Effects of deep tillage and fertilization on the population, growth and yield of soya during an exceptionally wet season on a compacted sandy loam, Santa Cruz, Bolivia

Abstract

Many of the soils in the tropical lowlands of Santa Cruz, Bolivia are compacted. A trial using a split-plot design and four replicates was initiated in 1985 to study the effects of deep tillage, and subsequently fertilizers, on soil properties, soya growth and yield in a compaction-susceptible sandy loam ustochrept with a compacted layer from 0.12 to 0.35 m depth. The main (tillage) treatments were: (1) conventional (CN), generally two passes of a heavy disc harrow to 0.12 m depth followed by two passes of a light disc harrow; (2) disc ploughing in 1985 (DP/85), one pass of a disc plough to 0.30 m depth in 1985 only followed by CN tillage; (3) subsoiling in 1985 (SS/85), two overlapping passes of a subsoiler to 0.40 m depth to give 0.37 m tine separation distances in 1985 only, followed by CN tillage; (4) subsoiling every year (SS/YR), depth as for SS/85, followed by CN tillage. The two subtreatments were: (a) without fertilizers; (b) with fertilizers, an application of 20 kg N ha −1 and 35 kg P ha −1. The following results are for the exceptionally wet 1987/1988 season. The SS/YR, SS/85 and DP/85 gave deeper maximum rooting and faster root growth rates compared with CN. Nevertheless all treatments displayed marked reductions in root growth rates below 0.19 m depth in the original compacted zone, indicating recompaction of the SS/85 and DP/85 treatments since deep loosening in 1985. Root growth rate reductions in SS/YR show the inability of even annual subsoiling to ameliorate completely the detrimental effects of compaction on root growth. The SS/YR and fertilization treatment gave taller plant heights owing to internodal elongation. Fertilizer application also resulted in improved foliar nutrition, increased maximum depths and weight of roots, number of leaves and branches. Significantly higher yields were obtained with deep-tillage treatments and fertilizer application but no interaction effect was found. However deep tillage, unlike the fertilizer treatment, also resulted in significantly higher plant populations compared with the CN tillage owing to improved drainage, associated with lower penetrometer resistances, during the exceptionally wet germination — emergence period. After adjusting soya yields for population differences by covariance analysis, no significant tillage effect on yield was found although a highly significant fertilizer effect on yield persisted. Thus, although deep tillage, and in particular SS/YR, resulted in taller plants and deeper maximum rooting, it had no direct effect on soya yield, other than through its influence on plant population owing to improved drainage.