Soil sampling in the big data era

Satellite images. Yield maps. Drones. Gamma rays. Electrical conductivity. RTK elevation. Farmers and agronomists have more data available to them than ever before. Has soil testing kept up? After all, the beginnings of soil testing started on the Prairies in the late 1950s and early 1960s.

“The soil tests and protocols we developed in the 1960s are still being used now and are still valid. The one exception is for nitrogen; we don’t have a good test to predict the amount of mineralization that will occur,” says Geza Racz, soil science professor emeritus at the University of Manitoba. “We need to couple the nitrate soil test with a way to predict mineralization to more consistently account for soil organic matter, tillage, crop rotations, fertilizer management and manure applications.” (See the sidebar below this article for more on the nitrogen test.)

All three Prairie Provinces established publicly funded soil test labs in the early 1960s. In Manitoba, for example, Robert Soper was hired in 1958 by soil science department head Bob Hedlin to develop soil fertility tests for Manitoba soils. Racz was hired in 1961. The three soil scientists along with colleagues and graduate students went on to develop sampling protocols, test extraction methods and calibrate lab results to fertilizer response in the field.

In the late 1980s and early 1990s, the provincial soil test labs were either sold to private soil testing companies or closed, but the standard tests they developed are still widely used.

In Manitoba, the sodium bicarbonate (Olsen) test was developed for analyzing phosphorus soil fertility, and was well calibrated by Soper and colleagues on the higher pH soils of Manitoba. At the University of Alberta, Jim Robertson found the Miller Axley extraction method worked better on the low pH soils around Edmonton. It was used until the 1990s when Alberta Agriculture soil scientist Ross McKenzie verified that the Modified Kelowna method of extraction provided more dependable results on a wide range of soil pH levels across Alberta. Saskatchewan also used a Modified Kelowna test.

Today, Rigas Karamanos, senior agronomist with Koch Fertilizer Canada at Calgary, Alberta, who managed the Saskatchewan Soil Test Lab and subsequently EnviroTest Laboratories Saskatoon from 1989 through 1997, cautions that some soil test labs are using extraction methods different than those developed and calibrated on the Prairies.

“Some labs are now using extraction methods that aren’t calibrated on the Prairies and have no value to farmers,” says Karamanos. (See table below.)

Zoning in on fertility

When soil test protocols were first developed for Western Canada, entire fields were sampled to develop fertilizer recommendations across the field. For uniform fields, this sampling protocol worked well but could be time consuming. For example, Racz says that for nitrogen, a composite test of 30 samples across a field to a four-foot depth provided the most reliable results. But researchers realized that sampling to four feet was impractical, and Alberta, Manitoba and Saskatchewan settled on sampling to depths of 0-6”, 6-12” and 12-24”. Eventually, soil test protocols evolved to 0-6” and 6-24” and most composite samples consist of 20 core samples per field.

Then precision farming came along. With development of GPS guidance and the technology to vary rates across different management zones, the ability to develop different fertility recommendations for different parts of the field became reality.

“In a sense, you are still doing a composite sample but just in more management zones to try to understand differences in soil characteristics and fertility,” says McKenzie.

Getting to those different management zones is a hotly debated process among precision farming companies. In its simplest form, McKenzie says zone maps can be developed by looking at changes in elevation, since topography is a major influencing factor in how soils are formed. Upper slopes are typically lower in organic matter and nutrients. Mid-slopes are generally average in soil nutrients, texture and organic matter. Lower slopes are usually highly fertile and productive. Elevation maps can be collected with GPS and converted into topography maps with a free software program called LandMapR. These zones can then be ground-truthed with soil sampling. This approach is supported by research by Raj Koshla at Colorado State University that found permanent soil characteristics combined with the farmer’s experience resulted in the best management zones.

“Developing zones and prescriptions is about dealing with probabilities. You can’t control the weather, so the purpose is to increase the probability of growing the largest and most profitable crop.”

