Chapter 9
EXPERIENCES IN ESTABLISHING VEGETATION ON ACID SULPHATE POND DIKES

by

H.L. Cook, Umpol Pongsuwana,
L.D. Rajamanickam and Ramli bin Khamis

1. INTRODUCTION

Aquaculture ponds at the Brackishwater Aquaculture Research Centre, Gelang Patah, Johor, Malaysia have been constructed on acid sulphate mangrove soil. Problems connected with the acid sulphate soils have resulted in low pond production. The primary problem is that when it rains acid is produced on the dikes and runs into the ponds. Entry of the acid water (pH less than 3) carrying large amounts of iron sets in motion a series of complex events which result in mortalities to the cultured species. The problem is not unique to this station as problems with ponds constructed on acid sulphate mangrove soil occur in many places (Ruddle, 1982; Singh, 1981; Ti and Rajamanickam, 1981). The problem at this station is perhaps more acute since the dikes are large: the height of the main dike is 2.5–2.8 m above pond water level, while that of interior dikes is around 1.5 m. The dikes are constructed of heavy clay. Although a number of dead mangrove roots are present, organic matter in the soil itself is only 5 to 6 percent by weight. Soil pH is in the range of 1.5 to 3.5. There is about three to five percent iron and exchangeable cations are low (see Chapter 2 of this report).

The station's pond dikes are essentialy bare of vegetation. Some plants have started spontaneously in a few places. The most common are sedges. Of these the most abundant is lesser fimbristylis which grows in large clumps right at the water's edge. Most of the vegetation other than sedges became established in places where dead mangrove roots and other debris were burned. A list of the most common plants is given in Table 1.

Table 1

TYPES OF WILD VEGETATION GROWING ON ACID SULPHATE DIKE SOIL

One of the methods suggested to improve the harmful effects of acidic rain water runoff is to establish vegetative cover on the dikes (Potter, 1976; Camacho, 1978). One way this helps is by preventing erosion. When erosion is stopped, the weathering process would rapidly reduce acidity of the surface soil. It is probably not necessary to completely raise pH to neutrality in order to obtain significant benefits. It may only be necessary to raise pH to the point where iron is no longer soluble as many of the problems experienced, especially with shrimp, are iron-related. Potter (1976) and Camacho (1978) suggested planting African star grass (Cynodon plectostachyus) and Bermuda grass (C. dactylon). Stocks of these and digit grass (Digitaria decumbens cv transvals) were obtained from the Institute Haiwan, Kluang, Johor.

For the initial planting, strips about 25 cm wide were tilled by hand along the sides of the dike. The strips were laid diagonally, 2 m apart in a cross-hatch pattern. Lime was added to the tilled strips at a rate of 8 t/ha. Two weeks after liming the three grasses and a ground cover (Erecthites valerianifolis) were planted by sprigging.

At first the vegetation grew well, especially the digit grass. After a few months, however, the leaves of all the plants started to turn reddish brown and the plants began to die. Application of additional lime and fertilizers did not help. Eventually, there were only a few patches of vegetation left, mostly near the water's edge. Visiting experts from the Universiti Pertanian Malaysia observed that the soil was too hard and dry for new shoots and runners to penetrate and become established. Realizing then that perhaps soil acidity was not the only factor limiting growth of vegetation on the dike an investigation was started to find out more. A review of the important factors follows.

2. FACTORS AFFECTING GROWTH OF VEGETATION ON THE DIKES

Soil acidity: Acid soils usually are not productive for several reasons. High levels of iron and aluminium can be toxic. Levels of exchangeable cations (calcium, magnesium and molybdenum) might be too low. There is more leaching of nutrients than in non-acid soils (Donahue et al., 1971).

Available water: The wilting point is the point at which soil moisture content is so low that plants cannot extract water. Water in the zone between the wilting point and field capacity is available to most plants. When moisture content exceeds field capacity it is available only to water-loving plants. When soils become saturated plants generally cannot live because of lack of oxygen. The wilting point for clay soil is 14.7 percent and the field capacity is 22.6 percent (Donahue et al., 1971). If soil moisture content falls outside that range most vegetation cannot grow well.

