Variable-rate technology is based on a simple premise.
“Basically, we’re aiming for the right rate at the right place in the field,” Ross McKenzie, a soil scientist with Alberta Agriculture, told attendees at the Agronomy Update event in Red Deer.
That starts with knowing your soil, but that’s a lot harder than most people think, he said.
“In my mind, we still have a fair ways to go to really be able to learn how to accurately prepare prescription fertilizer maps that will be economical for fertilizer on the farm,” he said.
Variable-rate technology, or VRT, starts with the producer identifying unique areas of a field, taking soil samples, and then trying to determine the ideal fertilizer rates.
But determining the different soil areas within the field, and deciding which soil factors to look at, is challenging. So is deciding how to manage fertilizers on these different soil areas, which is complicated because weather and growing conditions play such a huge role in yields.
Producers don’t have a lot of control over soil-related factors, but they can apply proper fertilizers to adjust soil nutrient levels.
“How do these areas vary across your field? Can you check for soil nutrient levels?” McKenzie asked. “This is the first thing I want to look for. The second thing is soil moisture content. How does soil moisture vary? Moisture is probably the biggest, most important factor in achieving optimum yield.”
Other factors include salinity, soil organic matter, texture, chemical exchange capacity, pH, surface water drainage, and past history of erosion on a field.
Risks versus benefits
Producers should also consider how much soil variability is needed to make VRT pay.
“What are the risks versus the benefits?” asked McKenzie. “The benefits are obvious. If you can put on different rates of fertilizer and cut back on your fertilizer costs to increase your yields, then you’ve got it. But VRT can be expensive. It can be anywhere from $3 an acre to $10 an acre.”
The range of variability across a field is also a factor as changes in topography can affect optimum fertilizer rates.
“The most important thing is to identify soil management zones,” McKenzie said. “One simple place to start is looking at topography and how soils change with topography.”
Producers should try to remember yield patterns they have seen over the years. Soil and salinity maps are useful in this process, as are crop yield maps and satellite imagery.
However, McKenzie said he prefers to examine the soil itself using ground truthing. Different parts of a slope have different soil properties. Upper slopes are characterized by thinner topsoil, and less soil organic matter which results in more soil crusting and reduced crop emergence.
“It has less water-holding capacity, so after a significant rain, you might get a bit more run-off and less stored water,” said McKenzie. “Organic matter is a storage of nutrients, so if we have lower organic matter, we will have lower cycling of nutrients released for plant growth.”
Upper slopes have higher soil pH levels.
Lower slopes show almost the opposite characteristics of higher ones, with deeper topsoil, higher soil organic matter, better water infiltration, and a higher water table. Soil pH levels are much lower, which could result in salinity problems. All of these factors have a deep impact on yield potential.
McKenzie’s advice to any producer considering VRT is not to do the entire farm right off the bat. Producers should use VRT on the fields that are the most varied, particularly in terms of topography, using yield and topography maps to determine slope positions. Then consider how nutrients such as nitrogen and phosphorus vary with slope positions, and how the moisture and salinity varies in different soil conditions.
“Start simple,” said McKenzie. “If you’ve got 5,000 acres, don’t do all 5,000 acres. Start with a couple or three quarter sections. Start off slow and get a feel for it and develop a knowledge of what you’re actually doing.”