Reclamation of saline soils through biological approaches
Business Recorder, S M MUJTABA, SAJIDA MUGHAL AND MAZHAR H NAQVI,
ARTICLE (June 30 2003): The total geographical area of Pakistan is 79.6lmh(about 197m acres) of which only 25% or 19.82mh is currently under cultivation.
The whole country has more than 5.727mh of salt affected land, which is mainly situated in Indus plain, where about 4.2mh of land is affected by salinity and water-logging .3 million acres falling in the canal commanded area, while 11 million acres outside it.
Of all the salinized lands 4.7 million acres are saline, 7.2 million acres saline-sodic and 0.07 million acres sodic.
The province wise salt affected areas are Punjab 2.56mh; Sindh 2.32mh; NWFP 0.5l6mh and Balochistan 0.304mh.
Salinity stress is a major environmental factor that drastically affects the crop productivity throughout the world. It is a menace to both agriculture and the soil body.
The accumulation of topsoil changes its physical, chemical and biological characteristics.
These soils have a preponderance of Na+on the soil complex resulting in compaction, decreased permeability to water and exclusion of air from the soil pores.
Such condition are inimical to the growth of plants and even other useful organisms such as bacteria, fungi and other micro flora and fauna which are so essential for fertility and productivity of a soil.
Such soils are also devoid of organic matter, have a high pH (ie they are alkaline) which influences nutrient transformation and uptake and cannot thus sustain any crop or microbe and are biologically "dead soils."
Numerous secondly affects of the saline and sodic condition of the soil adversely affect the ecology and the environment.
The salinity stress problem arises when semi-arid or arid lands are subjected to cultivation either because the soils are already saline and/or irrigated with saline water, which adds to the salinity of the soil.
In addition the excessive use of chemical fertilisers and irrigation have turned hundreds of hecters of cultivated fertile lands in to saline lands-plants responses to salinity depend on the kinds of salts (SO42- And chlorides) contributing to salinity as well as the total electrolyte concentrations.
The aim of reclamation is to remove salinity and saline/sodicity from the root zone within possible limits so as to restore the productivity salt affected soils.
Reclamation technology involves four essential steps: i) Adequate lowering of the water table, ii) Leaching excessive salt out of the root zone, iii) Attainment of satisfactory water infiltration, iv) Scientific management of soil and water.
The effect of reclamation of salt-affected lands. (a) Mechanical method, (b) Biological method.
A permanent solution to the problems seems to be to provide a comprehensive drainage system throughout the irrigated area to stall the rising water table.
In addition to drainage, the seepage needs to be stopped and minimised to area where groundwater recharge is desired or is desirable.
This call for concrete lining of the canals, dredging deep of the rivers and construction of an extensive network of drainage of canals which is a very capital intensive, costing billion of dollars.
If the top soil in the area is sodic or saline sodic, just bringing the water table down is not sufficient to reclaim it because the excess of sodium has changed the soil structure to make it impermeable, the surface salt cannot be leached down into the soil.
To counter excess of sodium, excess of calcium in the form of powdered gypsum rock is added to the soils, which restore the soil structure, and the salts can then be leached down.
Further that a liberal supply of sweet irrigation water is required for leaching of top salts.
However, despite all the problems and drawbacks the drainage leaching combination as being practised has continued. For example: - If the soil has an average salt level of 18dsm-1 in the top foot and the salt level is to be reduced to 2dsm-1, the amount of leaching water needed is calculated in the following.
Amount to be reduced 18 - 2 = 16
Reclamation of these soil consist of simply applying enough high-quality water to leach the soil thoroughly, but now these days, due to shortage of irrigation water, it is to much difficult to facilitates enough high quality water practice in reclamation processes.
The water applied should be low in sodium but can be fairly saline (1500-2000 ppm total salts) as this helps to keep the soil permeable during the leaching process.
Generally about 12 inches of water are required to remove the 70-80% of the salt for each foot of the soils.
