Australia produces around 3% of the world's cotton fibre, and is the third largest exporter after Uzbekistan and USA.  The area sown to cotton in Australia is about 550,000 ha.  The major soil types used for growing cotton are clayey Vertisols (clay content >50%) which are relatively fertile, have low organic matter contents and can be sodic at depth.

Soil organic matter plays a key role in maintaining soil structure and fertility.  At the same time, intensive farming causes a rapid decline in soil organic matter content.  Research conducted in sandy soils has suggested that this can be reversed by applying organic waste products as soil amendments.  Information on the effects of organic waste products on Vertisol quality is, however, sparse, particularly for irrigated cotton production systems.  The objective of this study, therefore, was to evaluate the effects of three easily available organic amendments on soil quality of a Vertisol sown with irrigated cotton. 

A field study was conducted at the Australian Cotton Research Institute (ACRI), near Narrabri, New South Wales during the growing season of 2004-05.  The experiment was laid in a 4 RCB design using three different organic amendments viz., cattle manure, composted cotton gin trash and “Tryton” vermicast liquid.  The soil was sampled on three occasions from the 0 to 0.1 m depth and analyzed for selected physical, chemical and microbiological properties including aggregate stability, pH, EC, nutrient dynamics both cations and anions, particulate organic matter, non-exchangeable potassium, microbial biomass and soil respiration.  Physiological data such as plant height, numbers of nodes, flowers, bolls etc. of the cotton plant were also recorded during its active growth stage. 

Addition of liquefied vermicast increased lint yield by 6.8%, but there was no increase in yield due to application of other amendments.  Microbiological analysis of the samples collected after cotton picking suggested that manure increased the soil microbial biomass and respiration, whereas vermicast increased only microbial biomass.  There was no difference in plant height and other physiological characteristics among the treatments during the active growth stage.  There were no significant short-term effects, beneficial or adverse, on soil physical and chemical properties.  Further work will examine potential longer term effects, other organic amendments, and the influence of application rate.