DIVISION S-6-NOTES A METHOD FOR STUDYING THE EFFECTS OF SOIL AGGREGATE SIZE AND DENSITY P. L. DE FREITAS, R. W. ZOBEL,* AND V. A. SNYDER Abstract Soil macroproperties, such as bulk density, porosity, water conduc- tivity, and aeration, have been utilized in order to evaluate soil struc- ture effects on crop yields. It has been suggested that many of the limitations to crop growth can be explained by the physical properties of the aggregates. To explore some aspects of this, a method to simulate soil aggregates of different sizes and densities was developed and tested. A Collamer silt loam soil (fine-silty, mixed, mesic Glossoboric Hapludalf) was dried, crushed, sieved, and compressed to different densities (1.4,1.6, and 1.8 Mg m ~ 3 ). The resulting 25- and 50-mm-thick cylindrical soil slices were cut to two different cubical sizes (25- and 50-mm sides). Cylinders with continuous horizontal and vertical mac- ropores were assembled from these artificial aggregates to make six different treatment combinations of aggregate size and density. Corn (Zea mays L.) seedlings were transplanted into the cylinders and grown in a growth chamber for 27 d. Results show that high aggregate density limits crop growth and that, at intermediate densities, aggregate size assumes more significance as a limiting factor. P.L. De Freitas, EMBRAPA/CNPS, Rua Jardim Botanico, 1024, 22460- 000 Rio de Janeiro, RJ, Brazil; R.W. Zobel, USDA-ARS-PSNL Rhizobo- tany Project, 1017 Bradfield Hall, Cornell Univ., Ithaca, NY 14853; and V. Snyder, Agronomy and Soils, Univ. of Puerto Rico, Mayaguez PR 00708. Received 16 Dec. 1994. "Corresponding author (r.zobel® cornell.edu). Published in Soil Sci. Soc. Am. J. 60:288-290 (1996). S OIL STRUCTURE is considered one of the most im- portant properties controlling plant growth. This property largely determines soil behavior in relation to air and water (aeration, water retention and conductivity, nutrient flow and diffusion, etc.) and is strongly related to plant-root growth and function. Measurement of mac- roscopic soil properties as indicators of soil structure has been reported extensively in the literature (Veihmeyer and Hendrickson, 1948; Phillips and Kirkham, 1962; Schuurman, 1965; Letey, 1985). The value of measuring macroscopic properties such as soil strength, hydraulic functions and gas dynamics is that they provide indices of the most important structure-related processes that affect root development and crop yields. However, the relatively large scales of measurement that are commonly used provide values that are insensitive to small-scale variation of these properties within the root zone. An increasing body of evidence is developing which suggests that microscopic variation in soil physical properties due to soil structure is very important in determining root growth and function. In particular, it appears that discon- tinuities in soil properties associated with interaggregate voids and cracks can have significant effects (Edwards etal., 1964; Seneetal., 1985). Methodologies are needed to study such systems under sufficiently controlled condi- tions that the processes involved can be better defined and quantified. We designed an experimental system to study the interaction of aggregate size (or more precisely the fre- quency of occurrence of interaggregate zones of weak- ness) and aggregate density in determining root growth patterns under conditions closely resembling undisturbed soil in the field. We also studied the effects of the relation- Published January, 1996