Rainfall: It is not only the frequency of rain which is important; the type of rain is also significant. Light rains of long duration supply water which can be taken up by the soils gradually. Heavy rains from short squalls usually run off rapidly with little water seepage into the soil. Rainfall in Gelang Patah is usually in the form of short violent thunder storms.

The larger raindrops in most short storms also contribute to rapid runoff. A raindrop falls at a speed of about 27 km/h. When large raindrops hit clay soil they beat it into a following mud which seals the pores and cracks. The result is that less water enters the soil. When the surface layer of mud dries it forms a soil crust. This crust hinders the emergence of seedlings and penetration of new roots from grass runners. Formation of crusts is very serious on acid soil high in silt and clay which contain little organic matter (Donahue et al., 1971). It has been observed that even after a rain of one or two hours, soil just below the surface is still dry.

After a crust forms, this usually cracks into small pieces after only a short period of drying. During the next storm the rapid water flow over the surface washes these small pieces off the dike into the pond. Thus, the soil surface never has a chance to leach. Also, the dry soil particles have been fully oxidized and so exert maximum effect on lowering the pH of the pond water.

Organic matter. Soils containing a large amount of organic matter have a high available water capacity due to the water-absorbing capability of the organic matter. Thus soils with a lot of organic matter can withstand dry periods for a longer time than those with little organic matter. There are additional benefits to a high content of organic matter. Coarse organic material on the surface reduces the effect of falling raindrops and allows clear water to seep into the soil. Surface runoff and erosion are reduced and there is more available water for plant growth. There are no crust formed. Loss of water due to evaporation is reduced. Bacterial decomposition of the organic matter produces slimes and gums which contribute to a desirable soil structure (Donahue et al., 1971).

Slope: Steep slopes cause water to run off rather than infiltrate the soil. Erosion is increased.

Salt: pond water infiltrates the dikes. For brackishwater ponds this means the soil at and below the pond water level will be saline.

Transpiration: Loss of water from the surface of the plants themselves is an important source of water loss to the soil. This is particularly true on pond dikes which are subject to the direct force of wind coming off the ponds. On the dikes at the Centre wilting usually occurs by midday when the sun shines.

3. EXPERIMENTATION AND OBSERVATIONS

As described earlier, newly planted vegetation usually grows well at first and then dies back. It was hypothesized that the initial preparation makes the soil suitable for plant growth, but then conditions change. As the plants grow more water is required. Due to the increased uptake of water by the plants the percentage of water in the treated surface layer drops below the wilting point. This causes two things to happen. The plant roots try to penetrate deeper into the dike where conditions are unfavourable (low pH and waterlogged condition); at the same time there is movement of water from the moist deeper zone into the drier surface layer. The low pH and high iron content of this water makes it toxic. The result is that the plants are deficient in water and exposed to toxic forms of iron and aluminium. The leaves turn red, wilting occurs, and eventually the plants die.

In order to test this hypothesis, several experiments were conducted: addition of MnO₂ to counter the effects of iron; planting grass in plastic tubs with soil which had received various treatments; measurement of percentage moisture in the dike soils and adding an organic much to increase the water-holding capacity of the soil.

Addition of MnO². Rice production in flooded acid sulphate soils was found to be much higher when MnO² was added with lime, and it was postulated that the MnO² counteracts physiologically the toxic effects of iron in water satured soil (Ponnamperuma and Solivas, 1981). Experimental plantings were made to test whether or not addition of MnO² would have any effect on growth of vegetation on the station's dikes. A terrace was made about 0.5 m down the side of a 1.5 m high dike. Soil on the terrace was tilled by hand and wood ash applied at a rate of 8 t/ha; MnO² was added to one section at a rate of 100/kg/ha to a portion of the terrace. Sprigs of buffalo grass, carpet grass and Erecthites valerianifolia were planted in both sections. Growth of all three plants was markedly greater on the portion to which MnO² had been added. Growth of buffalo grass was exceptional; it completely covered the side of the dike, from the water line to the crown. Also, the colour of the leaves was deep green, while that of those without MnO² had reddish streaks.

