How does phosphorus affect plant growth

More uniform and earlier crop maturity. Increased nitrogen N-fixing capacity of legumes. Improvements in crop quality. Increased resistance to plant diseases.

Supports development throughout entire life cycle. Phosphorus deficiency is more difficult to diagnose than a deficiency of nitrogen or potassium.

Crops usually display no obvious symptoms of phosphorus deficiency other than a general stunting of the plant during early growth. By the time a visual deficiency is recognized, it may be too late to correct in annual crops. Some crops, such as corn, tend to show an abnormal discoloration when phosphorus is deficient. The plants are usually dark bluish-green in color with leaves and stem becoming purplish. The degree of purple is influenced by the genetic makeup of the plant, with some hybrids showing much greater discoloration than others.

The purplish color is due to accumulation of sugars that favors the synthesis of anthocyanin a purplish-colored pigment , which occurs in the leaves of the plant. Phosphorus is highly mobile in plants, and when deficient, it may be translocated from old plant tissue to young, actively growing areas. Consequently, early vegetative responses to phosphorus are often observed. As a plant matures, phosphorus is translocated into the fruiting areas of the plant, where high-energy requirements are needed for the formation of seeds and fruit.

Phosphorus deficiencies late in the growing season affect both seed development and normal crop maturity. The percentage of the total amount of each nutrient taken up is higher for phosphorus late in the growing season than for either nitrogen or potassium. The photo at left displays a P deficient corn plant.

Older leaves are affected before younger ones because of the redistribution of P in the plant. Corn may display a purple or reddish color on the lower leaves and stems.

This condition is associated with accumulation of sugars in P-deficient plants, especially during times of low temperature. The photos above are a sample of a greater collection, which provides a comprehensive sampling of hundreds of classic cases of crop deficiency from research plots and farm fields located around the world. For access to the full collection, you can visit IPNI's website. The total phosphorus content of most surface soils is low, averaging only 0.

This compares to an average soil content of 0. The phosphorus content of soils is quite variable, ranging from less than 0. Phosphorus exists in large quantities in most Iowa soils; however, much of the P is present in mineral and organic forms that are not immediately plant available.

Phosphorus becomes plant available as minerals weather or by microbial degradation. Over the years, P fertilizer and manure have been used to augment the amount of plant-available P in soils and, subsequently, improve crop yields. When reasonable P soil test levels have been achieved, producers have some flexibility in their management of P inputs.

A buildup of plant-available P has been accomplished on many soils through continued use of fertilizers and manure. But management systems that do not add supplemental P will eventually experience a decline in plant-available P, and, as a result, reduced crop yields. Phosphorus uptake total amount in plant material and crop removal removed in harvested crop are large for agronomic crops. The portion not taken off the field in harvested grain or forage is returned in crop residues and available for future crops.

Examples of P removal in harvest crops include the following: corn, 38 lb P2O5 for each bu of grain harvested; corn silage, 35 lb P2O5 per 10 tons chopped; soybean, 40 lb P2O5 for each 50 bu of grain harvested; and alfalfa, 63 lb P2O5 for each 5 ton of forage harvested.

Continual cropping with no replacement of this P results in reduction of plant-available P in soils. Phosphorus is chemically reactive with the soil. However, compared with the nitrogen cycle, the P cycle is less complex and P less easily lost from soils.

Phosphorus is strongly adsorbed by soil particles and readily retained in soil. Due to this retention, high applications of P, in excess of P removal in harvested crops, push soil test levels and available P above agronomic need. Soil testing is the research-based method for monitoring crop-available P levels in soil and the need for P fertilization. Although much is known about P and its interaction with soils, there is still much to be learned about the relationships among soil management, P management, and P movement to surface water systems.

As water quality criteria for P are refined, specific field, soil, and P management requirements need to be clearly defined for producers to maintain optimum P and production levels in their fields. Cold temperatures retard root growth and reduce the phosphorus uptake in plants. Symptoms diminish, however, as the soil warms up. Factors such as soil compaction, herbicide injury, insect pressure, and poor soil health also can cause phosphorus deficiency.

Plant tissue analysis can serve as a valuable tool to diagnose phosphorus deficiency or other potential fertility problems. While soil testing is performed to predict the nutrient availability in soils, plant tissue analysis provides information on the nutrients taken up by the plants.

The phosphorus concentration in plant tissue might be in deficiency range, sufficiency range, or excess range. Tissue phosphorus in the deficiency range causes yield reduction when soil test P is low. Excess levels of phosphorus in plant tissue may not affect yield but may induce iron, zinc, or manganese deficiencies. The sufficiency range of phosphorus for various crops is presented in table 1.

The sufficiency range of phosphorus varies among crop, plant part, and growth stage. Phosphorus is present in soil in organic and inorganic forms. However, the amount of phosphorus available for plant uptake is very low compared to the total amount of phosphorus present in the soil.

For example, total soil phosphorus may be pounds per acre, but the plant available amount in soil solution might be 0. Plants take up phosphorus from soil solution in two forms only: H 2 PO 4 — or HPO 4 2- , commonly referred to as orthophosphates. Orthophosphates are very reactive and can form stable complexes by binding with iron, aluminum, calcium, or magnesium that may be present in the soil. Availability of orthophoshates also depends on soil pH and is greatest when soil pH values are between 6 and 7.

The lack of available phosphorus in soil solution necessitates phosphorus applications via organic sources such as manure or inorganic sources such as synthetic fertilizers. Phosphorus in commercial fertilizers is expressed in oxide form P 2 O 5 rather than the elemental form. This system of expressing phosphorus as oxide is conventional shorthand. Several inorganic phosphorus fertilizers are available that differ in nutrient analysis. While one phosphorus fertilizer product might work better than the other in certain situations, the phosphorus recommendations are the same regardless of the P fertilizer source.

It should be noted that these are severe phosphorus deficiency symptoms and crops may respond well to phosphorus fertilization without showing characteristic deficiencies. In addition, the reddish-purple color does not always indicate phosphorus deficiency but may be a normal plant characteristic.

Red coloring may be induced by other factors such as insect damage which causes interruption of sugar transport to the grain. Phosphorus deficiencies may even look somewhat similar to nitrogen deficiency when plants are small. Yellow, unthrifty plants may be phosphorus deficient due to cold temperatures which affect root extension and soil phosphorus uptake.