Reclaiming sodic and saline and sodic soils: Amendment should be used only when needed or when past results justify their use they may be useful where soil permeability is low due to low salinity excess sodium or high carbonate / bicarbonate in the water.
However, they will not be useful if poor permeability exists due to problems with soil texture, soil compaction, restrictive layers (hardpans, claypans) or high water tables.
If sodic soil contains no source of calcium (gypsum of free CO3) then gypsum or a soluble Ca source should be applied Ca amendment include gypsum (hydrated calcium sulphate) and calcium chloride. Gypsum is moderately soluble in water.
It requires about one acres foot of water/acre to dissolve one ton gypsum/acre.
Calcium chloride is highly water-soluble and fast acting, but it generally is too expensive to use.
Gypsum found in very fine particles, the particle gypsum reacts more quickly to replace sodium because they are more soluble then the coarse gypsum usually applied.
Gypsum is the slowest of the all amendments.
If the soil contains lime, then acid or acid-forming materials can be used.
Examples of acid or acid-forming materials include sulfuric acid, elemental sulfur, ferric sulfate and lime sulfur acid reacts immediately with the soil calcium carbonate (line).
To release the soluble calcium to exchange with the sodium. Elemental sulfur must be oxidised by soil bacteria and react with water to form sulfuric acid (H2SO4).
The formation of appreciable amount of H2S04 from elemental sulfur may take several months to several years.
Elemental S takes several year to completely oxidise into sulphates and gypsum is the slowest of all amendments.
Amendment for soil and water and their relative affective ness in supplying Ca are shown in the table below.
A question commonly asked by growers is since elemental Sulfur is too slow, iron sulfate is very expensive and sulfuric acid H2SO4 so dangerous, what is the maximum %lime and ppm calcium our test could have where we can still use gypsum.
-- %Lime ppm calcium
-- Iron sulfate effective on soils with lime, without lime, with Ca>4,000; with calcium < 4000; with Na>230 and with Na<230.it is an ideal treatment for crusting.
-- Sulfuric acid and phosphoric acid are excellent products for testing a crust problem on salt affected ground. Their role in alkaline soil is to acid in the preventation of cementation of salt affected ground.
-- Manure and green manure crops, if left close to the surface, will aid in the physical separation of silts and clays, preventing then from plating and causing a crust.
Deep ploughing can be benefit the surface soil structure if a sandy material is turned up and mixed with the platy surface soil.
This soil reduces the surface crusting potential or soil high in silts and clays.
However the reclamation process is not complete until most of the sodium is removed from the soil at least a depth of 3-5 feet.
Even then more time is required for restoration of good soil productivity. Once the soil structure is completely destroyed, it is slow to return to a desirable condition.
BIOLOGICAL METHOD: It was thought that introduction of such plants species into salt affected lands would not only provide green matter (biomass) for various uses but will also improve the land.
It was considered to be possible to evolve salt -tolerant plants, which can grow on the worst saline lands, even when irrigated with brackish ground water.
Such plants could be introduced on barren saline waste lands where no sweet irrigation water was available and ground water was brackish.
The green matter (biomass) produced on these lands could be utilised in numerous ways such as forage, manure and for making pulp for paper.
Using biotechnological methods it could also be converted into other value added products such as some chemicals, CH4 (biogas) or even alcohol for fuel and solvent purposes.
Cultivation of a crop also results in creating acidic condition in the soil and green manuring enhances it.
It was therefore felt that cultivation of salt tolerant plants and their green manuring could create the desired acidic conditions to improve the soil structure and its permeability, which will help leach salt down.
Field crops, particularly barley, weed, sorghum, cotton and sugarbeet have been used extensively in bioremedation of saline-sodic sites.
By utilising more water on these crops then is actually needed, salt and sodium can be need beyond the reach of roots, and the soils can be prepared for later plantings of more extensive crops (Oster-200 1).
Bermuda grass, a halophyte, and dry grass have been successfully used as bioremedation crops in agricultural settings.
Purity Amendment Suitable Tons equivalent to one ton of 100% gypsum
100 Gypsum Soil/H2O 1.00