Soil Physical Properties as Affected by Soybean and Other Cropping Sequences1

Infiltration, water stability of soil aggregates, and water retention characteristics were measured on a Sharpsburg silty clay loam (Typic Argiudolls) in a 4-year cropping sequence. The cropping systems include soybean (Glycine max L. Merr.), sorghum (Sorghum bicolor L. Moench), corn (Zea mays L.), and fallow, in various sequential cropping combinations. The influence of cropping systems on size distribution of waterstable aggregates is indicated by the values of geometric mean diameter (GMD). The rank order of GMD for the soybean sequences was: soybean after fallow > soybean after sorghum > soybean after corn > continuous soybean. The low GMD of continuous soybean reflected the negative effect of soybean roots in building a stable soil structure. Cumulative infiltration after 4 hours of water application was 6, 13, 29, and 41 cm for continuous soybean, sorghum after soybean, fallow after soybean, and corn after sorghum, respectively. The low infiltration was associated with low macroporosity and decreased aggregate stability. Both Kostiakov's and Philip's equations fitted the infiltration data reasonably well statistically, but Kostiakov's equation was a better fit for the early and late stages of infiltration. Additional Index Words: Glycine max L., infiltration, aggregate stability, water content. Fahad, A. A., L. N. Mielke, A. D. Flowerday, and D. Swartzendruber. 1982. Soil physical properties as affected by soybean and other cropping sequences. Soil Sci. Soc. Am. J. 46:377-381. T increasing importance of soil and water management on plant growth has demonstrated the need for integrated investigations of the plant's role in altering soil properties. Physical properties for a particular soil condition do not remain constant during the time a plant develops (Trouse, 1971). Harris et al. (1966) pointed out that the pressure exerted by actively growing plant roots has been associated with aggregate breakdown and formation. They indicated that a stable soil structure and active humus result primarily from the activity of rhizosphere bacteria in utilizing root excretion and dying root hairs as an energy source. Johnston et al. (1942) reported that the size distribution of soil aggregates has been influenced materially by the cropping system. Larger aggregates were associated with a carbon/nitrogen (C/N) ratio of 13 and the smaller aggregates with a C/N ratio of 11 (Saloman, 1962). The cropping system can alter the soil porosity, which affects the water content and water transmission properties of soil (Skidmore et al., 1975). Page and Willard (1946) found that cultivation ultimately decreased pore space and correspondingly increased soil bulk density. 1 Contribution from ARS-USDA, in cooperation with the Nebraska Agric. Exp. Stn. Published as Paper no. 5976, Journal Series, Nebraska Agric. Exp. Stn. Received 5 Jan. 1981. Approved 29 Oct. 1981. 2 Soil Scientist, Biology and Agric. Dep., Atomic Energy Comm., Tuwaitha, Baghdad, Iraq (formerly Graduate Student, Univ. of Nebraska-Lincoln); Soil Scientist, ARS-USDA: and Professors, Agronomy Dep., Univ. of Nebraska-Lincoln, Lincoln, NE 68583, respectively. Information on the effect of soybean on soil properties is sparse. Strickling (1951) showed a seasonal decrease of soil aggregates under soybean. Browning et al. (1942) and Wilson and Browning (1945) indicated that soybean and corn had the same negative effect on aggregation. Kidder et al. (1943) studied the effect of corn and soybean residue on infiltration and showed that the infiltration flux after 60 min was 1.2 and 3.2 cm/hour under soybean residue and com stover, respectively. The purpose of this investigation was to assess the influence of different cropping systems on infiltration, water content, and porosity of soil, and the influence of soybean cropping systems on size distribution of aggregates. MATERIALS AND METHODS The field site was located near Mead, Nebr., in a subhumid area receiving an average of 690 mm of precipitation annually. The soil is a Sharpsburg silty clay loam (Typic Argiudolls). The experimental area received 50 kg/ha of P in 1972, and corn and sorghum treatments received 112 kg/ ha of N annually, whereas the soybean and fallow treatment received no supplemental fertilizer. Each block of the randomized, complete block design consisted of 15 treatments with soybean (Glycine max L. Merr.), corn (Zea mays L.), sorghum (Sorghum bicolor L. Moench), and fallow, from which the plots were chosen for the selected sequences (Fig. 1). The treatments were replicated three times. Tillage was uniform for all treatments each year and generally included chisel plowing to the 20-cm depth in the fall and double disking about 8 cm deep in the spring. Eight of the 15 cropping sequences were selected in 1976 for measuring infiltration rate and water retention characteristics (Fig. 1). Treatments selected were: continuous soybean (B), continuous corn (C), continuous sorghum (S), corn after soybean (C-B), sorghum after soybean (S-B), fallow after soybean (F-B), corn after sorghum (C-S), and sorghum after corn (S-C). The size distribution of water-stable aggregates was measured in the soybean treatments: continuous soybean (B), 015 OU 013 012 Oil 010 009 006 007 006 005 004 003 002 001 1977 76 75 74 73 72 c r c S C F B S C R+ Ct S F R c F C* B S F C s rs s* B F C S R-f S F C R S F S* C B F S R+ r B C B F C B S B C F S B F B 5 C F R r F S B r F S Ft B C S F R* F C S R F Alley Rt r B C B F S B S B C F S B F B 5 C B c F S B C F S F* B C 5 F R+ F C S R F Alle C F