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        Liquid fertilizers part 2 (polyphosphate liquid fertilizers)

        The study of efficiency of polyphosphates as a phosphate fertilizers began globally in the 60s, when the technology for their production was developed. Many studies have also been performed on the comparative efficiency of liquid and solid ammonium polyphosphate and other phosphate fertilizers in the former USSR. Numerous studies have shown that liquid polyphosphates are more efficient than solid granular fertilizers, especially applied on carbonate and heavy soils due to greater mobility of phosphorus.

        Ammonium polyphosphates are liquid complex fertilizers (RKDs) used in agriculture of some countries (most commonly in the United States and Australia). Phosphoric acid and ammonia are the starting compounds for the production of liquid ammonium polyphosphates. Polyphosphates combine all compounds with two or more orthophosphoric acid residues (pyrophosphates, triphosphates, tetraphosphates, pentaphosphates, hexaphosphates, etc.).

        In the soil, polymer chains of polyphosphates are degraded to simple orthophosphate molecules under the influence of enzymes produced by soil micro-organisms and plant roots (pyrophosphatases). Some polyphosphates may also be degraded as a result of non-enzymatic reactions.

        The enzymatic activity of the soil is higher, the better the conditions for the microflora vital activity: humidity, temperature. In most cases, half of the polyphosphates are converted to orthophosphates within 1-2 weeks, but in cold and dry soils this process may take longer.

        Polyphosphates are available in two main grades: 10-34-0 and 11-37-0. These two fertilizer grades differ only in density (1,396 and 1,426 g / cm3), crystallization temperature (-17.8 and 0oC) and water content. Both fertilizers contain 65-75% of phosphorus in the form of polyphosphates and pH of 5.8-6.2.

        Advantages and disadvantages

        The practical advantages of polyphosphates are as follows:

          • Polyphosphates are a basic fertilizer for producing triple RKDs of different composition. They are carried out by cold mixing, adding urea, ammonium nitrate or potassium chloride. When mixing, it is necessary to follow the mixing instructions, and it is better to perform a miscibility test, as crystallization and precipitation are possible.
          • The advantage of polyphosphates is the possibility of producing fertilizers with a high content of nutrients in a clear liquid that is stable over a wide temperature range and with long-term storage possibility.
          • Polyphosphates form complex compounds combined with micronutrients, thus keeping them in a more accessible form for plants. However, the solubility of zinc, manganese and copper sulfates in ammonium polyphosphates is not high. Zinc sulfate can be added in an amount not exceeding 2% of solution, since the boundary line between the liquid solution and the precipitate is very small; manganese is 10 times less soluble in ammonium polyphosphate. Whereas zinc and manganese chelates of EDTA can be dissolved in it in sufficient concentrations. However, in the soil, ammonium polyphosphate is able to convert these micronutrients into a more accessible forms and release them from insoluble compounds. It’s clear that chelated forms of micronutrients are more expensive than inorganic forms. But you also have to consider the higher efficiency of chelates. But in any case, the presence of micronutrients in RKDs increases the efficiency of all fertilizer components.
          • Since polyphosphorus fertilizers contain a combination of ortho- and polyphosphates, plants are able to use phosphorus more efficiently. Polyphosphates can be considered as phosphate fertilizers of prolonged effect. Immediately after application, the plants can use orthophosphates that are present in their composition, but polyphosphates become available after decomposition in the soil. Most polyphosphate fertilizers contain 40% — 60% of phosphorus in the form of orthophosphates, which is sufficient to ensure a rapid plant response to the application.
          • Polyphosphate-based RKDs can be mixed with microbial preparations when applied as a starter.

        However, field experiments rarely demonstrate any benefits of polyphosphate application. The majority of tests for the comparison of ortho- and polyphosphate efficiency show that when the norms are equal, the same increase of crop yield is provided. This is due to the following reasons:

        1) polyphosphates become available to plants only after conversion to phosphate (orthophosphorus) form, which takes several days at normal soil temperature and humidity level;

        2) Only 10-20% of phosphorus contained in fertilizers (any types) is absorbed by plants in the first year after application. Although some tests show a higher absorption rate of the phosphorus from polyphosphates compared to orthophosphates, this had little effect on crop yields.

        3) Phosphate fertilizers based on polyphosphates usually contain up to 50% of the phosphorus in the form of orthophosphates;

        4) In cases of severe phosphorus deficiency in the soil, polyphosphates are not able to release inaccessible to plants soil reserves of nutrients sufficiently;

        5) without zinc application polyphosphates increase its deficiency for plants to a greater extent than orthophosphates, and if zinc is added, polyphosphates and orthophosphates guarantee the same increase of crop yields;

        6) The mobility of polyphosphates in the soil is no higher than that of orthophosphates.

