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A Field Study of Soil Water Depletion Patterns in Presence of Growing Soybean Roots: III. Rooting Characteristics and Root Extraction of Soil Water¹

ABSTRACT

Rooting characteristics and water extraction patterns in the root zone of a mature soybean (Glycine max L.) crop were determined in the field. Although rooting density for liter samples of soil varied from 0.07 to 5.0 cm/cm³, nearly 60% of the root zone exhibited densities in the 1.0 to 2.0 cm/cm³ range. Densities near the plant row and at depths of 20 to 45 cm in the interrow space tended to be well above average. Elsewhere densities varied in a random manner. Total root length per plant was about 680 m. Mean root diameters ranged from 120 to 500 µm with the 150 to 300 µm range being the most common. Maximum fresh weight concentration was in the 30- to 40-cm zone. The dry root weight monotonically decreased with depth. The ratio of fresh to dry weight increased with depth.

Rates of water extraction by the roots in the various 10-cm intervals were computed over an irrigation-drying cycle, 81 to 91 days after planting. In the 10- to 30- and 60- to 70-cm layers the extraction rate decreased with time. In the 30- to 60-cm layer an initial decrease was followed by a substantial increase later in the cycle. Total extraction between 40 and 240 hours after irrigation decreased with depth—1.32 cm in the 10- to 20-cm layers and 0.25 cm in the 60- to 70-cm layers. Extraction rate for the 10- to 70-cm soil during the 47- to 56- and 81- to 91-days growth periods showed an initially rapid decrease following irrigation but reached constant levels of 0.48 and 0.39 cm/day, respectively.

In the light of Philip's (1957) extraction model, our rooting characteristics and water uptake measurements suggest that soil-water pressure between roots (_B) and on root surfaces (_R), effective conductivity of the soil (), sink strength (r_z), and rooting density (Lv) are related by

Estimates of
were negligibly small suggesting that rhizosphere resistance was not a significant factor in our experiments.

Patterns of water movement in the two-dimensional root zone space show considerable deviation of flow direction from the vertical. These deviations suggest that with row crops more sophisticated analyses and more detailed soil and root measurements will be needed for the meaningful interpretation of field data.

¹ Contribution from the Minnesota Agricultural Experiment Station, Univ. of Minn., Scientific Journal Series Paper 8662, and the North Central Region, ARS, USDA, St. Paul, 55108. This investigation was supported by funds provided in part by the US Dept. of Interior, Office of Water Resour. Res. under act of 1964, P.L. 88-369.

² Research Assistant, Professor of Soil Science, and Soil Scientist (ARS, USDA), Univ. of Minn., St. Paul. The senior author is now in the Dept. of Soil Sci., Univ. of the West Indies, St. Augustine, Trinidad. Present address of the third author is Div. of Soils, CSIRO, Glen Osmond, South Australia 5064.

Received for publication April 11, 1974. Accepted for publication January 8, 1975.