Practices for raising Prosopis plantations in saline soils - Gurbachan Singh

Central Soil Salinity Research Institute, Karnal 132001, India

Introduction

High soil salinity and alkalinity have degraded about 8.5 million ha of once productive land in India. The severity of the problem is increasing, and several million ha of the canal irrigated area in the arid and semi-arid regions of the country run the risk of being degraded. Increasing human and livestock pressures on available land necessitates protracted efforts to reclaim lands which are already degraded, and to prevent their further degradation. Providing vegetative cover to such lands with suitable woody and herbaceous species can put the these lands to optimal use and also increase the forest cover of the country. The Central Soil Salinity Research Institute, Karnal, has found that trees of the genus Prosopis (mesquite) can be used for the afforestation of salt affected lands. These trees are highly salt tolerant and excellent biomass producers. The technology has been developed for their cultivation in problem areas. This paper deals with various aspects of Prosopis cultivation for the rehabilitation of salt-affected lands. The most widely cultivated species of Prosopis in the world are; P. juliflora, P. alba, P. chilensis, P. cineraria, P. pallida and P. tamarugo.

The nature of salt affected soils

Salt affected soils differ from normal soils in respect of soil reaction (pH) and soluble salt content. Visually, they are recognised by the presence of a white or greyish-white efflorescence of salts on the soil surface during dry months. Commonly they are devoid of good natural vegetation. Being poorly drained, water stagnates on the surface for long periods. Indicator plants of these soils include: Prosopis juliflora, Acacia nilotica, Capparis aphylla, Salvadora persica, Butea monosperma, Sporobolus spp., Desmostachya spp., Suaeda maritima, Kochea indica, Leptochloa fusca, Cynodon dactylon and Bracharia mutica.

Salt affected soils in India are broadly classified into groups of alkaline or saline soils. Soluble salts in alkali soils are mostly carbonates and bicarbonates of sodium. The exchangeable sodium percentage (ESP) of these soils often exceeds 15%. In barren alkali soils the exchange complex may be largely occupied by sodium ions, and the presence of large amounts of exchangeable sodium dispersed in the soil resulting in their poor physical condition. The presence of sodium carbonate and hydrolysis of exchangeable sodium increases the soil pH, which in highly deteriorated soils may be as high as 10.5. Alkali soils are known as ‘Usar’ in Uttar Pradesh and ‘Kallar’ in Punjab and Haryana. The factors inhibiting tree growth in alkali soils are:

a) A high pH throughout the profile which causes problems of nutrient availability.

b) A highly deteriorated soil structure, with poor water transmission characteristics leading to water stagnation and reduced aeration of roots.

c) A hard calcium carbonate layer at about one metre depth in the profile acting as a physical barrier for the vertical penetration of tree roots, although the location of this layer in the profile and its thickness varies in different soils. Often compact sub-surface horizons also restrict root penetration in alkali soils.

Saline soils contain an excess of neutral soluble salts, generally chlorides and sulphates of sodium, calcium and magnesium. Saline soils rich in such divalent cations have low ESP and pH and a good physical condition. Owing to the flocculating effect of neutral salts, saline soils are very permeable and can be reclaimed by leaching with good quality water provided the ground water table is deep. The factors inhibiting tree growth in saline soils include:

a) The salinity induced high osmotic pressure of soil water.

b) The toxic effect of specific ions.

c) Nutritional disorders occur due to competitive uptake of ions.

d) A high water table and therefore regular and/or prolonged water logging is associated with such soils.

e) The ground water found in saline areas is often of poor quality, while fresh water is scarce.

