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        Liquid fertilizers Part 1. (General review)

        Liquid fertilizers (liquid complex fertilizers RKD) are quite popular in the world: in the US liquid ammonia accounts for up to 52% of all nitrogen fertilizers, 22% of complex fertilizers are used in liquid form. Canada is the world’s second largest consumer of liquid fertilizers. Liquid fertilizers are less popular in Europe, due to considerably smaller fields and farms and poor infrastructure for their storage and application.

        Liquid complex fertilizers reached an unusual development after the Second World War. The development of industry was driven by the synthesis of superphosphoric (polyphosphoric) acid, production and technological innovations and the development of suspension fertilizers. The latter “gained momentum” with the highest speed: In 1974, suspension RKDs accounted for 25% of all liquid fertilizers, and in 1984 for up to 40%. However, their popularity has declined significantly in the following years, giving way to pure solutions. RKDs were first manufactured in the U.S. in 1950, in England – in 1957 , in France – in 1960, and later in other European countries. In the former Soviet Union, RKDs have been produced since 1980. That year, farms received the first 26 thousand metric tons of RKD 10-34-0. In 1984, the first million metric tons of RKDs were manufactured in the USSR.

        Unfortunately, Ukraine lost the technology of RKD production in the 90s. And the experience of use is forgotten. Only 10 years ago, RKDs gradually began to appear on the market as a starter fertilizer. However, their amount used cannot be compared to the real demand of the agricultural sector for liquid phosphate fertilizers.

        Over the last five years, Ukrainian farmers’ interest in liquid phosphorus-containing fertilizers has increased rapidly. Fertilizers are now firmly entrenched in the crop production among agricultural holdings, as well as medium and small farms that are ready to use modern efficient technological solutions for plant nutrition.

        Types of liquid fertilizers

        Liquid fertilizers include aqueous and anhydrous ammonia, UAN, liquid complex fertilizers.

        Aqueous ammonia NH3 + NH4OH + H2O (ammonia water) is a highly effective nitrogen fertilizer for many crops. It is one of the most efficient, low cost, high quality nitrogen fertilizers.

        Anhydrous ammonia NH3 (anhydrous liquid ammonia, liquefied ammonia, refrigerated liquid ammonia) is the most concentrated and cheapest nitrogen fertilizer containing 82.3% nitrogen, balance – hydrogen.

        UAN (urea-ammonium nitrate solution) is a solution of urea and ammonium nitrate in water, the mass fractions of which are 31-36% and 40-44% respectively.

        Polyphosphate liquid complex fertilizers are liquid phosphorus-containing fertilizers based on polyphosphoric acid, which contain nitrogen and phosphorus. The polyphosphate fertilizers include NP 10:34 and NP 11:37.

        Orthophosphate liquid complex fertilizers are liquid phosphorus-containing fertilizers based on orthophosphoric acid. They are produced with two, three or more components and different composition ratios. Meso- and microelements can be added to the complexes.

        Forms of RKDs

        There are two main types of liquid complex fertilizers: suspension and solutions. Substances in solutions are completely water soluble, whereas in suspension partially dissolved, and some components, potassium in particular, are suspended in water. Suspending agents (mainly clay materials) are used in Suspension RKDs to prevent the latter from precipitation. Liquid fertilizers can be stored as solutions for a sufficient period of time, while suspensions can be applied for a short period after mixing.

        Suspension RKDs combine the advantages of solid and liquid fertilizers — high nutrient content and preparing the mixtures with different ratios of nutrients. The total nutrient content of Suspension RKDs may exceed 40% with different ratios of nutrients, but such fertilizers have less attractive physical properties.

        There are some restrictions on mixing Suspension RKDs with pesticides. Some contact herbicides, such as paraquat, are inactivated in suspension by interacting with negatively charged clay particles. Suspending agents that do not contain negative charges (e.g., xanthane gum) are recommended, although they are generally more expensive.

        The choice in favor of suspension liquid fertilizers is made if high potassium content is required. There is no specific grade of suspension RKDs, as they can be customized with a wide range of basic nutrients.

        There are two main forms of liquid phosphorus-containing fertilizers: based on ammonium orthophosphate and ammonium polyphosphates. The difference between the different RKDs is the grade of phosphoric acid used in the production process. Most of phosphoric acid for liquid fertilizers is produced by the wet method (extractive phosphoric acid), where pulverized phosphoric ore is treated with acids. But some RKDs are based on phosphoric acid that produced by the dry method — thermal phosphoric acid (it is chemically purer, with fewer impurities, but the price is higher). Polyphosphates are produced by condensation of orthophosphoric acid obtained by both methods (more often by the first one).

