Salinity is one of the most severe environmental stress and affects crop production in arid and semi-arid lands.
Introduction
The salinity problem in arid and semi-arid regions is more serious than that in other areas. An increasing proportion of the world’s food supply is produced in semi- arid regions with irrigation (Shannon et al., 1994) and the reductions in production of agricultural crops in these areas sometimes reach up to 50% because of salinity problems (McWilliams, 1986; Evengelou, 1994) due to the rapid expansion of irrigated agriculture, efficient use of the limited water resources in arid and semi- arid regions is becoming more and more vital. However, water salinity is a major problem due to its negative influence on the yields of many crops.
Except for halophytes, water salinity partially inhibits the growth of most plants. Four reasons that are usually introduced as solely responsible for reduction of plant growth under salt- stressed conditions are briefly the (i) Osmotic stress caused by reducing the availability of external water, (ii) Specific ion toxicity effects caused by metabolic processes in the cell, (iii) Nutritional imbalance caused by these ion-toxicity effects, and (iv) A combination of any two of the above- mentioned factors. Citrus is salt-sensitive. Its response to salinity depends on several factors like rootstock – scion combinations, irrigation system, soil type and climate, etc.
Changing one or more of these factors (with the same irrigation water) could produce entirely different results. In all plants, salinity reduces growth and causes physiological disorders. For instance, salt-stress lowers net CO2 assimilation, stomatal conductance, and water potential of citrus tree leaves. These effects are associated with accumulation of excessive concentration of chloride or sodium in leaves, in addition to reduction of cell size and foliar damage.
The accumulation of chloride in leaves occurs only above a certain threshold of irrigation water salinity. It is rootstock dependent. Accumulation of Sodium in shoots and leaves is more scion-dependent (Banuls et al., 1990). When data on relative yields of citrus, expressed in relation to the controlled treatment, were plotted against soil salinity paste extract (ECE), it indicated tolerance threshold values of different rootstock-scion combinations and yield decline for each salinity unit increase. However, few studies provide information on fruit-yield.
Perspectives of Salt-Affected Soil in Pakistan
Agriculture sector is performing a vital role in the economy of Pakistan (GDP 21 %). It is the source of livelihood of millions of people of Pakistan. In the last few Decades with the explosive increase in world population, sustainable agriculture and food security are becoming a serious threat but our land resources are same and deteriorating day by day. The major aspect of soil degradation includes poor soil fertility, soil salinity, water logging, and soil erosion, and soil contamination, scarcity of essential minerals and steadily depletion of organic matter contents.
Soil salinity is one of the major issues in the agriculture sector of Pakistan. The salt-affected soil is categorized into three types (saline, sodic and saline-sodic soils).
Soil types and their properties
- Saline Soils are those soils that contain a high amount of dissolved salts (e.g. NaCl, NaSO4, CaCl2, CaSO4 etc.) in the soil solution. Its Electric conductivity is (EC) > 4 dS/m PH < 8.5 Exchangeable Sodium Percentage (ESP) < Sodium Adsorption Ratio (SAR) < 13.2.
Sodic soils contain an excess amount of exchangeable Sodium (Sodium carbonate and bicarbonate) EC is < 4 dS/m pH > 8.5 ESP > 15
SAR < 13.2. Dispersive effect of exchangeable sodium disturbs soil physical properties and results in plant growth retardation
Saline-Sodic soil contains soluble salts as well as exchangeable sodium. EC > 4 dS/m pH > 8.5 ESP > 15 SAR < 13.2.
At present, the total salt affected soil on the surface of the earth is about 954 million hectares. Such types of salt affected soils are distributed throughout the world. More than 80 million hectares of such soils are in Africa, nearly 147 million hectares in Central, North and South America, 357 million hectares in Australia as well as 50 million hectares in Europe. Similarly, a large bulk of about 320 million hectares land in South and Southeast Asia is under the grip of salinity (Dr. Alam S. M. et al., 2000).
Out of 79.7 million hectares (7, 96096 Km2), 22 million hectares area is under agriculture. It is estimated that out of this 22 million hectares,6.28 million hectares (28.54 %) area is salt affected (total 7.88 %). About 1.89 million hectare is saline, 0.28 million hectare is sodic and 3.05 million hectare is Saline sodic. (Dr. Alam. S. M. et al., 2008)
Soil salinity and sodacity problems are common in arid and semiarid regions, where rainfall is insufficient to leach salts and excess sodium ions out of the Rhizosphere. Nearly 10% of the total land surface is covered with different types of salt-affected soils. At present, there are nearly 954 million hectares of saline soils on the earth’s surface. All these salt affected soils are distributed throughout the world. More than 80 million hectares of such soils are in Africa, 50 million hectares in Europe, 357 million hectares in Australasia, nearly 147 million hectares in Central, North and South America. Similarly, a large bulk of about 320 million hectares and land in South and South East Asia is under the grip of salinity.
The total geographical areas of Pakistan is 80.0 million hectares or 197.0 million acres, with a very good canal irrigated system of about 62,400 km long and mainly confined to Indus plain covering an area of 19.43 million hectares (48 million acres). The salt affected soils are mainly situated in this plain. In Pakistan, about 6.30 million hectares of land are salt-affected and of which 1.89 hectare is saline, 1.85 million hectare is permeable saline-sodic, 1.02 million hectare is impermeable saline-sodic and 0.028 million hectare is sodic in nature.
It is estimated that out of 1.89 million hectares saline patches, 0.45 million hectares present in Punjab, 0.94 million hectares in Sindh and 0.5 million hectares in NWFP. Out of 19.3 mha area available for farming, irrigated agriculture is practiced on about 16 mha. The irrigation water is mainly supplied through the world’s largest canal system arranged through dams. Intensive and continuous use of surface irrigation has altered the hydrological balance of the irrigated areas, especially Indus basin. The substantial rise in the water table has caused salinity and water logging in large areas of Sindh, Punjab, NWFP and Baluchistan. The magnitude of the problem can be gauged from the fact that the area of productive land was being damaged by salinity at a rate of about 40000 hectares annually.
Causes of salt accumulation:
The location of Pakistan is in arid and semi-arid climatic zones. Generally high evapo-transpiration accumulation on the soil surface. The average summer temperature is about 40°C and the minimum winter temperature remains between 2°C to 5°C. The annual rainfall varies between 100 mm to 700 mm throughout the country. The evaporation rate is generally very high and exceeds that of precipitation. Thus, the insufficient rainfall followed by high evaporative demand and shallow ground water depth, enhances the movement of salts towards soil surface.
Salinity is an important problem affecting irrigated agriculture of Pakistan. Improper irrigation practices and lack of drainage have generally led to accumulation of salts in the soil in concentrations, which are harmful to the crops. There is a major imbalance in the amount of salt entering and leaving the soil in Pakistan. Each year about 120 million tons of salts are added to the land in canal water and brackish underground water. Only about 1/5th of this salt finds its ways to the sea. The remainder accumulates in the soil, it continues to decrease the growth and survival of crops.
The soils with electrical conductivity of less than 4 dSm-1 are generally considered as salts free, where almost all crops can be grown. As the salt concentration increases, the choice becomes limited and one has to go for tolerant plants suited for specific conditions.
Soil analysis and water Analysis
- Soil sampling
- Sample preparation
- pH of soil solution
- Preparation of soil extract
- EC of soil extract
- Determination of carbonates and bicarbonate
- Determination sodium Na
- Determination of chloride Cl
- Determination of sulpHates