Soil Phosphorus

Soil Phosphorus is provided by the weathering of certain minerals, primarily the mineral apatite, which is a calcium phosphate mineral. As apatite weathers it gives off a group of anions that can be used by plants. These ions are primary orthophosphate and secondary orthophosphate.

Ionic Charge of Phosphorus

Ionic Charge= -3

Ionic form of Phosphorus is HPO4-2, H2PO4,

Ion Name= Orthophosphates

pH of Phosphorus

In strongly acidic soils (pH 3.5-4.5) insoluble iron phosphates form. Between pH 4.0-6.5 much of the phosphorus reacts with aluminum. Calcium phosphates are important between pH 7.0-9.0. Maximum availability lies at pH 6.5 in mineral soils but the range 6.0-7.0 is satisfactory for most crops.

Sources of Phosphorus

Rock Phosphate; mineral fertilizer, finely ground, spread on acid soils

Superphosphate; containing 20% phosphate, results from the reaction of rock phosphate with sulfuric acid

Treble Superphosphate; 46% phosphate, treated rock phosphate

Phosphoric Acid; 53% phosphate, highly corrosive liquid in ure form, used to prepare fluid fertilizers

Ammonium phosphates; made by mixing phosphoric acid with ammonia, make MAP and DAP.

Bone Meal and Manure; organic sources of phosphate.

Phosphorus fertilizers are obtained by the mining of rock phosphate in Florida and other states.

Functions of Phosphorus in Plants

Phosphorus spurs growth. It is part of the genetic material and so is involved in plant reproduction and cell division. It is part of the chemical that stores and transfers energy in all living things. Without it, all biological reactions come to a halt. Phosphorus helps young plants develop roots; speeds maturity; aids blooming and fruiting; and improves quality of grains and fruit.

Phosphorus Deficiency

A shortage of phosphorus can cause stunting but the plant remains green. Phosphorus deficient plants often have a purple tint to the leaves and stems starting on the lower leaves. A shortage of phosphorus may delay the maturity of several crops including corn, cotton, soybeans, and others. Some crops like carrots develop poor root systems. On the other hand, an excess of phosphorus in the soil ties up several plant nutrients such as iron. Soil testing will indicate the amount of phosphorus a soil needs. To correct a deficiency, add phosphorus.

Phosphorus Application

Ground rock phosphate can be applied directly to soil, but is usually treated with acid to break down the apatite into simpler compounds.

A split-application of fertilizer is recommened. Apply before planting and then again 30 days after.

Phosphorus moves very little in mineral soil.