Correlation of a Soil Health Assessment Tool and Phosphorus Loss with Surface Runoff in Agricultural Fields
The procedures and analyses for assessing the degree to which a soil is healthy continues to be investigated, and there is no widespread agreement among scientists about the value or use of some measurements. Scientists, nutrient management planners, and farmers are wondering how soil measurements used for soil health assessment or the soil heath ratings relate to crop productivity and water quality, mainly concerning nitrogen and phosphorus loss to water resources. The most widely used set of measurements in production agriculture in Iowa and the Midwest to evaluate soil health is the Soil Health Tool, commonly known as the Haney soil health test. However, no research has correlated the P component of the Soil Health Tool or the overall soil health rating with P loss from fields for a wide range of soil-test P levels and management practices.
The objectives of this study are to identify how the overall soil heath ratings provided by the Soil Health Tool relate to dissolved P loss with surface runoff for several soils, management practices, and soil P levels; and to assess how results of the P components of the Soil Health Tool correlate to dissolved and total P loss with runoff compared with routine soil-test P methods.
These objectives will be accomplished by analyzing soil samples with the Soil Health Tool and the use of surface runoff P data (dissolved and total P) from completed or ongoing experiments funded by other sources. The soil and runoff samples will represent field experiments managed with natural or simulated rainfall that included different soil-test P levels and management practices for corn-soybean rotations. The management practices include fertilizer and manure P sources, no-till and chiselplow/disk tillage, amendments for soil or manure (alum and gypsum), corn harvesting systems (grain and grain plus stover), and conservation practices involving use of a cover crop and buffer grass strips. About 450 soil samples will be analyzed.
During the January-June period all data collections and soil analyses were completed, and work began to manage data and summarize results. The management practices for these experiments with corn-soybean rotations were two P fertilizer placement methods (broadcast and planter-banded) and two tillage systems (no-till and chisel-plow/disk). Surface runoff samples associated with this set of management practices were analyzed for total solids, dissolved-reactive P (DRP) and total P. The need for a better understanding of how soil-test P relates to differences in DRP and TP led to the correlation analyses with select soil properties. The Mehlich-3 soil test showed the best relationship with the difference between DRP and TP, whereas the other routine soil tests Bray-1 P and or Olsen showed slightly poorer correlations. This partial data set showed no clear relationships between the soil health rating resulting from using the Soil Health Tool and runoff DRP, total P or soil loss. Organic acid-extracted P (the H3A test used in the Soil Health Tool) correlated more poorly than the Bray-1 P, Mehlich-3 P or Olsen P tests commonly used for crop production and the Iowa P Index. Accounting for P mineralization, as estimated by the Soil Health Tool, did not improve the correlation with runoff P loss. Interestingly, however, iron (Fe) extracted by weak organic acids, showed the best relationship with DRP in runoff than any other component of the soil health test. Oxidation-reductions that control iron speciation in surface soils may explain the results. Data management and summarization of results from the study will be completed by the end of the year.
During the second quarter, site and sample selection was finalized and analyses initiated. Soil samples representing a subset of the spectrum that will be used for the study have been analyzed for the Soil Health Tool and all tests included within the scope of the tool. The most relevant soil measurements (range in parenthesis) include pH (5.0-7.2), organic matter (3.6-5.85 %), Bray-1 P (9-62 ppm), Mehlich-3 P (10-66 ppm), Olsen P (4-31 ppm), Solvita CO2 24-hr burst (65-195 ppm C), water-extractable C:N ratio (7.3-15.7), H3A P colorimetric (6-35 ppm), and H3A P inductively coupled plasma (20-55 ppm).
The management practices included in the field experiment with corn-soybean rotations from where this subset of soil samples (54 samples taken in the fall of each year from 0-2- and 2-6-inch depths) were collected included two P fertilizer placement methods (broadcast and planter-banded) and two tillage systems (no-till and chisel-plow/disk). Surface runoff samples (191 samples from 26 runoff events from 2013 to September 2018) associated with this set of management practices were analyzed for total solids, dissolved-reactive P, bioavailable P, and total P.
Preliminary findings for this experiment identify no clear relationships between the soil health rating resulting from using the Soil Health Tool and runoff dissolved-reactive P (DRP), bioavailable P (BAP), total P loss (TP), or total-solids loss (TS). Organic acid-extracted P (the test used in the Soil Health Tool) did not correlate better than the commonly used Bray-1 P, Mehlich-3 P, or Olsen P tests for either sample depth. The application of a P mineralization factor estimated by the Soil Health Tool did not improve the correlation of the H3A P extractant with any runoff P loss measurements. No firm conclusions can be made at this time about the general relationship of the Soil Health Tool metrics with runoff P measurements. The study still needs to be completed for many other soil and management practices across multiple sites.
The project complements recent and ongoing research studying phosphorus (P) loss with surface runoff in Iowa. The July – September quarter was the first for this new project. During this period, initial work consisted on beginning to identify and organize already collected soil and runoff samples that had been stored to be used in the project. The work also included studying field information and available data from several recent or ongoing field surface runoff projects from which soil and surface runoff samples will be selected. These studies use a methodology under natural rainfall with small watersheds or large plots and some use a field rainfall simulation technique. The studies include different tillage management, conservation buffer strips, soil amendments such as gypsum, and P application using inorganic fertilizer or animal manures. A careful selection of the most relevant samples encompassing a wide range of soils and management practices is essential to achieve the project objectives with the planned budget. The objectives of the project are to:
- Identify how the overall soil heath ratings provided by the Soil Health Tool relate to dissolved P loss with surface runoff for several soils, management practices and soil P levels.
- Assess how results of the P components of the Soil Health Tool correlate to dissolved and total P loss with runoff compared with routine soil-test P methods.