Linda Benedict, Mascagni, Jr., Henry J., Tubana, Brenda S., Wang, Jim Jian
Brenda S. Tubaña, Jim Wang and Henry Mascagni Jr.
Testing for soil phosphorus is an important tool to effectively manage phosphorus fertilizer in crop production. It provides an estimate of plant-available phosphorus – both in solution and a readily soluble form in the soil – and fertilizer requirements. Commercial fertilizer and organic materials, such as manure and compost, can be applied to soil to ensure an adequate supply of available phosphorus for plant use. Phosphorus, however, is reactive with different soil components, which can make it immobile within the soil and unavailable for plant use.
The amount of phosphorus fixed in the soil varies with soil type and pH. The higher the clay content of the soil, the higher the amount of fixed phosphorus. It is generally believed that crops growing on soils with a pH below 6 and above 7 may encounter phosphorus deficiency. Ironphosphate and aluminum-phosphate compounds form at lower pH levels while calcium and magnesium can precipitate phosphate at a higher pH; both groups of compounds are unavailable for plant uptake.
Louisiana crops are mostly produced on soils of loess origin or on alluvial plains along major rivers. These soils are highly diverse because they were formed from different parent materials that originated in the Rocky Mountains, the northern Appalachian Mountains and the upper Midwest. Abrupt changes in physical and chemical properties occurring within and across production fields result in uneven crop stands and varying degrees of responses to fertilizer.
An experiment conducted in 2008 as part of ongoing soil fertility calibration research evaluated several soil types commonly planted with corn. These soils from different areas vary in texture and chemical properties (Table 1). They are slightly acidic to strongly acidic with a wide range of soil-test phosphorus levels. In the experiment in which corn plants were grown in pots, the plants’ response to increasing phosphorus fertilizer rates demonstrated how soil tests can show if the level of plant-available phosphorus is adequate. In Louisiana, the established critical soil phosphorus level is 35 parts per million (ppm). Soil test levels near or below 35 ppm indicate a risk for phosphorus deficiency while levels above 35 ppm would more likely not require phosphorus fertilization.
Because the soil test level of Perry clay at 12 ppm is much lower than 35 ppm, corn clearly responded to phosphorus fertilization. Without phosphorus, corn plants showed typical symptoms of deficiency – purpling of older leaves and stunted growth. Significant improvement in color, height and biomass occurred when phosphorus fertilizer was applied. A slight response to phosphorus fertilizer occurred in corn planted on Commerce silt loam, which has a soil test value slightly lower than 35 ppm. Sharkey clay soil with a 64 ppm soil test level had an adequate supply of available phosphorus, so growth was similar with or without phosphorus fertilizer.
Because phosphorus is reactive with different soil components, not all applied fertilizer will be available for plant use. The increases in soil test levels in Commerce silt loam and Perry clay soils did not proportionately increase with the amount of added phosphorus (Figure 1). For example, 15 ppm of added phosphorus raised soil test levels by 8 ppm in Commerce silt loam but only 3 ppm in Perry clay.
In the laboratory, scientists can estimate this proportion by using the “soil phosphorus buffer coefficient,” which measures the rate of increase in plant-available and readily available phosphorus for every unit of fertilizer added. On a practical application, the buffer coefficient can tell how much fertilizer should be applied to reach a desired level of plantavailable phosphorus in a soil.
The laboratory estimates of phosphorus buffer coefficient of different soils collected from corn-producing areas in Louisiana are shown in Table 1. Values ranged between 0.446 to 0.749 micrograms per milliliter of soil. The general premise is that the higher the buffer coefficient, the higher the amount of added phosphorus that will become available for plant use. Because the clay contents of the coarse-textured Sterlington silt loam and Calhoun sandy loam soils are lower than the heavy-textured Sharkey and Perry clay soils, smaller amounts of applied phosphorus are expected to be fixed and held unavailable for plant use.
Commerce silt loam in northeast Louisiana with a high buffer coefficient of 0.74 and initial soil test level of 24 ppm of phosphorus will need less fertilizer than Perry clay with a buffer coefficient of 0.446 and a soil test level of 12 ppm to raise the soil test level to 35 ppm. Figure 1 shows that 30 ppm of added phosphorus was required to raise the Commerce silt loam soil test level above 35 ppm while Perry clay needed 60 ppm of phosphorus.
The results of this study showed that initial soil-test phosphorus values are useful in evaluating the capacity of soil to supply plant-available phosphorus. The buffer coefficient can be used to estimate the amount of phosphorus needed to reach the desired soil test level and the corresponding fertilizer requirement with a higher degree of accuracy. Phosphorus fertilizer guidelines based on soil testing and the phosphorus buffering coefficient offer the opportunity to improve phosphorus fertilizer-use efficiency.
Brenda S. Tubaña is an associate professor and Jim Wang is a professor in the School of Plant, Environmental & Soil Sciences. Henry Mascagni Jr. is a professor at the Northeast Research Station, St. Joseph, La.
(This article was published in the spring 2013 issue of Louisiana Agriculture magazine.)