Unit 13 - Fertilizers and Fertility

Chapter 4 - Phosphorus Fertilizers

Phosphorus does not exist in a gaseous state and therefore does not circulate in the atmosphere. Phosphorous cycles through water, the crust of the earth, and living organisms. Phosphorus is released by rock and mineral weathering. Phosphorus is a limiting factor for plant growth in many ecosystems and leads to eutrophication when abundant in others. The phosphorous cycle is a very long cycle associated with the rock cycle

Phosphorus does not occur in nature in its elemental form, but in phosphate rock. The mineral apatite (Ca₅(F,Cl,OH)(PO₄)₃) , a calcium fluoro-phosphate is the source of fertilizer P. It is a phosphate compound of calcium containing chlorine or fluorine, or both, that is transparent to opaque in shades of green, brown, yellow, white, red, and purple. Apatite is a minor constituent in igneous and metamorphic rocks. Yellow-green asparagus stone and blue-green manganapatite are used in jewelry. Large deposits of this rock can be found in the Bone Valley area of Florida, also Tennessee, Idaho, and South Carolina. Large deposits are mined in Northern Africa. Most phosphorus rock minerals are converted into phosphorus compounds. Environmentalists in Florida are concerned about the impact of phosphorus mines on water quality (Florida's phosphate_mining_crisis ).

Phosphorus plays an important role as a part of the molecule ATP (adenosine triphosphate) which fuels all biochemical work in living cells. Additionally, sugar phosphates form the "rails" of the nucleic acids DNA and RNA (which N-containing bases forming the "rungs"). Phospholipids are an important constituent of membrane chemistry and phosphoproteins are essential for life functions.

Phosphorus is phloem-mobile and the physiological results of P deficiency are spread more or less evenly around the plant, usually with minimal visual deficiency symptom except for stunted growth, late maturity, poor seed development, and dark green coloration. Grassy species, including corn, will show purplish coloration of leaves if P is severely deficient.

Soil Phosphorus

The Phosphorus compound most commonly found in the soil is some form of calcium phosphate. When the H₂PO₄- ion is free in the soil solution, it reacts quickly with other ions to become unavailable to plants. P can also be absorbed by organisms, so that it has an organic cycle, similar to nitrogen. The result is that the soil solution has a very low concentration of phosphorus at any one time because of the low solubility of the fixed forms. Even when phosphorus is applied as a fertilizer, it can become unavailable, especially at high pH where it is fixed by calcium, and at low pH where it is fixed by iron and aluminum.

Phosphorus fertilizers that dissolve in the soil solution are, therefore, subject to fixation. For this reason, the phosphorus from fertilizers may move about 1 inch from the place of application. Therefore, it is necessary to try and get the phosphorus fertilizer as near plant roots as possible to insure maximum contact. Thus phosphorus is often applied as a band at planting time. This concentration of nutrients reduces phosphorus fixation with the soil. Non-mobile nutrients such as phosphorus are now available for quick uptake by the crop even in cold growing conditions. See Chapter 6 for Fertilizer Applications.

Soils in Minnesota generally are low in P in the west and high in P in the northeast. This is mainly due to the more alkaline conditions in the west and the lower natural levels of P in the parent materials.

Proper use of phosphorus fertilizer according to soil test recommendations will maintain healthy plant growth without contaminating surface waters; however, excessive applications of phosphorus fertilizer on high phosphorus testing soils may result in significant water degradation if it leaves the site via runoff. Phosphorus fertilizer recommendations are based on the kinds of plants being grown and the phosphorus soil test level.

Two different soil tests are used to determine available soil phosphorus. The Bray-P1 test is used when the soil pH is 7.4 or less and the Olsen-P test is used when the soil pH is greater than 7.4. It is important to note that interpretations for phosphorus fertilizer will change, depending on which test is used. In other words, the amount of extractable phosphorus using the Bray-P1 test is not equivalent to the amount of extractable phosphorus using the Olsen-P test. Relative levels of phosphorus based on each soil test are presented below

*ppm x 2=lbs/acre

Nearly all of the phosphorus absorbed by plants is taken up as two ions. Phosphorus is not absorbed in an organic form. The HPO₄-- ion dominates in calcareous soils and is the form of phosphorus absorbed when crops are grown on these soils. The H₂PO₄- ion dominates in acid soils and is the ion absorbed when the soil pH is less than 7.0.

For most soils in Minnesota, the amount of these ions dissolved and accessible for crop uptake is a fraction of a pound per acre. The amount of phosphorus that is dissolved and accessible is in equilibrium with the phosphorus in the solid phase. This solid phase phosphorus is both organic and inorganic.

Crops need more phosphorus than is dissolved in the soil solution at any one time, therefore, this phosphorus in the solution phase must be replenished many times during the growing season. The ability of a soil to maintain adequate levels of phosphorus in the solution phase is the key to the plant available phosphorus status of the soil.

Phosphate Fertilizer Manufacture

The manufacture of most commercial phosphate fertilizers begins with the production of phosphoric acid. Phosphoric acid is produced by either a dry or wet process. In the dry process, rock phosphate is treated in an electric furnace. This treatment produces a very pure and more expensive phosphoric acid used primarily in the food and chemical industry. Fertilizers that use white phosphoric acid as the P source are generally more expensive because of the costly treatment process.

The wet process involves treatment of the rock phosphate with acid producing phosphoric acid and gypsum which is removed as a by-product. The impurities which give the acid its color have not been a problem in the production of dry fertilizers. Either treatment process (wet or dry) produces orthophosphoric acid— the phosphate form that is taken up by plants.

