Top fertilization is called fertilization after the seed germinates in the soil and rises to the soil surface or after planting seedlings in annual plants.
In perennial plants such as vineyards, olives and fruit trees, the fertilization made after the budding of the shoots, flowering or fruit set is called top fertilization. Top fertilization is done once in some plants, while it may need to be done 2-5 times in some plants. The main factors that require top fertilization are the growth rate of the plant grown in the development period and the duration of the development period. In addition, the availability of the nutrients in the fertilizers used depending on the soil, climate and irrigation conditions and the loss of nitrogen from the soil in various ways are also factors that affect the necessity of top fertilization. It is important to use the right dosage of fertilizer according to the age and variety of the plant when top fertilizing.
Nitrogen fertilizers are generally used as top fertilizer. However, if the soil is poor in potassium or if the plant removes too much potassium from the soil, potassium fertilizers can also be used as top fertilizer from time to time.
Nitrogen fertilizers are mostly preferred in our country for top fertilization. In this direction, the use of HDF Fertilizer 30-0-0 organomineral fertilizer will be correct. Nitrogen loss occurs in various ways from top fertilization. Nitrogen loss should be understood not only as the loss of nitrogen from the soil, but also as the nitrogen remaining in the soil and not being taken up by plants. Nitrogen loss occurs by fixation or absorption, and nitrogen loss by nitrate (NO3) leaching.
Use of nitrogen by soil microorganisms
Microorganisms in the soil need nitrogen to maintain their vital activities. Bacteria need mineral nitrogen so that both organic materials added to the soil (eg animal manures) and harvest residues (eg stubble) can be decomposed by bacteria in the soil and turned into humus. Soil microorganisms prefer nitrogen in the soil, especially in the form of ammonium (NH4), to absorb it and perform their functions. For this reason, ammonium present in the soil or some ammonium of nitrogenous fertilizers given in the form of fertilizer passes into the soil microorganisms before they can be taken by plants. This is not a loss of nitrogen from the soil, but the conversion of nitrogen from mineral form to organic nitrogen form for a certain period of time. For this reason, when the animal manures are well matured and the stubble is mixed with the soil, adding some (3-5 kg/decare) Ammonium Nitrate to the soil ensures easy decomposition of the stubble. Since the carbon/nitrogen ratio (C/N) of the stubble residues is large, it is useful to apply nitrogen fertilizer to the soil in the fall of the stubble.
Nitrogen conversion through nitrification
The ammonium of nitrogen fertilizers applied to the soil in the form of ammonium (NH4) is converted to nitrate (NO3) by the nitrifying bacteria in the soil. This event occurs as a result of enzymatic reactions of bacteria in the soil.
For example, if Ammonium Sulphate fertilizer (NH4)2SO4 is applied:
As explained above in the chemical formula, as a result of the application of Ammonium Sulphate to the soil, ammonium nitrogen is converted to nitrate nitrogen and free hydrogen ion (H) is formed, causing a slight decrease in pH in the soil. The pH decrease is not caused by sulfate (SO4), but by hydrogen released by the conversion of ammonium to nitrate. Nitrification occurs in calcareous and high pH soils. Nitrification is very low in waterlogged, cold (less than +50C) soils and low pH soils.
Nitrogen loss from nitrate fertilizers via denitrification
Depending on soil conditions such as humidity, temperature and pH, and especially in soils with a pH value above 7, nitrogen loss from nitrate (NO3) fertilizers occurs as a result of enzymatic reactions of nitrifying bacteria in the soil. This situation occurs when the nitrogen in the nitrate is blown into the air in the form of nitrogen gases as follows.
Nitrogen loss from ammonium and urea
When urea fertilizer is applied to the soil surface, it changes from solid to liquid with very little moisture. If the air temperature is high, the soil conditions are calcareous and the pH value is high, the urea fertilizer is hydrolyzed and ammonium carbonate (NH4)2CO3 or ammonium bicarbonate is formed. These two compounds are unstable and turn into ammonia (NH3) and carbon dioxide (CO2) in a short time.
The nitrogen in the urea fertilizer passes into the air as ammonia (NH3). The loss in this way is too great. For this, urea application should be done before the rains or by mixing it with the soil. This loss is less than ammonium sulfate. Nitrogen loss is close to zero due to ammonia volatilization from urea fertilizer applied by mixing in low pH acidic soils and at least 5 cm into the soil.
Points to consider in nitrogen fertilizer application in top fertilization:
As explained above, nitrogen loss occurs in the fertilizers that are used by sprinkling on the soil surface and cannot be mixed into the soil for plants with dense sowing, such as wheat and barley, due to the volatilization of ammonia. For this reason, the top fertilizer application should be coincided with before the rain. Since this loss is too much in urea fertilizer, 26% N CAN fertilizer should be preferred in such cases. In regions where rainfall is sufficient and balanced, top fertilizer should be given in two separate periods (bringing and stalking periods). Time is needed for all of the urea fertilizer to be useful and available to plants. Since the plants grow fast during this period, 33% N Ammonium Nitrate or 26% N CAN fertilizer should be used instead of urea.
Those who do not have the opportunity to use top fertilizer try to meet this deficiency with foliar fertilization. With foliar fertilization, it is never possible to fully meet the nutrients the plant needs. Foliar fertilization is effective on yield and quality when done in addition to soil application.
Top fertilizing should be done several times in irrigated and hoeed plants. Before hoeing and irrigation, 26% N CAN, 33% N Ammonium Nitrate, Urea can be mixed with the soil between rows, and if potassium is insufficient in the soil, Potassium Nitrate and Potassium Sulphate fertilizers can be applied during hoeing or before the first water at the latest.
In fruit trees, fertilizers should be mixed with the soil to reach the crown projection of the trees, and 50-60 cm to the right and left of the vine in the vineyard. In fruit trees, top fertilizers should be applied twice, at fruit set and fruit growing period. The most important point that should not be forgotten in top fertilization is that the plants prefer the nitrogen form nitrate, which they take more easily during the rapid growth and laying period. For this reason, nitrate fertilizer should be used in top fertilization.
Nitrogen leaching occurs with excessive irrigation, especially in nitrate fertilizers, in soils with a very permeable sandy beach and sandy-national structure. In such soils, 26% N CAN fertilizer should be preferred instead of 33% N Ammonium Nitrate and Urea fertilizer. At the same time, the amount of fertilizer should be divided into 2-3 parts instead of a single top fertilization.
Top fertilization can be done together with the sprinkler irrigation system. In this case, fertilizer irrigation should be done at the beginning of irrigation and then only water should be given. In this way, it is ensured that the fertilizers dissolved in the water pass into the soil with irrigation.
The amount and types of fertilizers to be used in base and top fertilization should be determined by soil analysis. If this is not done, a specialist should be consulted.