The Potential For Variable-Rate Fertiliser Application in Cotton

by Craig Stewart

Introduction

    Recent progress in precision agriculture has seen the development of many new technologies that permit the identification of spatial variability within agricultural fields, thus allowing the implementation of site-specific management strategies. At present the technology to apply variable-rate inputs already exists for growers in the Australian Cotton Industry. However, the development of nutrient management schemes modeling growth response functions with a view to fertiliser recommendations has been limited to a holistic approach, using average soil, plant and climate values to predict the mean crop growth of an entire field, or more commonly, a farm or district. Much of the research to date into determining variable-rate fertiliser has been conducted on grain cropping systems (Kitchen et al., 1995; Vetsch et al., 1995; Mulla and Schepers, 1997) although the same principles will apply for managing cotton. In summary, the three main points that need to be addressed when developing a method for varying fertiliser application within a field are:

• the cost, time and complexity of variable-rate fertiliser application
• advantages of a proactive (anticipating total before sowing) versus a reactive (response to crop uptake) approach to fertiliser application
• the merit of increasing yield versus reducing fertiliser usage in cotton farming systems

• To quantify the degree of spatial variability of soil nutrients within and between cotton fields
• To apply fertiliser on a site-specific basis using variable-rate technology to maximise economic returns and minimise environmental impact
• To develop a suitable program for the prediction of site-specific growth response functions given localised variations in soil properties within a field

    There are currently two core research sites where this study is being carried out. Both are irrigated fields of approximately 100 hectares, located in two of Australia's major cottoning growing regions, the Gwydir and Namoi Valleys.

    In this project, advances in mobile electromagnetic (EM) mapping, yield mapping and remote sensing are being employed to determine potential management zones. The information gathered from these techniques is being integrated, to generate ‘Equifertiles’, representing areas that equate in their potential productivity. This allows directed soil sampling of each equifertile to be carried out to determine available soil nutrients and soil physical properties impacting on cotton growth. Fertiliser rates are then being adjusted accordingly for each ‘zone’, using the recommendations of NutriLOGIC, a decision-support tool released by the Australian Cotton CRC as part of cottonLOGIC. The success of this approach will be measured by remotes sensing with ground truthing throughout the growing season and yield monitoring at harvest.
 

Preliminary Results

    Progress to date suggests that the potential for variable-rate fertilisation of cotton fields is very high. Variation in yield (Map 1) and soil nutrient levels (Table 1) are occurring at magnitudes and in spatial patterns that indicate that fertiliser should be applied at varying rates throughout a single field. Electromagnetic (EM) mapping is being used (map 2) to aid in the identification of soil physical differences, as EM is a function of texture, soil mineralogy and soil moisture.
 
 

Two methods are being tested to determine a suitable protocol for the application of variable-rate nitrogen fertiliser. Directed soil sampling of the field based on the yield and EM data has enabled a nitrogen fertiliser requirement map to be developed based only on the soil samples and the recommendations of nutriLOGIC (Map 3). An alternative approach being tested is the use of multivariate clustering of all data layers to determine homogeneous zones within the field which receive fertiliser based on yield potential (Map 4).
 
 

    Both methods of variable-rate fertiliser application are being trialled over the 1999/2000 cotton growing season in Australia.

References
 

• AUSCOTT MIDKIN
• OAKVILLE - NARRABRI
• INCITEC FERTILISERS
• JOHN TRIANTAFILIS and ANDREW HUCKEL

THE UNIVERSITY OF SYDNEY
© 2003 - Australian Centre for Precision Agriculture