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May 13, 2022

How Accurate Are Soil Tests for Sulphur?

Automated probe for soil samples

North American farmers increasingly rely on soil testing to determine nutrient levels in their fields. These tests deliver a relatively high degree of accuracy. Yet, when it comes to soil tests for plant-available sulphur, soil testing may not provide an entirely reliable measurement.

As we will explain, there are several reasons behind this. Due to the uncertainty, many agronomists will proactively advise customers to begin an elemental sulphur program to ensure S is available for current and future crops. This can provide a cost-effective insurance policy for crops with high sulphur demand (such as alfalfa, canola, and corn), or soils prone to S deficiency (eg. lighter, sandier soils or soil low in organic matter).

Soil testing for sulphate sulphur.

Soil test results for sulphate sulphur levels are typically included as part of the standard soil analysis package. Sulphate sulphur is the plant-available form of sulphur.

However, not all sulphur resources in the soil exist in a sulphate form at the time of testing. Elemental (or organic) sulphur can also be found in the soil. Over time, microbial activity will oxidize this sulphur, converting it into sulphate. So, while the test for sulphate-sulfur will account for available sulphate at the time of the test – it may miss available reserves that plants can draw on throughout the season.

Challenges measuring sulphur levels through soil testing.

Field Variability Makes Sulphur Testing Difficult.

Sulphur and sulphate levels can vary dramatically from one part of the field to the next. This is due to several factors, including organic matter, microbial populations, soil texture, environmental conditions at the time of testing (excessive moisture or drought), and topography.

So, a soil test may indicate sufficient sulphate sulphur levels in one part of a field, but this may not reflect the reality across the entire field. In terms of topography, sulphur deficiency tends to be a greater concern on hilly terrain or knolls.

To ensure a representative sample, between 20 to 30 samples per field (or soil management zone) are recommended.

Sulphate Sulphur Can Fluctuate in the Soil.

Another challenge is that sulphate sulfur is extremely mobile in the soil and is susceptible to leaching. As with nitrogen, this makes testing a challenge. For this reason, it is generally recommended that soil tests be taken at three depths: 0”-6”, 6-12”, and 12”-24”. However, if sulfate sulfur is below the root zone, it won’t be accessible to the plant in the early growth stages.

The results can also vary depending on the time of year you test. The presence of excessive moisture in the spring may result in low levels of sulphate. This can be caused by the downward movement of sulphate in the soil with water. Alternatively, sulphate could potentially rise with upward water movement, and as organic matter mineralizes during the season.

There Is No Obvious Correlation Between Sulphur and Yield.

While we know that plants require sulphur for healthy development and plant functions that ultimately impact yield – the correlation is not as apparent as it is with other key nutrients. In areas where sulphur deficiency is not an issue, sulphur fertilizer may not be widely (or ever) used. In places where sulphur deficiency is a known issue, it tends to be part of the nutrient management plan.

The research team of Sawyer and Barker conducted extensive testing in Iowa between 2000 and 2009, largely in corn and alfalfa. They found that extractable sulfate-S did not correlate to yield response against the check plots. Sulfate concentrations higher than 10 parts per million (ppm) were considered nonresponsive. So even though the research suggested a non-responsive sulphur “condition” there were still responsive sites. This reinforces the soil variability of sulphur, and the need to agronomically evaluate a crop’s need for sulphur on a situationally specific basis. It’s an example of the importance of making decisions based on trusted agronomic advice.

A Test Is Only as Good as the Interpretation.

As we have seen, with so many factors involved, soil testing is not an exact science. Interpretations are made based on the data received. There is a subjective element to it, so one interpretation may recommend adding sulphur, while another may not.

When should you apply sulphur as an insurance policy?

So, if sulphate sulfur testing is not entirely accurate, what should you do?

Not every field and not every crop requires sulphur. In many areas where there is known sulphur deficiency, farmers may regularly build up sulphur levels by adding a degradable elemental sulphur product to their fertilizer blend, supplemented by an immediate sulphur source. For example, it is common practice among potato growers in Idaho.

Ultimately, the best advice is to consult a reputable agronomist with local knowledge. They can advise on the best way to test for sulphur levels and can help you ‘interpret the interpretation.’

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