Tolerance of Prosopis

Prosopis is moderately tolerant to alkali conditions (Table 1), although growth may be restricted at very high pH (pH >10.0). In such situations special site preparation techniques and amendments (soil applications) may be needed for raising plantations. Recent greenhouse studies show that the nearly thornless and more erect formed Prosopis alba clone B₂V₅₀ is as tolerant to high pH soil as the naturalised P. juliflora. Prosopis spp. can grow in soil salinity regimes equivalent to sea water. Workers in Texas A&M University, Kingsville, U.S.A. found that all species of Prosopis tested tolerated 6,000 mg/1 salinity with no reduction in growth. P. velutina tolerated 12,000 mg/1 salinity, while P. articlulata, P. pallida and P. tamarugo grew successfully in salinity levels of 18,000-36,000 mg/l. Satisfactory growth of P. juliflora could be expected in soils with an electrical conductivity (EC) of 27 ds/m. All the species which grow in high salinity can fix nitrogen and thus have potential as nitrogen fixing halophytes.

Table 1. Performance of P. juliflora in alkali soils.

Prosopis plantations in alkaline soils

In order to facilitate rooting through the hard sub-surface layers, the augerhole technique has been developed for growing Prosopis in high pH soils, which involves the digging of pits (20-25 cm diameter and 100-130 cm deep) with tractor mounted diggers. These pits are refilled with a mixture of the original alkali soil, 3 kg gypsum and 8 kg farm yard manure, before 3 month old Prosopis seedlings are planted in these pits. Approximately 20 g zinc sulphate and 10 g B.H.C. powder are also mixed in the filling mixture. Four irrigations are given after planting at an interval of 4-5 days during the first month. Planting is generally carried out from July to September, and this technique results in >90% survival on highly alkaline soils where nothing else could be produced. In a field trial, growth and biomass production after 6 years was significantly higher when planted by this augerhole technique than by the traditional pit and trench planting methods (Table 2).

The amount of amendment is determined by the volume of soil extracted from the augerhole and the chemical status of the soil. Gypsum, pyrites, farm yard manure, molasses, sulphuric acid and even rice husk have been used for mixing with the alkali soil before refilling the pits/holes. However, maximum survival, height, girth and biomass of Prosopis was obtained when 3 kg gypsum plus 8 kg farm yard manure were used per augerhole (Table 3).

In experiments conducted from 1985 to 1988 at CSSRI, Karnal it was found that during the first two years, growth was far better when irrigated than in plantations which depended on rainfall alone. The nature and growth habit of the tree species and the purpose for which a plantation is raised, determine the spacing at the time of planting. Close spacings allow for the risk of high mortality so common in salt affected soils. In a soil of 10.4 pH, maximum biomass was obtained with a 2 x 2 m spacing (Table 4). Lopping is a necessary practice in Prosopis plantations, all side branches should be removed up to 1 m of stem height at the first lopping, and to 2 m above ground level in the second. As a rule, about one third of the stem above ground level should be kept branch free, but lopped only when the tree is in the dormant stage.

Table 2. Response of P. juliflora to 3 planting techniques in an alkali soil after 6 years.

Table 3. Response of P. juliflora to amendments in an alkali soil after 6 years.

Root growth

Root growth of P. juliflora was studied in relation to different methods of site preparation and amendments used. Seedlings were planted in pits of 30 cm³, trenches of 30 cm wide and 30 cm deep dug across the plots, augerholes of 15 cm wide and 90 cm deep, and augerholes 15 cm wide and 15 cm deep after mixing gypsum in the surface 10 cm soil of the whole plot area. The root systems were exposed after two years, and it was found that Prosopis roots were mainly in the soil mass amendment mixture in the augerhole and roots which grew beyond the hard pan were close to the water. Horizontal lateral root growth was however, quite poor. With trench planting, the majority of roots remained in the amendment treated trench soil and followed a horizontal path, instead of growing vertically, with no root penetrating beyond a depth of 30 cm. Such a shallow root system can expose the plant to strong winds and moisture stress. In the pit planting method, the whole root system was confined to the pit soil and only one or two roots reached the hard pan. Where gypsum was mixed in the whole surface soil, as is done for growing crops, roots were spread uniformly in the whole amendment treated surface soil. Root extension downwards was lacking, thus site preparation for planting Prosopis in alkali soils is more important in the vertical, rather than in the horizontal direction. The augerhole method allows roots to breach hard sub-surface layers and as roots reach the lower less deteriorated and coarser soil depths, they proliferate there and boost the above ground biomass production.

