Soil Carbon and Physiological Responses of Corn and Soybean to Organic Amendments

Field research was conducted to assess the effects of organic and inorganic soil amendments on soil humified carbon content and plant physiological properties. Long term applications of various types and rates of compost, poultry litter and inorganic fertilizer were assessed for their effects on corn [Zea mays (L.) Pioneer 31G20] and soybean [Glycine max (l.) Delta Pine 4933RR] grown in a Virginia (United States) Piedmont soil in 2004 and 2005. Treatment differences in leaf antioxidant activity were only observed in the corn plots. Corn fertilized with amendments supplying the crop's nitrogen needs, regardless of the source, had greater leaf nitrogen (+29%), chlorophyll (+33%), and protein contents (+37%), lower superoxide dismutase (-29%) and ascorbate peroxidase (-17%) activities, and lower malondialdehyde (-33%) contents than the control and low nitrogen treatments. Yield was strongly related to midseason leaf nitrogen contents (R-2=0.87, p<0.0001) and not with soil humified carbon (R-2=0.02, p=0.0543). Soybean grown in the organically amended soil had higher yields (9-21% increase), protein contents (4-9% increase) and seed weights (5-14% increase) than the inorganically fertilized and unfertilized treatments. Improvements in soybean yield and seed quality were due to organic amendment benefits other than plant available water or nutrient supply.