Essential Elements for Plant Growth


P deficiency in corn

The biological functions of P in living organisms is most notable in the ubiquitious ATP/ADP energy transport and storage compounds. 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 no glaring visual deficiency symptom except for stunted growth and late maturity. Grassy species, including corn, will show reddening of leaves if P is severely deficient.

The forms of P in soil are: soluble, mineral, and organic.

Soluble P in soils in often in the micromolar concentration range and is an almost negligible percent of total soil P except for the fact that it is the only P that plants can use! The forms of soluble P are H2PO4- and HPO42-, which the proportion between them dependent upon pH. At pH 7.2, they are present in equal concentrations. At pH 6.2, the ratio H2PO4-:HPO42- is 10:1, and at pH 8.2, 1:10.

Mineral P constitutes 20 to 85% of total soil P, depending upon the soil itself, particularly the soil organic matter content. Generally, formation of Al and Fe phosphates, either by precipitation or sorption, limits P solubility in acid soils and Ca phosphates limit P solubility in alkaline soils. This picture creates a "sweet spot" for maximum P solubility in equilibrium with P minerals at pH 6.5. By the same token, the minimum amount of P retention, or fixation, (within the range of common soil pH values) is at pH ~6.5.

Organic P constitute 15 to 80% of total P (and exactly complements the % mineral P since soluble P is numerically negligible.) Soil organic matter typically contains C/N/P/S in a ratio of 140:10:1.3:1.3, so that knowledge of the amount of organic matter will provide an estimate of the amount of organic P. Organo-P is cleaved by enzymes called phosphatases, which are present in many soil organisms and produced by some plants as well. In some soils, P is the most limiting plant nutrient, not N. In such soils, usually in the tropics, the pH is in the acid range, soils have a great P-fixing capacity, soil organic matter levels are very low, and no organic residues are returned to soils.

Another way of looking at P, particularly from a management point of view, is:

soluble-P <==> labile-P <==> nonlabile-P

Soluble P is fairly well defined, of course, but the chemistry of the labile P is more complicated, being a combination of mineral and organic P, and both absorbed and precipitated. In addition, the mineralization of organic-P is a particularly valuable source of bioavailable P, particularly in strongly-P-fixing soils. The soluble P may be thought of as an intensity and labile P as a quantity, so that the P buffer capacity is quantity/intensity.

P Inputs to soil:

P Outputs from soil:

P Testing

The most common methods of testing for P availability are the Bray-1, Mehlich III, and Olsen methods. Bray 1 uses a mixture of HCl and NH4F to dissolve adsorbed and poorly crystallized phosphate minerals into an acid extractant that increases the solubility of phosphate for purposes of extractant; fluoride is a good complexing agent for Al3+ to improve phosphate solubility. The Mehlich method is similar and uses a mixture of NH4F, acetic acid, and NH4Cl/HCl. Both are applicable for measuring labile phosphate in acid and neutral soils. For calcareous soils (containing free CaCO3, with pH>7.5) such acid extractants would simply dissolve the carbonates and consume the acid in the extractant. Instead, the Olsen method uses 0.5 M NaHCO3 at pH 8.5, which suppresses Ca2+ by both the high HCO3- concentration and high pH, allowing phosphates to dissolve out of calcium phosphate minerals (by the common ion principle). (All of these extractants differ from direct measure of the soluble phosphate in soil solution, which is present in analytically-difficult, but not impossibly low, concentrations and is only a small portion of the total plant-available phosphate in soil.)

Yet another P-test that is becoming increasingly popular is iron-oxide impregnated filter paper, which uses to advantage the strong sorption of phosphate on iron oxides. A solution of ferric chloride is neutralized to form iron oxides on filter paper. The iron-oxide impregnated filter paper can either be shaken with soil and water or buried in soil for a period of time to absorb soil phosphate and then recovered. The phosphate is then desorbed from the filter paper and analyzed. Results generally show a strong correlation between labile P by this method and by the more conventional methods.

Results of these analyses are used with tables of low, medium, high and very high values, based on probability and degree of plant response to added P. Such tables are collected soil-by-soil and crop-by-crop using field trials and may be distributed also as computer-computed recommendations.

Phosphorus Fertilizer Analysis

Fertilizer analyses of phosphorus in the U.S. is in the form of P2O5. [Remember: To convert %P to %P2O5, multiply by (2x31 + 5x16)/2x31 [= 2.29]. Analyses are divided into water-soluble P2O5 and citrate-soluble P2O5. Monocalcium phosphate, the major portion of superphosphate, is water-soluble. Dicalcium phosphate is largely citrate-soluble. Rock phosphate has low citrate-soluble P and very low water-soluble P.

