What Does Phosphorus Do For The Garden

Essential Role of Phosphorus in Plants

Phosphorus is an essential nutrient both as a part of several key plant structure compounds and as a catalysis in the conversion of numerous key biochemical reactions in plants. Phosphorus is noted especially for its role in capturing and converting the sun's energy into useful plant compounds.

Phosphorus is a vital component of DNA, the genetic "memory unit" of all living things. It is also a component of RNA, the compound that reads the DNA genetic code to build proteins and other compounds essential for plant structure, seed yield and genetic transfer. The structures of both DNA and RNA are linked together by phosphorus bonds.

Phosphorus is a vital component of ATP, the "energy unit" of plants. ATP forms during photosynthesis, has phosphorus in its structure, and processes from the beginning of seedling growth through to the formation of grain and maturity.

Thus, phosphorus is essential for the general health and vigor of all plants. Some specific growth factors that have been associated with phosphorus are:

Stimulated root development
Increased stalk and stem strength
Improved flower formation and seed production
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 in Plants

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.

Symptoms in Corn

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.

All photos are provided courtesy of the International Plant Nutrition Institute (IPNI) and its Crop Nutrient Deficiency Image Collection. 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.

Phosphorus in Soils

The total phosphorus content of most surface soils is low, averaging only 0.6% phosphorus. This compares to an average soil content of 0.14% nitrogen and 0.83% potassium. The phosphorus content of soils is quite variable, ranging from less than 0.04% P₂O₅ in the sandy soils of the Atlantic and Gulf coastal plains to more than 0.3% in soils of the northwestern United States.

Many factors influence the content of soil phosphorus:

Type of parent material from which the soil is derived
Degree of weathering and erosion
Climatic conditions
Crop removal and fertilization

Organic Phosphorus

Soil phosphorus is classified into two broad groups, organic and inorganic. Organic phosphorus is found in plant residues, manures and microbial tissues. Soils low in organic matter may contain only 3% of their total phosphorus in the organic form, but high-organic-matter soils may contain 50% or more of their total phosphorus content in the organic form.

Inorganic Phosphorus

Inorganic forms of soil phosphorus consist of apatite (the original source of all phosphorus), complexes of iron and aluminum phosphates, and phosphorus absorbed onto clay particles. The solubility of these phosphorus compounds as well as organic phosphorus is extremely low, and only very small amounts of soil phosphorus are in solution at any one time. Most soils contain less than a pound per acre of soluble phosphorus, with some soils containing considerably less.

Through adequate phosphorus fertilization and good crop/soil management, soil solution phosphorus can be replaced rapidly enough for optimum crop production.

Soil Phosphorus Availability

Soluble phosphorus, either from fertilizer or natural weathering, reacts with clay, iron and aluminum compounds in the soil, and is converted readily to less available forms by the process of phosphorus fixation. Because of these fixation processes, phosphorus moves very little in most soils (less than an inch), stays close to its place of origin, and crops seldom absorb more than 20 percent of fertilizer phosphorus during the first cropping season after application. As a result, little soil phosphorus is lost by leaching. This fixed, residual phosphorus remains in the rooting zone and will be slowly available to succeeding crops. Soil erosion and crop removal are the significant ways soil phosphorus is lost.

Factors of Phosphorus Availability

Soil pH

Precipitation of phosphorus as slightly soluble calcium phosphates occurs in calcareous soils with pH values around 8.0. Under acid conditions, phosphorus is precipitated as Fe or Al phosphates of low solubility. Maximum availability of phosphorus generally occurs in a pH range of 6.0 to 7.0. This is one of the beneficial effects of liming acid soils. Maintaining a soil pH in this range also favors the presence of H₂PO₄⁻ ions, which are more readily absorbed by the plant than HPO₄⁺ ions, which occur at pH values above 7.0.

Balanced Crop Nutrition

Adequate supplies of other plant nutrients tend to increase the absorption of phosphorus from the soil. Application of ammonium forms of nitrogen with phosphorus increases phosphorus uptake from a fertilizer as compared to applying the phosphorus fertilizer alone or applying the nitrogen and phosphorus fertilizers separately. Applications of sulfur often increase the availability of soil phosphorus on neutral or basic soils, where the soil phosphorus is present as calcium phosphates.

