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Plant Nutrition - Phosphorus

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Phosphorus is one of the main 17 nutrients essential for plant growth.   Phosphorus is the P in NPK.

Phosphorus is a component of the complex nucleic acid structure of plants, which regulates protein synthesis. Phosphorus is, therefore, important in cell division and development of new tissue. Phosphorus is also associated with complex energy transformations in the plant.

Its functions cannot be performed by any other nutrient, and an adequate supply of P is required for optimum growth and reproduction. Phosphorus is classified as a major nutrient, meaning that it is frequently deficient for crop production and is required by crops in relatively large amounts. The total P concentration in agricultural crops generally varies from 0.1 to 0.5 percent.

(P) is vital to plant growth and is found in every living plant cell. It is involved in several key plant functions, including energy transfer, photosynthesis, transformation of sugars and starches, nutrient movement within the plant and transfer of genetic characteristics from one generation to the next. Chlorophyll Photosynthesis = Carbon Dioxide + Water Sunlight Oxygen + Carbohydrates Phosphate Energy another as new cells are formed

Soil Phosphorus Management, (1/3)

photo.jpg Univ of Wisconsin Integrated Pest and Crop Management


Plant Energy Reactions

Phosphorus plays a vital role in virtually every plant process that involves energy transfer. High-energy phosphate, held as a part of the chemical structures of adenosine diphosphate (ADP) and ATP, is the source of energy that drives the multitude of chemical reactions within the plant.

When ADP and ATP transfer the high-energy phosphate to other molecules (termed phosphorylation), the stage is set for many essential processes to occur.

  • In every day  terms, phosphorus is very important to many aspects of plant growth.



The most important chemical reaction in nature is photosynthesis. It utilizes light energy in the presence of chlorophyll to combine carbon dioxide and water into simple sugars, with the energy being captured in ATP.

The ATP is then available as an energy source for the many other reactions that occur within the plant, and the sugars are used as building blocks to produce other cell structural and storage components.


Genetic Transfer & Seeds

Phosphorus is a vital component of the substances that are building blocks of genes and chromosomes.

  • Very necessary when making seeds.
  • It is an essential part of the process of carrying the genetic code from one generation to the next, providing the “blueprint” for all aspects of plant growth and development.
  • An adequate supply of P is essential to the development of new cells and to the transfer of the genetic code 

Large quantities of P are found in seeds and fruit where it is believed essential for seed formation and development.

  • Phosphorus is also a component of phytin, a major storage form of P in seeds.
  • About 50 percent of the total P in legume seeds and 60 to 70 percent in cereal grains is stored as phytin or closely related compounds.
  • An inadequate supply of P can reduce seed size, seed number, and viability.

Adding phosphorus to soil low in available phosphorus promotes root growth and winter hardiness, stimulates tillering, and often hastens maturity.


Nutrient Transport

Plant cells can accumulate nutrients at much higher concentrations than are present in the soil solution that surrounds them. This allows roots to extract nutrients from the soil solution where they are present in very low concentrations.

Movement of nutrients within the plant depends largely upon transport through cell membranes, which requires energy to oppose the forces of osmosis. Here again, ATP and other high energy P compounds provide the needed energy.

Uptake and Transport of Phosphorus Phosphorus enters the plant through root hairs, root tips, and the outermost layers of root cells.  Uptake is also facilitated by mycorrhizal fungi that grow in association with the roots of many crops. 

  • Phosphorus is taken up mostly as the primary orthophosphate ion (H2PO4 - ),
  • Can also be absorbed as secondary orthophosphate (HPO4 =),
    • this latter form increasing as the soil pH increases.

Once inside the plant root, P may be stored in the root or transported to the upper portions of the plant. Through various chemical reactions, it is incorporated into organic compounds, including nucleic acids (DNA and RNA), phosphoproteins, phospholipids, sugar phosphates, enzymes, and energy-rich phosphate compounds.

  • Example, adenosine triphosphate (ATP). It is in these organic forms as well as the inorganic phosphate ion that P is moved throughout the plant, where it is available for further reactions.  (For more information on ATP consult the plant physiology posting)


Phosphorus Deficiency

Adequate P allows the processes described above to operate at optimum rates and growth and development of the plant to proceed at a normal pace. When P is limiting,

  • Effects are a reduction in leaf expansion
  • leaf surface area,
  • as well as the number of leaves.
  • Shoot growth is more affected than root growth,
    • which leads to a decrease in the shootroot dry weight ratio.
  • Nonetheless, root growth is also reduced by P deficiency,
    • leading to less root mass to reach water and nutrients.
  • Generally, inadequate P slows the processes of carbohydrate utilization, while carbohydrate production through photosynthesis continues.
    • This results in a buildup of carbohydrates and the development of a dark green leaf color.
    • In some plants, P-deficient leaves develop a purple color, tomatoes and corn being two examples.
    • Sugars can accumulate and cause anthocyanin pigments to develop, producing a reddish-purple color.

