Scientific registration n : 1941 Symposium n : 2 Presentation : poster Integrating environmental soil P tests into nutrient management plans for intensive animal agriculture Mise en place dans les plans de gestion environnementale des nutriments de tests d appréciation des teneurs en phosphate du sol Application à l élevage intensif PAUTLER Maria, SIMS J. Thomas Department of Plant and Soil Sciences, 149 Townsend Hall, University of Delaware, Newark, Delaware, 19717-133, USA Introduction Non-agronomic environments, as well as agronomic settings are affected by soil P levels. Soils that are over-fertilized with P defy principles of sustainable agriculture and increase the risk of non-point source pollution of surface waters. Delaware especially Sussex County, site of a large, coastal watershed has one of the most intensive poultry industries in the eastern United States and long-term land application of animal wastes as fertilizer has elevated P levels in many soils to values well beyond those needed for crop production. To protect the quality of surface waters, it is necessary to minimize P accumulations to excessive levels in soils and the transport of P from soil to water bodies. Thus, establishing quantitative, predictive relationships between soil test P (Mehlich 1) and potentially desorbable P could be extremely useful in ongoing efforts to prioritize non-point source pollution control. Our specific objectives were to evaluate rapid analytical methods to assess desorbable P in soils and answer the following: Which rapid tests correlate well with the degree of P saturation (DPS) in soils? From an environmental standpoint, what is the best way to quantify the potential for P to be desorbed from soil? Methodology - Agricultural soils from Delaware (124) were taken from the archives of the University of Delaware Soil Testing Program (UDSTP); Mehlich 1 P (M1-P) had previously been determined. The number of soils studied was based on the percentage of soils that were rated as excessive (> mg P kg -1 ) by M1-P in each county. New Castle (NC; 12%) and Kent (K; 23%) soils were chosen over the range of M1-P, ph, and organic matter (OM) values. Sussex (S; 6%) soils were chosen over the range of M1-P values and also to correspond with NC and K or other S samples (e.g., same M1- P, but differing ph or OM values). Five soils received from The Netherlands were subjected to the same analytical methods as for the Delaware soils to determine similarities in P chemistry between soils with similar topography, agricultural system, climate, and hydrology. 1
The extraction methods used included: 1 - Mehlich 1 (M1-P; 1: soil:.n HCl /.2N H 2 SO 4 ; min reaction time); 2 - Readily desorbed (RD-P; 1:1 soil:.1m CaCl 2 ; 1 h reaction time); 3 - Oxalate-extractable (P ox ; 1:4 soil:acidified (NH 4 ) 2 C 2 O 4 ; 2 h reaction time in darkness); 4 Iron-oxide (Fe-oxide) strip (Strip P; 1:4 soil:.1m CaCl 2 and Fe-oxide coated filter paper strip; 16 h reaction time); and - Dutch water soluble (P w ; 1:22. soil:h 2 O; 36 h reaction time). Soils have a finite capacity to sorb P and as the concentration of sorbed P increases so does the equilibrium P concentration that is maintained in the soil solution. As soils become more progressively saturated with P, they pose greater risks for the release of soluble P to runoff and leaching. Changes in the extent of P saturation are likely to be slow, hence a method to rapidly quantify the degree of P saturation (DPS) would provide a means to identify areas in a watershed that are of greatest environmental concern. In The Netherlands and Belgium, DPS is estimated from the P ox test, which also extracts Fe and aluminum (Al). Since these countries also have coastal, sandy soils, the DPS approach was evaluated to determine if it would be useful for Delaware soils. DPS (%) = P ox x 1.(Al ox + Fe ox ) A critical DPS range of 2-4% is used in The Netherlands and Belgium to protect ground waters from non-point source pollution by P. This is the range at which environmentally significant increases in readily available P and potentially desorbable P occur. Adsorption isotherms and dialysis membrane studies were conducted to study P sorption/desorption over a longer period. The isotherms were used to relate the ability of a soil to adsorb P to its ability to maintain a given level in solution. i.e., an estimate of the soil s buffer capacity for P. Oxalate-extractable Al and Fe were also correlated to adsorption maxima to verify the value of using the P ox method to calculate DPS. Results and Discussion Selected properties of the Delaware soils included those in Table 1. We found that RD-P and Strip P were well correlated for all soils (Fig. 1) and collected a subset of soils for more detailed study. 2