Organic Matter

Soils high in organic matter contain considerable amounts of organic phosphorus that are mineralized (similar to organic nitrogen), and provide available phosphorus for plant growth. In addition to supplying phosphorus, organic matter also acts as a chelating agent and combines with iron, thereby preventing the formation of insoluble iron phosphates. Heavy applications of organic materials such as manure, plant residues or green manure crops to soils with high pH values not only supply phosphorus, but upon decomposition, provide acidic compounds, which increase the availability of mineral forms of phosphorus in the soil.

Type of Clay

Clay particles tend to retain or fix phosphorus in soils. Consequently, fine-textured soils such as clay loam soils have a greater phosphorus-fixing capacity than sandy, coarse-textured soils. Clays of the 1:1-type (kaolinite) have a greater phosphorus-fixing capacity than the 2:1-type clays (montmorillonite, illite, vermiculite). Soils formed under high rainfall and high temperatures contain large amounts of kaolinitic clays, and therefore have a much greater fixing capacity for phosphorus than soils containing the 2:1-type clay. High temperatures and high rainfall also increase the amount of iron and aluminum oxides in the soil, which contributes greatly to the fixation of phosphorus added to these soils.

Application Timing

Fixation of soil phosphorus increases with time of contact between soluble phosphorus and soil particles. Consequently, more efficient utilization of fertilizer phosphorus is generally obtained by applying the fertilizer shortly before planting the crop. This practice is especially effective on soils with high phosphorus-fixing capacities. On coastal plain areas, fertilizers may be applied several months before planting with little or no decrease in availability of the fertilizer phosphorus to the crop. Banding of fertilizer for row crops is also much more likely to increase the efficiency of fertilizer phosphorus on soils of high phosphorus-fixing capacity than on soils of low phosphorus-fixing capacity.

Soil Temperature/Aeration/Moisture and Compaction

Phosphorus absorption by the plant is decreased by low soil temperature and poor soil aeration. Starter fertilizers containing water-soluble phosphorus are much more likely to increase crop growth during cool weather. Excessive soil moisture or soil compaction reduces the soil oxygen supply and decreases the ability of the plant roots to absorb soil phosphorus. Compaction reduces aeration and pore space in the root zone. This reduces phosphorus uptake and plant growth. Compaction also decreases the soil volume that plant roots penetrate, limiting their total access to soil phosphorus.

Soil Test Phosphorus Levels

Crop responses to fertilizer phosphorus will be greater and occur more frequently on soils testing low in phosphorus than on high-testing soils. However, yields on soils with high P soil test levels usually are higher. The response to phosphorus fertilizer on high-testing soils is increasing, and it is important to maintain high soil phosphorus levels to support optimum crop production.

Phosphorus Placement

If a grower is looking for maximum return from high phosphorus investment on low-testing soils, band application is best. Where conservation tillage is practiced, combinations of band and broadcast applications of phosphorus may be needed. This ensures an early, accessible phosphorus supply for developing seedlings and a nutrient reserve later in the growing season, when phosphorus demands remain strong.

Advantages of Broadcast/Plow-down Phosphorus Applications

High rates can be applied without injuring the plant
Nutrient distribution throughout the root zone encourages deeper rooting, while band placement causes root concentration around the band
Deeper rooting permits more root-soil contact, providing a larger reservoir of moisture and nutrients
Practical way to apply fertilizer to forages
Helps ensure full-feed fertility to help the crop take full advantage of favorable growth conditions throughout the growing season

Dual application of anhydrous ammonia and ammonium polyphosphates at seeding of wheat has been found to be superior to broadcast or band applications of ammonium polyphosphates.

Placement directly under the drill row (band seeding) for forage crops has proven superior to broadcast or side placement.

Adapted from "The Efficient Fertilizer Use Manual",
Phosphorous chapter by Dr. Bill Griffith