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Phosphorus in the soil and determination of its availability to plants is a very complex problem. It is hard to predict the effects of phosphorus fertilizers upon crops for all kinds of soils and for different growing season lengths. The satisfactory utilization of phosphorus is dependent not only upon the phosphate concentration, but upon the concentration of the other plant food elements, as well as soil temperature, moisture, pH, and the soil micro-organisms.
All soils have some phosphorus reserve in compounds of different chemical form such as phosphates of iron, aluminum, calcium etc., and though these reserves may be measured in large amounts in the soil, plants may still suffer from phosphorus deficiency. The natural release of phosphorus from these compounds may be severely limited due to certain physiological and biological conditions of the soil resulting in the continuation of the compounds in insoluble or unavailable forms of phosphorus.
Plants absorb phosphorus primarily in the form of ions of ortho or dihydrogen phosphate {H2PO4). The difficulty in supplying enough of this available form of phosphorus is that the reactions of soils tend to make water soluble phosphates into water insoluble phosphates, thus adding to the phosphorus reserves which are not as available to plants. Acid soils containing excess iron and aluminum, and basic soils containing excess calcium, cause a chemical recombination of acidic available forms of water soluble phosphates into forms less soluble.
Much of the soluble phosphorous is built into the bodies of the soil micro-organisms and subsequently becomes part of the soil humus. Therefore, supplying the phosphorus needs of plants is partly dependent upon the amount of phosphorus ions released from the phosphorus reserves by the bio-chemical processes of the soil. To supply enough phosphorus for plant needs, a reserve of phosphorus in excess of soil biological needs must be maintained, as well as proper soil conditions for maximum biological activity.
Phosphorus does not leach easily from the soil and normally is not lost unless the whole soil body is removed. However, there is evidence a minor amount of phosphorus movement occurs in our sandier soils.
The addition of phosphorus to the soil may have a three fold purpose:
1. To furnish an active form of phosphorus as a starter fertilizer for immediate stimulation of the seedling crop.
2. To provide a continuing supply of available phosphorus for the crop during the entire growing season.
3. To insure a good reserve supply of phosphorus in the inorganic or mineral, the organic, and the absorbed forms. (Phosphorus storage in all these forms is possible and desirable.)
The first objective is best insured by applying readily available, acidic, or soluble forms of phosphate at the time of seeding, as a band application, as a mild starter solution, or both. The second objective requires the deeper incorporation of adequate amounts of available forms to supply the crop through the growing season. The third objective can be reached in some soils (pH 6.5 or less, and with adequate organic matter level) by using the highly pulverized natural mineral forms such as rock phosphate. In other soils of more basic pH, and in some acid soils, the only material of appreciable value is acid formulations or soluble forms.
There are several chemical methods by which to determine the amount of phosphorus in soils in terms of availability. Currently we are employing the sodium bicarbonate (NaHCO3) method which is the current standard method adopted by the Soil Improvement Committee of the Pacific Northwest Universities and the fertilizer industry representatives. Other methods are under investigation, especially for acid soils.
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