A Prosopis based silvopastoral model

A silvopastoral model comprising of P. juliflora and Leptochloa fusca grass has been found to be most promising in terms of firewood and forage production, and soil amelioration. Leptochloa fusca in association with P. juliflora planted at a spacing of 3 m x 5 m produced 46.5 t/ha green fodder, in 15 cuttings over a 50 month period without the use of fertilisers or soil amendments. About 80 t/ha air dried wood was produced by the P. juliflora after 6 years (Table 4).

Table 4. Biomass production by P. juliflora and Leptochola fusca grass under 3 different tree spacings, on an alkali soil after 6 years.

This silvopastoral system improved the soil to such an extent (Table 5) that it was possible to plough under the Leptochloa fusca grass after 4 years and grow less tolerant but more palatable fodder species such as Trifolium resupinatum, Trifolium alexandrinum and Melilotus parviflora under the P. juliflora trees. The green forage yield of these inter-crops were comparable to their yields in normal soils. The highest yield of 23.1 t/ha was obtained with Trifolium resupinatum (shaftal), followed by 21.3 t/ha with Trifolium alexandrinum (Berseem).

Prosopis plantations in saline soils

Unlike alkali soils there is no need to apply amendments or make deep augerholes for raising Prosopis seedlings in saline soils. Planting in channels gives improved results over flat planting and ridge planting methods, where seedlings are planted in a 30 cm deep channel, which is also used for irrigation. This planting technique ensures an improved micro-edaphic environment for the seedlings, as most soluble salts are deposited at the higher elevations/ridges with evaporation. Application of phosphorus and zinc aids growth in saline soils. Frequent irrigation with good quality water is highly beneficial for raising plantations in saline soils as this provides a low salinity environment to the young roots. When good quality irrigation water is not available, saline ground water can also be used in conjunction with available fresh water, but the electrical conductivity of the saline water cannot exceed 20 ds/m.

Biomass production in high density plantations

P. juliflora planted at spacings of 1 x 1 m and 1 x 2 m in 1985 under irrigated conditions, after applying 15 t/ha gypsum and harvested after 7 years had an above ground biomass (stem + branches + leaves) of 39 kg/tree at a density of 5,000 plants/ha and 32.2 kg/tree when the planting density was 10,000 plants/ha (Table 6). Growth and biomass production of P. juliflora was compared in 10 year old plantations with Acacia nilotica, Casuarina equisetifolia and Eucalyptus tereticornis in a soil whose original pH was 10.4. Mean plant height attained in 10 years was maximum in E. tereticornis followed by C. equisetifolia, P. juliflora and A. nilotica. Girth growth was better in P. juliflora and A. nilotica plantations (Table 7). Tentative biomass production in 10 years was 260 t/ha from P. juliflora, 215 t/ha from A. nilotica and 188 t/ha from C. equisetifolia and E. tereticornis.

Table 5. Effect of P. juliflora - Leptochloa fusca silvopastoral system on alkali soil properties after 6 years

Table 6. Performance of P. juliflora in high density energy plantations on highly alkali soil.

Table 7. Growth and biomass production of 4 tree species in a highly alkaline (pH 10.4) soil after 10 years.

Litter production and its effect on soil

P. juliflora litter falling on the ground adds to the humus content of salt affected soils. The organic acids from the decomposed litter react with the calcium carbonate in the soil and releases calcium which substitutes for sodium in the exchange complex and thus Prosopis helps in the reclamation of alkali soils. Stands of P. juliflora can yield 5-8 t/ha of air dried leaf litter after 4-6 years, containing 2.2% nitrogen, 0.2-0.4% phosphorus, 1.5-1.9% potassium and sodium content generally less than 0.2%. The annual turnover of macronutrients to the soil through litter would be 88-132 kg N/ha, 8-16 kg P/ha and 60-76 kg K/ha. Such nutrient additions through litter fall will raise the fertility status of salt affected soils which are otherwise deficient in essential plant nutrients. P. juliflora helps reclaim salt affected soils more effectively than other trees such as Acacia spp., Eucalyptus spp., Terminalia spp. and Albizia spp. of the same age and stocking rate. The long term effects of P. juliflora on highly alkaline soils are presented in Table 8.

