Soil fragment size distribution and compactive effort effects on maize root seedling elongation in moist soil

Distributions of soil fragments (a mixture of primary aggregates resulting from tillage fragmentation) in seedbeds are known to influence emergence and early shoot and root growth of crops. However, it is not clearly understood which distribution model parameters the roots are responding to when water imbibition and nutrient availability are not limiting factors. The objective of this study was to determine the effect(s) of variation in geometric mean diameter (GMD) and log of the geometric standard deviation (LogGSD) taken from a log-normal model of soil fragment size distribution on maize (Zea mays L.) root elongation over a range of soil bulk density (rho(b)) and air-filled porosity (AFP) levels. Root growth, determined 48 h after seedling emergence, was evaluated in a greenhouse experiment with artificially packed soil fragments sieved from a Maury silt loam (fine, mixed, semiactive, mesic Typic Paleudalf) under sod. Two experiments were conducted. The first consisted of a complete factorial combination of four compactive efforts (CEs) (0.0, 26.8, 53.5, and 107.0 kJ m(-3)) applied over four GMDs (4.3, 5.1, 6.8, and 9.0 mm) at a uniform LogGSD of 0.22. The second experiment was a complete factorial combination of the same four CE treatments applied over three LogGSD (0.22, 0.31, and 0.48) values at a uniform GMD of 5.1 mm. Increasing CE, GMD, or LogGSD caused the rho(b) to increase. Maximum root elongation occurred at intermediate (5.1-6.8 mm) GMD values when 26.8 kJ m(-3) of CE was applied, corresponding to an average rho(b) of 1.15 Mg m(-3). There was no direct effect of the spread in soil fragment size (LogGSD) on root elongation. Total root length density (RLD) showed a quadratic response to rho(b), relative bulk density (rho(b)), bulk density divided by maximum Proctor density), or AFP levels reaching maximum elongation at values of 1.12 Mg m(-3),0.78 m(3) m(-3), or 0.187 m(3) m(-3), respectively. These results suggest that maximum radicle elongation depends more on the size of soil fragments rather than on the spread in their size distribution. Bulk density seems to be a more relevant parameter than size or distribution of soil fragments in characterizing compacted seedbeds. Loose or highly compacted seedbeds are inadequate for maximal early growth of maize roots.