SOIL STRENGTH AND WATER-CONTENT INFLUENCES ON CORN ROOT DISTRIBUTION IN A SANDY SOIL

Initial field observations revealed a shallow corn (Zea mays L.) root system on a Zimmerman fine sand in a corn/soybean (Glycine max L.) rot ation. Since toot distribution influences crop water and nutrient abso rption, it is essential to identify factors limiting root growth. The objective of this study was to determine the factor(s) limiting corn r ooting depth on an irrigated fine sand soil. Bulk density, saturated h ydraulic conductivity, and soil water retention were measured on undis turbed soil cores. Corn root distribution assessed at tasseling over a 3-yr period showed an average of 94% of total. root length within the upper 0.60 m of soil with 85% in the upper 0.30 m of soil. Mechanical impedance was estimated with a cone penetrometer on two dates with di ffering water contents. Cone penetrometer measurements greater than 3 MPa indicated mechanical impedance in soil layers extending from 0.15 to 0.35 m deep. Penetration resistance decreased as soil water content increased. However, soil water contents greater than field capacity w ere required to decrease penetration resistance below the 3 MPa thresh old. Such water saturated conditions only occurred for short periods i mmediately after precipitation or irrigation events, thus roots usuall y encountered restrictive soil strengths. The soil layer from 0.15 to 0.60 m had high bulk density, 1.57 Mg m(-3). This compacted soil layer , with slower saturated hydraulic conductivities (121 to 138 mm hr(-1) ), held more water than the soil above or below it and reduced water m ovement through the soil profile. Crop water use occurred to a depth o f approximately 0.75 m. In conclusion, a compacted soil layer confined roots almost entirely to the top 0.60 m of soil because it had high s oil strength and bulk density. The compacted layer, in turn, retained more water for crop use.