An Evaluation of Seedbed Preparation Methods for Growing Irrigated Cotton in Grey Clays

Abstract

Declining cotton yields led to doubts about the long term viability of irrigated cotton growing in the lower Macquarie Valley, N.S.W., only 10 years after the industry was established. Soil limitations to cotton growth were implicated as the main reason for the yield decline in the absence of insect and disease outbreaks. Following unsuccessful screening for soil chemical deficiencies, soil physical conditions were studied. A series of experiments have since shown that poor soil physical conditions in Macquarie Valley vertisols can be improved by using crops to dry the soil, deep tillage and gypsum. The next step, which this project addresses, is to study soil management systems which minimize degradation of soil physical conditions. Aims of this project were to: i) Assess the viability of the permanent bed system for irrigated cotton production in which the hills on which cotton is grown are left in the same place for a number of years. This was done by comparing soil physical properties and cotton growth on areas prepared using conventional seedbed preparation in which the hills are knocked down and reformed each year with areas of cotton under permanent beds. ii) Study cotton growth in response to a range of soil physical conditions created by different tillage systems, and the relationship between cotton growth and some measures of soil physical condition. From these relationships, we should gain a clearer picture of how deep tillage ameliorates poor soil structure, and also how it can cause the yield depression observed in some prior experiments. iii)Combine information from this project with other, similar, projects, to establish guidelines to predict which soil management practices are best suited to a given situation. The main part of the project was a field experiment, monitored over two and a half years. Three tillage treatments, ripping to 0 .45 m, chisel ploughing to 0 .25 m, and a permanent bed system were imposed in a randomized complete block design with three blocks (replicates) at the start of the trial in May, 1984. Cotton, wheat, and maize crops were grown over the next two years, then deep tillage treatments were reimposed prior to a second cotton crop.