Phosphorus in the soil is an essential macronutrient required for plant nutrition. It is involved in metabolic processes such as photosynthesis, energy transfer, synthesis and breakdown of carbohydrates.
Phosphorus is found in the soil in the form of organic compounds and minerals. However, the amount of readily available phosphorus is very small compared to the total amount of phosphorus in the soil. Therefore, in many cases, phosphate fertilizers must be applied to meet the needs of crops.
Phosphorus is found in soils in both organic and inorganic (mineral) forms, and its solubility in soil is low. There is an equilibrium between solid-phase phosphorus in the soil and phosphorus in the soil solution. Plants can only take up phosphorus dissolved in soil solution, and since most soil phosphorus exists in the form of stable chemical compounds, only a small amount of phosphorus is available to plants at any given time.
When plant roots remove phosphorus from the soil solution, some of the phosphorus adsorbed to the solid phase is released into the soil solution to maintain balance. The types of phosphorus compounds that exist in soil are mainly determined by soil pH and the type and amount of minerals in the soil. The mineral compounds of phosphorus usually contain aluminum, iron, manganese and calcium.
In acidic soils, phosphorus reacts with aluminium, iron and manganese, while in alkaline soils fixation with calcium predominates. The optimum pH range for maximum phosphorus availability is 6,0-7,0. In many soils, the decomposition of organic material and plant residues contributes to the available phosphorus in the soil.
Plants absorb phosphorus from the soil solution in the form of orthophosphate ion: either HPO4-2 or H2PO4-. The proportion in which these two forms are taken up is determined by soil pH, with higher soil pH taking up more HPO4-2. The mobility of phosphorus in the soil is very limited, so plant roots can only absorb phosphorus from their immediate environment.
Since the concentration of phosphorus in the soil solution is low, plants use predominantly active uptake against the concentration gradient (i.e., the concentration of phosphorus is higher in the roots than in the soil solution). Active uptake is an energy intensive process, so conditions that inhibit root activity, such as low temperatures, excess water, etc., also inhibit phosphorus uptake.
Phosphorus deficiency symptoms include stunting and dark purple coloration of older leaves, inhibition of flowering and root development. In most plants, these symptoms appear when the concentration of phosphorus in the leaves is below 0,2%.
Excess phosphorus mainly interferes with the absorption of other elements such as iron, manganese and zinc. Over-fertilization with phosphorus is common, and many growers apply unnecessarily high amounts of phosphorus fertilizer, especially when using NPK compound fertilizers or when acidifying irrigation water with phosphoric acid.
The allowable concentration of phosphorus in nutrient solutions is 30-50 ppm, although it has been found that this can be reduced to 10-20 ppm. In nutrient solutions that flow continuously, the concentration can be as low as 1-2 ppm.
In soilless environments, as in soil, phosphorus builds up with each addition of phosphorus, and the minerals of phosphorus and calcium or magnesium begin to precipitate. The types of minerals formed depend on the pH of the medium.
The soil test does not measure the total amount of phosphorus in the soil because the available amount of phosphorus is much less than the total amount. It also does not measure phosphorus in soil solution because the amount of phosphorus in soil solution is usually very low and does not properly reflect the amount of phosphorus that plants can potentially take up during the growing season.
The soil test for phosphorus is actually a metric that helps predict a crop's need for fertilizer. Fertilizer recommendations are based on numerous field trials in many soils and crops. Different test methods result in different values, which must be interpreted accordingly.
But the confusion doesn't end there - different labs using the same testing method may interpret the same values differently. Proper soil sampling is very important to get results that truly reflect the level of available phosphorus.
Since phosphorus is immobile in the soil, samples taken from the topsoil usually show more phosphorus than samples taken from the ground.
Most of the phosphorus applied to the soil stays within 1-2 inches of the application. Thus, the exact location where the samples are taken from can significantly affect the result.
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