Salinization is an emerging challenge to agriculture now a day as it is making its jaws strong over the cultivated land and decreasing the space for crops production round the globe. The accumulation of high contents of salts in cultivable soils is a serious socio-economic and environmental problem worldwide. It is a natural revolutionary process that occurs by the accumulation of salts in the root zone of plants and plants become unable to grow. As salts are the natural components of soil and thus, are good for both plants and soil health but if remain within certain limit. However, in excessive concentration they hinder the plant growth and ultimately yield. About 7% (one million hectare) of the world area is under salt stress either directly or indirectly and is going to increase with time. So, salinization has been declared as a major threat to agriculture that needs immediate consideration otherwise, it is hitting hard to crop production. Pakistan is also suffering a lot from this malady and it is spreading continuously rather than sequestering. About 9.97 million hectares (mha) of Pakistani land is also under salt stress and this is going to be increased day by day.
Salts may develop from various sources including natural and/ or anthropogenic acting either alone or in combination but major factor involved behind it, is the process of weathering and parent material. Soil minerals weather over a longer period of time and release salts. Besides mineral weathering, salts may also deposited via dust, sea water, lacustrine and marine deposits, fossil salts, aridity, uneven distribution of rainfall, and physiographic unevenness. In dry regions salts may accumulate on the surface after weathering, leading to naturally saline soils. Anthropogenic sources which can cause addition of salts may include through irrigation water, chemical fertilizers and waste materials, flood waters and waste effluents. Unwise use of fertilizers in fertile cultivated lands has converted hundreds of hectares into saline lands. Poor drainage patterns for the leaching process are also a part of salt accumulation. Flat topography, porous soil, low rainfall, high evaporation, and gradually increasing water table due to poor natural drainage in some areas are also adding to it.
These soils have a prevalence of salts on the soil exchange sites resulting in damage to physical nature of soil by increasing compaction. Such conditions are imperfect for plant growth as well as for the organisms like fungi, bacteria and other beneficial flora and fauna which are taking part for increasing the soil fertility. Such soils have low organic matter and usually have high pH values which have severe impacts on soil environment and ecology, which also disturbs the nutrient balance and uptake by plants. Long term prevailing conditions of salinity often results in decrease of production level or sometimes no production at all. Under extreme conditions these salts can cause the soil surface sealing which lower down the infiltration rate, decrease permeability to water and soil aeration. In arid regions it is decreasing the cultivated land by 1-2% per year.
Economically it is affecting by creating severe decrease in soil productivity and crop yields along with the increase in requirement and use of inputs i.e. seed, water, fertilizers, etc. Farmers have less choice in cropping system and are forced to look towards the salt-tolerant crops due to it. There is low crop yield per unit of input and less water use efficiency in these conditions. Salinity causes about 25% of yield loss in major agricultural crops by affecting nutrient and water uptake. It is reducing effectiveness of nutrient cycle, therefore saline soils are frailer and are susceptible to degradation i.e. erosion, etc. It influences the plant by delayed germination, stunted growth, and foliar damage. Under saline conditions there is decrease in germination rate and emergence in cereal crops. Usually there is necrosis at leaf tips and older leaves tend to die off. Plant growing patterns usually seem poor and thin patchy salt crusts are formed on the surface layer of fallow lands. Drained water of salt-affected soils can contaminate other sites if not disposed of properly.
No doubt it is a harsh natural process but it can be controlled by managed strategies using some proximal techniques. The objective of reclamation is to reduce the concentration of salts from the root zone area to recover the productivity status. In case of soluble salts it is easy to reclaim the soils but for sodic soils it needs some amendments. It comprises following strategies; proper infiltration and drainage system, controlled scientific management of soil, enough good quality irrigation water, and then leaching of excessive salts from root zone. A long-lasting solution for this problem is the provision of vast and comprehensive drainage system to the affected soils with enough good quality irrigation water. It helps in leaching down of excessive salts from soil surface when drainage system is good but lowering of ground-water table is also mandatory for this process. Among the farm management practices is the addition of amendments which include powdered gypsum which replaces the sodium ion from the exchange site and make it capable for leaching. It also improves the structure and restores the productivity status of the soil besides alleviating the salt stress from plants. Besides gypsum, other amendments include sulphur and sulphuric acid but special care is taken in case of latter ones due to its corrosive nature. It boosts up crop growth rate by decreasing soil pH which ultimately enhances the crop yield and revenue. Further agronomic practices are land leveling, sanding, deep ploughing, strip cropping, and scrapping of salty layers which facilitate the recovery of salty soils. The areas where it is difficult to lower down the salts we can look for some salt tolerant crops and forestation as it is the best solution to harvest profit from such barren soils. It also supports economically as well as remediates the soils with the passage of time because fallow lands are more prone to become salt affected than cultivated ones. Most of these plants release root exudates (i.e. organic acids) which help in lowering of pH besides enhancement in soil structure and support the biotic factors to sustain bio-life of soil. Among these plants are various crop plants (sorghum, cotton, barley and sugar beet, etc.), forages, green manures and numerous others used for phyto-remediation of saline-sodic soils. This obtained biomass from these useless lands can be used as raw material for many industrial products like for making pulp in paper industry, for making bio-fuel, biogas (CH4), alcohol products and for solvent purposes, etc. besides remediation of soil.
There is no ambiguity in this statement that such salt conditions of soil are deteriorating the production of agriculture commodities but these can be made under control by using integrated management practices and great economical benefits can also be harvested from these soils. For this, there is just need of some attention rather than leaving this land fallow and a great part of soil can be added to the cultivated lands. There is need of some public awareness and farmer education in this respect and great benefits can be obtained from uncultivated salt affected soils.
This article is collectively authored by Muhammad Umair, Muhammad Zia-ur-Rehman and Hinnan Khalid.