Among the main plant nutrition elements (NPK), agronomists from all over the world pay the most attention to phosphorus. Unlike many other elements, phosphorus applied to the soil practically does not move from the application zone. Its distinctive feature is rapid and fairly strong fixation by the soil, as a result of which plants can absorb only a small share of the applied fertilizers (on average 25-30 when applied by the continuous method and 40-50 when applied locally).

In Ukraine, the problem of phosphorus in agriculture is complicated by the constant underapplication of phosphorus fertilizers, as well as the violation of the optimal ratio between phosphorus, nitrogen and potassium.

The importance of phosphorus for plants is difficult to overestimate, since it takes part in the key functions of the plant organism. First of all, this is energy metabolism: phosphorus is a component of the "energy currency" of the plant - ATP. Normal provision of plants with phosphorus is necessary for the formation of seeds, stimulating the accelerated ripening of crops, in particular cereals. At the initial stages of plant growth, phosphorus affects the development of the root system, its resistance to diseases and pests, as well as adverse abiotic conditions (for example, it reduces the risk of winter crops freezing).

Phosphorus is quite mobile in the plant; if there is a deficiency in the soil, it moves from older to younger plant organs and to reproductive organs, as a result of which deficiency symptoms appear on old leaves (as a dark green with a purple tint).

PHOSPHORUS EFFICIENCY OF CROPS

Not all plants absorb phosphorus from soil and fertilizers equally. And this must be taken into account when administering phosphorus fertilizers. Phosphorus-efficient crops and varieties have an increased ability to absorb phosphorus. This is due to a number of reasons, such as the size and morphology of the root system (distribution of roots in the soil); the ratio between the underground and aboveground parts of the plant; the formation of mycorrhiza (except for the Brassicaceae family and some others); changes in the pH of the rhizosphere (can reach 2 units, due to the balance between the absorption of cations and anions, increased release of protons in response to phosphorus deficiency); an increase in the phosphatase activity of the rhizosphere (hydrolysis of organic phosphorus-containing compounds in the soil); secretion of organic acids by roots (form citrate, malate, fumarate, oxalate, etc. anions, increasing the solubility of inorganic phosphorus compounds in the soil; this property is especially evident in crops such as lupine, buckwheat, and rapeseed).

Phosphorus efficiency differs not only for different crops, but also for different varieties/hybrids of the same crop. The characteristics of the root system of a variety play an important role in the absorption of phosphorus from the soil. For example, some varieties have a well-developed root system that explores a large volume of soil and “gets” to a larger amount of phosphorus. Other varieties are characterized by a more aggressive reaction of root secretions, which allows them to mobilize soil phosphorus reserves and use them more efficiently. Some varieties form longer root hairs, which increases the area of ​​contact between the root and the soil.

Crops such as onions, tomatoes, and beans are characterized by low levels of phosphorus absorption from the soil and a low root to aboveground mass ratio. They can be characterized as crops with low phosphorus efficiency. At the same time, rapeseed and spinach, for example, have an increased ability to absorb phosphorus from the soil, and rye and wheat have a high ratio of roots to above-ground mass, which transfers them to the category of phosphorus-efficient crops.

In the soil, phosphorus is the least mobile among NPK, which often puts it in the category of limiting factors: phosphorus deficiency is the second most common after nitrogen. More than 80% of soil phosphorus is directly unavailable to plants. And its high retrogradation (conversion into poorly accessible compounds) is a “stumbling block” to successful technology for applying phosphorus fertilizers.

HOW MUCH AND WHEN TO APPLY?

Where does successful phosphorus management begin? The very first question an agronomist looks for an answer to is: is there a need to add phosphorus, and if so, how much should be added? As for the first part of the question, theoretically there is always a need to add phosphorus. Firstly, it is necessary to replenish the removal of the element by the crop; secondly, as numerous studies show, even on well-provided soils, a number of crops, as a rule, respond well to the application of even small rates of phosphorus during sowing; thirdly, optimization of plant nutrition implies a certain ratio between the elements in the soil solution, which changes when other elements are added.

There are a number of calculation methods for establishing the rates of phosphorus fertilizers, the results of which sometimes differ slightly. These include the method of recommended rates, and the calculation of phosphorus fertilizer rates for saturation and/or maintaining the optimal content of mobile phosphorus in the soil, and normative calculation methods, and balance-calculation methods. When choosing any method, it is necessary to conduct a soil analysis at least once every 3-4 years to control the phosphorus content: a decrease in the content of available phosphorus in the soil indicates depletion of soil reserves, while a strong increase may indicate phosphatization of the soil. Plant diagnostics are also effective, helping to understand how well the plants are supplied with phosphorus during the growing season, and therefore how correctly the fertilizer application rate was calculated.

