Crop Response to Irrigation and Fertilizers
The requirement regarding the number and their timings vary widely for different crops. It has been observed that water requirement of crops vary with the stages of its growth. When the water supply is limited, it is necessary to take into account the critical stages of crop growth with respect to moisture.
The critical stages of crop growth is commonly used to define the stage of growth. Certain critical stages at which if there is shortage of moisture, yield is reduced drastically. When there is shortage of water, it is better to take care of the critical stages first to obtain increased water use efficiency.
(i) Water and fertilizer – Water is the key factor in all the three mechanisms (mass flow, diffusion, transpiration pull) of nutrient uptake. Root intercepts more nutrient ions when growing in a moist soil than dry soil. In moist soil, the effective root zone area will be more and extensive which in turn absorbs more water and nutrients. This is especially important for calcium and magnesium. If the applied fertilizer uptake is more, it enhances the growth and increases the yield under irrigated condition than dry condition which in turn increases the water use efficiency.
Hence, it is concluded that there is a close relationship between soil moisture and nutrient uptake by plants. The application of fertilizer or nutrients without adequate moisture in root zone is not useful to plants. Similarly, mutual benefits are also obtained from fertilizer. For e.g., in drought situation balanced fertilized crops is able to withstand drought, than relatively low fertilized crop. Even well balanced fertilized crop may not show its normal growth and development unless adequate moisture is available. This is not only due to poor uptake, but also due to poor ET and which in turn reducing the use of absorbed nutrients for photosynthesis.
(ii) Fertilizer use efficiency can be increased by :
• Soil test to evaluate nutrient deficiency and use of proper quantity of the needed fertilizer. Applying fertilizer based on soil test values.
• Placement of fertilizers rather than broadcasting.
• Split does of application at suitable time interval rather than bulk application.
• Controlled application of water to avoid leaching of fertilizers to deeper layers.
In most cases there is significant correlation between soil moisture regime, fertilizer requirement and the availability of fertilizer for plant use.
(a) Nitrogen – Mineralization of nitrogen increases as the water content of soil increases from PWP to FC and to saturation. When the fertilizer is applied to the surface soil, its uptake is inhibited when the soil dries.
(b) Phosphorus – Increase in soil moisture to an optimum level is generally possible because of reduced aeration and root penetration or the increased activity of sesqioxide fraction on ‘P’ fixation under reduced condition. In dry areas ‘P’ applied close to the seed is more effective than the broadcast application. The availability and uptake of P is less in dry or rain fed condition.
(c) Potassium – Soil moisture content affects the level of exchangeable ‘K’ in the soil. In high soil moisture zone, availability of k is increased. The results of studies on fertilizer-irrigation relationship lead to the following conclusions.
• Water use efficiency is raised by fertilizers by increased DMP (DRY matter production) and yield
• The response of fertilizer is generally of a higher order under irrigated condition than under unirrigated condition.
Response to frequent irrigation is generally enhanced by increased levels of fertilizer application, particularly crops grown for its vegetative plant parts.
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