3.3 - Soil Surveying - on the go pH

CHECKLIST FOR VARIABLE RATE LIME 

• Use a rapid sampler on a 1ha grid or alternatively sample zones derived from imagery or yield maps. 

• Collect at least 3 calibration samples to send to the lab. 

• Determine lime requirements using soil test results and convert them to an application map to be loaded into the spreader controller

Acid soils affect some of the most productive agricultural land in Australia.

The symptoms of soil acidification are not easily recognised as they are less specific than other soil constraints such as salinity and erosion. In grain crops, soil acidity impairs root growth and reduces the plant's ability to access water and nutrients in the soil profile. This is more significant in low rainfall regions, where the topsoil tends to dry out in the late growth stages of the crop.

Soil acidification often causes a gradual decline in crop production and this decline is frequently overlooked or attributed to other causes such as seasonal effects. The application of surface lime to ameliorate topsoil acidity is a common practice in the southern and western grain-growing regions of Australia.

Research shows that surface lime applications will slowly reduce acidity at lower levels in the soil profile. Although deep placement of lime is more effective, it is costly. A more viable approach is to prevent the development of subsoil acidity by the application of lime.

PA technology can help to address soil acidity by enabling rapid field tests for pH, followed by variable application of lime. In many cases, variable-rate lime applications have delivered significant cost savings for the grower (25-30% is typical) over a uniform application rate.

The rapid soil pH meter is simple to use. With an easy push mechanism, the pH probe is inserted into the soil for a few seconds. An integrated GPS-enabled data logger references the sample site. The pH electrode is automatically cleaned after each sample. Many lime spreaders are capable of variable rate applications or can be upgraded to do so. A controller changes the belt speed on the spreader or controls the height of the trapdoor, thereby changing the application rate according to a prescription map.

Below shows the process of collection bottom left, interpolated layer top left, and the prescription map on the right.

INTERPRETATION OF pH MAPS

• pH is the measure of acidity. The Veris and other pH detectors measure pH in a water solution.

• The pH data points need to be interpolated to produce a soil pH map from which a zonal application map is derived.

• Significant changes in soil pH often indicate significant changes in soil type and yield potential.