Influences of degree of compactness and matric water tension on some important plant growth factors

The degree of compactness (D) has been defined earlier as the dry bulk density of a soil in percent of a reference bulk density obtained by a standardized uniaxial compression test on large samples at a stress of 200 kPa. It was primarily aimed for use in annually disturbed soil layers. Field experiments have demonstrated its usefulness for characterizing the state of compactness from a crop production point of view, but knowledge is lacking regarding the relation between D and various plant growth factors. While the bulk density or porosity optimal for crop growth has varied considerably between soils, the optimal D-value has been virtually independent of soil texture. This led to a hypothesis that critical limits of penetration resistance (3 MPa) and air-filled porosity (10%, v/v) are similarly related to the D-value in most soils. With the objective to test this hypothesis, the positions of these limits as functions of the D-value and the matric water tension were studied in four soils with clay content ranging from 70 to 220 g kg−1. In all of them, the positions of the critical limits were similar. The higher the D-value, the more limited was the tension range offering adequate conditions for crop growth, the higher was the water tension (the lower the water content) at which aeration became critical, and the lower was the water tension (the higher the water content) at which penetration resistance became critical. The effects of critical soil conditions were reflected in increased stomatal resistance of plants grown in soil with a high D-value. The results provide basic information for modeling the relationships between soil compactness and plant growth.