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Seven tips to getting more from soil tests

Farmers can get more value out of soil test reports by digging deeper, 
according to an agronomic soils specialist

Look for some hidden values out of that soil report. Don’t just look at the obvious,” said Mandy Huska, an agronomic sales specialist at Taurus Technology. Huska spoke to farmers at Cavalier Agrow’s plot tour in July.

Here are seven tips for sampling soil and interpreting test results.

1. Look for patterns

Weeds can signal specific problems, or high production areas, in fields.

“So, for example, something like a sow thistle could be where you have excess magnesium in your field,” said Huska. Lamb’s quarters thrives in high organic soils and is a sign of high fertility.

Huska said there is also a correlation between nutrient deficiencies and disease, as well as insects and fertility problems.

“So a sucking insect like a leafhopper actually is attracted to areas of the field, or plants, that produce a sugar called asparagine. And actually that happens when you’re low on potassium,” she said.

Huska also suggests looking for high production areas using yield maps, and soil testing those areas.

Huska said she wasn’t necessarily suggesting farmers invest in variable rate technology. But farmers could sample from high production areas, based on yield maps, and also test areas with production problems.

2. Soil sampling depth

Huska said the zero to 12 inch samples dilute some numbers. For soil under zero or minimum till, Huska recommended sampling at zero to six inches.

“If you can take two, that would be a zero to six (and a) six to 12 to look for your mobiles.”

Mobile nutrients such as sulphur, nitrogen and boron show up at six inches and lower. Sodium is another issue to watch for at depth.

“If you’ve got areas where the water table’s been really elevated, your sodium levels at depth could be higher and you could run into some problems later on,” said Huska.

PH levels and nutrients can also vary with depth. For example, the pH levels may be fine in the zero to six inch range, but either drop off or climb at depth.

Huska said some consulting firms are starting to sample at zero to three and three to six. She said if farmers have been using zero till for years and not working in trash, many of the nutrients will be in the first three inches.

“If you want to start getting a little more in tune with where nutrients are located, how much stratification is happening, maybe we have to look at flipping the soil or rotating it once every five or 10 years.”

3. Understand pH

Les Henry’s Handbook of Soil and Water states that, “soil pH is to a soil scientist as blood pressure is to a medical doctor.” A pH score lower than seven means soil is acidic, or sour, while numbers higher than seven mean the soil is alkaline, or basic. A pH level of seven is neutral. Chemical intervention isn’t usually tried for soils with a pH between six and eight, Henry writes.

Soils with very low pH levels may have aluminum tying up phosphate, Huska said. Henry notes that soil phosphorus availability drops sharply at pH levels below six. Acid soils also affect soil organisms.

Calcium binds phosphate in high pH soils, Huska said. Henry writes that alkaline soils also reduce micronutrient availability, including zinc, manganese, copper and iron.

Huska went through soil tests from the Cavalier area, in northwest Saskatchewan. Some fields had pH levels as low as 4.5. “You guys can grow blueberries there. It’s really low,” she said.

Other fields in the area had pH levels of 7.5, which Huska said was “pretty decent.”

“Just having this huge variance here makes me say you need to soil test,” she said.

4. Understand CEC

Cation exchange capacity (CEC) tells you whether you’ve got sand or clay. Sand and clay react differently to moisture and applied nutrients, Huska said.

“If you’ve got a sand, you want to control some of the nutrients that we have. You want to be able to hold on to nitrogen through the season,” she said. She suggested farmers with sandy loams consider products controlling nitrogen loss.

“There’s nothing really there to bind those nutrients,” she said.

Higher CEC scores mean heavier clays. Many Saskatchewan soils have CEC in the 25 plus range, Huska said.

5. Know your organic matter

Huska said organic matter is the lifeblood of the soil. “It has the ability to hold or release nutrients all through the growing season.”

For every percentage of organic matter, a field could release between five and 13 lbs. of actual nitrogen in a year, Huska said.

“If you’re over-applying because you don’t know what your organic matter is, not only have you spent more money on nitrogen than you needed to, maybe you’ve got lodging issues. Maybe it’s not going to mature quick enough. Maybe you’ve got an imbalance of something in your field as well,” she said.

Huska points out that if soils have a huge amount of available nitrogen, farmers may want to spend part of their fertilizer budget on other needed nutrients, such as potassium.

Organic matter can vary greatly within a field. For example, one quarter in the Cavalier area had organic matter ranging from 1.2 per cent to 4.9 per cent

6. Look at micronutrients

Huska said 80 per cent of the tissues tests she’s seen this year are low on boron. “I think for the most part you have to be at a certain level, fertility wise, to get a response (to boron).”

Huska cautions against applying micronutrients without soil and tissue testing, as they could cause unintended reactions.

Henry writes that some crops are more prone to certain micronutrient deficiencies. For example, alfalfa is sensitive to boron deficiencies. Wheat is sensitive to copper deficiencies, oats to manganese and beans to zinc. Soil types also affect micronutrient deficiencies.

7. Look at macronutrients

Calcium promotes root growth and gives roots the ability to push into the lower levels of soils. Huska noted some fields in the Cavalier area had very low calcium levels.

“I’m not an advocate of liming because it’s very hard to find in this part of the world. But there are certain areas of our province that we need to start looking at adding calcium,” she said.

Plants need the right balance of nitrogen and sulphur for optimum protein. Severely sulphur deficient canola causes cupping leaves and purpling, especially on the leaf’s underside, Henry writes.

“The only time I trust a sulphur recommendation… is when it’s low,” said Huska. “You can pick up sulphates of any kind that can throw your numbers off so always apply sulphur, especially to canola, regardless of what your soil test says.”

Henry writes that Saskat-chewan’s grey wooded soils are well known for their sulphur deficiency. But sulfur deficiencies in other soils are much more common than they used to be, so canola producers should always apply sulfur.

Potassium helps with photosynthesis, nutrient management and disease resistance. It also helps prevent lodging, especially in high organic matter soils.

“If we were going through some sort of a dry spell, your plants would be able to take in water much better if they have potassium available to them,” said Huska.

Farmers might assume potassium is abundant in Saskatchewan soils, but Huska said it’s not always available when the plant needs it. Much might be available early in the growing season, but as plant growth ramps up, the plant might not have enough potassium within the root zone. Magnesium can also influence potassium uptake,.

Henry writes that though fertilizer potassium isn’t needed on many Prairie farms, there are deficient areas. Soil tests are a reliable indicator of potassium deficiency, and soils below 121 lb. per acre in the top six inches require potassium fertilizer, he writes. Soil with 121 to 180 lbs. per acre will still probably require a low rate of seed-placed potassium. Sandy soils need less potassium than clays, as potassium isn’t tied up as much in sandy soils, said Huska.

“If you guys soil test and find that you’ve got some low K levels — which I know you do, I’ve seen your soil tests — here’s an opportunity for you to go and apply some on. Spread it on. It’s money in the bank.” †

About the author

Field Editor

Lisa Guenther

Lisa Guenther is field editor for Grainews based at Livelong, Sask. You can follow her on Twitter @LtoG.

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