All soils contain salts, and all irrigation waters, whether from canals or underground pumping, including those considered of very good quality, contain some dissolved salts. Hence, the process of soil salinization is dramatically exacerbated and accelerated by crop irrigation. The overall effect of irrigation in the context of salinity is that it “imports” large quantities of new salts to the soil that were not there before.
Removal of salts from the root zone (reclamation) is perhaps the most effective and longer lasting way to ameliorate or even eliminate the detrimental effects of salinity. However, in addition to being slow and expensive, the process requires large quantities of water and effective soil drainage. Consequently, it is not always possible or feasible to carry out a “true reclamation” operation. A number of different approaches involving removal or reducing the salts may be considered.
1. Soil Reclamation: The process of “true reclamation” involves replacing sodium ions in the soil with calcium. The released sodium ions are then leached deep beyond the root zone by using excess water and finally carried out of the field in the drainage water. The most commonly used method for replacing the sodium ions is by applying large quantities of gypsum (calcium sulfate) to the soil and followed by water ponding. The applied gypsum slowly dissolves in the water releasing calcium ions which replace sodium ions from the soil into the downward moving water. Lime (calcium carbonate) is not used as saline soils are sometimes already high in carbonate salts and are therefore alkaline. The reclaimed soils can become saline again unless appropriate management practices are followed.
2. Various management practices based on reducing the salt zone for seed germination and seedling establishment: The early seedling establishment and tillering phase are generally the most sensitive stages to salinity. Any management practice that could provide an environment of reduced salt concentration during these stages would mitigate the salinity effects and benefit the crop by promoting plant densities and early seedling growth. A number of approaches have been used.
2.1. Scraping and removal of surface soil: Due to continuous evaporation the salt concentration is the highest in the surface soil. The top soil can be scraped and transported out of the field. The practice has been used in many areas of the world (Qureshi et al., 2003).
2.2. Pre-sowing irrigation with good quality water: Where available, irrigation with good quality water prior to sowing helps leach salts from the top soil. This helps in promoting better seed germination and seedling establishment. The benefits of this practice were documented in a long-term study by Goyal et al (1999 a,b).
2.3. Appropriate use of ridges or beds for planting: The impact of salinity may be minimized by appropriately placing the seeds (or plants) on ridges. Where exactly the seeds should be planted on the ridge or bed will depend on the irrigation design. If the crop planted on ridges would be irrigated via furrows on both sides of the ridge, it is better to place plants on the ridge shoulders rather than the ridge top because water evaporation will concentrate more salts on the ridge top or center of the bed. If the crop is irrigated via alternate furrows, then it is better to plant only on one shoulder of the ridge closer to the furrow that will have water. For additional benefits, this approach may be combined with pre-irrigation (2.2) via furrows or sprinklers which will help reduce salt concentration in the area where seeds or plants are to be placed.
2.4. Planting into a pre-flooded field: An interesting approach has been widely used in the San Joaquin Valley of California to grow safflower crop on salt affected soils. Prior to planting, the field is flooded with good quality water. Just as most of the water has percolated into the soil and only a few millimetres of standing water is left, the seeds are flown over the field via an aircraft. The seeds traveling under the force of gravity get imbedded into the muddy soil surface where the salt concentration is expected to be the lowest. The approach has provided good seed germination and seedling establishment (Goyal et al., 1999 a,b).
3. General management practices to reduce the impact of soil salinity on crop performance: In addition to the management practices mentioned above, the following approaches may help reduce salinity impacts.
3.1. Mulching: Mulching with crop residue, such as straw, reduces evaporation from the soil surface which in turn reduces the upward movement of salts. Reduced evaporation also reduces the need to irrigate. Consequently fewer salts accumulate.
3.2. Deep Tillage: Accumulation of salts closer to the surface is a typical feature of saline soils. Deep tillage would mix the salts present in the surface zone into a much larger volume of soil and hence reduce its concentration and impact. Many soils have an impervious hard pan which hinders in the salt leaching process. Under such circumstances “chiseling” would improve water infiltration and hence downward movement of salts.
3.3. Incorporation of Organic matter: Incorporating crop residues or green-manure crops improves soil tilth, structure, and improves water infiltration which provides safeguard against adverse effects of salinity. In order for this to be effective, regular additions of organic matter (crop residue, manure, sludge, compost) must be made.
Irrigated agriculture can be sustained by better irrigation practices such as adoption of regulated deficit irrigation (RDI) or partial root zone drying methodology, and drip or micro-jet irrigation to optimise use of water. Current levels can be controlled by leaching fractions, where fresh irrigation water is available, and by drains.
The leaching fraction is the fraction of the applied water that passes through the root zone; this carries salts below the root zone. The smallest leaching fraction that maintains maximum crop productivity is called the ‘leaching requirement’. It depends on the salt content of the irrigation water and the salt tolerance of the crop.
If more than 30% passes through the root zone, the cost of drainage, or the risk of rising water tables, becomes too great. Hence, increasing the salt tolerance of crops is desirable. Salt tolerant crops are also needed if the drainage water is to be reused.
The disposal of saline drainage water from salt-affected irrigated land has been a controversial issue, and recycling of such waters has been considered for further crop irrigation. Feasibility studies indicate that re-use of drainage water is suitable for irrigation of moderately salt-tolerant crops. Up to 4 dS/m can be used for irrigation of moderately tolerant crops provided that the ground is leached with fresh water before sowing. With the more salt tolerant crops like sugar beet and cotton, the use of water up to 9 dS/m can be sustained for three years, but for a longer period the salinity must be reduced to 5 dS/m (Goyal et al. 1999 a,b).
No comments:
Post a Comment