Phosphorus
Posted: Sunday, December 12, 2010
by Robert Wright
UsaveHydroponics
Molecular Form:
Soil phosphorus originates from the weathering of minerals such as apatite, calcium phosphate minerals. As apatite weathers, it releases anions that can be used by plants:
Primary Orthophosphate (H2Po4)
pH of Mineral:
Most soils have large amounts of phosphate, but much of it remains unavailable to plants. Available of phosphate depends on soil p H.
- In strongly acid soils, insoluble iron phosphate is formed.
- Calcium Phosphates are most important between 7.0 and p H 9.0, with secondary orthophosphate more common as the pH increases.
- Maximum availability is at pH 6.5, but most plants have access to satisfactory levels of phosphates between pH 6.0 - 7.0.
- Apatite Mineral: Slow weathering process.
- Organic Matter: Contains phosphates recycled from decayed plant residue and animal manures. Provides 25- 90% of soil phosphorus storage.
- Calcium, iron, and aluminum phosphates: Bound to other minerals, made available by changes in soil pH or the actions of microorganisms.
- Only a small amount of phosphorus is available in the soil solution at any one time. Plant growth depends on the low availabilty of phophorus, many plants have developed adaptation to improve access to it. These include Mycorrhizal associations, specialized root systems, high root length densities, longer root hairs, and exudates given off by roots or mycorrhizae that free fixed phosphorus.
Phosphorus affects plants growth in many ways:
- Phosphorus is require for plant reproduction and cell devision.
- Phosphorus is part of adenosine triphosphate (ATP), the chemical that stores and transfers energy in all living things.
- Phosphorus is an important component of plant cell membranes.
- Phosphorus spurs early root growth, helping a young plant develop its roots. This improve root development allows plants to use water more efficiently.
- Phosphorus helps plants resist cold and disease, speeds crop maturity, aids blooming and fruiting, and improves the quality of grains and fruits.
A phosphorus shortage can cause stunted plants with few or small leaves. Plats will remain green and can even become darker green than normal. Leaves can turn purple because of the increase in anthocyanins, the pigments that are responsible for red plant colors. Phosphorus shortage will delay maturity and hinder root development in most plants.
Application Recommendations:
Crops will only use 10 to 30 percent of phosphate fertilizer applied to it. The remaining phosphates are fixed in the soil and may be used by future crops. On land that has undergone many past fertilizer applications there may be significant stores of phosphorus. Phosphorus does not easily leach through the soil, but excess phosphorus can cause environmental problems if it is transported to waterways through surface runoff or wind erosion. Excess phosphorus can also cause other important nutrients, such as iron, to become unavailable to plants. A soil test is the only reliable way to determine the phosphorus nee of a soil.
Author: Robert R. Wright
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