    • 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.

  • Since P is readily mobilized in the plant, when a deficiency occurs the P is translocated from older tissues to active meristematic tissues, resulting in foliar deficiency symptoms appearing on the older (lower) portion of the plant.
    • However, such symptoms of P deficiency are seldom observed in the field.
  • Other effects of P deficiency on plant growth include
    • Delayed maturity,
    • reduced quality of forage, fruit, vegetable, and grain crops,
    • and decreased disease resistance.

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. When the soil warms, deficiencies may disappear.

These symptoms usually only persist on extremely low phosphorus soils. It should be noted that these are severe phosphorus deficiency symptoms and crops may respond well to phosphorus fertilization without showing characteristic deficiencies.

Phosphorus Cycle Explanation- A biogeochemical cycle

photo.jpg MooMoo Math and Science

Phosphorus in the soil

Phosphorus is absorbed by plants in the ionic forms H2PO4– and HPO4=. General knowledge of ion exchange in soils would predict that these anions are not retained by the negative charged soil colloids, but move in the soil similar to nitrogen.

  • However, phosphorus does not leach. In fact, it moves very little, even with large amounts of precipitation or irrigation.

The reason for this apparent anomaly is that the soil solution contains only a very small amount of available phosphorus in these ionic forms at any one time. In fact, most soils contain less than 0.00005 grams phosphorus per liter or 0.0000068 ounces phosphorus per gallon of soil.

  • It has been estimated that the phosphorus in the soil solution must be replenished on an average of about twice every day for normal crop growth.
  • This is the basic phosphorus problem — to adequately re-supply the soil solution as the crop roots remove available phosphorus from the soil solution.
    • It is the soil’s ability to re-supply the soil solution that dictates whether the crop will need additions of fertilizer phosphorus and whether those additions will be effective in the forms applied.
  • The ability of the soil to re-supply the soil solution with phosphorus is dependent on the complex chemistry of the soil system.
  • However, the system can be viewed very simply with the following diagram:

Slowly Soluble or Insoluble P Form

 Soluble or Plant Available P Forms

Relatively Unavailable ————>
P minerals and         <———— Soil Solution P
Compounds of Ca, Fe,
and Al
Organic P

This is an equilibrium reaction.

  • As soil solution phosphorus is removed by crop roots, more phosphorus becomes available from the slowly soluble sources.
  • However, if soluble fertilizer phosphorus is placed in the soil,
    • it reverts into slowly soluble or insoluble forms, removing soluble phosphorus from the soil solution.
  • This phenomenon is often called “fixation.” 
    • Fixation is the primary reason why placement of phosphorus fertilizer is important.

Placement of phosphorus is an attempt to limit fixation.

  • This is done by banding the phosphorus fertilizer near the seed or by dual placement with anhydrous ammonia bands.
  • The goal is to limit soil-fertilizer contact, while placing available sources of phosphorus from the fertilizer in a position of a high probability root contact.

The above relationship is sometimes shown in terms of labile and non-labile phosphorus forms according to the following relationship:

Non-labile P  <—>  Labile P  <—>  Soil solution P

In this relationship, non-labile phosphorus refers to slowly available forms, while labile phosphorus is an intermediate form that is rather weakly absorbed or bound to various compounds and clay in the soil (solid phase).

  • This is the primary phosphorus source supplying the soil solution.

The equilibrium relationship shown above between non-labile or insoluble phosphorus forms and labile phosphorus is affected by many factors,

  • such as size of the slowly available pool,
  • soil temperatures,
  • kind of compounds in the pool,
  • kind and amount of clay in soil,
  • and the pH of the soil solution. 

Figure 6.1 shows the general relationship between soil pH and phosphorus availability, which is based on the kinds of phosphorus compounds associated with the various pHs.

  • At high soil pH, most phosphorus is in the form of calcium compounds.
  • At low or acid pH, phosphorus is combined with iron and aluminum compounds.
  • Maximum phosphorus availability occurs at a soil pH between 6.5 to 7.0.
    • This is why one of the most important benefits of liming acid soils is improving phosphorus availability.
    • Reducing the pH of calcareous soils would also increase the availability of phosphorus in the soil solution by changing some of the solid phase compounds into compounds of higher solubility.
    • Sulfur will reduce the soil pH; however, the cost is prohibitive for field crops because of the high sulfur rates required.
Figure 6.1. 
Soil phosphorus compound in relation to soil pH.


Figure 6.2 characterizes phosphorus additions and removals from the soil system in addition to the inorganic minerals.

