1988 Annual Report for LNC88-016
Assessing Soil Phosphorus Availability in Low-Input Systems
Summary
Rationale:
Low-Input and sustainable systems require reliable knowledge of a soil's phosphorus supplying
ability for effective management of phosphorus nutrition. For soils that do not regularly receive
fertilizer-phosphorus input, current soil tests do not reliably predict phosphorus availability.
When inputs of inorganic P are reduced, the readily available soluble pools of orthophosphate
decline and crops will depend more on the mineralization of organic P to provide adequate
nutrition. Under these circumstances, a test which evaluates only the inorganic status of the soil
will likely be inadequate to describe the P supplying power of a soil. The purpose of this project
is to develop a new soil test that will account for both the inorganic and organic fractions of P in
the soil which are available for plant uptake during the course of the growing season. A
successful soil test needs to emulate the plant uptake process either by a) quantifying the labile
organic phases or b) measuring the rate of mineralization.
Objectives:
1) Develop a soil test capable of predicting the phosphorus supplying power of a soil in a
low-input system.
2) Evaluate this new soil test on a wide variety of soil types, for its ability to predict which soils
will respond to fertilizer inputs.
Methods:
Fourteen Kansas soils, ranging in bicarbonate-extractable inorganic phosphorus from 0.2 to 26.1
mg P kg -1, were enriched with a C+N source to raise microbial activities and create a biological
sink for the bioactive treatment or left untreated. After 7 days of incubation, the quantity of
NaHCO3-extractable inorganic P, total P, and inorganic plus microbial P was measured. This
data was then used to describe the size of the labile organic P, microbial P, immobilized P,
potentially mineralizable P, total bioavailable P, bioavailable organic P, and residual extractable
P pools.
Results:
Bioactive soils always had less in organic P than untreated soils after incubation, most likely
because of microbial immobilization. Labile organic P in the bioactive soils average 2.4 time
greater than in untreated soils. After incubation, labile organic P averaged 2.8 times greater than
the P level prior to incubation in bioactive soils but had remained essentially the same in
untreated soils. Clearly, microbial activity was enhancing the availability of certain organic P
forms. This increase in P availability would not be apparent in typical soil tests using chemical
extractions, which measure only inorganic P. The inorganic, organic, and microbial P pools were
combined to create a bioavailable P indexes. This index was not as closely correlated to the
typical soil test P index (pre-incubation inorganic P) for the bioactive as for the untreated soil.
These data suggest that this extraction procedure can enhance understanding about the P
supplying power and P nutritional dynamics for certain soils.
Potential Contributions and Practical Applications:
Organic P constitutes a significant portion of total soil P, ranging from 15 to 80 percent, but
quite normally 30 to 50 percent in most soils. Including organic P in P soil tests would provide
information useful for avoiding excessive fertilizer P application, enhancing long-term
maintenance of soil fertility, monitoring P dynamics in soils receiving little or no inorganic P
fertilizer, and quantifying the rate and extent of organic P mineralization.