A Soil Quality Framework for Evaluating the Impact of CRP

The book entitled “Soil and Water Quality: An Agenda for Agriculture” by the U.S. National Academy of Sciences caused people to ask whether soil quality assessments could be used to evaluate the impact of public policies such as the Conservation Reserve Program (CRP). However, differences in scale, perception of soil quality, and the inability to directly measure soil quality led to significant uncertainty among several potential users. A major challenge was determining how to evaluate and combine information from different indicators to make an overall soil quality assessment that is meaningful. Our objectives are to present a structured approach for interpreting soil quality indicator data and to introduce a conceptual framework that can be used to link the various scales of evaluation, including those needed for assessing effectiveness of public policies such as the CRP. The framework and its use are discussed and demonstrated using soil quality indicator data from published and unpublished studies. On-farm measurements suggest that biological indicators such as microbial biomass and respiration were affected most quickly and to the greatest extent when cultivated land was converted to grassland. Applying the conceptual framework to this data suggests that enrolling fragile lands into CRP had a positive soil quality effect. It also indicates that using no-till practices to return CRP land to row-crop production will preserve soil quality benefits of the CRP, but tilling to prepare a seedbed will destroy the benefits almost immediately. Research QuestionHow can indicators of soil quality be combined to assess the effectiveness of public policies such as the Conservation Reserve Program (CRP)? Literature SummaryPublication of the book entitled “Soil and Water Quality: An Agenda for Agriculture” by the U.S. National Academy of Sciences caused politicians, administrators, and scientists to ask whether soil quality assessments could be used to evaluate the effectiveness of public policies such as the CRP. Similar questions in Canada during the past decade resulted in partnerships between federal and provincial governments, universities, and the private sector focused on the development of a Soil Quality Evaluation Program. Through these efforts, soil quality has been defined simply as “the capacity (of soil) to function” and a consensus has evolved that soil quality should be viewed as a composite picture of a soil's physical, chemical, and biological properties and of the processes that interact to determine soil condition. However, soil quality per se cannot be measured directly. It must be inferred or estimated by measuring key indicators, including properties such as texture, pH, nutrient availability, electrical conductivity, salinity, porosity, aggregate stability, water retention, bulk density, organic matter, microbial biomass, respiration, or soil enzymes; through observations of soil conditions such as color, structure, topsoil and plant rooting depth, erosion, infiltration rates, earthworm populations, and invertebrate populations or communities; or by evaluating the efficiency of processes such as crop production or nutrient cycling. A major challenge that has not been adequately addressed, however, is how these various indicators will be evaluated and combined to make an overall soil quality assessment that is meaningful and useful to several different groups of people. This is important since soil quality can be assessed with varying levels of accuracy and precision at the point, plot, field, farm, watershed, or larger areas. Our objectives are to present a structured approach for using soil quality indicator data and to suggest how the approach can be used to link the various scales of evaluation, including those needed for assessing the effectiveness of public policies such as the CRP. Applied QuestionHow has the CRP changed soil quality? The CRP affected several critical indicators of soil quality including resistance to erosion. Indicators used to measure this change at the plot scale included runoff and soil loss from simulated rainfall studies. Runoff from undisturbed CRP land averaged <1%, compared with 2% for recently tilled CRP land, 42% where the tillage occurred 10 mo earlier, and 47% for long-term cropland in Mississippi. In Iowa, runoff averaged 11, 8, or 38% for undisturbed CRP land, former CRP land where no-till corn was planted, or former CRP land that was plowed and planted to soybean, respectively. Corresponding soil loss after 0.5 h of rainfall (1.9 in.) was 0, 0.05, and 1.4 tons/acre for CRP, no-till, and plowed treatments, respectively. Paired comparisons of several soil quality indicators for CRP and cropped sites in Iowa, Minnesota, North Dakota, and Washington showed that soil microbial biomass and respiration were affected more quickly and to a greater extent than organic carbon, organic N, nitrate N, ammonium N, and pH. This sensitivity resulted in more statistically significant differences when averaged across sampling sites in each of the four states, suggesting that microbial biomass measurements should be seriously considered when selecting a minimum number of measurements for soil quality evaluation. Summary and ConclusionsOur evaluation of data collected from tilled, CRP, and no-till post CRP treatments using a structured framework that can be applied to assess soil quality at various scales indicates that the CRP generally increased soil microbial biomass, organic C and N, long-term infiltration, and aggregate stability. Quantities of inorganic N and bulk densities were generally reduced. Assuming more-is-better relationships for the first four soil quality indicators and less-is-better relationships for the latter two, we conclude that enrolling fragile lands into CRP did have a positive soil quality effect. Furthermore, we also conclude that if CRP land is returned to row-crop production, no-till practices should be used to preserve the soil quality benefits that have been purchased through a 10-yr, $20 billion public investment.