Michigan State University Extension
Soils & Soil Management - Fertilizer - 06209701
07/10/97
destroy previous editions
by R. E. Lucas and M. L. Vitosh, Department of Crop and Soil Sciences
Fluid fertilizers are increasing in popularity in Michigan. There are many misconceptions about fluid and dry fertilizers being promoted in the fertilizer industry. This bulletin aims to sort out the facts and highlight important differences between fluid and dry fertilizers.
Fluid Fertilizers
A fluid fertilizer contains one or more available plant nutrients. Two types are recognized: liquids in which all ingredients are in solution, and suspensions, which require periodic agitation of the solution to keep certain ingredients from settling out. Liquids may be clear or dark. The color is an indication of the amount of impurities in the liquid. Suspensions contain both soluble and solid fertilizer and usually contain colloidal clay to prevent rapid settling of the solid fertilizers. This allows the formulator to make a higher analysis N-P-K fertilizer.
Fluid-mixed fertilizers, either clear liquids or suspensions, are made by two types of processes, generally known as hot-mix and cold-mix. Traditional cold-mix fertilizers are made by mixing ammonium polyphosphate (10-34-0), nitrogen solutions (28 to 32 percent N) and soluble potash (62 percent K2O). There is no heat of reaction when these materials are mixed. The hot-mix process often involves the neutralization of orthophosphoric or superphosphoric acid with ammonia to make ammonium polyphosphate and then the addition of other materials to get the desired formulation.
There are essentially two grades of phosphoric acid used in the production of fluid mixes: furnace acid (white) and wet-process acid (green, black or merchant grade). Wet-process phosphoric acid is used most frequently because of its lower cost, but it does contain some impurities, such as iron, aluminum and magnesium. In the process of making ammonium polyphosphate, however, the impurities are sequestered and held in solution, thus eliminating any problems of precipitation or settling out. Furnace acid, which is more costly to produce, is sometimes used to make a higher analysis ammonium polyphosphate (11-37-0). Certain clear liquid mixes are made with 1137-0 using potassium hydroxide (caustic potash). The additional cost of both furnace acid and potassium hydroxide often makes these mixes two to three times more expensive than comparable fluid fertilizers made from wet-process acid and soluble potassium chloride.
Agronomically, all liquid mixes are equivalent when applied to the soil in equal nutrient amounts, regardless of the process by which they are made or the materials they are made from. It's possible to make slightly higher analysis clear liquid mixes with potassium hydroxide than with potassium chloride, but identical fertilizer grades can be made if potassium chloride is added to a clay-base suspension. Suspensions must be periodically agitated to prevent settling or salting out and therefore should not be stored for long periods of time. Cold temperatures promote settling or salting out. If settling occurs and agitation does not appear to dissolve the crystals, it may be necessary to add additional water and heat the solution.