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Re: [SANET-MG] Phosphorus availability (was Loss of Minerals) Phosphorus in Soil
Edna Weigel wrote:
Dale, LL, and any one else interested:
So, if VAM help make P available, what cultural processes encourage VAM?
Does this get back to the concept of using cold compost and/or organic
mulch? Am I correct in thinking greater organic matter content helps VAM?
Does tillage slow down VAM activity? How does soil temperature (excessively
high or cold) affect VAM? How does (in my case) excessive calcium carbonate
in soil affect VAM (if at all)?
Warm thoughts, Edna
Thanks to Heide for this link:
LESSON 34 Phosphorus Management: Bridging the Interface between Agriculture and Environment
MODULE D Land Application and Nutrient Management
Phosphorus in Soil
Soil P exists in inorganic and organic forms (Figure 34-2). Each form is
a continuum of many P compounds, existing in equilibrium with each other
and ranging from solution P (taken up by plants) to very stable or
unavailable compounds (the most typical). In most soils, 50% to 75% of the
P is inorganic.
Inorganic P is usually associated with aluminum (Al), iron (Fe), and
calcium (Ca) compounds of varying solubility and availability to plants.
Phosphorus has to be added to most soils so adequate levels are available for
optimum crop growth and yield. However, P can be rapidly fixed (also
referred to as sorption) in forms unavailable to plants, depending on soil pH
and type (Al, Fe, and Ca content). Conversion of unavailable to available
forms of soil P usually occurs too slowly to meet crop P requirements
(dashed line on Figure 34-2). As a result, soil P tests were developed to
determine the amount of plant-available P in soil, and from this, how much P
should be added as fertilizer or manure to meet desired crop yield goals. The
estimated amount of plant-available soil P is subsequently referred to as soil
Organic P compounds range from readily available undecomposed plant
residues and microbes within the soil to stable compounds that have become
part of soil organic matter. Biological processes in the soil, such as microbial
activity, tend to control the mineralization and immobilization of organic P.
Mineralization, the breakdown or conversion of readily available organic P to
inorganic solution P, occurs in most soils, but it is usually too slow to provide
enough P for crop growth. Immobilization is the formation of more stable
organic P, which is resistant to breakdown.
In most soils, the P content of surface horizons is greater than subsoil
(Figure 34-3). Except in special situations, added P tends to be fixed by the
soil where it is applied, allowing for little movement down through the soil.
In reduced tillage systems, fertilizers and manures are surface applied with
little or no mechanical incorporation, thus increasing P buildup in the top 2 to
5 inches of soil.
Overall, soil pH is the main property controlling inorganic P forms,
Figure 34-4. Approximate representation of the fate of P added to soil by
sorption and occlusion in inorganic forms, as a function of soil pH.
In most soils, the P content of surface horizons is greater than subsoil.
although Al, Fe, and Ca content determine the amounts of these forms.
In acid soils, Al and Fe dominate P fixation, while Ca compounds fix P in
alkaline soils. As a result, P availability is greatest at soil pH between 6
and 7 (Figure 34-4). Immobilization of inorganic P by these processes renders
a portion of the added P unavailable for plant uptake (Figure 34-5). Mehlich-
3 soil P decreased with time after application of P to a clay and silt loam soil.
At the same time, more inorganic P was fixed with Al and Fe (Figure 34-5).
This illustrates why crop removal of inorganic P from soil is generally low. In
the United States, an average 29% of P added in fertilizer and manures is
removed by harvested crops, ranging from < 1% in Hawaii to 71% in
Wyoming (National Research Council 1993). The low recovery reflects the
predominance of high P-fixing soils in Hawaii.
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