Mapping soil variability for variable rate application

21 Jan 2015

Earlier this month I visited a farming couple as part of NGRMGs efficient farming program. They are growing Rhodes grass to be sold as high quality hay feed. Initial investigation of the paddock which is irrigated by a centre pivot showed some obvious yield variability. Further investigation using their soil map supplied by NGRMG, showed the paddock had some good soils but there was an area of Penman soil type which is a sodic soil. This was confirmed by soil tests. Sodic soil is high in exchangeable sodium ions, symptoms include: alkaline pH, highly erodible, crusting of soil, impeded infiltration and drainage which lead to reduced yields. Classically these areas are boggy when water logged. To ameliorate the sodic areas, requires the addition of a soil amendment such as gypsum while the rest of the paddock does not. Consequently accurate mapping of these areas is useful for targeted soil amendment application. The most accurate soil mapping for the area is the comprehensive QLD Department of Natural Resources and Mines MDIA 1:25 000 soil mapping. But, this clearly did not agree with what we were seeing on the ground. For that reason I surveyed the paddock using an EM-38 ground conductivity meter. This useful device, maps soil to a depth of around 1 metre (Picture 1 & 2). This can be converted to a soil map, after some strategic soil sampling together with some spatial analysis using a Geographic information system (GIS). This is planned for later in the year. A preliminary investigation shows that about 10 ha (Picture 2) will require gypsum application regularly. This is half of the area compared to a blanket application of over 20 ha. This is a direct saving of input costs for the grower. Variable rate fertiliser application will also be investigated.

This is one of the many professional services offered through the efficient farming program to farming enterprises in the Mareeba-Dimbulah Water Supply Area. Participating growers may also access a grant of up to $5000. Our specialists will work one-on-one with growers to deliver:

  • Detailed property and soil mapping services

  • Pump and irrigation system assessments

  • Nutrient management audits

  • Water quality, soil fertility and soil leachate monitoring

  • Irrigation scheduling training

The primary focus of the efficient farming program is practical outcomes that improve sustainability and benefit the grower, the environment and the community. I am passionate about Natural Resource Management (NRM) and I enjoy applying my GIS and surveying skills to help growers preserve and improve our natural resources. I believe NRM outcomes are compatible with production outcomes. This is evident through the efficient farming program where growers and the environment benefit. Examples are plentiful such as simple upgrades to an irrigation system, for instance, correct intake pipe diameter can reduce energy costs. The efficient farming program is funded through the QLD Regional NRM investment program and Federal Government CFOC program and supported by ESRI not for profit program, Irrigation Australia and the rural water use efficiency program.

 EM38 survey using quad

Picture 1: Another hard day in the office for Trevor Parker surveying a paddock using the EM-38 connected to a Trimble RTK GPS, allowing accurate soil imaging together with fine scale topography.

EM38 results centre pivot

Picture 2: EM-38 survey results show the variability of soil which can be used as a basis of variable rate application. The blue areas are suspected sodic.