Soil Tests for Phosphates

Plants Need Phosphorus

• Plants need phosphates for seed, root, flower, and fruit growth. It is essential for membrane formation and the transfer of energy. Phosphorus deficiency results in slow or stunted plant growth, poor flower development and poor fruit set. Phosphorus deficient plant leaves turn dark green with tip burn. Excess phosphorus in soil restricts plant ability to use iron, manganese and zinc. Soil becomes depleted of the nutrients needed for healthy food crops.

Soil Tests

• Soil tests that identify phosphates exclusively have been developed to prevent the overuse of synthetic phosphate in fertilizers. Agricultural phosphate runoff in waterways greatly accelerates the growth of unwanted plants and algae. Excess algae reduce available oxygen for fish and severely unbalance aquatic ecosystems. Excessive phosphate content in public drinking water is unhealthy and requires additional chemicals to remediate.

Types of Tests

• Soil tests differentiate between the total amount of phosphorus in a soil sample and the amount of phosphorus usable by plant life. They provide "an indication of the level of soil P (phosphorus) that is available to the plant," according to an Ohio State University School of Environment and Natural Resources fact sheet on soil testing. Soils often contain several hundred to several thousand pounds of phosphate per acre, but not all of it is plant-available. Much of it becomes a pollutant in agricultural runoff.

Test Process

• A small amount of soil is mixed with an extracting solution to determine plant-available phosphorus content. The acid or complexing agent removes some of the phosphate from soil particles. The extracting solution and the soil are then separated by filtration and the amount of phosphorus is determined. This calibration is used to recommend the amount of synthetic fertilizer to use to produce optimum crop growth without dangerous excess.