Soil management for food security
THE cultivated area is decreasing due to salinity/sodicity, water-logging, erosion and urbanization of agriculture land due to population growth. The decrease in cultivated area seems to widen the already existing gap between the food production and requirements.
Agriculture productivity is based on soil’s potential to support crops on sustainable basis which is defined in terms of soil quality. According to the US National Research Council, soil quality is the capacity to promote the growth of plants; protect watersheds by regulating infiltration and partitioning of precipitation; and prevent water and air pollution by buffering potential pollutants. It is necessary to know how soil responds to agricultural use and practices over time for successful management of quality.
Soil’s capacity to promote crop growth is determined by the environment which it provides to plant roots to grow. Plants absorb water and essential nutrient from soil through roots. Water holding capacity, cation exchange capacity (CEC), pH and soil bulk density attributes quality. These factors directly or indirectly affect the plant growth.
Soil bulk density which is a measure of compactness, greatly affects the plant growth by restricting root penetration. Soil bulk density is increased due to use of heavy machinery, erosion and loss of soil organic matter (SOM).
Fertility is one of the most important determinants of soil quality. It is affected by SOM, pH; texture, depth and water holding capacity. Soil quality is affected due to different deteriorative processes. The way, it is managed for different uses, is affecting the quality.
Poor management degrades the quality through salinity/sodicity, water-logging, erosion, compaction, nutrient depletion, OM loss and pollution due to different organic and inorganic pollutants particularly heavy metals.
Plants require nutrients for growth and production. Nutrients supply to plants is a complex function of different processes which are affected due to above-mentioned factors. Salinity/sodicity is the most important factor which adversely affects productivity.
About seven million hectares is salt-affected which is increasing due to shortage of good quality water and use of brackish groundwater (having high EC, SAR and RSC). Shortage of good quality water, low and variable rainfall pattern compels farmers to use brackish groundwater for irrigation.
About 70-80 per cent underground water is brackish and unfit for irrigation. Water-logging results in salinisation due to solublization of the salts present in lower layer and movement to surface. Water-logging adversely affects crop growth.
The mismanagement on steep slopes, deforestation and loss of soil cover due to unsystematic grazing of fragile rangelands results in water erosion. Water erosion has affected about 7.2 million hectares. Erosion leads to the loss of top-layer productivity and decreases potentials of crop growth. Wind erosion is common in drought hit areas and about two million hectares is affected due to it.
Nutrients which indicate soil quality are being depleted due to intensive cropping, imbalanced and low fertilization and poor return of crop residues. Quality demands the maintenance of soil fertility status. Farmers use high rates of nitrogen, low rate of phosphorus and no potassium. The NPK ratio in Pakistan is 1:0.26:.01 against the 1:0.4:0.27 in Europe and 1:0.4:0.2 in China. As a result soil nutrients are depleted and result in poor crop stand and low yields.
The amount of nutrients applied and removed by crops constitutes nutrient balance. Positive nutrient balance is indicative of over-fertilization while negative necessitates the use of fertilizers for optimizing yields. Intensive cropping without proper and balanced fertilizer use leads to deficiency of nitrogen, phosphorus, potassium, iron, copper, zinc and boron. Poor nutrient status reduces potential to support crop growth thereby decreasing crop residues to the soil.
Poor return of crop residue and low or no use of organic fertilizer result in extremely low organic matter (OM) which is insufficient to maintain productivity. The OM is lifeblood as it improves soil’s physical, chemical and biological properties. It also improves water holding capacity and permeability for both water and air, and nutrient holding capacity by contributing to the cation exchange capacity (CEC). In addition, it is also a source of different essential nutrients and supplies nutrients on mineralization.
Organic and inorganic pollutants are another reason of degrading soil quality. City waster water is an important source of these pollutants which is used for irrigation around big cities. The pollutants make soil less productive by disrupting many biological cycles.
All factors which modify soil environment and adverse plant growth affect quality. Poor quality soils have low potential to produce economic yield. Thus food security is threatened.
The management strategies can be enumerated as follows:
* Using adequate and balanced fertilizers.
* Improving fertilizer use efficiency.
* Improving water use efficiency.
* Economical and safe use of brackish water for growing crops.
* Improving organic matter contents of soils.
* Reclamation of salt-affected and waterlogged soils.
* Reducing water erosion by better harvesting of water during rainy season and maintaining soil cover.