SOIL CARBON, ROOT HEALTH AND NEMATODE PESTS IN SUGARCANE SOILS. 1. ROOT AND SOIL HEALTH AND ITS RELATIONSHIP TO SOIL CARBON LEVELS

A SURVEY OF farms where sugarcane was grown using a farming system that incorporated legume rotation crops, minimum tillage, crop residue retention and controlled traffic showed that all soils were highly stratified with depth. The soil immediately below the trash blanket had much higher carbon levels than the soil a few centimetres down the profile and this had flow-on effects that influenced root health and the soil biology. Root biomass, the number of fine roots, microbial activity and numbers of beneficial nematodes were all much higher in the surface soil than at 5–10 cm, whereas populations of plant-parasitic nematodes showed the opposite distribution with depth. A pot experiment in which sugarcane was grown in soils collected from different depths in the soil profile confirmed the observations made in the field. Plants grew much better in surface soils, with aboveground biomass 35–100% higher than in soils collected from depths of 2.5–10 cm. Assays in the laboratory suggested that suppressiveness to nematodes was greatest in the topsoil and declined with depth, while the results of the pot experiment showed that there was an inverse relationship between soil carbon levels and the number of root-lesion nematodes (Pratylenchus zeae)/g root. Data from two quite different soil types showed that increasing the soil carbon content by 0.5% decreased the number of P. zeae/g root by about 80% and 35% in sandy loam and clay loam soils, respectively.