Using soil samples to determine nutrient levels can inform field input decisions

Generally, soils in Wyoming feature a lack of development, an expected find in dry environments. There is often poor drainage, but because low precipitation levels correlate with lower incidences of leaching, soils are often relatively fertile, with the exception of nitrogen.

“High calcium carbonate contents tend to flocculate soil particles and create good structure and drainage, but it tends to tie up a lot of phosphorous,” explains Jay Norton, University of Wyoming Extension soil fertility specialist.

Low levels of organic matter are also common in Wyoming soils, which is natural for the arid environment.

“We can’t say that’s a problem, but repeated irrigation and tillage tend to drive it even lower, and we lose the benefits of organic matter, which hold soil particles together, creating good soil structure, increased water holding capacity and nutrient cycling,” he describes.

One benefit of low organic matter in the soil is increased nutrient access for growing plants, although Norton notes that increased organic matter can improve overall soil health.

Using various soil tests, nutrient levels can be estimated in growing areas to help determine which fertilizers have the most beneficial impact.

Soil sampling

“If we have an area we want to sample, how many cores do we take to put in our bucket?” Norton asks.

One core sample usually won’t account for high levels of variability in a field, but taking hundreds of samples requires a lot of time and money.

“Fifteen to 20 soil samples from one area is where we really start to see the benefit, although it depends on the variability of the field,” he comments.

Those 15 to 20 samples are combined in a composite sample to send off to the lab.

A number of different methods can be used to decide where soil for each composite sample comes from.

“The traditional method is by soil map and topography. We use these soil survey maps to delineate and divide the area into fields that have been managed differently, had different crops, different soil nutrient management practices, old feedlots or other things we know will affect changes,” describes Norton.

For each segregated area, 15 to 20 samples are taken for the composite test.

This method may or may not be accurate, depending on the area, as it assumes that the chosen areas have more variability than the variability seen within each area alone. Long-term management or natural geological features may affect the site and, therefore, the results of the testing.

Grids and zones

“Grid sampling is the other extreme. We lay a grid out on the whole field, and those 15 to 20 samples come from either each intersection of the grid or from within each grid cell,” he explains.

This particular method can be very expensive, so many producers use zone sampling, which combines ideas from both a traditional method and the grid system.

“Zone sampling tries to take advantage of all the tools we have available now. First, we develop zones on real information that we have, then we look at the yields and soil fertility to adapt the zones,” he continues.

Once results come back from the lab, the goal is to adjust inputs to match the needs of the crops, so despite the method, producers should try to gather soil that provides a representative overview of the area.

Variability

Reviewing the difference between nutrients in the soil and those actually available to the plants also impacts how fertilizers are used in the field.

To illustrate an example, Norton explains, “Our soil tests often come out high or very high for potassium, but we’ll still use potassium starter fertilizer because of our geology. Early in the season when it’s cold, the plants are very small, and they haven’t exploited the root zone. Starter fertilizer near the seed can give them a little bump, and the plants will actually respond, even if there is adequate potassium in the soil.”

Norton also reminds producers that soil science can be complicated, and nutrient levels can change throughout the season.

“Nitrogen is very dynamic,” he mentions. “Plant-available nitrogen is like a wildcard.”

One method Norton suggests is applying half of a recommended fertilizer prescription pre-planting to evaluate how the plants respond as they start to come up. Once they begin to grow, the second half of the fertilizer can be adjusted accordingly before application.

Plant tissue tests, soil sampling, remote sensing and stock irrigation tests can all be used to help determine the nutrient levels in a field and how plants respond to those nutrients.

“It’s difficult to know exactly how we should sample. It mainly depends on how much money we have,” he says.

Jay Norton spoke in Worland on Feb. 19 at WESTI Ag Days.

Natasha Wheeler is editor of the Wyoming Livestock Roundup and can be contacted at natasha@wylr.net.