Soybean Growth and Seed Yield Response to Tillage and Compost

Plant response to organic amendment often varies and may interact with tillage. The objectives of this research were to determine if soybean [Glycine max (L.) Merr.] growth, nutrient uptake, and soil water use could explain whole-plant and seed yield responses to tillage and compost amendment. A corn (Zea mays L.)-soybean-wheat (Triticum aestivum L.)/clover (Trifolium spp.) rotation, with or without fall compost application was initiated in 1998 in plots that had been managed with moldboard plow (MP), chisel plow (CT), or no-tillage (NT) since 1988. No tillage by amendment interaction for yield was detected. Yield was similar in MP and CT. In 2003, yield was similar among tillage systems, but NT yielded 15% greater than MP and CT in 2004 and 8% lower in 2005. Mean yield across the 3 yr was similar. Compost amendment increased yield (9%) compared with nonamended soil only in 2004. Mean yield was 2% higher with compost than without. Water depth from the 0 to 90 cm soil depth was not affected by tillage or amendment. Seed P and K uptake were 18 and 16% greater in NT compared with MP and CT and 16 and 13% greater with compost than without in 2004. Soybean growing on compost-amended soil had less seed protein compared with nonamended soil. Alternatively, seed oil content was 2 and 1% lower in nonamended than amended soil in 2003 and 2004. The mechanisms responsible for compost-enhanced plant growth are difficult to quantify in the field under natural conditions because effects are short-lived or too extreme.