  • Organic phosphorus in the form of residues, manures, or from the soil organic matter can contribute greatly to the phosphorus in the soil solution for crop growth.
  • In some soils organic phosphorus can contribute 50 percent of the available phosphorus.
  • Since availability of organic phosphorus is dependent on decomposition of the organic matter,
    • soil temperature and moisture are important factors regulating how fast organic phosphorus is made available.
Figure 6.2.  
Relation of additions and losses of phosphorus in a soil system.


As previously indicated, available or soil solution phosphorus can revert to slowly soluble mineral forms. This fixation may also occur when available phosphorus is used by microorganisms in the decomposition of residues.

  • This type of fixation is called immobilization and can be either long- or short-term.


Agricultural Management Practices for Phosphorus, (2/3)

photo.jpg Univ of Wisconsin Integrated Pest and Crop Management


The Plant Problem

While the soil system limits the amount of phosphorus in the soil solution at any one time and limits its re-supply, the plant root also has its problems. The concentration of roots in the soil volume is relatively small. It has been calculated that roots contact only about one percent of the soil volume.

Phosphorus enters the root primarily by diffusion (90-98 percent), which can occur only if the phosphorus is very close to the root.

  • Very little phosphorus enters the root by mass flow in the water (one percent).
  • Root growth is essential for adequate phosphorus uptake or the soil solution needs to be replenished frequently.
  • Actually since roots contact such a small amount of the soil, the soil solution in the areas of root contact must be replenished more often than twice a day or phosphorus deficiencies will occur.
    • This makes the labile forms (those weakly bound to compounds or clay) very important in soil phosphorus supply.

Research has developed valuable models which predict phosphorus plant uptake and the factors that influence it. One of the most commonly known models has been developed by Dr. Barber at Purdue University.

  • His model indicates phosphorus uptake is largely a function of
    • size and nature of the root system,
    • rate of water absorption,
    • amount of phosphorus in the soil,
    • and ability of the soil to supply phosphorus to the soil solution. .


Application Methods

There is little producers can do to change the basic soil and climatic characteristics that affect crop response to applied fertilizer. However, one can control phosphorus availability by managing the soil pH (acid soils), increasing organic matter, and by proper placement of phosphorus fertilizer.

  • Research has shown that band application of phosphorus is much more efficient than broadcasting.
  • Wheat studies in Nebraska have shown that profits from application with the seed are double those of broadcasting.
    • This is because each pound of applied phosphorus with the seed increased yield much more than a pound broadcast. 

Another banding method (dual placement) applies liquid phosphorus (10-34-0) at the same time as anhydrous ammonia with a separate tube delivery for each fertilizer.

  • Dual placement has been found to be equal to seed application on wheat and equal to or better than row application for corn and soybeans.
    • While band applications of phosphorus require special application equipment and require extra time at planting, these methods are generally economically superior to broadcast phosphorus.
    • The primary exception being broadcast phosphorus applied to growing
      • alfalfa,
      • grass,
      • or in no-till farming systems.

When residues remain on the soil surface, research studies indicate broadcasting phosphorus can be nearly as effective as dual placement. This is attributed to increased root activity in the residue-soil interface where soil moisture and mineralizing nutrients from the residues stimulates root development.

  • This is believed to give a broadcast application the advantages of a band application. This is sometimes referred to as a “horizontal band.” The horizontal band, which is unincorporated, has limited soil-fertilizer contact and is in a position of increased root activity.

Seed placement is another method of banding that can be very effective.

  • The problem with seed application is that starter fertilizer contains salts from the nitrogen and potassium sources;
    • when applied in excessive amounts, reduces seed germination.
  • Phosphorus fertilizer without nitrogen has little effect on germination,
    • but mixed fertilizers containing potassium, sulfur, and nitrogen are very damaging, unless water moves the fertilizer from the seed.
      • A major factor affecting salt concentration in the seed row is row spacing.
  • Since wheat is planted in 7- to 12-inch rows, the concentration of 18-46-0 fertilizer is only one-third of the concentration in a 30- or 36-inch corn row.
    • Phosphorus fertilizers, even with nitrogen, can be safely used on wheat at normal phosphorus application rates.
    • For row crops, such as corn, sorghum and soybeans, rates must be limited, because germination will be decreased about one percent for each pound of salt applied (pounds of nitrogen + potassium + sulfur) for corn.
    • Soybeans are more susceptible to germination damage, and so any fertilizer should be kept from contacting the soybean seed.

Row application to the side and below the seed is favored over seed application for row crops,

  • even though this method requires more expensive application equipment than seed applications.
  • This method is also referred to as a “starter” method for row crops and is more effective than broadcast incorporation methods on soils low in available phosphorus.
  • It is, however, important to remember that increased early growth from starter fertilizer application does not always indicate increased yields at harvest.


