Land-based farms - conflicts
Sites generally available for coastal pond farms are tidal and intertidal mud flats in protected areas near river estuaries, bays, creeks, lagoons and salt marshes including mangrove swamps. The traditional and, in many cases, the most economical method of water management for a coastal farm is through tidal flow, and so one of the essential pieces of information is the tidal amplitude and its fluctuations at the site. The tidal range along the shore line may be more easily obtained from tide tables or other sources, but in estuaries and other water bodies away from the coast the figures will be different: the mean tidal level generally becomes higher; the duration of the ebb tide becomes longer and the flood tide shorter. The diurnal tidal range, that is the difference in height between the mean higher high and the mean lower low waters, becomes less. In order to determine the relation between tidal levels and ground elevation at the proposed coastal farm site, tide measurements will have to be made on the site with a tide gauge or tide staff over a period of time. The relationship of tides between the nearest port and the tide gauge placed at the site has to be determined first for this purpose. The tide curves and other necessary tidal data at the site can be calculated from the highest astronomical tide (HAT), mean high water springs (MHWS), mean high water neaps (MHWN), mean low water neaps (MLWN) and mean low water springs (MLWS).
The construction of ponds in areas reached only by the high spring tides would require excavation, leading to high construction costs and problems in disposal of the excess soil. If the dikes are made higher than necessary to deposit excess earth, the productive water area in the farm will be reduced. Excavation may also affect efficient drainage using tidal energy. Further, the removal of fertile top soil, which is important to induce the growth and maintenance of benthic food organisms in coastal ponds, will result in the loss of much time in reconditioning the pond bottom to stimulate such growths. However, in certain mangrove areas, particularly those under the red mangroveRhizophora, the top layer may contain peat or a very dense mass consisting of rootlets of mangroves, which in any case will have to be excavated to make the pond bottom productive.
The selection of suitable sites, based on tidal fluctuations and elevation, is shown in fig. 4.4.
A tidal fluctuation of around 3 m is considered ideal for coastal ponds. However, it has to be remembered that if the tidal energy can be replaced by other forms of energy for water management, the limitations indicated would not apply. As mentioned earlier, the main consideration then would be the cost involved and the economics of operation. Gedrey et al. (1984) estimate that the construction and operation of a farm with a pumped water supply system can be more economical than that of a tidal water farm.
The quality of soil is important in pond farms, not only because of its influence on productivity and the quality of the overlying water, but also because of its suitability for dike construction. The ability of the pond to retain the required water level is also greatly affected by the characteristics of the soil. It is therefore essential to carry out appropriate soil investigations when selecting sites for pond farms. Such investigations may vary from simple visual and tactile inspection to detailed subsurface exploration and laboratory tests. Because of the importance of soil qualities, detailed investigations are advisable, particularly when large-scale farms are proposed. Sandy clay to clayey loam soils are considered suitable for pond construction. To determine the nature of the soil, it is necessary to examine the soil profile, and either test pits will have to be dug or soil samples collected by a soil auger at regular distances on the site. To obtain samples, rectangular pits (1.0–2.0 m deep, 0.8 m wide and 1.5 m long) are recommended. If available, a standard core sampler or soil auger of known capacity (e.g. 100 cm3) can be used for collecting samples of soil from each soil horizon.
Texture and porosity are the two most important physical properties to be examined. Soil texture depends on the relative proportion of particles of sand, silt and clay. The size limits and some general characteristics of the soil constituents are given in Table 4.1. By touch and feel one can roughly determine the texture. A sample of the soil should be kneaded in the hand (to make it somewhat drier, if it is wet and sticky; if the sample is dry, add some water to make it moist but not sticky). If the kneaded sample can be rolled into a bar (about 6 mm thick) and bent to form a ring around the thumb, without any cracks, the soil must be clayey. If it cannot be made into a bar and remains separate with visible grains when dry, the sample is sandy. If the sample does not fall into either of these categories it can be classified as silty or loamy. Sand grains can be felt distinctly, even when not readily visible in loamy soils. Silty soils feel like flour or dough between the fingers. There are, of course, intermediate categories depending on the proportions of the constituents.
Because of their cohesive properties, the finetextured soils (clay,silty clay,clay loam,silty clay loam and sandy clay) are more suitable for pond farms. They have a greater surface area and can therefore absorb more nutrients and retain and release them for organic production in ponds; they are also less subject to erosion and other damage. The soil structure or the arrangement of soil particles is of special importance in determining the compactness, and therefore the porosity, of the soil. Light-textured soils, particularly in close proximity to open drains can cause high seepage and percolation. Pond farms built on such soils may, however, improve in the course of time due to the blocking of interstitial pores by organic sediments produced in the pond, or introduced with the water supply or derived from manuring. Puddling is an efficient means of sealing ponds. In this process, fine particles clog the most permeable parts and in due course the bottom of the pond may be completely sealed. Compaction of soil by mechanical means during pond construction can also assist in reducing seepage. Suitable linings such as polyethylene sheets have been used on pond bottoms and water supply channels to prevent seepage with some success. But it is difficult to prevent damage to the lining and it often turns out to be too expensive for practical use. It may also greatly reduce the contribution of the pond bottom to natural productivity in the pond,even if the initial and continuing costs of the lining are acceptable.