About six months after planting a prolonged dry spell occurred. During this time all the grass died back. When rains started again the vegetation grew slowly. This time no difference could be seen between the vegetation planted on the different sections. Evidently the benefit of the MnO² had been exhausted.

Planting in plastic containers. Two experiments were conducted to grow grass in 30 cm diameter plastic basins. Soil was taken from one of the pond dikes and the basins were filled to a depth of 10 cm. Initial soil pH was 2.6.

In the first experiment 20 basins were used: 10 had holes punched in the bottom for drainage and 10 did not. Soil in the basins was treated with varying combinations of lime (8 t/ha), wood ash (8 t/ha), chicken manure (5 t/ha), inorganic fertilizer (1 part 15:15:15 and 1 part urea at 350 kg/ha) and MnO₂ (100 kg/ha). Seven sprigs of buffalo grass were placed in 16 containers, digit grass in two and carpet grass in 2.

The plants were not watered regularly during this experiment. The basins were uncovered and water was supplied by rain. The soil did dry at times and many of the plants died. Consequently, all that could be concluded was that without lime or wood ash all plants died.

During a second experiment lasting 4 months, the plants were watered daily. Two types of grass were used; buffalo and digit grass. Seven sprigs of each grass were planted in three basins for each soil treatment, except for the control which had only one basin. Soil treatment was as follows: no treatment; lime; lime plus manure; lime plus manure plus MnO₂. Rate of application was the same as in the previous experiment. In this experiment all the grass grew well, even those in the untreated soil. The grass had to be trimmed back to about 10 cm on three occasions to prevent the runners from taking root outside the basin. At the termination of the experiment soil pH of the control was 3.0 and 3.6. That of the other treatments varied from 6.7 to 7.3. A small difference could be observed between the controls and the other treatments. The other treatments were all similar, their leaves were green and had very little red colour.

Plants in the untreated control were smaller than the others, but they also had very little red colour. Root growth was good, there were heavy mats of roots in all the containers. These results indicate that iron toxicity alone is not responsible for the grass dying back on the dikes, but that addition of lime or wood ash does help and that sufficient water is essential. It is not surprising that no effect was observed due to the addition of MnO₂. All the grass grew well; the MnO₂ is only effective in water-logged soil, and soil in all the containers was well drained.

Measurement of percent moisture in dike soils. To determine the percentage moisture in the dike soil, samples were taken from just below the surface and at different depths below the surface. The samples were dried in an oven at 105°C for 24–36 hours. The following calculation was used.

Two sets of observations were made. The first was a more or less random collection after a dry spell. Results (Table 2) showed that there was a great difference between surface and deeper samples. Moisture content of some of the surface samples was below the wilting point and that of some of the deeper samples was above field capacity. There were also intriguing differences in pH and iron content.

These results led to the decision to take a second series of samples. Three sites were selected. The first was on the top of a dike in a place with no vegetation. The second was on top of a dike on which grass had been planted. The soil at this location had been tilled and wood ash and MnO₂ applied about one year earlier. During sampling it was noted that the grass roots were heavily mattted near the surface, with very few extending below 10 cm and none below 20 cm. The third site was on top of a dike where a dense patch of wild grass had been established for at least three years. Here, the roots extended more or less evenly to 30 cm and some went to 40 cm. There had been a light rain the day prior to this sampling. Results of analysis are given in Table 3.

Table 2

PERCENT MOISTURE, pH AND IRON CONTENT OF DIKE SOIL

Table 3

SOIL MOISTURE, pH AND IRON CONTENT
OF DIKE SOIL AT FOUR DEPTHS

In the first sample there was great difference between iron content of the soil under vegetation and bare soil. This difference was even more pronounced in the second series of samples where depth was considered. The sample from under the well-established grass where the roots extended to 40 cm had very little iron content at all depths sampled. The sample from under the newly planted grass had a low level of iron in the top 10 cm in which the roots were heavily matted and a high level below that. The samples from the area with no vegetation had a high iron content at all depths. It appears that the vegetation is taking iron up from the soil. It had been noted that when grass on the dikes was burned it left a reddish residue.

