“Factors that affect crop yield are intrinsically linked to the physical and chemical characteristics of the soil.”
Roger Lawes says variable-rate applications are not widely adopted because “farmers have struggled to use the technology and they have been overwhelmed with information.” Part of the problem, says the agronomist with Australia’s CSIRO, is that farmers and agronomists have struggled to identify when and where favourable economic outcomes exist.
Lawes recommends you answer these two questions before adoping a variable-rate application system: What are the major factors that influence your yield? And do those factors vary enough within a field to make a variable rate system pay off?
Lawes says the major factor in Australia is soil variability within a field. “Factors that affect crop yield are intrinsically linked to the physical and chemical characteristics of the soil,” he says. “In Australia, this often comes back to the plant available water holding capacity (PAWC) of the soil.”
PAWC of soils in a field can vary by up to 100 mm (four inches), says Lawes, which can cause a yield difference within the field of 35 bushels per acre.
“Soils with high PAWC of greater than 75 mm (three inches) have better reserves of soil moisture and will respond to higher levels of nutrients. They are “responsive” soils. Soils with very low PAWC (30 to 40 mm) require an almost perfect season to yield well. They are “unresponsive” soils,” he says. So it makes sense for Western Australia farmers to record and map the different soil types within a field. And then, put more fertilizer investment only into the soils that will provide a good return on investment.
Lawes provides this example of how he’d break a field into three zones: Zone 3 produces acceptable yields and is responsive to favourable seasons. Zone 2 produces acceptable yields but is unresponsive to favourable seasons. Zone 1 produces poor yields and is unresponsive to favourable seasons.
CSIRO research has shown a link between Gamma K and EM38 measurements of the soil and the yield potential of that soil. (Les Henry has written about EM38 in past columns, and he touches on this issue in his article on the next page.)
In conclusion, “Soils with higher water holding capacity can produce more grain and should receive more nutrients. In contrast, soils with low water holding capacity rarely produce high quantities of grain. High levels of inputs are wasted on these soils and they therefore require lower levels of fertilizer.”
Jay Whetter is the editor of Grainews.