Liquid complex fertilizers (LCF)
Liquid complex fertilizers (LCF) — complex fertilizers that are aqueous solutions or suspensions containing major nutrients, sometimes with additions of micronutrient fertilizers, pesticides, and plant growth stimulants.
Compared with solid fertilizers, the advantages of liquid complex fertilizers are simplicity of manufacture and lower capital and operating costs. The ratio of nutrients in LCF can be adjusted within wide limits. Unlike liquid nitrogen fertilizers, LCF do not contain free ammonia.
Tests have shown that the use of solid and liquid complex fertilizers is equivalent. Somewhat greater effectiveness of LCF has been noted on carbonate soils and base-saturated soils.
Liquid complex fertilizers are among the promising types of fertilizers. The production scheme consists in neutralizing phosphoric acid with ammonia to pH 6.5. There are two types of LCF, whose production differs in the acid used: orthophosphoric and superphosphoric.
Ammonium nitrate, urea, or a mixture of them are used as a nitrogen source for LCF. Urea makes it possible to obtain a more concentrated fertilizer, especially when potassium is present in the solution, since potassium nitrate formed when ammonium nitrate is added to the solution is the least soluble salt in liquid fertilizers.
Liquid complex fertilizers based on thermal orthophosphoric acid are almost transparent liquids; those based on extraction orthophosphoric acid are turbid solutions (due to the formation of aluminum and iron phosphates and silicic acid). The concentration of nitrogen-phosphorus LCF based on superphosphoric acid is higher than that based on orthophosphoric acid.
When thermal orthophosphoric acid is used, liquid complex fertilizers are obtained with a nutrient ratio of 9:9:9, totaling 27% N, P2O5, and K2O. Crystallization of the solution does not allow the nutrient content to be increased. A typical composition of grade 9:9:9 is: (NH4)2HPO4 — 12–15%, NH4P2O4 — 2–4%, (NH2)2CO — 12–13%, KCl — 13–14%. Amide nitrogen constitutes 61–66% of the total. These fertilizers can also be obtained from extraction phosphoric acid. Because of the low nutrient content, their local use is economically feasible. A good economic effect of LCF is provided by their application with irrigation water, including in orchards, berry plantings, and vineyards.
Table. Ratio of major nutrients in liquid fertilizers obtained from orthophosphoric and superphosphoric acids.
| N:P2O5K2O | Orthophosphoric acid | Superphosphoric acid |
|---|---|---|
| 4:1:0 | 16-4-0 | 24-6-0 |
| 3:1:0 | 18-6-0 | 24-8-0 |
| 2:1:0 | 16-8-0 | 22-11-0 |
| 1:1:0 | 13-13-0 | 19-19-0 |
| 1:2:0 | 9-18-0 | 15-30-0 |
| 1:3:0 | 8-24-0 | 12-36-0 |
When polyphosphoric acid is used, owing to the high solubility of ammonium polyphosphates, base solutions and balanced fertilizers with higher concentration are obtained. In LCF based on polyphosphoric acid, micronutrients can be introduced; they are chelated by polyphosphates, remaining available to plants, whereas orthophosphates of micronutrients, except boron, form insoluble compounds. Micronutrients are added in the form of oxides, since this ensures high solubility and stability of the solutions. Micronutrients are introduced into base solutions (8:24:0; 10:34:0; 11:37:0) at a temperature of 50–90°C. Base solutions obtained on the basis of polyphosphoric acid can be applied directly as fertilizer or used for further mixing with nitrogen and potassium components.
The source of potassium for LCF is potassium chloride. Because of insufficient solubility, it reduces the concentration of the liquid fertilizer. Potassium nitrate is less soluble; it is formed when ammonium nitrate or a urea-ammonium nitrate mixture is used as an additional nitrogen component. Urea somewhat increases the overall solubility of the system.
In the USA, potash fertilizer is applied separately in autumn or introduced into liquid complex fertilizers as suspensions. Therefore, LCF of composition 10:34:0 is better used on soils sufficiently supplied with available potassium. In this case, potassium fertilizers in crop rotation are applied once every 2 years under potassium-demanding crops.
The introduction of stabilizing additives into the solution, for example colloidal clay or silicic acid, protects the supersaturated solution from crystallization. To prepare 1 t of fertilizer, 9–22 kg of dry clay is consumed. A 28% clay suspension in pure form is recommended for use; first a 10:34:0 solution is introduced into it, then a urea-ammonium nitrate mixture, and lastly potassium chloride. Red flotation potassium chloride with a particle size of 0.8–1 mm is suitable for suspensions. The total nutrient content in suspended LCF reaches 40–45%. Attapulgite or bentonite clays (1.0–1.5%) are used as stabilizing additives for suspended LCF.
Liquid complex fertilizers are manufactured by hot and cold mixing methods. In hot mixing, at a temperature of 210–250°C, phosphoric or polyphosphoric acids are neutralized with ammonia; this is carried out at large enterprises, producing base solutions of ammonium ortho- and polyphosphates. The cold mixing method at 35–45°C is used at small plants near the areas of application; fertilizers with a specified nutrient ratio are produced by introducing urea, ammonium nitrate, and potassium salts into the base solutions.
Liquid complex fertilizers do not contain free ammonia, therefore they can be sprayed over the field surface with subsequent incorporation, and transportation is not necessarily in hermetically sealed containers.
Liquid complex fertilizers are nonflammable, non-explosive, and non-toxic.
With special machines, liquid complex fertilizers are applied locally, in bands, under any crops. They are used on irrigated lands and with irrigation water.
