Actually, this should be titled "Phosphate Fertilizers" since phosphorus (P) is commonly available only as a part of the compound with oxygen called phosphate which has the chemical formula of P2O5 .

One of the challenges of phosphorus fertilizers is their level of water solubility. The way nutrients are absorbed by plants is when they are in water solution. So, the more soluble the nutrient, the easier it is to move it into the plant. However, solubility also means that the nutrient is likely to leach through the soil faster also. Of the three nutrients commonly found in commercial fertilizers, nitrogen is the most water soluble, followed by potassium (potash) with phosphorus (phosphate) being the least soluble.

Phosphorus is strongly attracted to clay particles in the soil. Each year, only a small percentage of the phosphorus applied will be used during that season. For this reason, it will build up over time in such soils so periodic soil tests should be taken. After several years of adding phosphorus to the soil, you may find that you do not have to use any for several years.

Phosphate Fertilizers
  • Complete Fertilizers - These products contain all three of the major plant nutrients and the percentage of each must be limited on the label. For example, a typical fertilizer might have an analysis of 18-24-12. This means that it contains 18% nitrogen, 24% phosphorus in the form of phosphate (P2O5) and 12% potassium as potash (K2O).

  • Rock Phosphate - This is the mineral product extracted from the earth and used in early forms of commercial fertilizers. It is not very soluble in water and is generally not found for sale. Rock phosphate is now "processed" and combined with other elements in modern fertilizers.

  • Normal Superphosphate (0-20-0) - The original type of superphosphate fertilizer (ordinary superphosphate) has generally been replaced by newer, higher analysis products. Normal superphosphate contained sulfur content which made it useful for plants such as boxwood, rhododendron, azaleas and pin oak which need an acidic soil.

  • Concentrated Superphosphate (0-46-0) - Commonly sold as triple superphosphate, this product is often used for encouragement of roots in transplant type of fertilizers.

  • Diammonium Phosphate (18-46-0) - This form of phosphate is very water soluble so it works its way into the soil well. It also contains nitrogen so it is a fertilizer that can be used if potassium is not needed. The ammonium form of nitrogen is good for acid loving plants.

  • Monoammonium Phosphate (11-48-0) - The key difference between this and diammonium phosphate is a lower level of nitrogen which might be needed by a particular crop. Other than that, the impacts are the same.

  • Polyphosphates - Most dry fertilizers have what are called orthophosphates while nearly all liquid fertilizers use a polyphosphate form of phosphorus.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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