CALGARY — A good understanding of a farm’s soil is the foundation to accurate variable rate fertilizing, and soil sampling is key.
But how much sampling is required to obtain reliable results that can be used for a precision agriculture management zone?
A lot, says Raj Khosla, a Colorado State professor and president of the International Society of Precision Agriculture.
Khosla, keynote speaker at the Feb. 22-23 Precision Ag 2.0 conference, related results of a colleague’s field research showing soil variability exists in every square foot of a field.
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In his own research, he has found variability while sampling a quarter section in grids of 75 metres by 75 metres, taking four soil cores for every 7.5 acres.
Some soil scientists recommend sampling in seven metre grid squares, which would mean 10,000 soil samples in a 136 acre field.
Obviously, such sampling is labour and cost prohibitive, said Khosla.
“The take-home for you is, I know a lot of you are doing grid soil sampling. I’m not a great proponent of that. Grid soil sampling is great when you do it at the right scale. If you’re not doing it at the right scale … it has no meaning.”
Khosla said research has found that electrical conductivity (EC) soil mapping is as good as grid-based soil sampling about 87 percent of the time, so that can be an option to extensive core sampling.
Ross McKenzie, a research scientist with Alberta Agriculture, told the conference that grid soil sampling is useful, and there is little benefit in being overly precise.
He said it isn’t reasonable to farm every square foot of the field differently, regardless of what variable rate fertilizing field maps may suggest.
If producers can accurately identify three or four management zones within a field, “that’s probably a step ahead of what we have been doing.”
McKenzie advised taking 15 to 20 core samples from each zone at six-inch depths up to 24 inches deep. Use proper equipment and handle the samples properly.
Send the samples to a reputable soil testing lab, preferably in Western Canada so the extraction methods used are calibrated to western Canadian soil types.
Analysis of samples from six and 12-inch depths should include nitrogen, phosphorus, potassium, sulfur, pH, electrical conductivity and micronutrient content. The deeper cores should be analyzed for nitrogen, sulfur, pH and EC.
Khosla said in an interview that the limits of grid soil sampling must be understood by those planning to use VRF technology.
“People are aware of the variability in the field. We think it’s a continuum, but it changes dramatically,” Khosla said.
“I was showing you the limitations associated with large scale grid soil sampling. There’s nothing wrong with grid soil sampling if it is done at the right scale. I’m not quite sure if most people realize that it does take that kind of soil sampling to capture accurately what’s out there.”
McKenzie also spoke about the value of small plot research as opposed to field-scale trials that were praised at various conference sessions.
“If you can’t get response from fertilizer on small plots, carefully measured, then how can you do it with big equipment? If you think so, you’re kidding yourself.”
