THE EFFECT OF COMPACTION ON THE GROWTH OF PIGEONPEA ON CLAY SOILS .2. MECHANISMS OF CROP RESPONSE AND SEASONAL EFFECTS ON AN OXISOL IN A HUMID COASTAL ENVIRONMENT

Field experiments were conducted over two seasons (1986/1987 and 1987/ 1988) on a coastal oxisol to investigate the effects of soil compaction on the growth and seed yield of pigeonpea (Cajanus cajan). Combinations of deep ripping and roller compaction were used to establish a range of compaction treatments. The effects of seasonal conditions on plant response to compaction were investigated using irrigation treatments and a rainout shelter to exclude rain during early growth. Despite some reduction on compacted soil, infiltration of water remained high on all treatments and compaction had no effect on subsoil water storage. Plant response was related to the ability of the root system to overcome the soil strength limitations of the compacted soil. The distribution of rainfall during the early growth period was critical in determining plant response. When rain or irrigation maintained low soil strength, compaction increased early shoot growth, presumably as a result of increased volumetric water and nutrient supply. In dry conditions, root penetration was restricted by high soil strength thus reducing water uptake and shoot growth. These early growth restrictions could be compensated by rain or irrigation during later growth stages. The accurate prediction of compaction effects on crop growth and yield requires a modelling framework which accounts for the temporal variability in soil strength, and its effect on root growth and water uptake by plants.