requirements

Usually, calcium and magnesium are not deficient in tropical peats. Calcium deficiency is unlikely wherever the total Ca exceeds 0.5 percent. However, high levels of K, Na and/or NH₄ can induce Ca and Mg deficiencies. The Ca/Mg ratio can also influence deficiency, and even when the lower threshold value of 0.5 percent is surpassed, calcium deficiency can still develop. In coastal peats of South East Asia, where the magnesium contents are relatively high, this aspect is particularly important.

Experimental evidence indicates that the benefits from liming are more a result of the increase in pH and the de-acidifying effects of this than to rectification of any calcium or magnesium deficiencies. This is certainly the case with maize which requires a much higher pH than the usual value of 3.5 found in tropical oligotrophic peats. The relative growth of maize is therefore not a good indicator of the nutrient status of organic soils, although it is often used as such for mineral soils. Lim et al. (1973) advocate the use of grasses as indicators of nutrient status in organic soils though grasses can be insensitive to deficiencies in micro-nutrients.

8.6.8 Micro-nutrients or trace element requirements

Copper, iron, boron, manganese, molybdenum and zinc have been shown to be deficient in organic soils for many crops.

Copper

Copper (Cu) deficiency, especially in cereals, has been reported to occur on peat soils throughout the world and tropical peats are no exception. The reclamation disease found in oats and rye grown on freshly reclaimed peat soils in Germany and the Netherlands in the late nineteenth and early twentieth centuries is well known. A similar failure to produce grains was found to occur in Indonesia and Malaysia (Polak 1941; Coulter 1957; Driessen and Sudewo 1977). Remarkably, only wet rice appeared to be affected and not dry rice. The cause of sterility is still not fully understood. Driessen and Sudewo (1977) tentatively attribute it to the presence of certain organic compounds, notably polyphenolic lignin degradation products, which hinder directly or indirectly (through copper fixation) one or more essential enzyme-catalyzed carbohydrate transformations. The fact that dry rice seems to be unaffected even when growing on the same peat on which wet rice shows severe deficiency, coupled with findings from Japan where drainage tends to cure the sterility (Miyake 1982), may indicate that wet conditions somehow appear to be related to the release of such toxic organic compounds. Ennis and Brogan (1968), have shown that humic acids are likely toxic compounds.

In Malaysia, other crops as well as cereals, are prone to copper deficiencies; notably oil palm, sugar cane, tapioca and coconut (Kanapathy and Keat 1970). “Green die-back” commonly encountered in pineapple appears to be caused by Cu deficiency. The different response of plants to Cu is associated with the type of enzyme in the plant. Cereals respond to Cu because of the ascorbic acid oxydase which requires Cu in order to function in photosynthesis, whereas in forage crops the relationship between Cu and molybdenum content is important for livestock feeding. Molybdenum is taken up in toxic amounts when the Cu content is low and the molybdenum is higher than 3.0 ppm.

It is difficult to assess the need for copper by soil analysis. Much depends on the method of extraction and total copper values are poor indicators. The need for copper can be predicted, to an extent, by the Cu content of the foliage. Under deficient conditions the content of the plant is usually less than 6 ppm (Lucas 1982).

Copper deficiency can be corrected by several copper compounds but copper oxide and copper sulphate are mainly used in agriculture. Lucas (1982) recommends the use of about 10 kg/ha for low- and medium-response crops and 20 kg/ha for highly responsive crops. Oligotrophic peats in the tropics require larger quantities up to 35 kg/ha but this dosage will last for at least 5 years as the residual effect is good. The best method of application for wet rice is uncertain. Spraying of Bordeaux mixture (CuSO₄-solution) in the generative phase of the wet rice is however promising.

Iron

Iron (Fe) deficiencies arise in peats with a notably low Fe content. Its occurrence appears to depend partly on the crop. In many wet mineral soils Fe-content can become excessive due to reducing conditions but generally the oligotrophic peats in the tropics are so low in iron, particularly the centres of the peat domes, that iron deficiency is common in a range of crops including pepper, coffee, tapioca, grasses and legumes. Severe chlorosis is the common symptom. Iron deficiency can be overcome easily by foliar sprays of ferrous sulphate solution (0.5-1.5 percent w/w). Drilled iron sulphate at the rate of 50-100 kg/ha prevents chlorosis in cereal crops (Lucas 1982).

Boron

Boron (B) deficiency occurs in both alkaline and acid peats. It is commonly found in highly sensitive temperate vegetables such as cauliflower, beet and celery, but in the tropics boron deficiency is not so common. Alfalfa appears to be affected as also does oil palm and boron deficiency has been noticed in coffee (Tie and Kueh 1979). Deficiencies can easily be corrected by foliar sprays (sodium borate) of not more than 0.1-0.4 kg/ha of boron. Soil application of tetraborate effectively controls boron deficiency in oil palm.

Manganese

Manganese (Mn) deficiency in tropical peats is rare, because it very much depends on soil reaction. It is usually found only in eutrophic peats with a pH of over 5.5. Most tropical peats are oligotrophic, with a pH of less than 4.5, and frequently 3.5. Generally, manganese deficiency will therefore only occur after heavy liming. If this happens, it is easily corrected by applying manganese bearing materials as a foliar spray or mixed with other nutrients and broadcast at rates not exceeding 5 kg/ha of manganese. Another option is to acidify the soil with a sulphur compound.

