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SWEEP Report #53

Phosphorus Movement in Soil as a Function of
Phosphorus Solubility and Reactivity

Researchers: 
L. J. Evans, Dept. of Land Resource Science, University of Guelph, Guelph, Ont.

Executive Summary

Evaluation Summary (Tech. Transfer Report Summaries)

View/Download Report [554 KB pdf]  [040621]

Associated SWEEP/LSP Research

 

 

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Completed: May, 1992

Key Words:

phosphorus, crop residue, organic acids, conservation tillage

Executive Summary

To investigate the possible role of crop residues on the release of soil phosphorus and its effect on crop growth, a series of three experiments were performed. An acidic soil (Welland Series), pH 5.3, from the Niagara region of southern Ontario was limed to pH's 6.4 and 7.5 by addition of two rates of lime, CaCO3. Corn residues were either added directly to the soil surface or were incorporated into the soil. The experiment consisted of nine treatments: three levels of soil treatment (control, surface applied residue and incorporated residue) were combined with each of three pH values (5.3, 6.4, 7.5). Barley was grown in the various soils in a greenhouse. The plants were harvested at two different growth periods, corresponding to Feeke's stages 3 and 6. Both plant and soil analyses were performed.

There were differences in the yield of barley between the control soil and the two residue treatments. These differences in yield however were not significant at the first harvesting period. Final yields at the later harvesting were significantly lower in the control than in the two residue treatments at the two lower pH values but the effect of the residue treatments on final barley yields disappeared in the soil limed to a pH of 7.5.

The increased yield for the residue treatments did not appear to be due to an increased content of tissue phosphorus. The increased plant yields appeared to be more directly related to increased tissue potassium content at the earlier date of harvesting and to an increased tissue nitrogen content at the later harvesting date.

The increased yields also corresponded to increased soluble carbon contents in the extracted soil solutions and therefore it is possible that the degradation of the corn residues either at the soil surface or directly incorporated into the soil released soluble nitrogen and potassium, in addition to soluble phosphorus. Although there was some evidence for increased soluble phosphorus contents in the extracted soil solutions these increases were not significant.

In a second experiment, the soils with the three different pH values were incubated with incorporated corn or alfalfa residue over a period of forty-eight days. At various time intervals the soil solutions were extracted and analyzed for soluble organic carbon and phosphorus.

Incubation with either corn and alfalfa residues lead to an increase in soluble organic carbon, soluble total phosphorus and ortho-phosphate. The maximum amounts of each of the constituents occurred after two to three weeks, with contents of the constituents being released over a longer time period for alfalfa than for corn. Because of uncertainties in the exact nature of total phosphorus measured by the ICP-AES technique, it was not clear whether this increase in phosphorus content is due to the release of mineral-bound phosphorus or due to an increase in organic phosphorus. An automated procedure for determining both inorganic and organic forms of soluble phosphorus was developed in the latter stages of this work - see Appendix I.

In the third experiment, the soils were shaken with phosphate in the presence and absence of different naturally occurring organic acids. It was shown that retention of phosphorus was less in the presence of these acids and that those organic acids that form strong bonds with the surfaces of soil particles were the most effective in reducing specific adsorption of phosphate.

This study has shown that there is an increase in crop yield if soils at three different pH's are amended with crop residues either applied at the soil surface or incorporated into the soil. Incubation of the soils with either corn or alfalfa residues increases the amount of soluble phosphorus in the soil solution after about two to three weeks of incubation. With increased time however the content of soluble phosphorus decreases. This increase in soluble phosphorus is probably due in part to release of inorganically and organically bound phosphorus from the crop residues and, as the final part of the study has shown, to release of soil bound phosphorus by the increased concentration of released organic acids.

