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August 23, 2023

Tissue Testing for Sulfur: What You Need to Know.

Part 1 of 2

Ask just about any agronomist or crop advisor if the nutrient levels in your soil are sufficient, and most will come up with the same response:

“Don’t Guess. Test!”

When it comes to testing for sulfur (S), a tissue test will often provide a more complete picture of available sulfur reserves than a soil test.

While a soil test will give an accurate reflection of sulfate sulfur, it doesn’t capture convertible organic/elemental sulfur reserves in the soil. As a result, you may just get part of the picture.  

That’s why many experts recommend tissue testing for determining overall sulfur levels: particularly if the purpose is to build or evaluate a sulfur fertility plan.

In Part 1 of this series, we’ll explain what you need to know when testing for sulfur. In Part 2, we’ll share some broader advice around tissue testing best practices.

What does a tissue test reveal about S levels?

A tissue test will provide a percentage measurement of sulfur and other macronutrients like nitrogen (N), phosphorus (P) and potassium (K) in the plant. Micronutrients will typically be measured in parts per million.

According to Purdue University, the N:S ratio in grain proteins is 15:1… which can serve as a general guideline for sulfur requirements. That’s why the N:S ratio is 15:1 in many crops. This ratio can be lower in crops with high sulfur demand, such as canola (which can range from 10:1 to 7:1).

The laboratory results will reveal the percentage of sulfur in the sampled plant, as well as an explanation of what that number means, based on the crop. For example, Laboratories label the nutrient level as Very Low, Low, Sufficient, High or Very High.

A rating of very low or low will indicate S deficiency or borderline deficiency.

“Often including S in a fertilizer program to avoid S deficiency is more efficient and less costly than correcting an S deficiency once it occurs.”

Jim Camberato and Shaun Casteel, Department of Agronomy, Purdue University

Yellow leaves are your green light to test

If you see yellow leaves, you have a deficiency problem. But the question is… is the crop deficient of sulfur or is it a mild nitrogen deficiency?

Both types of deficiency exhibit similar symptoms in most crops. The primary difference is that with sulfur deficiency, generally only the top leaves will yellow. This is due to sulfur’s limited mobility within the plant. With N deficiency, bottom leaves will be affected as well.

Given what we learned about N:S ratios, the last thing you want to do is mistakenly increase nitrogen fertilizer if sulfur deficiency is the real issue. This will only worsen the problem. A tissue test will clearly identify which nutrient is deficient and by how much.

Which tissue test is recommended for S?

There are two common types of tissue testing: Total Nutrient Analysis and Petiole Nutrient Analysis. When testing specifically for sulfur levels and signs of deficiency, the Total Nutrient Analysis will be recommended in most situations. This test may involve the collection of the whole plant, petioles, or whole leaves and petioles (see below for collection by crop).

What plant parts to collect and when?

Every lab will have its own guidelines on which parts of the plant to collect at which growth stage and how many samples are required.

Below are the guidelines from AgVise Laboratories when collecting samples for a Total Nutrient Analysis:

  • Potato
    Submit entire leave (petiole and leaflets) from 20 plants
  • Canola
  • Seedling to rosette stage: Collect entire above ground plant. Sample 20 plants.
  • Rosette to pod development stage: Collect 5th leaf from top without petiole, from 30 different plants.
  • Corn
  • 0-12 inches tall: Cut stalk off about 1/2 inch above ground level. Submit 20-25 whole plants.
  • > 12 inches but prior to tasseling: Submit first fully developed leaf from top (first leaf below whorl). Cut leaf at its base where it joins sheath. Sample 20-25 plants.
  • Tasseling to pollination: Submit leaf below and opposite ear. Cut leaf at its base where it joins sheath. Sample 20-25 plants.
  • Sugarbeets
  • All growth stages: Submit 25 petioles for nitrate analysis. Submit 25 leaf blades of a fully expanded, recently mature leaf for complete nutrient analysis.
  • Alfalfa
  • All growth stages: Submit top 6 inches or top half of plant if less than 8 inches tall. Sample 20-25 plants.
  • Soybeans
  • 1st to 3rd trifoliate: Cut plant 1 inch above surface and submit entire plant. Sample 25 plants.
  • Early bloom to podset: Submit first fully developed trifoliate leaf from top. Sample 25 plants.
  • Sunflower
  • 4-8 leaves: Cut plant off 2 inches above ground. Sample 25 plants.
  • Bud to Bloom: 3rd mature leaf from top. Sample 20 plants.
  • Grains & Grass
  • Seedling to tillering: Cut plant off about 1/2 inch above ground. Sample 50 plants.
  • Boot to heading: Top leaf or flag leaf. Sample 50 plants.

When collecting tissues, you’ll want to get a representative sample from various sections of your field. We’ll cover some sampling strategies in Part 2.

Why are you testing? And why timing matters.

Generally, there are three reasons for tissue testing.

1. To test for deficiency and correct:

If you have a sulfur deficiency, it’s important to identify it early so you have time to address it with a rescue treatment of a sulfate fertilizer (for immediate plant availability). According to AgVise Laboratories, testing can detect “hidden hunger” – plants that suffer from nutrient deficiency without showing visible symptoms. This can result in yield losses between 10-15%.

When to test: Early Vegetative Stage

2. To evaluate a fertility plan

Was your fertility plan a success? Did it provide the required nutrients, or did it come up short? Or is there maybe another issue at play? Tissue testing can provide many of the answers.

When to test: Late Vegetative Stage

3. To trouble shoot problem areas and diagnose deficiency

When the plant is at the reproductive stage, you will have an accurate idea if the plant is getting the required sulfur throughout the growing season. This makes it an ideal time to test for deficiency. Grid samples can allow you to evaluate problem areas, which can guide your fertility strategy for future crops.

When to test: Reproductive Stage

Knowing is the Key to Growing

As you aim higher to achieve top yields, you don’t want to miss the mark due to a nutrient deficiency (an easily diagnosed and preventable problem.)

If sulfur is your focus, tissue testing allows you to capture the big picture to help you ensure your crops are getting ample amounts throughout the growing season.

In Part 2, we’ll cover some do’s and don’t’s and need-to-knows around tissue testing.

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