Author: Mr. Guy Sela, Agronomist, an international expert in plant nutrition and irrigation.
Timing of fertilizer application has a significant effect on crop yields. Proper timing of the fertilizer application increases yields, reduces nutrient losses, increases nutrient use efficiency and prevents damage to the environment.
Applying fertilizers at the wrong timing might result in nutrient losses, waste of fertilizer and even damage to the crop. The mechanisms by which losses occur depend on the properties of the nutrient and its reactions with the surroundings and will be discussed further in this article.
Plants need different nutrient ratesand ratios at different growth stages. In order for the nutrients to be available when the plant needs them, fertilizers should be applied at the right timing. The optimum timing for fertilizer application is, therefore, determined by the Nutrient Uptake Pattern of the crop. For the same crop, each nutrient has an individual uptake pattern.
Example of NPK uptake distribution
Different crops have different salt tolerance levels. When salinity level exceeds the salt tolerance of the crop, yield is affected and begins to decrease.
The maximum rate of fertilizer that can be applied at one application depends on the salinity threshold that the crop can tolerate.
Therefore, split fertilizer applications help to avoid salt damages to the crop and improves germination rate. Applying smaller amounts of fertilizers at shorter intervals reduce salt stress.
Soil type affects the timing and frequency of fertilizer application. Two major soil properties determine the frequency and timing of application:
CEC – Cation Exchange Capacity – this is a parameter that measures the capacity of the soil to hold and store positively-charged elements, such as calcium, magnesium and potassium. Soils with high CEC require a lower frequency of fertilizer application, and as a result, higher fertilizer rates are applied with each application. In soils with a low CEC splitting the fertilizer application into multiple applications is necessary to avoid loss of nutrients.
Soil Texture – soil texture is strongly related with CEC. Sandy soils usually have a low CEC, while clayey soils have a higher CEC. But while CEC gives an indication of the capacity of the soil to hold nutrients, soil texture refers to the particle size distribution of the soil. Sandy soils can hold less water than soils with a fine texture. Irrigation frequency is usually higher in sandy soils and, as a result, leaching of nutrients is stronger. Therefore, splitting fertilizer application in sandy soils is necessary.
Nitrogen requires careful management, as it is very susceptible to being lost from soils. Nitrogen can be lost from the soil through leaching, denitrification, erosion and surface volatilization. Nitrogen is more readily leached in sandy soils than in fine texture soils. If not properly applied, nitrogen loss can account for up to 50-60% of the applied amount.
For example, if nitrogen is applied too early, before the plant really needs it, a significant portion of the nitrogen may be lost before the crop takes it up.
Therefore, the time nitrogen is in the soil before the plant takes it up should be minimized.Splitting nitrogen application is one way to do that. Splitting the nitrogen application reduces the risk of nitrogen loss and improves the efficiency of the application.
Movement of phosphorus in the soils is very slow. Therefore, roots can uptake phosphorus only from their very close surroundings.
When first added to soil with fertilizers, phosphorus is in its soluble and available form. However, it quickly becomes unavailable for plants in a process called "Fixation".
Since phosphorus applied remains at the top soil layer, main losses are through surface runoff and soil erosion.
It is important to take the above factors into consideration when deciding on the timing of phosphorus application and on the frequency of applications. For example, applying a high rate of phosphorus, especially right before rain or heavy irrigation might cause loss of phosphorus through runoff and erosion.
Splitting phosphorus application may also be considered.
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