Soil Compaction in Conservation Tillage: Crop Impacts Dilraj Sidhu and Sjoerd W. Duiker* ABSTRACT Soil compaction effects on maize (Zea mays L.) plant population, height, and yield were studied from 2002–2005 in a no-tillage/in-row tillage study on a Hublersburg silt loam soil (Typic Hapludult) in Pennsylvania. Soil was compacted annually with a three-axle truck with 10-Mg axle load mounted with road tires (700 kPa inflation pressure) or flotation tires (250 kPa). In another treatment, soil was only compacted with road tires in the first year without subsequent compaction. Re- mediation treatments were deep (40 cm) in-row tillage before or after compaction with road tires and shallow (10 cm in 2002–2003 and 22 cm in 2004–2005) in-row tillage after compaction. Significant yield re- ductions averaging 17% in 3 yr out of 4 were observed for annual compaction with road tires compared with control (no-tillage without compaction). Compaction with flotation tires reduced yield signifi- cantly in 1 yr only. Yield reductions due to compaction disappeared after 1 yr. Deep tillage after compaction increased yield (17%) in 1 yr only, whereas shallow tillage did not increase yields. Yield improve- ments due to deep tillage were lost if it was followed by heavy traffic. Deep tillage and no-tillage without compaction gave similar yields in the first 3 yr, but no-tillage had higher yield in 2005. In-row tillage substantially reduced residue cover. Our results suggest little need for in-row tillage to manage compaction in long-term no-tillage when axle loads are no more than 10 Mg and flotation tires are used to keep inflation pressures below 250 kPa. W ITH GROWING FARM SIZE in the U.S., the use of heavy machinery is also increasing to improve labor use efficiency and timeliness of field operations. Average tractor weight has increased threefold from 1950 to 2000 (Soane and Ouwerkerk, 1998). Axle loads of 10 Mg are common in many countries (Hakansson and Reeder, 1994). Single-axle grain carts in the U.S. now have axle loads of 15 to 45 Mg (Schuler et al., 2000). One of the potential threats of this increase in equipment size is soil compaction. Soil compaction is the process by which the soil grains are rearranged to decrease void space, thereby increasing bulk density (SSSA, 1997). Subsoil compaction is a threat due to the increase in axle loads (Hakansson and Reeder, 1994). Although it usually causes smaller yield reductions than surface compaction, subsoil compaction can have much longer lasting effects on subsoil physical properties and crop productivity (Voorhees, 1983). Compaction up to 50 cm was reported when soils varying from sandy loam to clay textures were compacted with 10-Mg axle load at field capacity (Alakukku and Elonen, 1995; Hakansson, 1985; Hakansson and Reeder, 1994; Etana and Hakansson, 1994). Wheat (Triticum aestivum L.) yield reductions up to 38% on a sandy clay loam soil (Ishaq et al., 2001) and maize yield reductions up to 50% in a poorly drained silt loam soil (Gaultney et al., 1982) have been observed. Effects of deep compaction on soil physical properties and crop yields have been found to persist from 6 to 11 yr (Etana and Hakansson, 1994; Gameda et al., 1994; Alakukku and Elonen, 1995). Surface compaction, caused by high contact pres- sure (Hakansson and Reeder, 1994), can be alleviated by freezing–thawing and wetting–drying cycles, biolog- ical activity, and tillage (Larson and Allmaras, 1971; Voorhees, 1983). Therefore, although the yield reduc- tion due to surface compaction can be large, it is typi- cally of short duration. In the absence of tillage, however, the threat of surface compaction is significant. Low ground contact pressure exerts less stress on surface soil and causes less damage to topsoil physical properties than high-contact pressure (Reeves and Cooper, 1960; Wood et al., 1991). Soil tillage can partly alleviate compaction, but it also has negative effects. Benefits of no-tillage systems compared with conventional tillage are reduced soil erosion, increased infiltration (Lal and Van Doren, 1990), higher biological activity (Gantzer and Blake, 1978), and lower labor and equipment costs (Duiker, 2004a). Soil compaction is a concern in no-tillage be- cause of absence of alleviation through tillage. Negative effects of compaction on plant growth (Nevens and Reheul, 2003), soil physical properties (Botta et al., 2006; Abu-Hamdeh and Al-Widyan, 1999), and maize yields (Lal, 1996) have been reported in no-tillage systems. On the other hand, long-term use of no-tillage results in increased surface organic matter contents, more stable soil structure, and increased hydraulic conductivity due to worm holes and stable biochannels (Mahboubi et al., 1993; Dick et al., 1991). Reduced compactability due to high organic C contents in the surface soil suggests lower surface-compaction threat in long-term no-tillage (Thomas et al., 1996). Subsoiling is the process of deep tilling to a depth ranging from 30 to 90 cm (Roa-Espinosa, 1998). It has shown some success in alleviating compaction and im- proving yields on soils with compacted subsoil (Chen et al., 2005; Box and Langdale, 1984; Reeves et al., 1992; Abu-Hamdeh, 2003), sometimes in combination with the use of cover crops (Schwab et al., 2001). However, all soils do not respond to deep tillage (Raper et al., 2000a, 2000b). To be considered conservation tillage, deep-tillage methods need to leave at least 30% residue Abbreviations: AFTC, annual flotation tire compaction; ARTC, annual road tire compaction; DTAC, deep tillage after compaction; DTBC, deep tillage before compaction; DTNC, deep tillage no com- paction; FRTC, first-year road tire compaction; STAC, shallow till- age after compaction; WAP, weeks after planting. D. Sidhu, 2101 Raven Tower, Ct. Apt. 202, Herdon, VA 20170; and S.W. Duiker, Dep. of Crop and Soil Sci., Pennsylvania State Univ., 116 ASI Bldg., University Park, PA 16802-3504. Received 8 Mar. 2006. *Corresponding author (swd10@psu.edu). Published in Agron. J. 98:1257–1264 (2006). Tillage doi:10.2134/agronj2006.0070 ª American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Reproduced from Agronomy Journal. Published by American Society of Agronomy. All copyrights reserved. 1257 Published online August 3, 2006 Published September, 2006