P Fertilizers

Rock Phosphate: source of all mineral P for fertilizers. Ore grade contains 6 to 15% P. Mined from deposits in Florida, Tennessee, North Carolina, Morocco, and elsewhere. Very insoluble. Effectively used as fertilizer material only on very acid soils or if acidulated or partially acidulated.

Single Superphosphate:aka, normal, simple, or ordinary superphosphate (although how can anything "super" be both normal or ordinary?) Basically the product of the reaction of rock phosphate, usually a fluorapatite, with sulfuric acid according to the reaction:

2 Ca5(PO4)3F + 7 H2SO4 --> 3 Ca(H2PO4)2 + 7 CaSO4 + 2 HF

producing a mixture of monocalcium phosphate, a water-soluble phosphate, and gypsum, along with the volatile, and very nasty, hydrofluoric acid gas as a by-product. (Superphosphate plants are regulated with regard to HF and SiF4 emissions.) The commercial product contains between 7 to 9.5% P, or 16 to 22% P2O5.

Phosphoric acid, H3PO4: Produced by the same process as single superphosphate, but using more sulfuric acid.

2 Ca5(PO4)3F + 10 H2SO4 --> 6 H3PO4 + 10 CaSO4 + 2 HF

Phosphoric acid is then separated mechanically from the gypsum for use. Phosphoric acid is rarely used directly as a fertilizer except for purposes of fertigation and commercial hydroponics, particularly when the source water contains (calcium) bicarbonate alkalinity (=high pH) that would otherwise cause phosphate precipitation. This material is, however, the acid used to manufacture the ammonium phosphates and the next fertilizer:

Pure phosphoric acid is a solid that melts at 42C. Phosphoric acid of commerce is typically ~23%P, or 53-54% 16 to 22% P2O5.

Triple Superphosphate: The active ingredient is calcium monophosphate, as in the single superphosphate, but the product carries less gypsum content because it was made with phosphoric acid instead of sulfuric acid:

2 Ca5(PO4)3F + 12 H3PO4 + 9 H2O --> 9 Ca(H2PO4)2+ 2 HF

This material contains 17 to 23% P, 44 to 52% P2O5. It was the most popular P fertilizer until surpassed in usage by ammonium phosphates. Much of the gypsum generated as a byproduct is diverted into construction.

Ammonium phosphates: MAP, monoammonium phosphate, NH4H2PO4, typically 12-52-0, and DAP, diammonium phosphate, (NH4)2HPO4, typically 20-50-0. Formulated by reacting various quantities of ammonia with phosphoric acid. More often regarded as P fertilizer than N fertilizer since fertilizer application of DAP to achieve desired N rates exceeds usual P rates, in most cases. Completely water-soluble P.

Superphosphoric acid: A phosphoric acid mixture containing a substantial proportion of one or more polyphosphoric acids (pyrophosphoric acid, H4P2O7; tripyrophosphoric acid, H5P3O10; tetrapyrophosphoric acid, H6P4O13. Manufactured by "roasting" phosphoric acid to remove some water, thereby polymerizing two or more phosphates. Typically 30 to 36% P, or 68 to 83% P2O5

Ammonium polyphosphates, APP: Product of reacting polyphosphoric acid, (see above) with ammonia to form the final product, with a typical fertilizer composition of 10-34-0, 10% N and 15% P. The product is very water-soluble, and has the capability to chelate some micronutrients, such as Fe and Zn.

Organic sources of P: The very same sources of organic N mentioned previously, for example, manure, whey, sludge, etc., also contain organic P that can be serve as a P source as the organic matter mineralizes. Typical values are 4 lbs P/1000 gal in (solid) manure and whey and 6.3 lbs P/1000 gal in sewage sludge. The use of organic sources as both N and P sources is sometimes problematic because the amount required to meet N needs may also add P far in excess of crop needs; the extra P in the top soil may increase the hazard of P runoff to surface waters.

Other so-called "organic" P sources are bonemeal, in which the active P ingredient is closely related to a carbonatic rock phosphate, and rock phosphate (which is not really organic at all). Both are generally acceptable to organic farmers and gardeners. The availability of P in both is increased by acidulation, either partial or complete, to convert the apatite minerals to more soluble phosphates, although this process is frowned upon by organic farmers because of its use of harsh chemicals in the process. Guano, bird and bat dung accumulations, are also popular as organic fertilizer but wreak a heavy cost to the environments from which they are extracted. This outcome should be no surprise considering the history of the guano trade, as recounted in the Trade Environment Database.

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This page was last modified by Phillip Barak, Univ. of Wisconsin, on 5 Jan 1999. All rights reserved.