The phosphoric acid produced by either the wet or the dry process is frequently heated, driving off water and producing a superphosphoric acid. The phosphate concentration in superphosphoric acid usually varies from 72 to 76%. The P in this acid is present as both orthophosphate and polyphosphate. See the diagram on the manufacturing of P fertilizer P Fertilizer manufacture diagram .

Polyphosphates consist of a series of orthophosphates that have been chemically joined together. Upon contact with soils, polyphosphates revert back to orthophosphates. This conversion is rapid and, with normal soil temperatures, can be complete in days or less. This conversion process is enhanced by an enzyme called pyrophosphatase, which is abundant in most soils.

Ammonia can be added to the superphosphoric acid to create liquid or dry materials containing both nitrogen (N) and P. The liquid, 10-34-0, is the most common product. The 10-34-0 can be mixed with finely ground potash (0-0-62), water, and urea-ammonium nitrate solution (28-0-0) to form 7-21-7 and related grades. The P in these products is present in both the orthophosphate and polyphosphate form. When ammonia is added to the phosphoric acid that has not been heated, monoammonium phosphate (11-52-0) or diammonium phosphate (18-46-0) is produced depending on the ratio of the mixture. The P present in these two fertilizers is present in the orthophosphate form.

Phosphate Fertilizer Terminology

Water-Soluble — Fertilizer samples analyzed by a control laboratory are first placed in water and the percentage of the total phosphate that dissolves is measured. This percentage is referred to as water-soluble phosphate.

Citrate-Soluble — The fertilizer material that is not dissolved in water is then placed in an ammonium citrate solution. The amount of P dissolved in this solution is measured and expressed as a percentage of the total in the fertilizer material. Phosphate measured with this analytical procedure is referred to as citrate-soluble.

Available — Available=water-soluble + citrate-soluble. This total is the percentage that is available to plants and is the amount guaranteed on the fertilizer label. Usually, the citrate-soluble component is less than the water-soluble component.

*Water-soluble data are a percent of the total P₂O₅

Organic Phosphorus Sources

Organic P fertilizers have been used for centuries as the P source for crops. Even with the advent of P fertilizer technology processes, organic P sources from animal manures, composts, and sewage sludge are still very important.

From a fertilizer/nutrient management perspective, the major differentiating factor is the availability of P. Phosphorus from manure or sludge should be comparable to P from inorganic fertilizer. Therefore, if a producer has a P recommendation for 30 lbs/A of P₂O₅ , applying approximately 65 lbs of 18-46-0 (DAP) or 6 tons of 11-6-9 (manure, 80% available P coefficient) should provide equivalent results.

The P contained in organic P sources is a combination of inorganic and organic P. Essentially, all of the inorganic P is in the orthophosphate form, which is the form taken up by growing plants. Generally, 45-70% of manure-P is inorganic P. The diet being fed to the animals will have some control on the chemical make-up of the manure P.

Much of the organic P is easily decomposable in the soil, but factors such as temperature, soil moisture, and soil pH all have a bearing on the P mineralization rate. The final decomposition product is orthophosphate P compounds. The combination of the organic/inorganic P ratios in the organic P sources and the soil environment affect the availability coefficient for organic P. For more information on Manure Management go to Manure Management - Minnesota Extension/Soils(on line).

Most animal manure research interpretations indicate that approximately 60-80% of the total P is available to crops in the first year. Due to the chemical composition of other organic P sources such as bone meal, lesser amounts of plant available P compared to total P are expected.

Phosphorus as a Pollutant

Because of its rapid fixation, phosphorus is seldom leached from soils. However, P can still become a pollutant to our waters especially when P levels exceed 15 ppm. Phosphorus becomes an environmental problem when soil particles containing P erode off site or run off contains organic P. Excessive phosphorus from runoff and erosion can fertilize surface waters. In this process, called eutrophication, microscopic floating plants, known as algae, multiply rapidly when fertilized by phosphorus.

These algae cloud the water making it difficult for larger submerged aquatic vegetation (SAV) to get enough light. Excessive plant growth in water bodies may also lead to the death of fish and other aquatic animals from lack of oxygen in the water. (Oxygen is used by plants for respiration and by micro-organisms in breaking down plants after they die.)

Phosphorus pollution comes from many sources in urban areas, mainly sewage treatment plants, storm sewers, and industrial sources. Most of these are point sources (i.e. easy to locate). In rural areas, the sources are sewage treatments from small towns, improper septic systems, storm sewers, manure runoff, nutrient runoff, milkhouse wastes, and eroded soil. Most of these are non-point sources (i.e. harder to pinpoint). Phosphorus from farmland has three sources: the farmstead, pastures near watercourses, and cropland.

See web page on eutrophication Phosphorus & Eutrophication

Recently the state of Minnesota has passed legislation to prevent the sale of P containing lawn fertilizers for the Twin Cities Metro area. Most soils are already very high in P and additional P in fertilizer would contribute to phosphorus pollution. See City of Plymouth, MN and Lawn Care and Water Quality with MDA for more information about this topic.

Soil Scientists are also doing research to develope a Phosphorus Index (P Index) that can identify farm fields that are a potential source of phosphorus (P) pollution of surface waters. Using the P Index can help a farmer identify fields and management practices that have the greatest potential to pollute bodies of water with phosphorus. The P index can help land users assess management strategies to minimize P loss from agricultural areas. Minnesota P Index & Wisconsin P Index

Reference: UM Extension FO-6288-GO George Rehm, Michael Schmitt, John Lamb, Gyles Randall, and Lowell Busman

Chapter 5 Potassium Fertilizer

Fertilizer Chapters

Lab Units

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