Sources of Phosphorous 

Understanding Garden Phosphorous: What it Does, Chemical vs. Organic, Availability & pH: TRG 2014

photo.jpg Gary Pilarchik (The Rusted Garden)



By - Grow It Organically, click to visit their site

Organic Phosphorus Fertilizers (P)—Links Go to Offsite Affiliates to 
Purchase Organic Soil Amendments
 Soil Amendment  N-P-K  Description  Lasts  Application Rate
Soft Rock Phosphate 0-18-0   Colloidal Phosphate has a clay base that makes it easier for plants to assimilate than phosphate rock. Releases over months and years in acidic and neutral soils, but breaks down poorly in alkaline soils (pH higher than 7). Peak availability in 2nd year.  2-3 Years  Up to 6lbs/100 sq ft
Bat Guano (High-P)ir?t=growitorganic-20&l=as2&o=1&a=B0001W 3-10-1  High-Phosphate guano from fruit-eating bats. Excellent P source for container vegetables and gardens.  2-3 Years  2-3lbs/100 sq ft
Steamed Bone Mealir?t=growitorganic-20&l=as2&o=1&a=B001H1 3-15-0  Made from ground cattle bones. P in bone meal is highly plant-available. Great mixed into the planting hole with bulbs. Good amendment for allium family plants (onions, garlic). May attract raccoons. P in bone meal not released in alkaline (pH greater than 7) soils.  1-4 Months  10lbs/100 sq ft
Fish Bone Mealir?t=growitorganic-20&l=as2&o=1&a=B0047B 3-18-0   Phosphorus from fish bone meal is readily assimilated by microorganisms and plant roots in the soil.  1-2 Years  1-2lbs/100 sq ft
Rock Phosphate 0-33-0  Very slow release P source. Releases over several years in acidic and neutral soils, but won’t break down in alkaline soils (pH higher than 7).  3-5 Years  Up to 6lbs/100 sq ft.
Rock Dust (Crushed Granite) 0—3-5—0, 
trace minerals
 Granite fines, the dust from rock grinding and sorting operations. Veryslow releasing P source, good source of trace minerals for plant immunity and tolerance of temperature extremes.  5-10 Years  Up to 8.5lbs/100 sq ft
Chicken Manure 1.1-0.8-0.5  Good manure source for P and some K.  3-12 Months  1/2-1” layer 

(5-10 5-gal buckets/100 sq ft)
Pig Manure 0.8-0.7-0.5  Good, balanced manure source of N, P, and K. Because some pig parasites and pathogens can infect humans, pig manure is not allowed in many organic protocols. If it is used, it must be hot-composted prior to use.  3-12 Months  1” layer 

(10 5-gal buckets/100 sq ft)

Understanding the different sources of P and how they break down and are absorbed by the plants will give you the wisdom to know how much to add for each source of P to ensure proper P levels in an absorption state that is available for the plant throughout its life span.



Soil phosphorus is relatively stable in soil. It moves very little when compared to nitrogen. In fact, this lack of mobility is due to the rather limited solubility of soil phosphorus compounds. Because of the limited solubility of these compounds, fertilizer phosphorus will become much less available as it reverts back to soil phosphorus compounds.

Fertilizer phosphorus that reverts back to soil phosphorus compounds is not lost completely, but becomes slowly available to crops over several years. The rate depends greatly on soil type. For most applying more fertilizer phosphorus than needed for optimum yields is probably not economically justified.

Phosphorus availability is controlled by three factors:

  • soil pH,
  • amount of organic matter,
  • and proper placement of fertilizer phosphorus.
  • Acid soils should be limed to bring soil pH up to nearly 6.5.
  • The pH of alkaline soils (over 7.0) probably cannot be practically lowered for better phosphorus availability.

Organic matter maintenance is an important factor in controlling phosphorus availability. Mineralization of organic matter provides a steady supply of available phosphorus.

  • Organic soil phosphorus may represent 30-40 percent of the phosphorus available and may be a major factor affecting phosphorus availability during wet, cold springs.

Placement of phosphorus is the best practical control of phosphorus availability.

  • Placing phosphorus with seed wheat has given much better results than broadcast applications.
  • Banding phosphorus two inches to the side and two inches below the seed of row crops provides a ready source of phosphorus for the young seedling;
    • however, soil phosphorus must be deficient before yields can be expected to be increased.

Understanding the different sources of P and how they break down and are absorbed by the plants will give you the wisdom to know how much to add for each source of P to ensure proper P levels in an absorption state that is available for the plant throughout its life span.  This understanding is the tool of knowledge that can make the difference from a poor to good crop and from a good to spectacular crop!






Plant & Soil Sciences eLibraryPRO

Functions of Phosphorus in Plants

photo.jpg Univ of Wisconsin Integrated Pest and Crop Management

photo.jpg MooMoo Math and Science

photo.jpg Gary Pilarchik (The Rusted Garden)

Grow It Organically, click to visit their site





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