Effect of soil management on avocados in a krasnozem soil

Abstract

Avocado trees, cultivar ‘Fuerte’, were grown under 7 soil management regimes on krasnozem soil at the Tropical Fruit Research Station, Alstonville. The regimes consisted of combinations of dolomitic, gypseous and nitrogenous amendments with cover cropping or a kikuyu sward, and a “bare ground” treatment as control. This paper describes the effects of these treatments on physical and chemical characteristics of the soil, on nutrient levels in the avocado leaves and on the occurrence and spread of Phytophthora cinnamomi in the trial site and on tree health, growth and yield of the avocado trees. Treatments receiving dolomite or phosphogypsum achieved the desired high cation-exchange levels but did not influence the behaviour of P. cinnamomi in soil leachates or incidence of Phytophthora root rot. Trees receiving phosphogypsum produced more fruit than trees receiving dolomite, although trees given dolomite grew faster. Inputs of fowl manure and cover cropping were the same in both treatments. Soil phosphate levels were significantly ( P < 0.01) higher in treatments receiving fowl manure and higher ( P < 0.05) in the treatment receiving phosphogypsum than in dolomite treatments, owing to the phosphate content of the gypsum. However, higher soil P levels were not reflected in higher leaf P levels. Soil pH in the top 15 cm was raised and exchangeable Al lowered by applying dolomite, and pH decreased with increasing soil depth. Trees developed Phytophthora root rot first where there was bedrock in the top 80 cm of the soil profile, but later root rot developed in trees growing mainly in the lowest part of the trial area. The principal factor affecting the incidence and severity of root rot in this trial was internal drainage, as determined by the presence of bedrock, weathering rock or high bulk density in the top 80 cm and by topography. Management treatments did not influence the incidence of root rot in this trial, or the production and size of fruit on affected trees. A comparison of leaf analyses for healthy and root rot-affected trees over all treatments showed a dramatic reduction in some leaf nutrient levels in the diseased trees.