Agronomic and environmental soil test phosphorus method comparisons and diet modification impacts on poultry litter phosphorus composition

Branly Eugene, Purdue University

Abstract

Phosphorus (P) loss from agricultural fields to freshwater ecosystems is of increasing concern due to elevated soil P levels brought on by long term and excessive application rates of manure and commercial fertilizers. Both Bray P1 and Mehlich-3 P (PM3) soil tests are used in Indiana to make fertilizer recommendations and to limit manure application rates. The Mehlich-3 P Saturation Ratio (PSRM3) has been proposed as an alternative to PM3 and the ammonium oxalate degree of P saturation (DPSOX) for assessing the risk of soluble P loss from soils. We assessed the correlations among agronomic soil test methods (PM3 and Bray P1), environmental soil test methods (soluble P: deionized water, DW; artificial rainwater, ARW; dilute salt extractable P, DSEP), ammonium oxalate P (POX), total P (TP), and P saturation methods from 565 Indiana surface soil samples. Significant correlations were found among the various STP methods evaluated, and Bray P1 and PM3 displayed the strongest coefficient of correlation (r = 0.93, p < 0.0001). Mehlich-3 P had stronger correlations with TP and POX compared to correlations between Bray P1 and TP and POX, and their correlations were all highly significant ( p < 0.0001). Additionally, all soluble P forms were significantly correlated with Bray P1, PM3, and POX, and the correlations between Bray P1 and all three soluble P measures were consistently greater than those between other soil test methods and the soluble measures. Significant correlations were found between PM3 and PSRM3 (r = 0.93, p < 0.0001) and between PSRM3 and DPSOX (r = 0.92, p < 0.0001), suggesting that PSRM3 can be as effective as DPSOX to identify soils with a greater potential to contribute bioavailable (not just soluble) P to surface and ground water. We conclude that both PM3 and PSRM3 can be integrated into more comprehensive P loss risk algorithms to mitigate elevated P concentration in surface and ground water. The PSRM3 can be used as an alternative to PM3 as it does account for extractable Al and Fe, the primary P sorbents in most soils. Although Bray P1 is an appropriate soil test based on the data generated in this study, it would not be the most practical soil test as it is not considered a multi-element extraction in most states. Since PM3 was strongly correlated with Bray P1 and the benefit of using PM3as a multi-element extractant from which the PSRM3 can be calculated, it is probably the most practical test to use for both agronomic and environmental soil P assessment. We also evaluated the impacts of diets containing different amounts of DDGS and dietary fumeric acid on P excretion and P transformations during litter storage. Total P and phytate P were significantly (p< 0.0001) affected by dietary inclusion of DDGS; where DDGS inclusion of 20% decreased TP by 16, 15, and 16% for day 0, 7, and 14 of storage, respectively compared to commercial diets. Phytate P, on the other hand, was reduced by 38, 37, and 47% for day 0, 7, and 14 of storage, respectively. Overall, DDGS influenced the forms of P in poultry litter with phytate P being the most impacted. The inclusion of DDGS in poultry diets seems promising as it can potentially decrease the levels of phytate P in poultry litters, which is of may be of environmental significance as phytate P can desorb inorganic P in soils.

Degree

Ph.D.

Advisors

Joern, Purdue University.

Subject Area

Aeronomy|Water Resources Management|Environmental science

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