Molybdenum

Molybdenum (Mo) deficiency is associated with soils of low pH (less than 5.5). Soils rich in free oxides are often deficient in available molybdenum. There are no reports on molybdenum deficiency on tropical peats in the literature studied. This may reflect the facts that tropical peats are commonly low in Fe and crops usually grown on peat soils are not Molybdenum-sensitive.

Zinc

Zinc deficiency is normally only apparent in soils with a pH greater than 6.5 or on peat soils which have been heavily limed. Large applications of phosphorus fertilizer and poor drainage can also induce zinc deficiency. As is the case with molybdenum there is no evidence that zinc deficiency is a problem in tropical peats.

8.6.9 Conclusions

Trace element requirements are most pronounced for copper (nearly always) and iron (frequently). Although levels of other trace elements present few problems there is little detailed information and it is possible that, when more tropical peat is brought under cultivation and the crop range is extended, deficiencies not observed at present may develop. The likelihood of such development will depend largely on liming practices and fertilizer use. Over-liming should be avoided, a pH of 4.5-5.0 is adequate for most crops. Nitrogen and phosphorus applications should not be excessive.

8.7 Crop Protection

8.7.1 Weed control
8.7.2 Pest and disease control

8.7.1 Weed control

Weeds enjoy the excellent growing conditions in peat soils and their abundance is a nuisance, particularly in freshly reclaimed peats. Weeds compete for space, light, moisture and nutrients and also act as hosts for pathogens, insects and nematodes. They reduce the quality and yield of crops and interfere with harvesting. Weed control therefore needs attention.

In temperate regions weeds are removed either mechanically or by herbicides. Manual removal is costly, but in some cases it is necessary because either the crop is too dense to allow machinery or herbicides affect the quality of the crop. This is particularly the case with vegetable growing where yields of leafy vegetables may be affected by broadleaf weedkillers. Some of the most effective weedkillers are highly toxic which influences not only the crops grown but also the broader environment. Peat absorbs chemicals readily and undesirable toxicity can accumulate. The choice of effective weed controls is thus very much for the individual farm manger to decide, because the economics of each particular technique depends on local conditions and crop.

In the tropics heavy weed infestation is also common. Wee, quoted by Tie and Kueh (1979), reports that weed infestation of pineapples on peat soils in Malaya decrease yields by 20-40 percent. For non-vegetable crops many weedkillers are effective but their use is often limited by economics. Crops on organic soils require larger applications of chemicals than those on mineral soils (2 to 3 times the amount given on mineral soils is not uncommon). The choice of the herbicide is important because many are short-lived. Pre-emergence types are more effective than incorporative ones.

In most tropical countries, even in Malaysia where labour costs are relatively high, it is still cheaper to carry out weed control by hand than by other means.

8.7.2 Pest and disease control

The control of soil-borne pests and diseases is particularly desirable on peat soils. In their natural state organic soils usually have low populations of anaerobic micro-organisms tolerant of the inherent acid conditions. Once the soils are drained, limed and fertilized an excellent medium is created for the rapid spread of new soil fauna and flora. Tie and Kueh (1979) indicate that many crops in the South East Asian peats are affected by fungal collar rot, root rot, white root and serious nematode attack. Bacterial wilt is common in crops such as chilli, tomato and ginger.

There are several methods of controlling soil-borne diseases and pests. Fumigation and sterilization by gasses or steaming are used in the Netherlands in intensive systems of horticultural cropping (vegetables, potplants and flowers). High costs are a disadvantage and steaming and some fumigants such as dichloropropene, methyl bromide, methyl isothiocyanate and chloropicrin destroy a large proportion of the soil micro-organisms responsible for supplying available nitrogen to the plant. More nitrifying bacteria are killed than the ammonifiers and therefore a build up of ammonia can occur after fumigation, which retards some crops, particularly vegetables.

Crop rotation, including a clean fallow, can be effectively used against soil-borne pests such as nematodes. The use of flooding to combat soil pests was studied in Florida (Genung 1976) with remarkably good results. Table 30 illustrates the effect flooding had on important arthropods and oligochaetes. Both prolonged clean fallow and flooding caused evident reductions of populations of all the organisms including such predatory forms as Carabidae, Dermaptera, Formicidae and Chilopoda. Flooding under the 4-2-4 weeks alternation shows a much larger reduction of both soil pests and predators as well as oligochaetes than did either of two clean fallow treatments.

The key factor in controlling pests by flooding appears to be the length of inundation. Soil-borne pests and diseases in tropical peats can probably be effectively controlled by flooding. Water control is one of the most important aspects of the agricultural management of peat soils and it is therefore often possible to artificially create floods to control pests. It is relatively cheap, highly effective, also eradicates some weeds, and unlike fallow systems no land has to be taken out of production.

Table 30 INFLUENCE OF FLOODING AND CLEAN FALLOW ON ERADICATION OF MAJOR SOIL-BORNE PESTS (source Genung 1976)

CWW = Corn wireworm	CW = Cutworms	Cbd. = Carabidae	Swbg. = Sowbugs
SPWW = Sou. potato wireworm	Cyd. = Cydniade	Dmpa. = Dermaptera	Pred. Dip. = Predatory Diptera
W.Gbs. = White grubs	Ten. = Tenebrionidae	Cent. = Centipedes	Olig. = Oligochaetes

¹ White grub (Bothynus subtropicus Blatchley) mortality percentages under simulated flooding in a replicated and randomized laboratory trial was as follows: 0 hrs flooded (check) 5%, 24 hrs flooded 7.5%, 48 hrs flooded 25%, 72 hrs flooded 65%, 96 hrs flooded 95%.