Growth and Nutrient Uptake of Soybean Roots Under Field Conditions1

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Abstract
Although soybean (Glycine max L. Merr.) root growth in soil has been measured, the nutrient flux characteristics of the roots during the growth of the plant in the field have not. Soybean root amount and distribution were measured using approximately biweekly samplings during the growth of ‘Williams’ soybeans on Chalmers silty clay loam soil (Typic Argiaquoll) near Lafayette, Ind., in 1973, 1974, and 1975. Root length and fresh weight were measured with depth and distance from the row. Shoot samples taken at each sampling were analyzed for P and K. Shoot growth rate averaged 135 kg ha−1 day−1 between plant ages of 50 and 100 days. Shoot concentration of P decreased until 60 days then increased as beans began to form. Shoot concentration of K decreased during the sampling period. Root length increased until 70 to 80 days, remained constant to 100 days, and then decreased. Maximum root length averaged 24 cm per cm2 of soil surface. Average proportion of the roots in the 0 to 15 cm layer increased with plant age after 36 to 49 days and reached a maximum of 59% at 113 to 120 days. Soybean root density was approximately one‐fifth that observed at the same location for corn (Zea mays L.). Influx of P and K into soybean roots was less than with corn during early growth but more than corn later in the season. This result indicates that soybeans may not respond to fertilizer banded near the seed at planting but would require higher P and K fertility in the whole rooting volume of soil than corn after 50 days in order to sustain the required nutrient influx into the roots.