—Remi Schmaltz

Other approaches use more data acquired from various sources. Decisive Farming uses multiple years of satellite imagery to directly connect actual field performance to the development of management zones. After zones are established, typically five to six zones in a field, the zones are benchmark soil sampled at 0-6” and 6-24” with analysis of over 20 different soil characteristics. From there, variable-rate fertility and seeding plans are developed.

“Developing zones and prescriptions is about dealing with probabilities. You can’t control the weather, so the purpose is to increase the probability of growing the largest and most profitable crop,” says Remi Schmaltz with Decisive Farming.

Cory Willness, president of CropPro Consulting at Naicam, Saskatchewan, uses a zone mapping system called SWAT (Soil, Water and Topography) that is developed by layering in RTK elevation, topography features, soil organic carbon, water flow paths and electrical conductivity maps.

Manitoba-based Farmers Edge uses satellite imagery to identify production zones in a field and confirms zone accuracy by ground-truthing and zone-based soil sampling. Variable-rate prescriptions are developed, and profit maps analyze return on investment.

A new company on the scene, Soil Optix in Ontario, uses a scanner to read gamma rays emitted by the soil. A sensor bar mounted 24” above the soil is driven around the field every 40 feet to measure Caesium-137, Uranium-238,
Thorium-232 and Potassium-40. Layered maps are developed that include soil texture, macronutrients and topography. Premium maps can include micronutrients, water availability and hydraulic conductivity. A 160-acre field will have up to 32 calibration soil tests done to ground-truth the gamma ray layered maps. Variable-rate maps can be developed for seed, fertilizer and other inputs.

“Soil Optix doesn’t develop management zones. We have 335+ data points per acre, or one every 10 square feet, and develop variable-rate inputs based on these high-resolution topsoil maps,” says Mitch Blyth with Crop Care Consulting in Manitoba. Soil Optix sells the sensors and runs the correlations to create the background soil maps. Crop Care is the provider in Western Canada, which does the fieldwork and provides the prescription maps.
These approaches and more used by other companies come with a cost of managing data and developing prescriptions. Costs generally range from $5 to $15 per acre.

Converting knowledge to profit

“The zones yielded differently, but the slope of the N response curve was about the same in each zone. The economics were about the same for the variable rate N and the blanket rate. There is a lot more that we need to know about fertilizer response on different soils and slope positions in the same field. Further, crop response differs in wetter versus drier years.”

—Ross McKenzie

Yes, we can collect a lot more data about our fields and create productivity zones within fields. But is the payback there? Alberta Agriculture ran a variable-rate fertilizer application research study for four years. McKenzie says that delineating management zones by topography was relatively simple to do and was better than using satellite imagery. The research found that N response was relatively good in each soil management zone.

“The zones yielded differently, but the slope of the N response curve was about the same in each zone. The economics were about the same for the variable rate N and the blanket rate,” says McKenzie. “There is a lot more that we need to know about fertilizer response on different soils and slope positions in the same field. Further, crop response differs in wetter versus drier years.”

Schmaltz says weather is the number one factor determining production. Precision farming is about managing that risk, he says, and that farmers who apply blanket rates across a field have a higher financial and production risk than farms using variable rate.

“Precision farming isn’t perfect, but it works to help manage the variability in a field and evaluate where the agronomic program can improve,” says Schmaltz.

Schmaltz also says that precision farming goes beyond simply yield and net return per acre. Other factors such as more uniform ripening, easier combining and logistics come into play. He recently had one farmer who expanded from 10,000 acres to 15,000 acres and was contemplating purchasing an additional tractor, seeder and air cart for $1.5 million. Decisive Farming was able to look at fertilizer applications and logistics and increased the number of acres per fill by 32 per cent – eliminating the need for an additional seeding unit.

Today, Schmaltz estimates that about 15 per cent of farms use precision farming and variable-rate application. For the other 85 per cent, perhaps the first step is to actually soil test. McKenzie says that as a general rule across Alberta, only about 10 per cent of dryland farmers regularly soil test. The estimate is 30 per cent or more for irrigated land. While times are changing, using your father’s and grandfather’s soil test is still the foundation of fertility programs.