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 r ainfall distribution critically determined the response of pigeonpea ( Cajanus cajan [L.] Millsp.) to soil compaction. This paper reports exp eriments conducted under controlled conditions to further investigate the influence of water regime and soil type on the response of pigeonp ea seedlings to compaction. Undisturbed cores, 23.5 cm in diameter and 60 cm deep were recovered from compaction treatments (control, modera te, severe) at field sites on an oxisol (Krasnozem, Uf 6.31, Rhodic Pa leustalf) and a vertisol (Black earth, Ug 5.16, Entic Pellusturt). Pig eonpea seedlings were grown for 30 days in two experiments under eithe r drying or well-watered conditions. Under drying conditions, the vert isol retained more water in the surface than the oxisol. This reduced soil strength in the vertisol and root and shoot growth were unaffecte d by compaction. The water applied to the oxisol drained to lower dept hs and the surface dried rapidly, increasing soil strength and reducin g root and shoot growth. Under well-watered conditions, compaction had no effect on plant growth in the vertisol, but in the oxisol growth i n 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 rainfal l distribution and soil hydraulic properties on plant response to comp action. Predicting yield losses resulting from compaction will require modelling approaches that incorporate the effects of compaction on ro ot growth and crop water use.