When establishing the application rates of phosphorus fertilizers, the following factors must be taken into account: the biological characteristics of the crop, the depth of penetration and the nature of the development of its root system, soil pH, the content of organic matter in the soil, soil structure, predecessor, etc.

The second issue following the calculation of the phosphorus rate is its distribution by application time. The fact of low phosphorus mobility in the soil should be key in the management of phosphorus fertilizers. There is no one universal method of applying fertilizers to all crops. The decision should be made based on the characteristics of the crop and the phosphorus status of the soil.

With low phosphorus content in the soil, the most rational is a combination of row application of phosphorus and broadcast application before sowing. With an average level of available phosphorus in the soil, phosphorus fertilizers can be applied either broadcast before sowing or during sowing in rows (at a reduced rate). With a high phosphorus content in the soil, the response to phosphorus fertilizers is low, the best option is to apply in rows during sowing. With a very high phosphorus content, there is no need to apply phosphorus under a traditional farming system. When using minimum and no-till technologies, the decision must be made in each specific case.

The choice of timing and methods for applying phosphorus fertilizers becomes especially relevant on soils characterized by a high capacity for binding phosphorus (primarily acidic soils, as well as alkaline and carbonate soils). To reduce the retrogradation of phosphorus in the soil, it is recommended to apply water-soluble phosphorus fertilizers in granular form (smaller contact area with the soil) locally closer to the period of plant consumption (pre-sowing and at sowing).

The starter is most effective when sowing seeds in cold, wet soil, regardless of the level of soil fertility. In cold and wet soil, the growth and development of the root system of plants slows down. Along with this, under such conditions, the mobility of nutrients in the soil decreases, and the mineralization of organic matter slows down (as a result of reduced activity of soil microflora). Such conditions most often develop in early spring or late autumn, although soils processed using resource-saving technologies are also usually colder and wetter compared to traditional processing technologies due to the presence of plant residues on their surface. Thus, when using the above technologies, the introduction of a starter becomes even more important. Phosphorus is the only NPK element that can be applied at the full required rate during sowing. In this case, the rate can be reduced compared to the main application. However, it is important to understand that the “shift” in favor of pre-sowing phosphorus may not always be successful: for crops with a well-developed root system that begins to develop the lower soil horizons, the phosphorus added during sowing may end up above the zone of primary root development; in addition, under unfavorable weather conditions, phosphorus remains in the upper dry soil layer, from where plants cannot effectively absorb it. The latter primarily concerns the traditional soil cultivation system. With resource-saving technologies, the situation may look different.

Today in Ukraine, pre-sowing fertilization often becomes the only source of phosphorus in the fertilization system of many crops. This measure often turns out to be quite effective, given the high retrograding capacity of our soils in relation to phosphorus. However, long-term application of fertilizers in rows ultimately leads to soil depletion, a decrease in its fertility. Even on highly fertile soils, the “Libig law” or “law of return” is not canceled.

From the point of view of maintaining soil fertility, long-term row application of phosphorus can lead to a decrease in the content of available phosphorus. The same can be expected with long-term application of phosphorus at low rates. In this case, periodic scattered application of increased rates of phosphorus is recommended to maintain the phosphorus content in the soil at an average and increased level.

The choice in favor of row or continuous application of phosphorus should also be made based on the characteristics of the crop being grown. Row crops, in particular corn, prefer conditions where phosphorus is evenly distributed throughout the root development zone. The best effect can be achieved by combining both methods of phosphorus application.

Cereals have a poorly developed root system, and therefore the ability to "explore the soil". In addition, they have a short growing season, which also occurs during the cold season. Therefore, the placement of phosphorus for these crops is more critical than for row crops and perennials. They often respond better to spot application, especially on low phosphorus soils and when retrogradation is high. On phosphorus-rich soils, spot and uniform application are almost equally effective.

For soybeans, pre-sowing broadcast application of phosphorus fertilizers has an advantage. This difference from cereals is explained by the difference in root system development.

Alfalfa and other forage crops, both cereals and legumes, form a large number of small roots near the soil surface, so they are able to effectively absorb phosphorus fertilizers broadcast annually. For them, pre-sowing application of phosphorus and annual surface broadcast application are the best practice.

A promising method of phosphorus application is foliar feeding. In this case, the utilization rate of phosphorus from fertilizers increases significantly. Phosphorus applied to the leaf is quickly involved in the plant's metabolism.

Foliar application of phosphorus has a number of limiting factors. Firstly, this is the solubility of the fertilizers themselves (until recently, most phosphorus sources had low solubility in water and a large amount of ballast impurities). Secondly, the volume of the solution falling on the plant cover and the concentration of phosphorus in it, which can be applied without a negative effect on the leaf (for example, urea can be applied to the leaf in much higher concentrations than many phosphorus sources, without harming the plant). Thirdly, at the beginning of the season, when the phosphorus problem is especially acute, the leaves of the plant still have a small design coverage, which makes foliar application ineffective.

