Improved Soil Physical Properties with Long-Term Application of Humic Product in Corn-Soybean Rotations.

Application of commercial humic products can increase the biomass of crop roots. Long-term application could therefore lead to improved soil physical properties through increased input of root C and possibly root exudate C. Anecdotal reports exist of improved soil tilth with humic product use, although no field evidence exists in the literature. We measured soil dry aggregate stability and bulk density (0-15 cm depth) and penetration resistance (0-30 cm depth) in two Iowa corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] fields having had replicated applications of a humic product for four or five previous years. In initial results, after five years product application at 18.7 L ha^-1 (2 gallon acre^-1), the largest aggregate class (>4 mm) constituted 50% of the soil mass, compared to 39% for the unamended control (P=0.075), while the proportions of all smaller size classes (2-4 mm, 1-2 mm, 0.25-1.0 mm, and <0.25 mm) decreased marginally with product application by 1-3% of soil mass. Application of 9.4 L ha^-1 (1 gallon acre^-1) did not affect aggregate stability. Soil bulk density decreased from 1.46 g cm^-3 for the control to 1.40 g cm^-3 with application of either 9.4 or 18.7 L ha^-1 (P=0.17 and 0.19, respectively). After four years of product application to a second field, soil penetration resistance decreased by 6-37% at 2.5-cm intervals (mean 17%) within the 8-23 cm depth with a product rate of 2.3 L ha^-1 (32 oz acre^-1) applied at the fourth vegetative leaf. No consistent benefit was observed at 0-8 cm or 23-30 cm, nor for most depths at rates of 2.3 L ha^-1 or 4.7 L ha^-1 both applied at pre-emergence. Results for other soil physical properties will be presented. These trends are encouraging, given the limited history of product application in the two fields. We will continue to monitor both fields for crop and soil responses to the humic product.