The application of suspensions requires a special set of machines different from mechanized means for applying ordinary LCF. The domestic industry produces liquid complex fertilizers of grades 8:24:0 and 10:34:0; production of a more concentrated solution, 11:37:0, has been mastered.
Table. Characteristics of some properties of @abbr[Liquid complex fertilizers|LCF].
| Fertilizer grade | Total nutrients, % | Specific gravity, g/cm3 | Amount of nutrients, kg/m3 |
|---|---|---|---|
| 9:9:9 | 27 | 1,24 | 335 |
| 10:34:0 | 44 | 1,35 | 594 |
| 11:37:0 | 48 | 1,40 | 672 |
| 12:12:12 (suspension) | 36 | 1,35 | 486 |
The use of LCF makes it possible to mechanize loading and unloading processes, eliminate losses during transportation, storage, and application to the soil. At the same time, manual labor is excluded and costs are reduced.
The advantages of liquid complex fertilizers also include: automated control of fertilizer distribution across the field, and the possibility of co-application of herbicides, insecticides, and micronutrients.
The economic effect is associated with a reduction in capital costs due to the elimination of some stages of the production process, for example, drying and granulation. Capital costs for the construction of shops for the production of LCF are 20–30% lower than for solid fertilizers. Even at equal cost, labor costs for their use are 3–3.5 times lower. Transportation and application of LCF are 2–2.5 times cheaper than solid fertilizers.
The introduction of LCF requires the creation of special machines. It must be taken into account that these fertilizers (especially suspended ones) are characterized by corrosive activity.
Liquid fertilizers interact with the soil more completely than granular fertilizers. The rate of interaction determines the nature of the compounds formed, their solubility, and their availability to plants.
Application features and effectiveness of liquid complex fertilizers
Application features of liquid complex fertilizers:
1. When using LCF based on orthophosphoric acid on acidic soil that fixes phosphorus, for example, red soil, at a low phosphorus content, as well as on poor acidic sod-podzolic soils, the effect of LCF is weaker than that of granular forms. This is noted when applying a complete LCF with a ratio of 1:1:1 and an additional nitrogen component (ammonium nitrate). When using an unbalanced solution with an N:P2O5 ratio of 1:4.5 or 1:3, no decrease in the effect of the phosphate component on acidic sod-podzolic soil is noted.
2. On limed sod-podzolic soil and chernozems, liquid complex fertilizers and granular fertilizers are equivalent.
3. On carbonate soils with an alkaline reaction, for example on carbonate chernozems, chestnut soils, and serozems, the agrochemical value of liquid forms is often higher than that of granular forms.
4. On acidic sod-podzolic soil, a short-term decrease in the content of mobile phosphorus is noted when the solution is applied, which is associated with the fixation of phosphates by sesquioxides. On chernozems this is not observed. On serozem, after the application of LCF, the amount of mobile phosphorus increases compared with the application of granular fertilizer.
5. The effectiveness of LCF is determined by the phosphorus and nitrogen components included in its composition. For example, LCF with ammonium nitrate on acidic sod-podzolic soil and red soil is less effective than solid granular fertilizer, while with urea it is equivalent. On typical chernozem with a slightly acidic reaction and on serozems, the form of the nitrogen component does not affect the action of the fertilizer: the effectiveness of solutions and granular fertilizers is equivalent. Solutions are a better source of phosphorus for plants than granular forms. The presence of urea in the fertilizer has a positive effect on the accumulation of mobile phosphorus in acidic soils and is of no significance on chernozems and serozems, which is due to the temporary alkalization of the medium during the transformation of urea.
The effect of LCF on product quality (grain, potatoes, hay) is also equivalent to solid fertilizers.
The effect of suspended fertilizers coincides with the effect of LCF and depends on the properties of the nitrogen and phosphorus components. The suspending agent does not affect the effectiveness of liquid fertilizers.
In LCF based on polyphosphoric acid, half of the phosphorus is in the form of polyphosphates. The effectiveness of such fertilizers is determined by the presence of orthophosphates, the rates of hydrolysis of polyphosphates to orthophosphates, and the properties of the compounds that are formed when applied to the soil. Patterns of action of polyphosphate LCF—solutions 10:34:0 and 11:37:0 with a phosphorus content of 45–65%:
1. On sod-podzolic soils, liquid ammonium polyphosphates create a phosphate regime the same as orthophosphates and affect the yield equally, both in direct action and in aftereffect. Liming of soils does not affect this pattern. On strongly acidic, phosphorus-poor red soil, the effect of liquid polyphosphates is somewhat worse than that of granular orthophosphates.
2. On typical and leached chernozems, the effect of liquid polyphosphates on cereal crops is equivalent to the effect of liquid and granular orthophosphates.
3. On carbonate chernozems, polyphosphate liquid complex fertilizers show a better effect on the yield of agricultural crops compared with granular fertilizers. This is explained by the fact that when polyphosphates are applied, a greater amount of available orthophosphates remains in the soil for a longer time and a reserve of soluble phosphates is formed than against the background of orthophosphate fertilizers. On carbonate soils, polyphosphates promote the supply of zinc to plants.
4. On serozems, liquid ammonium polyphosphates are assimilated better than orthophosphates. Their effect on yield is equivalent to or exceeds that of orthophosphates. In aftereffect, polyphosphates are a better source of phosphorus than orthophosphates.
Polyphosphates enriched with micronutrients are effective.
References
1. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. by B. A. Yagodin. Moscow: Kolos.
2. Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Ed. by Mineev V. G. Moscow: Publishing House of VNIIA named after D. N. Pryanishnikov.
