Among nitrogen fertilizers, the best for foliar fertilization is urea (urea) it does not cause such burns that occur on the surface of the leaves when they are sprayed with aqueous solutions of ammonium nitrate. Also, when using urea, metal parts of sprayers are not subject to corrosion. This is due to the fact that aqueous solutions of urea have a neutral reaction, and there is no free ammonia in this fertilizer.
The advantage is that, what is non-dissociated (organic) urea molecules penetrate into plant cells 1020 times faster than cations and anions of mineral salts. The high rate of penetration of urea into the cytoplasm of cells is explained by the presence of the most accessible form of amide nitrogen in it, which quickly and easily passes through biological membranes. Also, urea, as an organic substance, rather than inorganic nitrogen salts, interacts with other substances in the tissues and is intensively used by the plant in biochemical processes. Plants assimilate it not only after preliminary decomposition with the formation of ammonia under the action of the urease enzyme, but also when directly included in the conversion cycle of nitrogenous substances.
The process of assimilation of urea by plants, in contrast to from cations and anions, it passes only two stages in plant cells:
1) urea arginine 2) the substance of the cell.
On the contrary, ammonium and nitrate forms of nitrogen, before they are used by plants in synthetic processes, undergo a number of chemical transformations.
However, urea, along with its positive properties, has its drawbacks. Thus, the process of its dissolution in water is an endothermic reaction, which is accompanied by a decrease in the temperature of the solution. For example, the dissolution of 20 kg of urea in 100 liters of water reduces the temperature of the solution by 9 °C. Such cooling can cause thermal stress in plants, because the temperature of the working solution should not be lower than the air temperature by more than 1012 °C. In corn and sunflower crops, during foliar fertilization in the development phase of 1012 leaves, the concentration of an aqueous solution of urea should not exceed 8%. At more than high concentrations on the surface of the leaves will inevitably cause burns.
In the later phases of the development of these crops, when their leaves become coarser, they are already able to withstand even a 20% concentration of urea. In the case when plants have a high nitrogen deficiency and the concentration of an aqueous urea solution needs to be increased, then another component of magnesium sulfate should be added to its composition at the rate of 3 kg of MdSO4 for every 100 liters of solution. The addition of magnesium sulfate simultaneously with urea reduces the risk of burns and ensures the effective use of nitrogen, since magnesium is part of chlorophyll, and sulfur is up to sulfur-containing amino acids, which play an important role in biochemical processes.
It should also be noted that the determination of the concentration of the working solution should be approached creatively. So, if there is little precipitation, the weather is sunny, the air humidity is low, it is desirable to reduce the concentration of solutions.
The highly effective effect of nitrogen on the growth of grain yields is impossible without sufficient supply of sulfur to plants. A particularly large shortage of sulfur occurs in those fields where the predecessor was winter rapeseed, which greatly reduces the sulfur content in the soil. The ratio of sulfur to nitrogen should be 1:10.
Urea fertilization can be carried out with almost all sprays with fungicides, insecticides and insurance herbicides, if there are no special reservations about compatibility with these pesticides.
When foliar fertilizing, small-drop application of liquid to the surface of the leaf has undeniable advantages (compared to large-scale drip). Small drops cover a large area of the leaf surface of plants, with large-drop application of solutions, the drops hang down, drain, and after drying form salt crystals that can cause deadening of the leaf tissues.
The intensity of assimilation of cations and anions from an aqueous solution of salts is also affected by pH. Cations are better absorbed in an alkaline reaction of the solution (pH 8.3) than in an acidic one, and anions, on the contrary, in an acidic reaction of an aqueous medium. The same dependence exists for the root nutrition of plants.
When only nitrogen fertilizers are presented in the working solution for foliar fertilization, the plants fed by them reduce their tolerance to pathogens. In this regard, phosphorus should also be present in the composition of the tank mixture. The question arises: in the form of which compound is it more appropriate to use phosphorus?
Among the existing assortment of phosphorus-containing substances, potassium monophosphate( potassium dihydrophosphate)is the most effective for foliar fertilization Potassium monophosphate is one of the most highly concentrated and almost ballast-free fertilizers, which contains 52% P2O5 and 34% K2O in its composition. This fertilizer has a high solubility. For example, 226 g of KN2PO4 is dissolved in one liter of water at room temperature.
However, the rate of phosphorus and potassium intake through the biological membrane of the leaves is relatively low. It is possible to increase the intensity of penetration of potassium cations and potassium dihydrophosphate anion with the help of dimethyl sulfoxide.
What does the tandem of urea and potassium monophosphate give to plants? Due to potassium monophosphate in the plant, the synthesis of organic carboxylic acids is accelerated, in which the amino groups of urea are easily attached to form amino acids: in the future, they are used for protein synthesis, thanks to which the plant begins to grow intensively. In addition, potassium monophosphate phosphorus reduces the negative impact of excess nitrogen fertilizer, optimizes the use of nitrogen, increases the efficiency of urea used for foliar fertilization. In addition, phosphorus increases the resistance of plants to most fungal diseases, primarily to powdery mildew and even to root rot.
In addition, the plants increase the resistance of the stems to lodging. And potassium has a positive effect on the hydration of cytoplasmic colloids, helps to retain water better and promotes its rational use, increases the drought resistance of plants. The high content of potassium in the cell juice increases the turgor of cells and protects plants from wilting.
Along with nitrogen, phosphorus, potassium, magnesium and sulfur, microelements are also used for foliar fertilization of plants, which are introduced into the tank mixture in the form of micro-fertilizers in a chelated form. It is worth noting here that trace elements cannot be replaced by other nutrients.to the section