Interactive effects of exchangeable sodium and water content on soil modulus of rupture

The interactive effect between exchangeable sodium (sodium adsorption ratio, SAR = 1, 7, 15, 30 and 50) and water content (water saturated and field capacity) on soil hardsetting behaviour was investigated in a loamy topsoil of a Typic Natraquoll. Hardsetting was evaluated by the modulus of rupture (MOR) of dry aggregates, previously subjected to a static pressure simulating that imposed by an adult cow (400 kPa). A control receiving no pressure was included. In both control and water saturated treatments, MOR increased steadily (from 6.5 to 10.4 MPa) with SAR. This indicates that the effect of exchangeable sodium prevailed over that of water content when the soil was stressed at saturation. Exchangeable sodium and water content interacted significantly (F ratio = 4.56; α < 0.002) during the applied pressure, but this interaction was not synergistic. Soil stressing at field capacity prevented MOR increases above a threshold of SAR ≥ 7. As a consequence, an attenuation of hardsetting behaviour can be expected when the studied soil is trampled in moist field conditions.