Noticeable efficiency from foliar application of phosphorus can be achieved most often under conditions of element deficiency in the soil, as well as under unfavorable weather conditions, when the absorption of phosphorus by the root system is complicated (low temperature and low humidity, poor lighting).

In some cases, most of the plant's need for phosphorus can be met by foliar application. However, under conditions of insufficient soil supply with available phosphorus, its foliar application is not able to meet the needs of the crop.

THE BEST FORM FOR APPLICATION

The third question that worries every agronomist: in what form should phosphorus be applied? And the answer to this question depends on the characteristics of the soil, the application period, weather and climate conditions, the crop being grown and, last but not least, on the material and technical capabilities of the farmer.

Today, the domestic market offers two groups of phosphorus-containing fertilizers: water-soluble and acid-soluble. Water-soluble ones, which include superphosphates, ammophos, diammonium phosphate, nitroammophoska and other complex fertilizers, quickly release phosphorus into the soil solution. They are recommended for application at all times and on all types of soil, but it should be remembered that phosphorus from these fertilizers easily interacts with the components of the soil solution, which is especially important on soils with a high capacity for binding phosphates.

Acid-soluble phosphorus fertilizers, such as phosphate rock, are recommended for application to acidic soils during primary tillage, since the phosphorus in them is in the form of tri-substituted phosphates, which is inaccessible to plants, and can only be absorbed after their interaction with an acidic soil solution. It should be noted that this group of fertilizers is significantly inferior in popularity to water-soluble forms.

According to the formula, the main phosphorus fertilizers are currently presented in the form of salts of orthophosphoric and polyphosphoric acids. Phosphorus fertilizers based on orthophosphoric acid are mainly solid fertilizers, and polyphosphoric ones are liquid. Most experiments comparing the effectiveness of ortho- and polyphosphates indicate that, when applied at the same rate, they give the same increase in crop yield.

Phosphorus absorption by plants does not depend on the form in which the fertilizer is applied (solid or liquid). The absorption of the element from liquid and solid phosphorus fertilizer depends on such factors as the method of application, the characteristics of the crop and the development of its root system, soil analysis, and climatic conditions. The amount of water present in liquid fertilizer is insignificant compared to soil moisture. The liquid form of fertilizer allows for a more uniform distribution of phosphorus in the soil layer and quickly saturates the soil solution with available phosphates.

SYNERGISMS WITH OTHER ELEMENTS

To increase the efficiency of phosphorus fertilizers, it is necessary to consider its synergistic relationships with other elements. The application of nitrogen fertilizers increases the absorption of phosphates by the plant. It should be remembered that phosphorus itself has a low salt index and can be applied with seeds, while nitrogen and potassium, applied with phosphorus, have a high salt index and can cause damage to young seedlings. In this case, it is recommended to apply according to the 5x5 scheme (when fertilizers are applied 5 cm below and 5 cm away from the seeds).

When applied at sowing, the best effect is obtained from a combination of phosphorus with the ammonium form of nitrogen: ammophos (MAP), ammonium polyphosphate (APP). Ammonium polyphosphate is often used when applied with seeds (pop-up). Other liquid fertilizers are also effective, and pesticides and microelements can be added to liquid fertilizers if necessary (subject to a preliminary compatibility test).

INNOVATIONS IN THE FORM OF MICROGRANULES

To solve the problem of poor solubility of solid fertilizers when applied during sowing, many companies are taking the path of creating microgranules. For example, these are Easy Start fertilizers (by the German company Compo Expert), Physiostart (by Timac Agrо) and others, which are present on the Ukrainian market. The creation of microgranules (0.5–1.5 mm in size) improves the solubility of the fertilizer, its availability to the plant in the initial period, and ensures better uniformity of application. A larger number of granules per unit area of ​​application contributes to better contact of the roots with the nutrients. According to the manufacturers of microgranules, such a formulation of fertilizers, due to an increase in the utilization factor of nutrients, allows for a reduction in application rates, also increasing the economic component.

For foliar application of phosphorus, monopotassium phosphate, liquid fertilizers and a number of phosphorus-containing foliar fertilizers can be used. As for the latter, the amount of phosphorus contained in them at the recommended application rate will not be enough to eliminate the phosphorus deficiency in the soil.

It should also be noted that foliar fertilizers available on the market that contain phosphorus in the form of phosphite ion (HPO32-) rather than phosphate (PO43-) are not a source of macronutrients, but rather a growth stimulator with protective properties against various diseases.

Thus, successful phosphorus management should be based on understanding the processes of phosphorus interaction with soil and the requirements of the crop being grown. Guided only by the economic possibilities of purchasing and applying phosphorus fertilizers and relying on soil phosphorus, it is possible to significantly reduce the effectiveness of other fertilizers applied, disrupting the optimal ratio between the elements.