EFFECT OF COMPACTION ON THE GROWTH OF PIGEONPEA ON CLAY SOILS .3. EFFECT OF SOIL TYPE AND WATER REGIME ON PLANT-RESPONSE

Field studies reported in previous papers in this series showed that rainfall distribution critically determined the response of pigeonpea (Cajanus cajan [L.] Millsp.) to soil compaction. This paper reports experiments conducted under controlled conditions to further investigate the influence of water regime and soil type on the response of pigeonpea seedlings to compaction. Undisturbed cores, 23.5 cm in diameter and 60 cm deep were recovered from compaction treatments (control, moderate, severe) at field sites on an oxisol (Krasnozem, Uf 6.31, Rhodic Paleustalf) and a vertisol (Black earth, Ug 5.16, Entic Pellusturt). Pigeonpea seedlings were grown for 30 days in two experiments under either drying or well-watered conditions. Under drying conditions, the vertisol retained more water in the surface than the oxisol. This reduced soil strength in the vertisol and root and shoot growth were unaffected by compaction. The water applied to the oxisol drained to lower depths and the surface dried rapidly, increasing soil strength and reducing root and shoot growth. Under well-watered conditions, compaction had no effect on plant growth in the vertisol, but in the oxisol growth in both the control and severe compaction treatments was significantly lower than under the moderate compaction treatment. Reduced volumetric water and nutrient content in the control and low air-filled porosity in the severe compaction treatment are thought to be responsible for these effects. Our results indicate the potential influence of rainfall distribution and soil hydraulic properties on plant response to compaction. Predicting yield losses resulting from compaction will require modelling approaches that incorporate the effects of compaction on root growth and crop water use.