Amounts and Forms of Dissolved Phosphorus Lost with Surface Runoff as Affected by Phosphorus Management and Soil Conservation Practices

Sep 2017


Surface runoff accounts for the majority of phosphorus (P) from Iowa fields delivered to streams. Important fractions of runoff P are dissolved reactive P (orthophosphate P) and particulate (sediment-bound) P. Dissolved P is readily available to aquatic organisms and a large proportion of particulate P be comes available over a period of time depending on properties of the receiving water body. Recent surveys of Iowa streams and in the Lake Erie watershed suggest the amount of dissolved P loss from fields and its impact on water quality is greater than often assumed. Iowa research has shown higher orthophosphate P loss with fertilizer than with manure, and some conservation practices that reduce erosion and particulate P loss may not affect or may even increase dissolved P loss.


The goal of this project is to study dissolved P in runoff for a wide range of soil P levels, fertilizer and manure P management practices, and soil conservation practices. Specific objectives are to determine the amount of runoff dissolved P not measured by the commonly used dissolved reactive P method, and study how amounts of different dissolved P forms in runoff can be estimated by soil-test P methods recommended for crops, water-extractable soil P, and an index of soil P saturation.


About 900 soil and runoff samples from other ongoing and recently completed experiments will be analyzed. These samples represent field experiments managed with natural or simulated rainfall that included different soil-test P levels, crops, fertilizer and manure P placement methods and times of application, tillage systems, soil or manure amendments, corn harvesting systems, and the conservation practices cover crops and buffer grass strips.

Project Updates

June 2019

During the January-June semester all the chemical analyses were finished for a total of about 200 soil samples and 800 runoff samples. Across all soil samples, Bray-1 P was 2 to 222 mg/kg; water-extractable P was 2 to 61 mg/kg; organic matter was 22 to 60 g/kg; pH 4.6 to 7.5; and texture was sandy loam to clay loam and silty clay loam. The TDP runoff concentrations were greater than DRP concentrations, and as expected, the total P losses were much greater. Across all runoff samples, DRP and TDP concentrations were 0 to 69 and 0.05 to 96 mg P/L, respectively. The loss of DRP and TDP were 0 to 18 and 0 to 22 kg P/ha, respectively. Early results had indicated that TDP was about 20% higher than DRP with tillage but 13% with no-till. However, use of different P sources and application rates greatly affected the tillage system effects and both the total and relative amounts of DRP and TDP.

At this time, data management and both correlation and regression analyses ,including the recently analyzed soil and runoff samples, are being completed. More specific results cannot be provided at this time. Data management and a results summary will be completed by the end of the year.

December 2018

Advancements this quarter included completion of all soil and surface runoff analyses for the sites used for this project, with the exception of field-scale trials conducted in 2018 to evaluate the impact of prairie strips on total surface runoff P and dissolved P fractions. The soil analyses included soil P (water-extractable, Bray-1, and Mehlich-3), pH, extractable cations calcium and iron (to estimate soil P saturation) and several routine tests. The runoff measurements included runoff flow, totals solids, dissolved reactive P (DRP), total dissolved P (TDP) and total P. The different sites and management practices resulted in wide ranges on the runoff dissolved P forms for the available data set.

Currently, there are 340 soil and 689 runoff samples analyzed for all relevant metrics. This data set represents a wide range of management practices that captures those widely used in Iowa and the Midwest. A clear result of the preliminary correlations among the P soil tests is that water-extractable P correlates with three agronomic tests (Bray-1, Mehlich-3, and Olsen) more strongly for samples collected from the top 2 inches of soil compared with samples taken from the 2 to 6-inch depth. Another important result is that, as expected, TDP often is greater than DRP but the difference becomes larger as the soil-test P concentration increase (for both soil sample depths). Also, of the P-sources used (triple super phosphate, swine manure, poultry manure and alum-treated poultry manure) the TDP/DRP ratio is the greatest for the fertilizer and swine manure sources and the lowest for the alum-treated poultry manure. However, management practices such as the tillage system and the P placement methods (regardless of P-source) did not show a consistent influence on the TDP/DRP ratio across different sites, soils and timing of runoff events.

Next steps will be to complete all chemical analyses and begin detailed analysis of the impacts of management practices on the runoff P fractions and the relationships between the soil P and the runoff P measurements.

September 2018

Significant advancements were made in soil and surface runoff sample analyses during this quarter. Higher than normal rainfall since June resulted in large amounts of runoff at two ongoing experiments managed under natural rainfall located in northwest and central Iowa. Therefore, samples from this year were selected to be included in the project to complement well data from drier past years. Those samples have since been measured for dissolved-reactive phosphorus (DRP), total solids (TS) and total runoff P (TP). Water-extractable soil P (WEP) was measured for 191 soil samples that correspond with the previously reported runoff samples analyzed for DRP and total dissolved P. 

Water-extractable P values ranged from 1 to 60 ppm P. These values compare (for example) to a range of 2 to 278 ppm by the Bray-1 test. For the set of samples analyzed, there was a strong positive correlation between WEP and P measured previously with the three agronomic soil P tests (Bray-1, Mehlich-3, and Olsen), although the ratio between soil P measured by WEP and routine tests seems to change with some management practices and the general soil P level. These values will be used with other previously measured soil properties to relate differences in soil metrics with DRP and total dissolved P loss with surface runoff. The identification of specific relationships between different soil P tests and runoff P concentrations or losses (total, dissolved and particulate) will be possible only when all samples for the project have been collected and analyzed.

March 2018

There was snowmelt runoff during late February and March in two of the four field studies, and selected soil and runoff samples will be used for this study. Soil samples collected last fall are being analyzed for P by several methods. The snowmelt runoff samples will be analyzed for dissolved-reactive P (DRP), total dissolved P and total P. The team completed the identification of stored soil and filtered surface runoff samples, so these can be analyzed for needed properties that were not analyzed before. These analyses include water-extractable soil P and runoff total dissolved P. The samples from previous studies represent 45 site-years for surface runoff work with natural or simulated rainfall. Relevant management practices represented by this sample population include no-tillage and chisel-plow/disk tillage, runoff collection during or following the growth of corn or soybean with or without a cereal rye cover crop or grass buffer strips. The P application sources were inorganic fertilizer or animal manures (liquid swine, chicken-layer, chicken-broiler or turkey). Multiple soil and soil parent materials of Iowa are represented by these samples. The diversity of soil properties, management practices and runoff P loss will be augmented by samples collected from 2018 trials.

December 2017

There was no surface runoff this quarter in any of the field studies where new runoff samples could be collected. Researchers continued identifying and organizing soil and runoff samples collected earlier this year and last year. This included summarizing field and precipitation information available from several recent or ongoing field surface runoff projects. Most soil samples had already been analyzed for P. The team began analyzing the soil samples for water-extractable P and extractable Ca, Al and Fe cations concentrations needed to estimate soil P saturation.

September 2017

The goal of this project is to study dissolved P in runoff for a wide range of soil P levels, fertilizer and manure P management practices, and soil conservation practices. Work began by identifying and organizing already collected soil and runoff samples that had been stored. This also included studying field information and available data from several recent or ongoing field surface runoff projects from where the needed samples will be selected. These studies use a methodology under natural rainfall with small watersheds or large plots or field rainfall simulations. The studies include different tillage management, conservation buffer strips, soil amendments such as gypsum, and P application using inorganic fertilizer or animal manures.