Table 1. Selected properties of Delaware agricultural soils. All Soils Subset Soils Property Range Mean Range Mean All Soils M1-P (mg kg -1 ).1-1446 28 8.4-1146 194 ph 4.3-7.9 6. 4.3-6.9.7 OM. - 12.1 1.8. - 12.1 2.4 New Castle Soils M1-P (mg kg -1 ) 6.7-13 43 2-81 44 ph.6-7. 6.4.6-6.7 6.2 OM 1. - 3. 1.8 1.2-2.3 1.7 Kent Soils M1-P (mg kg -1 ).1-939 16 8.4-141 ph 4.8-7.3 6. 4.9-6.9.8 OM.6-4.4 1.6.6-4.4 2.1 Sussex Soils M1-P (mg kg -1 ) 17-1446 2 2-1146 268 ph 4.3-7.9.9 4.3-6.9.7 OM. - 12.1 1.9. - 12.1 2.7 RD-P (mg kg -1 ) 2 2 1 1 r 2 =.8** for All Soils r 2 =.98** for Subset Soils 2 7 1 12 Strip P (mg kg -1 ) All Soils Subset Soils Fig. 1. Correlation between Strip P and RD-P for all soils and the subset of soils selected for further study. Several rapid P soil tests were compared to M1-P to determine if M1-P is effective as a rapid, inexpensive soil P test that can identify soils high in potentially desorbable P. Mehlich 1-P was poorly correlated with RD-P (Fig. 2), but well correlated with P ox (r 2 =.7 ** ), Strip P (r 2 =.7 ** ), and DPS (Fig. 3). Strip P and DPS have been proposed as environmental soil P tests and as compared to M1-P, appear to be better predictors of potentially desorbable P. 3
RD-P (mg kg -1 ) 2 1 1 r 2 =.6** 1 1 M1-P (mg kg -1 ) Fig. 2. Correlation between M1-P and RD-P for 124 Delaware soils. DPS (%) 2 1 1 r 2 =.78** 1 1 M1-P (mg kg -1 ) Fig. 3. Correlation between M1-P and DPS for 124 Delaware soils. Mehlich 1-P, RD-P, Strip P, and P w were all relatively well correlated with DPS (Figs. 3,4). For these data, 2% DPS was attained at mg M1-P kg -1 ; thus the Dutch equation and the UDSTP delineation of excessive soil P for typical Delaware soils coincide. At ca. 12 mg M1-P kg -1, 4% DPS was attained; environmental concerns are raised at such a high level of soil P. These findings suggest that the range of 2-4% designated by The Netherlands and Belgium for DPS is appropriate for Atlantic Coastal Plain soils. 4
2 DPS (%) 2 1 1 RD-P (mg kg-1) Strip P (mg kg-1) Pw (mg kg-1) RD-P : r 2 =.71** Strip P : r 2 =.76** P w : r 2 =.7** 1 1 Desorbable P (mg kg -1 ) Fig. 4. Correlation between DPS and RD-P, Strip P, and P w for 38 Delaware soils. Conclusions - Strip P and DPS were the best predictors of potentially desorbable P. With the Fe-oxide strip acting as a sink for P, the immediately and potentially desorbed P is measured. In calculating DPS, with its P ox component (which measures the total amount of sorbed P present), the point at which a soil has become over-saturated with P is determined. Since DPS is well correlated with soluble P in soils and dissolved P in runoff and DPS can be rapidly measured, it should be possible to identify both those areas where P desorption into runoff or leaching waters will be highest and areas where soils will still have the capacity to retain additional P without creating environmentally excessive concentrations of soluble P in surface or ground waters. This would be extremely important in areas where intensive animal agriculture predominates and high DPS values are commonplace. Soil test P data from the M1-P test could be used as a screening approach to assess low, medium, high, and excessive P soils. A 2-4% DPS range should delineate soils of the most environmental concern. Future research will evaluate soil P tests inclusive of OM and ph values to determine if differences in the DPS critical range arise due to soil properties. Comparisons will be made between the Delaware and Netherlands soils. Adsorption maxima from P isotherm and long-term adsorption/desorption experiments will be assessed to determine the soil s buffer capacity for P. Results from these studies will contribute to efficient P management for crop production as well as environmentally sound P management in general and feasible solutions to the complex problem of non-point source pollution by soil P. Keywords : phosphorus, phosphorus desorption, soil testing, nutrients, water quality Mots clés : phosphore, désorption du P, test du sol, nutriments, qualité de l eau