Compost and Soil Quality: Residual Impacts of Five Years of Application.

Understanding the soil of organic farming systems is crucial to the benefits and existence of its ongoing success.  In general, soil organic farming has been related to “soil health sustainability” or in other words, related to the increase Soil Quality.  Soil quality (SQ) is the capacity of the soil to function. SQ can be evaluated through indicators of soil physical and chemical properties, as well as interactions between soil components. 

The International Federation of Agriculture Movements states that bio-organic agricultural systems rely on ecological processes, biodiversity and cycles adapted to local conditions, rather than the use of inputs with adverse effects.  These systems combine tradition, innovation and science to benefit the shared environment and promote fair relationships and a good quality of life for all involved.  The objective of this study was to measure the effect of prior compost application on soil quality.  The compost application study started in 2000 with the establishment 40 plots, each 1.4 x 2.4 m. The plots were limed individually as needed. Five compost fertility levels with four replications were randomly assigned and applied in the spring; the compost rates were 0, 5, 10, 20 and 40 tons.  In 2010, an intensive soil sampling was performed. Soil physical properties (bulk density (Uhland sampler), dry and wet aggregate stability, Geometric mean diameter (GMD), Mean Weight Diameter (MWD)), soil porosity) and Mehlich III plant available nutrients (organic matter, plant available P, K, Ca, Mg) were measured in the lab as SQ indicators.  Results show significant changes in bulk density due to the rate of prior compost application. Prior higher compost application rates resulted in lower bulk densities; the average decrease was from 1.39 g/cm³ to 1.27 g/cm³.  Increasing amounts of organic matter from 20.9 g/kg to 31.3 g/kg and phosphorus content from 10 ppm to 85 ppm were related to increasing compost rates. Soil phosphorus content rapidly increased after application of 10 tons of compost, and its residuality is shown in the data collected after 5 years of the last compost application.  The variability in soil quality fertility indicators, plant available P, K, Ca, Mg, Zn, B, Cu, total N, soil organic matter (OM) and pH was explained by prior compost application rate (p < 0.0001, R²  ranging from 0.48 – 0.92) as well as GMD (p < .0260, R² = 0.26) and MWD (p < 0.0121, R²= .30) . Simple linear correlation (Pearson correlation coefficient) found that based on compost application level, bulk density is negatively and statistically correlated with levels of P, K, Ca, Mg, Zn, B, total Nitrogen, organic matter, and pH (p < 0.0001 to 0.0102) and positively correlated with MWD, GMD and wet logGSD.  The results show significant differences in soil physical and chemical parameters due to residual fertility level consequence of prior compost rate applications. This suggest that even after five years from the latest compost application, soil quality was preserved showing elevated the nutrient levels and beneficial physical soil properties within the soil.