Phosphate accumulation: A MAJOR ISSUE

1. Some watersheds with a high percentage of cropland and pastureland receiving animal manure may require restricted P applications to avoid excess P loading of streams and lakes fed by that watershed. Maximum STP levels may be appropriate for these watersheds. Watersheds with a low population density of livestock and poultry farms may be able to tolerate higher STP levels on fields without harmful effects to water quality. However, to effectively manage P in every watershed, the factors affecting the movement of P from application sites to streams and lakes must be considered.

2. Growers should be encouraged to make commercial fertilizer applications formulated with N and K2O to meet the forage needs of fields where animal manure is no longer applied. It must be recognized that decreased fertility will result in a loss of forage cover and increased erosion, which could create a greater P problem in runoff than continued manure applications. Research has shown that when erosion is kept to a minimum, dissolved P is predominant in runoff water, but as erosion increases the percentage of particulate P in the runoff increases.

3. When applying commercial fertilizer on fields with STP at 100 or more pounds per acre, do not use fertilizer with P in the formulation. It should be N (ammonium nitrate or urea) or N-0-K2O, which is nitrogen and potash with no phosphorus.

4. When making N-based early season applications with manure, late season commercial fertilizer applications should be N or N-0-K2O with no P in the formulation.

5. Role of phosphate solubilizing microorganisms:

Phosphorous in soil is available chiefly as orthophosphate and rarely as phosphine and phosphonate is also available, the diverse soil P form can generally be categorized as soil solution P, insoluble inorganic P or insoluble organic P. Besides fertilization, the availability of P can be achieved by two pathways (a) the enzymatic decomposition of organic P compounds and inorganic P compounds and (b) the non enzymatic solubilization of different rock phosphate and inorganic phosphorous sources. Many fungi, bacteria, actinomycetes and cyanobacteria are potential solubilizers of bound phosphate in soil (Banik & Dey, 1983; Singh & Kapoor, 1992). Some important P solubilizing fungi, bacteria and actinomycetes are listed in table 2. These microorganism are found in all soil but there number vary with soil climate as well as history (Gupta et al. 1986).

List of some important phosphate solubilizing microorganisms.

Group Members

Fungi :Aspergillus niger, A. nidulans, Cephalosporium sp

Bacteria :Pseudomonas, Bacillus,

Cyanobacteria :Anabaena, Calothrix, Nostoc, Scytonema

Solubilzation of phosphorous takes place by at least two mechanisms (a) solubilization by production of organic acids and (b) solubilzation by action of phosphatase enzymes.

Biological nitrogen fixation depends appreciably on the available forms of P so the combined inoculation of nitrogen fixers and phosphate solubilzing microorganisms may benefit the plant growth better than either group of organism alone.

6.  Organic phosphate solubilization

To make this form of P available for plant nutrition, it must be hydrolyzed to inorganic P. Mineralization of most organic phosphorous compounds is carried out by means of phosphatase enzymes. The presence of a significant amount of phosphatase activity in soil has been reported .Important levels of microbial phosphatase activity have been detected in different types of soils .In fact, the major source of phosphatase activity in soil is considered to be of microbial origin .In particular, phosphatase activity is substantially increased in the rhizosphere.