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Reading soil test results

Get the most out of your soil test results by looking at the details 
on the results page and following these six tips

Farmers primarily use soil tests to figure out how much fertilizer they need to apply each season. But more value can be gained from those test results.

The first thing most farmers look at is the balance of nutrients. The soil test outlines which nutrients are deficient in the soil and which ones are sufficient.

The macronutrients nitrogen (N), phosphorous (P), potassium (K) and sulphur (S) are the most common limiting nutrients in Western Canada. Calcium and magnesium are generally abundant and so not much of a concern in Western Canadian soils. Most soils contain sufficient micronutrients for crop production but there are important exceptions and in some cases a micronutrient deficiency can be just as devastating as a macronutrient deficiency.

As well as the initial assessment of nutrient deficiency or sufficiency, a number of factors such as soil characteristics (pH, texture, etc), crop type and management should be considered to determine the right source, rate, timing and placement for fertilizer application.

Soil pH

This is arguably one of the most important things on the soil test. A pH measurement above 7.0 indicates alkaline soil. A pH number below 7.0 indicates acidic soil. For most crops on the Prairies the ideal pH is somewhere in the 6.5 to 8.0 range. The closer soil is to the ideal pH, the higher the yield potential. Western Canadian soils tend to have a pH in the range of 6.5 to 7.5. Soils with a pH of 8 typically contain carbonates (lime) which can tie up nutrients like phosphorus and copper.

The soil pH needs to be considered when determining phosphorus fertilizer rates, says Kory Van Damme, an agronomist with Farmers Edge. “We use the pH to tell us more about how available the phosphate is in the soil,” he says. “On the lower six to seven pH your P is more available in the soil than in a seven to eight pH soil. It’s another characteristic of the soil that helps us to decide how much P we should be putting down. If it’s a high pH we might be inclined to put more phosphate on just because we know it’s not as available as in a soil that has a lower pH.”

Cation exchange capacity (CEC)

The CEC is the measure of the soil’s capability to hold on to positively charged nutrients (cations) and is generally a good indicator of overall soil productivity. The higher the CEC the more clay and organic matter that is present and the more water, fertilizer and chemicals the soil can hold. This information can be important in determining the nitrogen fertilizer source, rate, timing and placement, particularly in sandy soils that have low organic matter.

“Most of our N fertilizer and manure N goes into the soil as ammonium or a source that changes into ammonium quickly such as urea or anhydrous ammonia,” says Dr. Danial Heaney of Random Cross Consulting. “When you are applying high N rates in coarse textured and low organic matter soils there may not be enough cation exchange to capture the ammonium.”

Clay and organic matter in the soil create negatively charged sites that positively charged ammonium particles can adhere to. The CEC measures how many of those cations are present and that determines how much nitrogen can be held in the soil. If there aren’t enough negatively charged sites the ammonium has nothing to bind to and can be converted back to ammonia (the gas) and be lost to volatilization. “In the context of the Canadian Prairies it’s not usually a huge issue,” says Heaney, “but if you get into soils that have a pH around eight, a sandy texture and low organic matter and high N rates, you can run into problems.”

Electrical conductivity and base saturation

Electrical conductivity (EC) is a measure of soil salinity. When the EC is above two, yield potential starts to decline, particularly in drier soils. Soils with an EC above four are considered saline and yields of most crops will be reduced.

Base saturation is a measurement of four exchangeable cations in the soil: calcium, magnesium, potassium and sodium. High base saturation is associated with better fertility and therefore better productivity. The two main areas of concern with base saturation are the percentage of sodium and potassium held on the CEC.

Exchangeable sodium percentages (ESP) should generally be no more than one per cent, but will vary depending on the topography and fluctuations in the underlying water table. Once exchangeable sodium gets to around seven per cent it will begin to cause structural problems with the soil. Once the concentration gets to 15 per cent sodium, it is considered to be a sodic soil, which will cause significant problems. “Once we get into a sodic soil we end up with an issue because once that soil structure is changed it does take quite a bit to remove all the sodium and get it back to where it should be,” says Van Damme.

