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Top and Root Growth of Irrigated and Nonirrigated Soybeans¹

Root depth and distribution in the field were correlated with easily measurable above-ground morphological paranfeters of soybeans (Glycine max L. Merr. ‘Williams’) at specific physiological stages. Previous research on soybean tooting depth and distribution was conducted with pots or field soil containing a barrier limiting full expression of root development. Other studies sampled a limited numberof times in a barrier free soil.

We investigated soybean root depth and distribution under irrigated and nonirrigated conditions in a deep barrier-free Muir silt loam soll profile. Irrigated plots received a total of 28 cm of water in three furrow applications. We measured plant height and dry weight, root depth, root distribution, and leaf area index each time roots were sampled. Each soil increment sampled (75 cm wide, 7.5 on thick, and in 15 cm increments to the 180 cm depth) was soaked and roots washed free of soil using a 35-mesh screen. We used neutron moderation and gravimetric sampling to obtain soil water measurements.

Soybean roots reached the 160 cm depth in both irrigated and nonirrigated plots. Root depth increased faster than plant height. At physiological maturity, 67% of the soybeans' root dry matter was in the 0–15 cm layer and 89% in the 0-90 cm layer of the irrigated soil compared with 51% in the 0-15 cm layer and 83% in the 090 can layer of nonirrigated soil.

A quick estimate of rooting depth after six node stage (V3) in nonirrigated soybeans, based on our work, would be twice the top height. With irrigated soybeans, root depth would be twice top height from V3 until pod development began (R5.5) and 1.4 x top height thereafter. The coefficient of determination (R²) for root depth vs. plant height was 0.994 and 0.987 for irrigated and nonirrigated soybeans, respectively.

Key Words: Available soil water • Leaf area index • Soil water potential

² Graduate student, associate professor, and assistant professor, Dep. of Agronomy, Evapotranspiration Laboratory, Kansas State Univ., Manhattan, KS 66506.

Received for publication April 4, 1975.

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