The differences in pH are of interest. There is a good relation between low iron content and higher pH values. Water content in the surface zone was close to the wilting point even though it had rained the previous day.

Conditions on the dike appear to be as follows. Roots of newly planted vegetation are restricted to a narrow surface layer in which the moisture content and quality are suitable for growth and survival. At times water content in this zone falls to below the wilting point due to evaporation and transpiration. Oxygen would be limiting at the high levels of moisture present at depths and iron would assume the Fe⁺⁺ state which is toxic to many plants. At times of low moisture in the surface zone the plants take up toxic amounts of iron, causing the leaves to turn red and die back. During the next moist period the plants, if still alive, grow back. The intermittent penetration of subsurface layers by the roots adds organic matter and removes iron thus gradually conditioning the soil.

Addition of organic matter. Test plantings were made on the pond dikes to determine if addition of organic material to increase water-holding capacity of the soil would be beneficial. It is difficult to obtain straw in this area, and sawdust - readily available from local sawmills - was used. It did not make a good surface mulch because it washed away too easily. It must be worked into the soil. Large amounts of seaweed were cropped from one of the station ponds and after it dried that was used also. The seaweed could be used on the surface or incorporated into the soil.

For the test plantings a layer of mulch about 5 cm thick was used on some areas and left off other areas. All other soil treatment was the same. A definite benefit was obtained from addition of the organic matter. Growth in sections without mulch was only about half that in areas with mulch. However, during dry spells wilting was observed even on the portions which had received organic mulch.

4. RECOMMENDATIONS FOR PLANTING

From the preceding discussion it is evident that dikes constructed from acid sulphate clay soils are hostile environments. To make them more suitable for growing vegetation it is necessary to reduce soil acidity and increase availability of good quality water. Numerous plantings were made with varying degrees of success. The following procedures are recommended based on the results obtained at this station.

4.1 PLANTING AT WATER LEVEL

For soils adjacent to the water there is little problem. Salt-tolerant plants can be grown without any soil preparation. Two types of grasses found in mangrove adjacent to the station were tried. Water couch grass, Paspalum vaginatum, grew most rapidly. Small sods planted 30 cm apart just above water level form a mat around the edge of a pond. It grows to a height of about 15 cm. Shoots from this plant extend down into the pond water. Siglap grass, Zoysia matrella, is a shorter, slower growing grass, but it forms a thicker mat and did not extend into the pond. Sea purslane (Sesuvium portulacostrum) was also tried. This grew very slowly and at the time of this writing did not appear to have as much value as the grasses. The sedge, lesser fimbristylis, grows abundantly right at the water's edge but is not as effective in preventing erosion of dislodged soil particles into the ponds as the grasses. Both of the grasses served well in trapping eroded soil particles before they could enter the pond. They also protect against erosion by wind waves. An important factor is that they only grow to about 20 cm above the pond water level. Thus, different plants must be grown higher on the dike if the dike is higher than that.

4.2 PLANTING HIGH ON THE DIKE

It is not as easy to establish vegetation high on the dike. The procedures described below were used with mixed results. Grass planted on the crowns of the dikes has established itself in some areas. However, it remains restricted to the tilled and treated portion. It has not become established on the sides of the dikes. During dry periods shoots extend outwards, but after even short dry periods, wilting occurs and the plants die back. It is hoped that with time the initial plantings will spread as the soil becomes further conditioned by the plants themselves.

Till the soil: Tilling the soil breaks the surface crust and permits water to enter the soil. Breaking of the soil into small pieces facilitates penetration of lime and fertilizer. It is much easier for roots to penetrate and become established in loose soil.

Add lime: Liming reduces the harmful effects of aluminium and iron. It improves the physical condition of the soil which results in increased infiltration of rain water. It adds calcium and makes phosphorus more available. It encourages growth of soil bacteria which help to condition the soil.

