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ARS Home » Northeast Area » University Park, Pennsylvania » Pasture Systems & Watershed Management Research » Research » Publications at this Location » Publication #134737
Title: THE EFFECT OF SOIL ACIDITY ON POTENTIALLY MOBILE PHOSPHORUS IN A GRASSLAND SOIL

Author
item MCDOWELL, R - AGRESEARCH LIMITED
item BROOKES, P - IACR-ROTHAMSTED
item MAHIEU, N - QUEEN MARY&WESTFIELD COLL
item POULTON, P - IACR-ROTHAMSTED
item JOHNSTON, A - IACR-ROTHAMSTED
item Sharpley, Andrew

Submitted to: Soil Use and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2002
Publication Date: 8/20/2002
Citation: Mcdowell, R.W., Brookes, P.C., Mahieu, N., Poulton, P.R., Johnston, A.E., Sharpley, A.N. 2002. The effect of soil acidity on potentially mobile phosphorus in a grassland soil. Journal of Agricultural Science. 139(1):27-36.

Interpretive Summary: Phosphorus in overland flow and subsurface drainage waters is related to soil P concentration. However, the level of phosphorus in a soil can change greatly due to how much and for how long phosphorus fertilizer is applied and what crops are grown on the soil. The solubility of this soil phosphorus is determined to a large extent by the pH of the soil, which can be made more acidic with the application of nitrogen fertilizer and made more alkaline by liming. Generally, as soils become more acidic, the solubility of soil phosphorus increases and conversely, as a soil become more alkaline, soil phosphorus solubility tends to decrease. This can in turn affect the potential for soils to enrich runoff waters with phosphorus. Even so, contradictory evidence shows that liming can both increase and decrease the concentration of phosphorus in soil solution. Our aim was to examine, on one soil type, the effect of soil pH on the potential mobility of phosphorus using soils from the Park Grass experiment at Rothamsted, England. On these plots various nitrogen and liming practices have been in place for over 100 years, with soils analyses taken over this period. A combination of laboratory and field plot experiments showed that changes in soil pH as a result of applications of nitrogen fertilizer or lime can greatly change the concentration of potentially mobile phosphorus. Although there was less potentially mobile phosphorus in soils with pH < 5.8 than in soils above this pH, it is usually advised in temperate regions to maintain soils about pH 6.5 for arable crops.

Technical Abstract: This study compared phosphorus (P) speciation and the relationship between bicarbonate extractable (Olsen) P and 0.01M CaC12 extractable P (a measure of potentially mobile P) in soils from plots of the Park Grass experiment started in 1856 at IACR-Rothamsted, UK and with and without nitrogen as (NH4)2SO4 and with and without calcium carbonate (CaCO3, lime). A point, termed the change point, was noted in Olsen P, above which 0.01M CaC12-P increased at a greater rate per unit increase in Olsen P than below this point. Soils given (NH4)2SO4 annually since 1856 and with lime periodically since 1903 mostly had a pH between 3.7 to 5.7; some of these (NH4)2SO4 treated soils were limed to pH 6.5 and above from 1965. Irrespective of their pH in 1991/92 all the soils had a change point (120 mg Olsen P kg-1) similar to that (112 mg Olsen P/kg) found for other soils with pH < 5.8. In a laboratory study lasting 30 days, the addition of CaCO3 to acid soils from the field experiment that had received (NH4)2SO4 had a similar change point to soils with pH < 5.8 irrespective of pH, suggesting soil P chemistry was controlled by the long period of soil acidity. This was not reversed by a short period at a higher pH. The effect of pH was attributed to the creation of P sorptive surfaces on aluminium precipitates compared with less acidic soils (pH > 5.8) where there was less exchangeable A1 to be precipitated. This was confirmed with solid-state 31P nuclear magnetic resonance, which indicated that for soils of similar total P concentration and pH, there was twice as much amorphous A1-P in soils given (NH4)2SO4 compared with those without. Changes in pH as a result of applications of (NH4)2SO4 or lime can greatly change the concentration of potentially mobile P due to the effects on A1 solubility. Although there was less potentially mobile P in soils with pH < 5.8 than in soils above this pH, it is usually advised in temperate regions to maintain soils about pH 6.5 for arable crops.