Soil compaction associated with inappropriate maneuvering of field equipment, and/or modern cropping system negatively affect soil physical properties, and thus, may limit microbial activities and biochemical processes, which are important to nutrient bioavailability. An experiment was carried out using the pot-culture technique to determine the effect of bulk density on soil microbial populations and enzyme activities in an Eutric Cambisol sandy loam soil (United Nations’ classification) planted with maize (Zea mays L.) in the Experimental Farm of Henan Agricultural University, Henan, China (34°49′N, 113°40′E). Numbers of bacteria, fungi, and actinomycetes and the enzyme activities of invertase, polyphenol oxidase, catalase, urease, protease, and phosphatase were determined at various stages during the plant growing season. Microbial numbers were negatively and linearly related to soil bulk density. With increases in soil bulk density from 1.00 to 1.60 Mg m-3, total numbers of bacteria, fungi and actinomycetes declined by 26-39%. The strongest correlations between the soil microbial population and bulk density occurred at the plant growth stages of the 6 fully expanded leaf (V6) and anthesis (R1), with R2 > 0.90 (P< 0.01) for all three microorganism categories. Increasing soil bulk density was related quadratically to the activities of soil invertase and polyphenol oxidase, protease and catalase. It appears that the greatest activities of most soil enzymes occurred at a bulk density of 1.0 to 1.3 Mg m-3, which are optimum for most field crops. The plant growth stages also had an important impact on soil enzyme activities and microbial populations, with strong positive associations between soil microorganisms and enzyme activities with crop growth. Key words: Maize, soil enzymes, microbial population, soil compaction, bulk density, Zea mays

There is increasing evidence that average field-crop yields on many fine-textured soils are starting to plateau, and perhaps even decline in some areas Brown and Kane 1994). In addition, average field-crop yields on fine-textured soils are still only about half their genetic and climatic potentials, despite continuing advances in cultivars, fertilizers, pesticides, and land management practices . Although many interacting factors are undoubtedly contributing to this situation, it is clear that the intrinsically poor physical quality of many fine-textured soils plays a key role. It has been estimated that if optimal soil physical quality could be achieved and (PAWC). Although soil conditioners derived from municipal, agricultural and industrial wastes are frequently used to improve soils, their effects on overall soil physical quality are still poorly understood. Hence, the objective of this laboratory soil core study was to determine for a Brookston clay loam the effectiveness of masonry sand, greenhouse rockwool waste, yard waste compost and swine manure compost for improving soil physical quality relative to "ideal" levels proposed in the literature, and relative to "benchmark" levels found in the soil under virgin conditions, long-term conventional tillage and long-term no-tillage. The virgin soil produced near-surface (0.05-0.15 m depth) values for ρ b (0.88 Mg m -3 ), AC (0.19 m 3 m -3 ) and PAWC (0.22 m 3 m -3 ) that fell within the optimal ranges proposed in the literature, while OC (68.2 g C kg -1 ) was slightly above optimal. The soil under long-term conventional tillage and no-tillage (cornsoybean rotation) produced below-optimal organic carbon content (21.9-22.5 g C kg -1 ), excessive ρ b (1.45-1.47 Mg m -3 ), insufficient AC (0.06 m 3 m -3 ) and low PAWC (0.14-0.19 m 3 m -3 ). Conventional tillage also produced below-optimal K s (10 -6 m s -1 ). Each conditioner could improve one or more of the above parameters, but not all five. Adding sand at 20-100 wt. % improved AC, but caused excessive reductions in OC and PAWC, and excessive increases in ρ b and K s . Greenhouse rockwool waste added at 2.5-10 wt. % improved AC and ρ b , but did not improve OC and PAWC. Yard waste compost added at 3.8-20 wt. % improved OC, ρ b and PAWC, but did not improve AC. Adding swine manure compost at 3.8-20 wt. % improved OC and ρ b , but did not improve AC or PAWC, and decreased K s . As no single conditioner could optimize all soil physical quality parameters, future studies using combinations of conditioners are proposed. For personal use only.