Submitted to: Soil and Tillage Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/26/2010
Publication Date: 1/1/2011
Citation: Nichols, K.A., Toro, M. 2011. A whole soil stability index (WSSI) for evaluating soil aggregation. Soil and Tillage Research. 111:99-104. doi:10.1016/j.still.2010.08.014 Interpretive Summary: Soil organic matter levels, biological activity, and soil functions such as water infiltration, water holding-capacity, nutrient availability, and soil structural stability are related to soil aggregation. However, a standardized methodology for measuring whole soil aggregate stability does not exist, and the term soil aggregation may refer to either dry soil aggregation or water-stable aggregation and may be an examination of only particular aggregate size classes. Aggregate size distribution – the amounts of large, medium, and small macroaggregates (>250 um) and microaggregates (<250 um) – influence pore size and continuity, soil carbon values, and soil fertility with the medium aggregate sizes being ideal. Therefore, a soil with more stable macroaggregates will be of a higher quality than a soil with more stable microaggregate with the following caveats: 1. if a soil consists solely of large macroaggregates, the pore size between these aggregates will be large with little water and nutrient retention resulting in a low quality microbial habitat, and 2.if all the aggregates in the soil are 100% stable, the soil surface will be sealed reducing water infiltration and soil function. Therefore, a combination of aggregate sizes and stabilities are needed for ‘ideal’ soil function and quality. The equations used to calculate the whole soil stability index (WSSI) take into account aggregate size, the proportion of aggregates in each size class, and water-stable aggregation percentages as well as a weighted factor for quality. The weighted factor was based upon soil function as well as management impacts on soil aggregation. These equations were tested on a number of benchmark soils and compared to other aggregation measurements, such as mean weight diameter (MWD), geometric mean diameter (GMD), and the normalized stability index. The results showed some uniformity among aggregation measurements, but the WSSI had the strongest relationship with expected results based on management.
Technical Abstract: Soil aggregate stability is an indicator of soil quality. However, there is no standard methodology for measuring soil aggregation or aggregate stability, particularly for determining a whole soil stability index. A whole soil stability index (WSSI) was developed here which combined data from dry aggregate size distribution and water-stable aggregation along with a ‘quality’ constant for each aggregate size class. The quality constant is based on the impact of aggregate size on soil quality indicators, such as porosity, water infiltration, water retention, and nutrient cycling. The WSSI was measured on soil collected from sites established on the same or similar soil types at the Northern Great Plains Research Laboratory in Mandan, ND. The WSSI was calculated for each sample and compared to the Mean Weight Diameter (MWD), Geometric Mean Diameter (GMD), and the Normalized Stability Index (NSI) for select samples from the Soil Quality Management (SQM) site. By utilizing dry aggregate size distribution, water-stable aggregation, and the quality constant, the WSSI was hypothesized to statistically demonstrate changes in soil quality with management. Improvements in soil quality due to management differences were identified with native plots having a higher WSSI than highly disturbed plots. The WSSI was strongly related to the MWD and GMD and loosely related to the NSI.