of greenhouse gases in the atmosphere (IPCC, 2001), and ability of our agricultural systems to sustain produc-Society is facing three related issues: overreliance on imported fuel, tion at rates needed to feed a growing world population increasing levels of greenhouse gases in the atmosphere, and producing sufficient food for a growing world population. The U.S. De-
Root length density (RLD) of corn (Zea mays L.) and two isolines (determinate and indeterminate) of Harosoy soybean (Glycine max. L. Merr.) in a Nicollet soil were measured from 1 July to 15 Aug. 1971. These measurements, along with water uptake sinks in these vertically developing root systems, were used to evaluate water relations of these three crops.Both soybean isolines had about 0.2 cm/cm3 RLD in the Ap layer with values < 0.1 below the Ap layer. This RLD distribution persisted with depth as the root system expanded vertically. Corn RLD were about 0.3 initially in the Ap layer, and as the root system expanded vertically, maximum RLD of about 0.3 was produced even to the 100‐cm depth on 9 Aug. Vertical extension rates of corn after 1 July were 1.3 cm/day as compared with 1.7 for soybeans, but maximum rooting depths on 9 Aug. were 145 and 120 cm, respectively.Modal specific water inflows were 0.03 cm3/cm root per day—maximal values for soybeans were 0.2; maximal values for corn were about 0.05. Maximum soybean rooting depths coincided with maximum depth of water uptake sink, but corn rooting depths ranged 15 to 30 cm deeper than the water uptake sink.Specific water inflow generally increased as soil hydraulic conductivity increased and RLD decreased, which agrees with models for steady‐state water flow into single roots. At low RLD, specific water inflows were either constant or increased as hydraulic conductivity decreased. This increased root resistance to water flow at the lower fringes of rooting may have resulted from low soil temperatures. Soybean root conductivities decreased with decreasing soil water potential. Corn root conductivities were largest at about −700 mbars and decreased as soil water potential decreased. Decreases of corn root conductivity resulted from potentials > −700 mbars when soil temperatures were low.
Wheel traffic of heavy farm machinery is causing increased concerns about soil compaction. To offset the increasing weight of large equipment, tires have increased in both size and numbers in an effort to keep constant the contact pressure on the soil surface. Recent research has indicated, however, that increasing axle load can cause increasing and deeper compaction irrespective of surface contact pressure. Axle loads of 9 and 18 Mg were applied in replicated field experiments initiated in 1981 and 1982 in Minnesota as part of an international cooperative study on the effects of high axle loads on deep soil compaction and plant response. Such axle loads are typical of modern harvest and transport equipment. These heavy axle loads were applied only at the beginning of the experiment, and are compared with check treatments where axle loads are limited to ≤4.5 Mg. Various soil parameters were measured to assess the extent and persistence of subsoil compaction. When the subsoil was relatively dry, axle loads of 9 and 18 Mg did not cause significant changes in bulk density, penetrometer resistance, or hydraulic conductivity deeper than about 20 to 30 cm, the normal depth of tillage. However, when the subsoil was relatively wet, bulk density increased about 0.08 Mg m−3 from about the 30‐ to 50‐cm depth. Saturated hydraulic conductivity decreased from 2 to 0.2 cm h−1 due to 18‐Mg axle load on a Webster clay loam (Typic Haplaquolls). A similar relative decrease was measured on a Nicollet clay loam (Aquic Hapludolls). Evidence of subsoil compaction persistence was measured 4 yr after initial heavy axle loading in spite of annual winter soil freezing to depths up to 90 cm.
Increasing size and weight of farm tractors is causing increasing concern about soil compaction. Controlled wheel‐traffic studies in Minnesota on a silty clay loam showed that wheel traffic of normal farming operations could compact the soil to a 45‐cm depth. Penetrometer resistance was a more sensitive indicator of soil compaction than was bulk density. Wheel traffic increased soil bulk density by 20% or less, whereas penetrometer resistance was increased by as much as 400%. Fall tillage essentially alleviated bulk compaction in the 0‐ to 15‐cm layer. Plowing was more effective than disking or chiseling in decreasing compaction in the 15‐ to 30‐cm layer. Compared with plowing, bulk density and penetrometer resistance values for chiseling or disking were about 5 and 40% higher, respectively. Compaction below the tillage depth was not completely ameliorated by annual freezing and thawing.Wheel‐induced compaction was more persistent in individual soil structure units than in bulk soil. Strength and density of wheel tracked clods were greater and average aggregate diamter was larger than that of nontracked clods, a difference which persisted overwinter.
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