Are fluid fertilizers agronomically better than dry fertilizers? Are polyphosphates (fluid) better than orthophosphates (dry)? These questions arise when considering the relative agronomic merits of various fluid and dry fertilizers. Unfortunately, non-agronomic factors such as ease and uniformity of application, and economics are included in these evaluations; benefits should be based solely on crop yield obtained when materials are compared using similar rates of plant nutrients under similar methods of application.

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Agronomic Comparisons of Fluid and Dry Fertilizers

Experimental data from a wide range of studies overwhelmingly supports the conclusion that there are essentially no differences among the liquid, suspension and dry fertilizers when they are compared over the long term under conditions of similar nutrient rates, placements and chemical forms. The last is particularly important when comparing phosphate fertilizers. For instance, it would not be valid to compare a highly water-soluble phosphate in fluids with a solid phosphate of low water solubility. However, when solids such as diammonium phosphate (DAP), monoammonium phosphate (MAP) or ammonium polyphosphate were compared with fluids such as 10-34-0, 8-24-0 or 11-37-0 under similar conditions, long-term studies have shown these to be essentially equal in nutritive value. Similarly, long-term studies have shown solid urea or ammonium nitrate to be virtually equal to nitrogen solutions, such as urea-ammonium nitrate. Essentially, the same conclusions would be reached with dry and fluid NPK mixes.

Cautions in Comparing Fertilizers

For valid comparisons, studies should be conducted for several years at the same location using the same experimental design to ensure that the variability inherent in field studies does not lead to faulty interpretations. If data are selected from one study, for one year, at one location, evidence can be cited to prove that solids are better than fluids, or vice versa, or that polyphosphates are better than orthophosphates, or vice versa.

Care must be exercised in comparing any solid or fluid fertilizers under field conditions. For example, concentrated superphosphate, 0-46-0 or CSP, can’t be compared directly with 10-34-0 solution or solid monoammonium phosphate, 11-48-0 or MAP, because the latter two contain nitrogen.

For valid comparisons, studies should be conducted for several years at the same location using the same experimental design to ensure that the variability inherent in field studies does not lead to faulty interpretations. If data are selected from one study, for one year, at one location, evidence can be cited to prove that solids are better than fluids, or vice versa, or that polyphosphates are better than orthophosphates, or vice versa.

Care must be exercised in comparing any solid or fluid fertilizers under field conditions. For example, concentrated superphosphate, 0-46-0 or CSP, can’t be compared directly with 10-34-0 solution or solid monoammonium phosphate, 11-48-0 or MAP, because the latter two contain nitrogen.

Fluids and Solids are Equal Agronomically

The relative equality of fluid and dry fertilizers should not be too surprising in light of the fact that the chemical constituents of the two physical forms are usually identical.

The matter of equality of various physical forms is even more predictable when one considers the limited variety of chemical forms presented to the plant root. Although a farmer may apply fertilizer nitrogen as anhydrous ammonia, urea, ammonium nitrate, urea-ammonium nitrate, calcium nitrate or several other forms, the same farmer may be assured that, within a fairly short time, the roots of his crops will be confronted mainly with nitrogen in the nitrate form (NO3). This is because various soil enzymes rapidly convert urea nitrogen to ammonium forms, and then soil microbiological processes fairly rapidly convert the ammonium forms to nitrate. So, for most of the growing season, plant roots "see" mainly nitrates unless a source of ammonium nitrogen is supplied during the season.

Despite the fact that farmers are offered a wide array of phosphorus-containing fertilizers, these farmers are assured that their crops are really confronted with a very limited variety of chemical forms of phosphorus. First, the phosphorus in most fertilizers is present in the orthophosphate form. When an orthophosphate-containing fluid fertilizer is applied or an orthophosphate-containing dry fertilizer dissolves in the soil solution, the plant roots are confronted mainly with two phosphate species (H2PO4- and HPO-2). If a fertilizer material containing polyphosphates is applied, the polyphosphate is fairly rapidly converted in most agricultural soils to the orthophosphate form. So, regardless of the physical or chemical form of phosphate fertilizer, after a short while in the soil, plant roots "see" only two very similar forms of phosphate.

Potassium fertilizers are even more uniform than either nitrogen or phosphate fertilizers. The dominant source of potassium for both fluid and dry fertilizers is potassium chloride. Even when other sources are used, such as potassium phosphate or potassium nitrate, it is the potassium ion (K+) that the plant root deals with in the soil solution.

Implications of Equality

Fluid and dry fertilizers of comparable chemical constituency are essentially equal agronomically when applied at equivalent nutrient rates under similar placements at the same time. This equality of the various forms of both fluid and dry fertilizers is a powerful management tool. It frees the farmer to choose from a wide variety of materials using a multiplicity of non-agronomic factors as criteria for the decision.

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