Abstract

The potential suitability of a soil to accommodate growing roots was assessed on the basis of: (1) the existing pore space available for unobstructed root growth; and (2) the obstruction offered to a growing root by the soil matrix. The former was evaluated by measuring the air permeability of undisturbed soil cores, equilibrated at a chosen water tension. The latter was evaluated by measuring the spectrum of the tip resistance encountered by a slowly penetrating fine probe. The influence of cropping history on this potential suitability was examined by comparing the same soil with two different cropping sequences (corn grown continuously for five years and forages grown for three years followed by two years of corn). Measurements were started shortly before the forages were plowed under. Air permeabilities were higher for the plots on which forages had been grown, indicating greater potential suitability for unobstructed root growth. The penetration experiments showed that the average resistance to deformation of root-sized pores was equal for the two cropping patterns. The average bulk density was higher in the continuous corn plots than in the plots on which forages had been grown. After analysis of our data we postulated that the effect of the higher bulk density was offset by lower aggregate strength in the continuous corn plots. Implications of this hypothesis are discussed.

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