Fertilizers
Phosphorus Fertilizers
Commercial phosphate fertilizers are manufactured using phosphate rock. Rock phosphate is insoluble in high and neutral pH soils so must be treated with acid before it can act as an active ingredient in fertilizers. It is also important to know that almost all phosphorus fertilizers sold today contain highly water-soluble phosphorus.
Monoammonium Phosphate
Monoammonium phosphate (MAP) has been an important granular fertilizer for many years. It’s water-soluble and dissolves rapidly in adequately moist soil. Upon dissolution, the two basic components of the fertilizer separate again to release ammonium (NH₄¯) and phosphate (H₂PO₄), both of which plants rely on for healthy, sustained growth. The pH of the solution surrounding the granule is moderately acidic, making MAP an especially desirable fertilizer in neutral- and high-pH soils.
Diammonium Phosphate
Diammonium phosphate (DAP), is the world’s most widely used phosphorus fertilizer. Diammonium phosphate fertilizer is an excellent source of phosphorus and nitrogen for plant nutrition. It’s highly soluble and thus dissolves quickly in soil to release plant-available phosphate and ammonium. It provides the correct proportion of phosphorous and nitrogen for crops such as wheat, barley, fruits, and vegetables.
Triple Superphosphate
Triple superphosphate (TSP) has the highest phosphorus content of dry fertilizers that don’t contain nitrogen. Over 90 percent of the total phosphorus in TSP is water soluble, so it becomes rapidly available for plant uptake. As soil moisture dissolves the granule, the concentrated soil solution becomes acidic. Triple superphosphate also contains 15 percent calcium, providing an additional plant nutrient. If TSP is used in strongly acidic soils, its phosphorus incorporating with iron and aluminum might convert into unavailable forms.
Superphosphate
Superphosphate (OSP) containing 20 percent phosphate, results from the reaction of rock phosphate with sulfuric acid. This material is half gypsum (calcium sulfate) and half calcium phosphate. Being lower in phosphate than other carriers, it is no longer used by most growers.
Ammonium Polyphosphate
Polyphosphate fertilizers were specifically developed to provide phosphorus and other essential nutrients to growing crops. The most commonly applied polyphosphate is ammonium polyphosphate (APP). Plants are only able to take up phosphorus in the orthophosphate form. As a result, users can usually expect 30 percent of their APP solution to be in orthophosphate form and immediately available to the plant. The remaining 70 percent will be in polyphosphate form at the time of application and will be converted in the soil to the orthophosphate form for the plant to use later in the season.
Orthophosphates versus Polyphosphates
Commercial phosphate fertilizers contain phosphorus as orthophosphates, polyphosphates, or a mixture of the two. When assessing phosphorus fertilizer options, some information sources may recommend using fertilizers with phosphorus available primarily in the orthophosphate form (i.e., monoammonium phosphate, diammonium phosphate, and triple super phosphate) while polyphosphate form (i.e., ammonium polyphosphate and urea ammonium phosphate).
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