Continuous Cotton and Cotton-Wheat Rotation Effects on Soil Properties and Profitability in an Irrigated Vertisol

Abstract

ABSTRACT The combination of cotton (Gossypium hirsutum L.)-wheat (Triticum aestivum L.) rotations and minimum tillage is known to be superior to cotton monoculture in maintaining soil quality in eastern Australian Vertisols. Furthermore, sowing cotton into retained wheat stubble can minimize erosion and runoff, reduce off-field movement of pesticide residues and nutrients, and protect cotton seedlings from attack by heliothis moths. However, the consequences of sowing irrigated cotton into standing wheat stubble on soil quality and profitability are unknown. The objective of this study was to quantify the changes in several soil quality indices, viz. soil organic C (SOC), salinity, sodicity, structure, exchangeable cations, and profitability due to sowing irrigated cotton into standing wheat stubble. The sodicity indices measured were ESP (exchangeable sodium percentage) and EC1:5/exchangeable Na, and structural indices were in situ macroporosity at field capacity and bulk density. Soil properties and profitability were measured between 2000 and 2003 in a furrow-irrigated experiment at the Australian Cotton Research Institute (ACRI), near Narrabri, in New South Wales, Australia. The treatments were continuous cotton sown after conventional (disc-ploughing and chisel ploughing, followed by ridging) or minimum tillage (slashing of cotton plants after harvest, followed by root cutting and bed renovation with a disc-hiller), and minimum-tilled cotton-wheat. Between 1985 and 1999 cotton was sown after wheat stubble was incorporated but after 1999 it was sown into standing wheat stubble. Conventional cotton was sown in all plots between 1985 and 1999, whereas Roundup-Ready™ cotton was sown from 2000 onwards. Continuous cotton sown after conventional tillage had lowest SOC, exchangeable K, in situ macroporosity at field capacity and EC1:5/exchangeable Na, and highest ESP and bulk density. The reverse occurred with cotton-wheat sown after minimum tillage. Differences in these soil properties were greater between tillage systems than between cotton-wheat and continuous cotton cropping systems. Nonetheless, minimum-tilled cotton-wheat had higher SOC and exchangeable K than minimum-tilled continuous cotton. Sowing cotton into standing wheat stubble improved salt leaching. By 2003, cumulative gross margins were similar in all three cropping systems. At the same time, in comparison with both continuous cotton systems, cumulative variable costs were about 20% lower with minimum tilled cotton-wheat. Similar profits can be realized with less intensive cotton production systems which at the same time degrade soil quality less and have lower production costs. Analysis of weed control costs suggested that over-the-top application of Roundup in Roundup-Ready™ cotton resulted in early season control of weeds in the short-term and decreasing weed control costs over the longer-term.