        Since ammonium polyphosphates are often produced from phosphoric acid obtained by wet method, they contain various impurities, such as Fe, Ca, Mg, Al, F, C etc. These impurities can significantly affect the properties of the fertilizer. For example, the presence of carbon and some metals affects the color and transparency of the end product (although these characteristics do not impair the quality of the fertilizer). Other impurities, such as magnesium, can eventually cause precipitation.

        Interaction with the soil

        Over half of the phosphorus contained in polyphosphoric acid-based RKDs is in the form of polyphosphates. The efficiency of such fertilizers is determined by the presence of orthophosphates, the hydrolysis rate of polyphosphates into orthophosphates and the properties of compounds formed by spreading into the soil.

        Liquid ammonium polyphosphates, as well as orthophosphates, create a phosphate regime on sod-podzolic soils, and has an equal effect and aftereffect on the yield. Soil liming does not affect this pattern. In strongly acidic, phosphorus-poor red soil, the effect of liquid polyphosphates is slightly worse than granular orthophosphates.

        In typical and leached chernozems, the effect of liquid polyphosphates on cereals is equivalent to that of liquid and granular orthophosphates.

        In carbonate chernozems, polyphosphate RKDs have a better effect on crop yields compared to granular fertilizers. This is due to the fact that, unlike orthophosphorus fertilizers, when polyphosphates are applied, more available orthophosphates are stored for a longer time in the soil, and a stock of soluble phosphates is formed. Polyphosphates provide plants with zinc on carbonate soils.

        In gray soils, liquid ammonium polyphosphates are better absorbed than orthophosphates. The effect on the crop is equivalent to or higher than orthophosphates. Polyphosphates are a better source of phosphorus than orthophosphates in the aftereffect.

        Polyphosphates enriched with micronutrients are effective.

        Polyphosphates are quite effective on carbonate soils. When growing agricultural crops on carbonate soils, plant analysis often shows a low phosphorus, although soil analysis may indicate a sufficient level. However, despite the high content of total phosphorus in carbonate soils, they show a very high absorption capacity relative to the applied phosphorus from fertilizers. Thus, carbonate soils are particularly critical of phosphorus.

        Orthophosphates applied into the soil quickly interact with the components of the soil solution, turning into less accessible to plants compounds. This is especially relevant to the soils with high pH, carbonate and acidic soils with high phosphorus retrograde potential. Polyphosphates do not react like this due to their polymer structure, which characterizes them as the fertilizers of prolonged effect.

        Alkaline soils are quite common worldwide, and account for the bulk of agricultural land. The problem with these soils is the high degree of phosphorus fixation. Granular phosphorus fertilizers rapidly retrograde (from easily absorbed compounds to elements that hard-to-absorb) in alkaline carbonate soils. Since a high concentration of phosphorus is formed in the area near the granule, it quickly interacts with the immobilizing cations.

        Unlike granular phosphorus fertilizers, which are firmly fixed in the soil, ammonium polyphosphates do not retrograde as much and are even able to mobilize soil phosphorus into more accessible forms for plants. Liquid phosphorus-containing fertilizers performed better than similar granular fertilizers in carbonate soils.

        Methods of application

        Terms and methods of polyphosphate application should be determined by the same criteria as the use of all phosphate fertilizers. Fertilizers should be available to the roots, and be applied as close as possible to the period of greatest nutrient consumption by plants, due to the high ability of phosphorus to interact with components of soil solution and transform into a form less accessible to plants.

        Polyphosphorus fertilizers are suitable for all crops and applied to all types of soils, in autumn or spring during plowing, in pre-sowing cultivation, for row- sowing, as root and foliar feeding, and through irrigation.

        Ammonium polyphosphates are most commonly applied as a starter fertilizer and for early plant feeding, as fertilizer has an ideal nitrogen-phosphorus ratio for many crops.

        Foliar feeding of winter crops is recommended during the tillering and until stem elongation, especially in the fields that have not received sufficient phosphorus level since autumn. The fertilizer is used at the doses of 25-50 l / ha, after diluting it with water.

        Spring is the most appropriate period for RKD fertilizers application in the areas with sufficient moisture, such as Polissya, where soils of light mechanical composition are common. As for the Forest-Steppe and Steppe areas, RKD fertilizers should be applied in spring season only if phosphorus fertilizers have not been applied in the autumn, and in case of sufficient soil moisture or irrigation. Localized placement is the best application method of RKDs.

