Evaluation of H3A for Determination of Plant Available P vs. FeAlO Strips

Phosphorus is an essential nutrient for plant growth but in excess is a source of environmental pollution. Fertilizer additions of P are recommended based on soil tests;however, the commonly applied P extractants are often applied outside of their design criteria (specifically soil pH). As a result, soil tests can produce inaccurate estimates of plant available P in the soil, which either increases P loss in runoff, contributing to eutrophication, or decreases crop production contributing to economic loss. In this study, 200 diverse soils from across the US were extracted with Mehlich 3, water, H3A-3, and FeAlO strips. Comparison with FeAlO was critical, as this method is accepted as the “gold standard” for plant-available P, but it is rarely used in commercial labs because of time and financial constraints. H3A-3 produced mean, median, standard deviations that are very similar to FeAlO strip results and low relative errors (<10%), as well as highly correlated regression relationships (r2 > 0.96 with slopes 0.95 - 0.98). Although Mehlich 3 and water were correlated with FeAlO, Mehlich 3 (strongly acidic) extracted much more P than FeAlO, and water (low buffering capacity) extracted much less P across the range of soil pH values. Thus, H3A-3 provides an improved methodology to accurately determine plant-available P by mimicking root exudate action in the soil, while avoiding the time-consuming and costly FeAlO procedure. In the face of high-profile water quality impairments with enormous economic costs, such advancements are critical to balance agronomic production with environmental concerns.

Impact of Management Practices on Water Extractable Organic Carbon and Nitrogen from 12-Year Poultry Litter Amended Soils

Water extractable organic carbon (WEOC) and nitrogen (WEON) are two key parameters of soil water extractable organic matter (WEOM). Proper management of manure application rate in combination with tillage and cropping management could maintain appropriate WEOC and WEON concentrations in soils while decreasing the risk of their runoff from cropland and pastures. The objective of this research was to determine the effect of poultry litter (PL) application on WEOC and WEON in soils under different crops, tillage regimes, and grazing strategies. From 2001 to 2012, PL was applied at multiple rates to cultivated fields in a corn-oat/wheat-hay rotation or to pastures grazed by cattle or ungrazed. Soil samples (0 - 15 cm) were analyzed for KCl-extractable mineral N, and WEOC, and WEON contents. In addition, Ultraviolet-visible (UV-vis) and fluorescence spectroscopies were used to characterize WEOC stability. Organic N levels were higher at the high PL application rates. The soil C:N ratio narrowed as the PL application rate increased. However, the soil from pastures which received PL tended to have a wider range of C:N ratios than soil from the cultivated fields, despite identical PL application rates. The spectral analyses indicated that WEOC properties were responsive to management and PL application rate;therefore, this parameter may be used as a guide to provide best management strategy for manure application.

Initial Field Evaluation of the Agro-Economic Effects of Determining Nitrogen Fertilizer Rates with a Recently-Developed Soil Test Methodology

Although agriculture is not the only contributor of excess nutrients to US waters, agriculture is an important contributor and should do its part to reduce nutrient loading. One important step in reducing agricultural contribution is to accurately account for all sources of plant available nutrients so that only needed nutrients are applied. In this study, three fertilizer rate treatments were evaluated: no fertilizer (control), traditional rate, and reduced rate based on a recently-developed enhanced soil test methodology. For each of nine sites in Texas, fertilizer data (formulation, rate, cost, and application date) and crop data (yield, price, and harvest date) were recorded, and economic throughput (profit) was determined. In this four year study, fertilizer rates were reduced 30%-50% (and fertilizer costs reduced 23%-39%) based on enhanced soil test methodology recommendations for wheat, corn, oats, and grain sorghum, but yields were not significantly reduced (0%-6%) and oat yields actually increased 5%. Profit decreased -18% for wheat, oats, and grain sorghum with reduced fertilizer rates. Although these changes were not statistically significant, they do represent benefit through increased profit potential and decreased input cost and production risk. In only 6% of the time was the traditional fertilizer rate the most profitable, compared to 51% for the unfertilized treatment and 43% for the enhanced soil test treatment. These results do not indicate that fertilizer application should be avoided but that fertilizer rates should be carefully chosen considering all sources of plant available nutrients (e.g., mineralization, irrigation water, nutrients deeper in the soil profile) to ensure that fertilizer is applied at the optimal rate.

Analysis Methods for the Determination of Anthropogenic Additions of P to Agricultural Soils

Phosphorus loading and measurement is of concern on lands where biosolids have been applied. Traditional soil testing for plant-available P may be inadequate for the accurate assessment of P loadings in a regulatory environment as the reported levels may not correlate well with environmental risk. In order to accurately assess potential P runoff and leaching, as well as plant uptake, we must be able to measure organic P mineralized by the biotic community in the soil. Soils with varying rates of biosolid application were evaluated for mineralized organic P during a 112-day incubation using the difference between P measured using a rapid-flow analyzer (RFA) and an axial flow Varian ICP-OES. An increase in the P mineralized from the treated soils was observed from analysis with the Varian ICP-OES, but not with the RFA. These results confirm that even though organic P concentrations have increased due to increasing biosolid application, traditional soil testing using an RFA for detection, would not accurately portray P concentration and potential P loading from treated soils.