        During the production of RKDs based on orthophosphoric acid, the total content of basic nutrients rarely exceeds 30%, as salts are crystallized and precipitated in more concentrated solutions at low temperatures. From the economic point of view, such concentration is low. During production based on polyphosphoric acid, it is possible to achieve a higher concentration of nutrients.

        The addition of the potassium component to the RKDs is the main «limiting factor», since potassium chloride has a low solubility and the content of all nutrients is significantly reduced by adding it. RKDs usually have a low potassium content (it is difficult to reach a degree above 11% K2O, 9% concentration — the most appropriate for production). Therefore, it is recommended that potassium fertilizers be applied separately. Higher potassium content can be achieved with the production of suspension fertilizers.

        RKDs application methods

        RKDs can be applied before plowing and cultivation, spreading them evenly over the soil surface, or placing locally while sowing, and for root and foliar feeding. The application of RKDs in the fertigation system is also promising.
        RKDs can be used for seed coating and pre-sowing treatment. RKDs application by fertilizer injectors is effective.

        Suspension RKDs are applied mainly by the broadcast method over the soil surface before its treatment.

        Foliar RKD fertilizers should be applied in late and cold spring, when plant uptake of phosphorus from the soil is difficult. If necessary, micronutrients, growth regulators, pesticides or microbial preparations can be added to the fertilizers. Foliar feeding of winter cereals is recommended during the tillering and until stem elongation, especially if little phosphorus has been applied in autumn.

        Feeding in the form of solutions is easily absorbed through the root system and the leaf surface. Root feeding is used while sowing and at the stage of seed formation, spraying should be carried out at the stage of leaf emergence and before harvesting. You can easily control plant development, combat disease, the effects of frost or drought, by changing the composition of liquid drugs. The effectiveness is due to the chelated form of nutrients.

        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.

        What is better: liquid or dry fertilizer?

        Very often the main advantage of liquid fertilizers is the presence of moisture, which allegedly in arid conditions turns out to be a «golden key», which opens the door for better absorption of nutrients from the fertilizers. In fact, the amount of water in a liquid fertilizer is incomparable with the amount of water that is already present in any soil. If the fertilizer is applied to very dry soil, there will be no liquid fertilizer remain in solution, and the solid fertilizer will not be dissolved. If the soil is well moistened, then the liquid fertilizer will remain in dissolved form, and the solid fertilizer will also be dissolved. Therefore, it is not correct to say that liquid fertilizer has some advantage due to the presence of water in it. Thus, even in dry years, there is no fundamental difference between liquid and solid fertilizers in terms of phosphorus availability.

        In this case, the advantage is that the nutrients are more evenly distributed in a certain layer of soil when applying liquid fertilizer, than when applying granular fertilizer. Since phosphorus has a very low mobility in the soil, the available phosphorus is localized near the granule once solid fertilizers are applied, and plants will be able to absorb it only when the roots reach the zone of element release.

        Low solubility of granulated fertilizers is a limiting factor that causes their low efficiency. Liquid fertilizers are diffused into the distance from their placement, making them more accessible than solid forms.

        However, numerous studies have shown that the absorption of phosphorus from fertilizers does not depend on whether they are applied in liquid or solid form. Factors such as application method, crop characteristics, soil phosphorus level and climate conditions have a greater influence on phosphorus uptake.

        In most cases, the choice of dry or liquid fertilizers should be based on nutrients price, transportation and storage possibilities, technological practices of the farm. There is no significant agronomic difference in these types of fertilizers. Most liquid fertilizers contain elements in the same form as solids. The efficiency of liquid fertilizers is determined by application method.

        The main advantage of liquid fertilizers over solids is that specific mixtures can be used, thus, the mixtures are homogeneous. It is not possible to achieve this uniformity when mixing solid fertilizers, as it depends on the homogeneity in particle size of individual components.

        In short, the advantage of dry fertilizers is that they provide a slow release of nutrients, and usually can be stored longer than some liquid counterparts without worrying if they will eventually «settle to bottom» over time. They are usually cheaper to buy in large quantities. Dry fertilizers are the better option for fields preparation befor planting, as the nutrients stay longer in them.

        Liquid fertilizers take up less storage space and provide more even distribution of nutrients. They are good for replenishment of specific nutrients throughout the life cycle of crops.

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