        RKDs are applied locally by large-drip sprayers to the field surface, in strips before plowing, followed by incorporation by means of plows and cultivators or disc harrows into the soil. There is also a so-called aerosol method of RKD application, or local-volume method, when fertilizers are applied to the soil by means of special devices in the form of a airborne chemical mixture. During such application RKDs are fragmented by an air flow into droplets and arranged along the movement area of paws of the unit, creating a source of nutrients in the active zone of root system, due to which the utilization rate of nitrogen and phosphorus by plants increases 1.5 times. There is also no variegation and evenness of crops.

        RKDs can be applied locally during the sowing, but it should be remembered that fertilizers are better placed 2-7 cm deeper than a row of corn, sunflower, beet seeds. If relatively high doses of RKDs are applied, it is recommended to apply them in two strips on both sides of the row at equal distances. In addition, the distance between fertilizers and sown seeds should be increased to 7-8 cm to prevent an increase in the salt index and increase the osmotic pressure of the soil solution around the sprouting seeds. The optimal depth of RKDs incorporation is 8-10 cm in loamy soils, and 10-12 cm in light soils, when a solid sowing method is used for grain crops. The depth of incorporation increases to 12-15 cm in the arid steppe area and to 15-20 cm for the tilled crops. In areas with sufficient moisture and irrigation, RKD application can be combined with a inter-row cultivation of tilled crops, and in the case of crops cultivation by a solid sowing method. Winter wheat, for example, can be fertilized with a tank solution by spraying on the sowing surface in the spring, and later, during tillering by means of boom sprayers with rubber tubes and drip nozzles that are installed instead of sprayers, which reach the soil surface. RKDs can also be applied with irrigation water if they are supplied to the fields by sprinkler units equipped with fertilizer metering devices.

        Application rates

        RKDs can be used at any stage of plant nutrition due to the liquid form and the content of ortho- and polyphosphates.

        During pre-sowing cultivation RKDs can be applied either on the surface or in furrows with the subsequent incorporation in soil. The application rate is 50-300 kg / ha.

        Liquid fertilizers can be applied as starter fertilizers directly in a row during sowing. Application rate — 20-40 kg / ha. It is recommended to avoid direct fertilizer-seed contact, as fertilizer can cause burns.

        The best method of applying RKDs during sowing is the technology of 5 x 5 cm, when liquid fertilizer is applied 5 cm deeper and away from the seedbed. The application rate is 50-150 kg / ha. T

        Application rate of RKDs during superficial or intrasoil fertilization is 30-100 kg / ha.


        The production of ammonium polyphosphates is based on the reaction of superphosphoric acid, produced primarily from extraction H3RO4, with anhydrous ammonia at a temperature of 310-370°C, wherein the polyphosphate content in the solution reaches 70-75%. After that, the melt is rapidly cooled, dissolved in water and then neutralized with ammonia to maintain a high polyphosphate content in the fertilizer.


        RKDs do not contain free ammonia, so they can be stored and transported in unpressurized containers. The product is inflammable, non-explosive, non-toxic. The formation of phosphate film on the surfaces of ferrous metals makes them virtually non-corrosive. It is transported in tanks as provided bythe shipping rules, which are valid in such mode of transport.


        RKDs can be stored in containers in the territory of the farms during the autumn-winter and summer periods. It should be remembered that the solutions of these fertilizers crystallize at a temperature of -18-20 ° C. Therefore, where the temperature of the coldest decade of the winter month does not fall below 20 ° C, RKDs can be stored in open storage in winter, covering pipes and valves with heat-insulating materials.

        At a temperature of -30 ° C, RKDs are able to supercool and form precipitation, which dissolves when the temperature rises to 20 ° C. At the same time, the chemical composition and properties of these fertilizers does not change. Long-term storage of RKD solutions at temperatures above 20°C results in a change in their chemical composition caused by significant hydrolysis of polyphosphates with the release of crystalline precipitate. If RKD solution is not mixed under these storage conditions, the formed precipitate solidifies and becomes completely insoluble. When a precipitate forms, the quality of RKDs deteriorates — the ratio of nitrogen to phosphorus changes. You can avoid such a negative phenomenon if you store RKDs in the autumn-winter period for a maximum of six months, and in the summer period — for three months. Micronutrient fertilizers in a chelated form, made on the basis of stock solutions, can be added to RKDs. RKDs and nitrogen- and phosphorus- balanced liquid fertilizers with the addition of micronutrient fertilizers can be stored only for 2-3 weeks. In the summer, containers should be covered with insulating material and light- reflecting coating, and their neck areas should be tightly closed.

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