Economics of raising Prosopis plantations

Planting 1 ha of alkali land with the augerhole technique at a planting density of 1,250 plants/ha, will cost approximately Rs. 12,600. The net income/ha/yr, based upon an 8 year rotation has been calculated to be Rs 8175 for alkali soils and Rs. 3587 for saline soils (Table 9). In addition to these monetary gains, Prosopis growth on saline soils results in their amelioration to such an extent that they can be used for arable farming after removing the trees.

Table 8. Ameliorating effects of plantations of 5 tree species on the pH and organic carbon (O.C.%) of an alkali soil after 20 years.

Table 9. Economics of raising P. juliflora plantations in saline/alkaline soils (US$1=Indian Rs.35 approx).

Planting materials and their handling

Seeds and seedlings are used for raising Prosopis plantations. In saline soils, seedlings are preferred over direct seeding because seeds, being sensitive to salt stress at the germination stage, either do not germinate at all or show stunted growth. Owing to the high sugar content of pods and the presence of a hard endocarp around the seed, separation of the seed is difficult. Small quantities of seed can be separated undamaged by using a pair of pruning shears and a pocket knife, although this method is time consuming, but workers at Texas A&M University, Kingsville, U.S.A. have developed a power operated machine. Scarification of seeds is required to break dormancy for rapid germination. The most effective method is to hand scarify the seed by nicking the distal end of the seed with a sharp blade. This produces over 95% emergence within 4 days, with a sowing depth of 1.5 cm and a temperature of 30-35^oC. P. juliflora seeds remain viable for more than 10 years at room temperature. The other common method is to feed entire pods to goats or sheep, seeds ejected by these animals after rumination being collected for sowing. To break seed dormancy, broken pods can be soaked in 95% sulphuric acid for 15-20 min and washed in tap water, giving 80-90% germination in 4-6 days.

As far as possible, nurseries should be located where the soil is of good quality. A nursery mixture comprising of good fertile soil, well rotten farm yard manure and silt in a 3:1:1 ratio should be prepared, and put into perforated polythene bags (11 x 22.5 cm). Scarified seeds can then be sown in February, but generally in September, and the seedlings develop quickly, attaining a height of 40-50 cm in 4-5 months.

Prosopis germplasm at CSSRI, Karnal

As a part of a Prosopis improvement programme, a large scale collection of seeds of superior trees is being obtained from both within India and from abroad. Mother plants of 20 Prosopis spp. have been raised in the CSSRI greenhouse, and a seed orchard has been established at the institute which will produce seeds in 3-4 years. The species planted in the orchard are; P. alba, P. articulata, P. chilensis, P.cineraria, P. glandulosa, P. juliflora (3 Indian accessions), P. tamarugo, P. velutina, and P. spp. (accessions PF 0450, PF 0457 and PF 1117). Most Prosopis species have thorns, a bushy architecture and slow growth rates. These characteristics hamper their widespread cultivation. In fact, although P. juliflora is highly tolerant of both saline and alkali conditions, its cultivation in India has not been popular on account of its prominent thorns and bushy growth, while P. cineraria is an ideal plant for the rehabilitation of desert lands but is very slow growing. There is an urgent need to modify and improve the undesirable features of Prosopis spp. to suit different situations, and in this context the following approaches are suggested:

a) selection of single stemmed plants in the field, and the collection of seeds from such plus trees, used to establish seed orchards,

b) methods need to be standardised to multiply superior germplasm through asexual methods, notably from stem cuttings as has already been achieved in the U.S.A. and India,

c) the use of inter and intra-species grafting to selecting root stock-scion wood for planting in different situations, for example in India, superior plantations could be raised by grafting fast growing, erect and thornless P.alba scions onto salt/alkaline tolerant P. juliflora rootstocks,

d) plant types can also be improved by manipulation of silvicultural practices such as planting at closer spacings to promote upward rather than spreading growth forms with the periodic removal of side branches producing a longer, clearer bole.