Soil Compaction and its Effect on Alfalfa in Zone Production Systems

Alfalfa (Medicago sativa L.) was grown under two zone production systems (no harvest traffic) (i) preplan! compaction and (ii) no preplant compaction and compared to two systems where traffic was applied (i) to 100% of the area during each harvest and (ii) to lanes simulating the traffic a grower would apply during harvest. The experiment was done in a semiarid area under irrigation on a Wasco sandy loam (coarse-loamy, mixed, nonacid, thermic Xeric Torriorthent), which is easily compacted. Without traffic the soil settled to a bulk density of 1.6 to 1.70 Mg m~\ Traffic increased the bulk density to the 1.8 to 1.9 Mg m~ range. Increases in bulk density at the 0.05and 0.15-m depths occurred only during the first five passes of traffic, but 10 passes were required to reach a stable bulk density at depths of 0.25 to 0.5 m. Traffic similar to what a grower would apply reduced yields by 10% compared with no traffic. Compacting 100% of the soil surface after each harvest reduced yields 17% compared with no traffic. Additional Index Words: bulk density, wheel traffic, alfalfa crown damage, Medicago sativa L. Z PRODUCTION SYSTEMS were defined by Taylor (1983) as those systems where the crop zone and traffic lanes are distinctly and permanently separated. Studies are being conducted at the U.S. Cotton Res. Stn. on zone production systems (Rechel et al., 1987) using alfalfa (Medicago sativa L.) and (Gossypium hirsutum L.) cotton. Other zone production system experiments have been conducted in England, the Netherlands, Israel (Hadas et al., 1983) and South Africa. In England, Soane (1975) measured large yield increases (24%) for potato (Solanum tuberosum L.) using controlled traffic. Zone production systems for the production of potato, wheat (Triticum aestivum L.) and sugarbeet (Beta vulgaris L.) in the Netherlands has resulted in small increases in yield but large savings in energy (Lamers et al., 1986). Modern crop production is a continuing battle between soil compaction caused by traffic and alleviation of this compaction by tillage. The degree of soil compaction resulting from traffic depends on force applied and soil properties, especially soil water. Harris (1971) reviewed the soil compaction process and presented stress-compaction relationships but stated that the possibility of developing rigorous relationships by analytical means is remote. Lambe (1962) measured the relationships between bulk density and soil-water content at the time of loading and found that the degree of compaction is strongly dependent on the water content. Texture and organic matter are two other soil properties that have a large influence on the compactability of a soil. Erickson et al. (1974) calculated the "degree of compactness" of a soil and compared soils with a wide range of textures and found optimum yields to be in the range of 85 to 90%. U.S. Cotton Res. Stn., USDA-ARS, 17053 Shafter Avenue, Shafter, CA 93263. Contribution from the U.S. Cotton Res. Stn., USDAARS. Received 30 Apr. 1987. "Corresponding author. Published in Soil Sci. Soc. Am. J. 52:232-236 (1988). Soil compaction can be characterized by bulk density (Canarache et al., 1984;Cassel, 1982), soil strength (Bauder et al., 1981; Swan et al., 1987), water infiltration rate (Douglas and McKyes, 1982; Patel and Singh, 1981), and root growth (Barley and Greacen, 1967; Taylor and Ratliff, 1969). Numerous soil compaction studies have been reported. Most field experiments have involved medium to heavy compaction levels because it was not possible to remove wheel traffic from areas that were of sufficient size to obtain accurate yields. The experiment described below was done using a wide tractive frame that allowed crop production under zero traffic. In alfalfa, production yields are affected both by soil compaction and damage to crown and shoot regrowth from harvest traffic (Sheesley et al., 1974; Sheesley and Grimes, 1977). This traffic may cover up to 70% of field surface (Grimes et al., 1978). Gifford and Jensen (1967) found that increasing bulk density from 1.44 to 1.53 Mg min a loam soil reduced yield, especially under a dry treatment. The objectives of this study were to define and characterize soil density and alfalfa yield changes resulting from traffic (under both conventional and 100% traffic) and compare these values to those obtained under zone production systems. METHODS AND MATERIALS The research was conducted at the U.S. Cotton Res. Stn., Shafter, Ca, at 35°32'00" N, 119° 16'40"W, and 111-m above sea level. The location receives an average of 159 mm of rainfall per year with little rainfall from May to September. The soil is a Wasco sandy loam (coarse-loamy, mixed, nonacid, thermic Xeric Torriorthent (Entisols). Unilateral compression at 200 kPa resulted in a bulk density of 1.72 Mg mat a soil moisture of 13% by weight (matric potential of -10 kPa). The bulk density was 1.96 Mg mat 11.8% soil moisture when compacted by the low-compaction procedure for soil material passing a 4.75 mm (no. 4) mesh sieve (ASTM D 698-58 T) (Felt, 1965). Alfalfa, nonwinter dormant cultivar 'WL514', was sown in October 1982. All plots were initially rototilled to 0.15 m; then chiseled with 0.33-m spaced shanks to a depth of 0.54 m in 0.18-m increments in August 1982. This tillage operation established a uniform soil condition for planting by eliminating any variation due to previous soil management. In February 1983 triple superphosphate was broadcast at 162 kg P ha '. Plant tissue analyses were conducted during the study to ascertain that P levels were adequate. The alfalfa was sprinkler irrigated in October 1982 (for germination), November 1982, and early April 1983. Plots were flood irrigated for the first time on 25 Apr. 1983, and all subsequent irrigations were by flooding using each plot as a basin. The alfalfa was irrigated when 50% of the available water was depleted in at least 25% of the plots at the 0.45m depth. Six replications of four traffic treatments were arranged in a randomized complete block design. Each plot was 8 m wide and 30 m long. The treatments were: No Traffic (NWT) Alfalfa was seeded into the loosened soil and all traffic was excluded. Preplan! Traffic (PR) The soil surface was allowed to dry, then compacted with an International TD9 crawler tractor