Compost Carryover As Measured by Yield, Mineralizable Nitrogen, and Fourier Transform Infrared Spectroscopy in Response to One-Time Applications in An Organic Field Trial.

Calculating appropriate application rates of compost for organic farming is challenging. It is widely assumed that the benefits of compost last many years; however, compost carryover is rarely taken into account when determining application rates. The purpose of this study was to investigate compost carryover, as measured against new applications of the amendment, in an irrigated organic corn-squash rotation on a silt-loam soil. Treatments consisted of five rates of a commercial steer manure compost (0, 10, 20, 30, and 40 Mg ha^-1) applied only once to any given plot during the trial period. Crop yield response and soil mineralizable N was used to determine compost carryover from year to year. From the yield data in year two, we estimated the carryover effect of 1 unit of compost applied in year one was equivalent to 0.28 units of compost applied in year two. Plots showed no response to treatment in year three, possibly due to lack of N mineralization or N loss in a very wet and cold growing season.  Compost carryover was further investigated using FT-IR Spectroscopy. Absorbance spectra were analyzed in soil from 0-10cm, and evidence of compost carryover was determined by identifying changes in spectra relative to the control.   Compost absorbance spectra were unique and could be identified in soil three years after an initial one-time compost application. Improved understanding of long-term contributions of compost to soil fertility and soil organic matter will help growers to better plan compost application rates.