Generally, the soil on sites selected for coastal pond farms is alluvial. It is usually porous with varying masses of fine roots of mangroves and other swamp vegetation. The preferred soils are clay, clayey loam, silty clay loam, silt loam and sandy clay loam. Sandy clay loam is the best for diking.
As mentioned earlier, one of the major problems in site selection for coastal pond farms in the tropics is the prevalence of acid sulphate soils or catclays. Even though such soils are also found in fresh-water swamps, the problem is more pronounced in brackish-water areas. The highly acidic conditions inhibit the production of fish and fish food organisms. Elements, particularly iron and aluminium, are released into the water in toxic quantities which render phosphorus unavailable, causing severe phosphorus deficiency for algal growth. Sudden fish kills during rains after long dry periods are a common phenomenon due to leaching of extremely acidic water from surrounding dikes into ponds built on such soils.
Acid sulphate soil results from the formation of pyrite which is fixed and accumulated by the reduction of sulphate from slat water. The process involves bacterial reduction of sulphate to sulphide, partial oxidation of sulphide to elemental sulphur followed by interaction between ferrous or ferric iron with sulphide and elemental sulphur. A sufficient supply of sulphate and iron, high concentrations of metabolizable organic matter, and sulphate-reducing bacteria (Desulfovibrio desulfuricans and Desulfo maculatum) in an anaerobic environment alternated with limited aeration are the factors that give rise to sulphate soils.
In mangrove swamp areas, the most favourable conditions for pyrite formation exist in the zones between the mean high water and mean low water levels which have limited periodic aeration due to tidal fluctuation. There is less pyrite in the better-drained parts of the marshes which are aerobic most of the time.
The reclamation of mangrove swamps for pond farms with drainage results in the exposure and oxidation of pyrite and causes acidic conditions. Ferrous iron (Fe2+) is released during atmospheric oxidation of pyrite under moist conditions at an optimum moisture content of 30–40 per cent. At low pH, oxidizing bacteria convert ferrous iron to ferric iron (Fe3+). It can remain in solution in appreciable amounts only at pH values in the range 3–3.5 and is a more effective oxidant for pyrite and elemental sulphur than free oxygen. At higher pH, almost all ferric iron is hydrolyzed and precipitated as ferric hydroxide. Basic ferric sulphate is also formed during pyrite oxidation. Elemental sulphur is oxidized to sulphuric acid by bacteria.
The most harmful effect of pyrite oxidation lies in the excessive amount of sulphuric acid produced, which if not neutralized by exchangeable bases creates strongly acid conditions. In selecting sites for pond farms, one has to take into account not only the existence of acid sulphate soils but also the potential for acid conditions to develop as a result of drainage after construction. The levels of pyrite and acid-neutralizing components such as calcium carbonate from mineral deposits and metal cations have to be considered. The use of combined criteria, as for example sedimentary relationships and sulphur sources, land form, vegetation and soil characteristics, has been suggested as a basic approach for recognition and prediction of potential and actual acid sulphate soils.Although it is desirable to have both field and laboratory investigations, it is considered possible to use certain simple criteria with confidence. Potential and existing acid sulphate soils are generally found in mangrove swamps and marshy back swamps, on the seaward side of river deltas and on marine and estuarine plains (fig. 4.5). Tidal brackish-water vegetation with dense rooting systems is usually related to accumulation of pyrite. Association with the red mangrove (Rhizophora), Nipa and Melaleuca stands is a fair indication of potential acid soils. Soils that are likely to become acidic have a high organic matter content, such as the fibrous roots of mangroves, and a grey subsoil with dark grey to black specks or mottles of partially decomposed matter.
The detection of actual sulphate soils is easy. They can be recognized by the pale yellow mottles of the top soil, overlying pyritic subsoil. The older acid sulphate soil shows the red-brown ferric hydroxide. Their pH is generally below 4. A comparatively easy method of estimating the extent of acid and non-acid soil layers is by implanting stakes coated with red-lead paint in the soil profile. Hydrogen sulphide generated in the layer with active sulphate reduction turns the red-lead marking black within about a week, leaving on the stake a record of the upper limit of the present sulphide accumulation.
As will be described later on the construction and maintenance of pond farms, it is possible to minimize the harmful effects of acid soils, but it is time-consuming and expensive. However, in many tropical areas, the available sites for pond farms may almost all have such poor soils and there may be little choice. In such cases sites that can more readily be reclaimed should be selected. Basically, reclaiming consists of removing the source of acidity by oxidizing the pyrite from the pond bottom and flushing it out of the 10–15 cm deep surface soil and preventing further diffusion of acids, aluminium salts and ferrous salts from the subsoil. Acid and toxic elements are also leached and removed. If this is feasible, the farm can be made suitable for aquaculture within a period ranging from three to five years, depending on the extent of the problem.