 

Evaluation Summary

(From Technology Transfer Report Summaries - A. Hayes, L. Cruickshank, Co-Chairs)

The rationale behind the research was that organic acids released during decomposition of crop residues on the soil surface of conservation tillage treatments would result in an increase in the amount of soluble phosphorus in the soil solution early in the growing season (when phosphorus nutrition is important) due to competition between phosphate ions and organic acid ions for soil adsorption sites (hence more available P). To investigate this hypothesis, a series of three experiments were performed. In the first, an acidic soil (Welland Series) of pH 5.3 from Niagara Region of southern Ontario was limed to pH's of 6.4 and 7.5. Corn residues were either added directly to the soil surface or were incorporated into the soil. The experiment consisted of nine treatments - three residue treatments - control, surface applied residue and incorporated residue and three pH values - 5.3, 6.4 and 7.5. Barley was grown in the various soils in a greenhouse. The plants were harvested at two different growth periods, corresponding to Feeke's stages 3 and 6 (approximately 20 and 40 days growth respectively). Both plant and soil analyses were performed.

There were differences in the yield of barley between the control soil and the two residue treatments but they were not significant at the first harvesting period. Final yields at the later harvesting were significantly lower in the control than in the two residue treatments at the two lower pH values but the effect of the residue treatments on final barley yields disappeared in the soil limed to a pH of 7.5.

The increased plant yields were more related to increased tissue potassium at the earlier harvesting stage and increased N tissue content at the later harvesting stage than to tissue phosphorus. The increased yields also corresponded to increased soluble carbon contents in the extracted soil solutions and therefore it is possible that the degradation of the corn residues either at the soil surface or directly incorporated into the soil released soluble nitrogen and potassium, in addition to soluble phosphorus. Although there was some evidence for increased soluble phosphorus contents in the extracted soil solutions these increases were not significant.

In a second experiment, the soils with the three different pH values were incubated with incorporated corn or alfalfa residue over a period of forty-eight days. At various time intervals the soil solutions were extracted and analyzed for soluble organic carbon and phosphorus.

Incubation with either corn or alfalfa residues led to an increase in soluble organic carbon, soluble total phosphorus and ortho-phosphate. The maximum amounts of each of the constituents occurred after two to three weeks, with contents of the constituents being released over a longer time period for alfalfa than for corn. Because of uncertainties in the exact nature of total phosphorus measured by the ICP-AES technique, it is not clear whether the increase in P content is due to the release of mineral-bound phosphorus or due to an increase in organic phosphorus. An automated procedure for determining both inorganic and organic forms of soluble phosphorus was developed in the latter stages of this work (see Appendix I).

In the third experiment, the soils were shaken with phosphate in the presence and absence of different naturally occurring organic acids. It was shown that retention of phosphorus was less in the presence of these acids and that those organic acids that form strong bonds with the surfaces of soil particles were the most effective in reducing specific adsorption of phosphate.

This study has shown that there is an increase in crop yield if soils at three different pH's are amended with crop residues either applied at the soil surface or incorporated into the soil. Incubation of the soils with either corn or alfalfa residues increased the amount of soluble phosphorus in the soil solution after about two to three weeks of incubation. With increased time however the content of soluble phosphorus decreases. This increase in soluble phosphorus is probably due in part to release of inorganically and organically bound phosphorus from the crop residues and as the final part of the study has shown to release of soil bound phosphorus by the increased concentration of released organic acids.

Comments:

The literature review contains a good overview of phosphorus forms and reactions in the soil.

Associated SWEEP/LSP Research:

  • SWEEP Report #23 - Processes Involved in Mobilization of Phosphorus in Different Farming Systems in Southwestern Ontario: Nutrient Levels in Plant Tissues and Soils

  • SWEEP Report #35 - Nutrient Distribution and Stratification Resulting from Conservation Farming

  • SWEEP Report #56 - Yield Reduction Effects of Crop Residues in Conservation Tillage

Future Research: ( ) indicates reviewers suggestion for priority, A - high, C - low.

(B) We need to develop an understanding of the cycling of carbon, phosphorous and nitrogen in cropping systems, particularly with a view to understanding the "allelopathic effect" observed by many no-till farmers.

 

 

 

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Created: 05-28-1996
Last Revised: Thursday, May 19, 2011 04:00:35 PM