The base saturation level of potassium may give a better indication of whether K needs to be applied even if the ppm listed on the results page seem to indicate that there is sufficient K for crop needs. “If there is a low percentage of 0.5 or one per cent it means there may not be enough potash on the cation exchange sites to supply the crop during cold, spring conditions or resupply the crop during the growing season even though the soil test says we might have a sufficient K content in the soil,” says Van Damme. Base saturation of K at four to eight per cent is generally an ideal level for most soils.

Organic matter

Organic matter can give something of a picture of the history of the field and past management practices. Land that has been under zero or minimum tillage for a number of years will have a higher organic matter content than soils that are regularly tilled. Organic matter, along with the clay content of soils is important in determining its water and fertilizer holding capacity. It’s important to remember that the natural breakdown of organic matter (mineralization) is an important source of nutrients like N, P, and S.

The percentage of organic matter will vary according to the soil type and it’s important, says Heaney, for farmers to bear in mind what percentage of organic matter would be considered the normal range for the soil zone that their field is in. In southern Saskatchewan an organic matter content of two to three per cent might be considered good, but not so in the clay soils of Manitoba’s Red River Valley, where organic matter should be considerably higher.

Micronutrients

Depending on the crop plan, it might be helpful to consider micronutrients like boron in the case of canola and chloride and copper for cereals.

Heaney says farmers need to be cautious with micronutrient soil test results in that they are not highly diagnostic. “They are less reliable at predicting whether you are going to get a crop response,” he says. “If you are testing deficient in macronutrients like N and P and you add the nutrient, you have a high likelihood that you are going to get a response. If you don’t get a response it’s typically because something else has happened, like a lack of moisture.”

Using boron as an example, Heaney explains that the soil test may show low boron levels, but the crops growing in that soil may not necessarily respond to the addition of boron fertilizer.

“For some of the micronutrients the soil test is just part of the diagnostic package so you have to look at other things as well like past management, the crops you intend to plant, are you getting the yields you expect, do they seem to be limited, is the soil of a type where you would typically expect to see that micronutrient deficiency? To make a proper diagnostic on micronutrients there are other things that you may have to bring into it.”

Six tips for relevant results

Farmers who go to the time and expense of getting soil tests can use these tips to get the most value out of the process.

1. Make sure the lab you (or your agronomist) choose uses testing procedures that have been calibrated for local soils.

“Soil samples can end up in a lab two or three provinces away or in the U.S. It’s not where it’s done as much as whether the tests that the lab uses are appropriate for the soils that the sample came from,” says Heaney.

2. Soil tests need to be done regularly to track trends and help plan for better long-term productivity on the land. Test for the basics (N, P, K, S, pH and EC) once a year to monitor short-term changes and make adjustments for residual nutrients.

3. Add tests for micronutrients to the package every three to four years, or more frequently if low values show up in the tests.

4. Soil organic matter, cation exchange and some of the other properties like soil texture either don’t change or change very slowly. Once they have been measured two or three times to establish a baseline they don’t need to be tested again for five to 10 years.

5. Make sure to attach a geo-reference to the soil test sites, so they can be sure to sample exactly the same spots in the field from year to year. This way, results give a clear picture of how things are changing over time.

6. Soil tests often include a computer generated recommendation for nutrient rates to target specific crop yields. Consider that recommendation a starting point. You can really add value by getting some first hand, professional help in interpreting the data. A local agronomist can help interpret soil tests and draw up a long-term plan of action based on what’s going on in the field and they should definitely be a part of the follow-up.

“We sit down with the grower and break it down for them and decide what we need to do,” says Van Damme. “There’s some benefits just being able to be one on one with the grower and I think there is more trust involved because we will stand behind what we give them for a recommendation.” †

About the author

Contributor

Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at http://alovell.ca or follow her on Twitter @angelalovell10.

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