Wood ash is a satisfactory substitute for lime. In addition to supplying an amount of calcium carbonate equivalent to lime it provides phosphorus, potassium, manganese and other plant nutrients.

Add organic matter: Organic matter is added to reduce reconsolidation of the soil after tilling and to improve water holding capacity. Sawdust is a good material to absorb moisture. Depending on its texture, sawdust is capable of absorbing 2 to 5 kg of water per 1 kg. Sawdust binds soil nitrogen and it is necessary to add 25 kg of nitrogen per 1 t of dry sawdust (Donahue et al., 1971). In the station experiments it was found that it is better not to use sawdust as a surface mulch since it tends to wash away. It is better to work it into the soil. A straw mulch could be used on the surface.

Add manganese dioxide (MnO₂): Experiments on the dikes showed that addition of MnO₂ at a rate of 100 kg/ha has a beneficial effect on growth of grass. It is thought that the MnO₂ counteracts physiologically the toxic effects of iron in water-logged soil.

Reduce surface run-off: On small dikes this can be accomplished by machine-tilling a row down the centre of the dike. The loose earth is then raked to each side so that a trough is formed to catch rain. This will not only increase water availability, but reduce loss of nutrients and erosion. On a wide dike a row is tilled along each edge of the crown. Loose soil is then raked toward the edge to form a small mound to prevent rain run-off. On high dikes it may be necessary to establish a terrace about halfway up the side.

In order to plant on top of the dike, the soil is broken up with a power tiller. This is best done after a period of rain when the soil is moist. The loose soil is raked to the sides and lime or wood ash applied to the tilled portion at a rate of 4 t/ha. This is left for two weeks to give the carbonate a chance to react with the soil. A second application of lime or wood ash (4 t/ha) and MnO₂ at rate of 100 kg/ha is applied to the area from which the loose soil had been raked. A 4–5 cm layer of sawdust or other mulch is then laid down and the soil tilled again. A week later the grass is planted. The grass establishes itself much faster if small sods are planted. After the grass has started growing well a top dressing of fertilizer can be added. An application of 50 kg urea, 200 kg superphosphate and 200 kg K₂O per hectare three to four times a year should be adequate. It is beneficial to keep the grass cropped to around 10 cm in order to reduce water loss due to transpiration. Digit grass and buffalo grass were the most successful of those plants which were tried for growth high on the dikes.

REFERENCES

Camacho, A.S. 1977 Implications of acid sulfate soils in tropical fish culture. In Joint SCSP/SEAFDEC Workshop on Aquaculture Engineering, edited by R.H. Gedney, SCSP, Manila, Vol. 2:97–102

Donahue, R.L., J.C. Shickluna and L.S. Robertson. 1977 Soil. An introduction to soils and and plant growth. (Third edition). Englewood Cliffs, New Jersey, USA, Prentice-Hall, Inc., 587 p.

Ponnamperuma, F.N. and J.L. Solivas. 1981 Field amelioration of an acid sulfate soil for rice with manganese and lime. In Proc. Bangkok Symp. On Acid Sulfate Soils, edited by H. Dost. Wageningen, the Netherlands, ILRI, Publication 31

Potter, T. 1976 The problems to fish culture associated with acid sulfate soils and methods for their improvement. Report of the ASEAN Seminar/Workshop on Shrimp Culture, 15–23 November 1976, Iloilo city, Philippines

Ruddle, K. 1982 Brackishwater aquaculture in south-east asia. Mazingira, 6(4): 58–67

Singh, V.P. 1981 Management of acid sulfate soils for brackishwater fishponds: experience in the Philippines. In Proc. Bangkok Symp. On Acid Sulfate Soils, edited by H. Dost. Wageningen, the Netherlands, ILRI, Publication 31, pp. 354–67

Ti Teow Loon and L.D. Rajamanickam. 1981 Observations on the effects of rainfall on the pH of pond water in Gelang Patah. Ministry of Agriculture, Kuala Lumpur, Malaysia, Fisheries Bull., (25): 19 p.