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Soil Test For Success

“Maybe your banker sets soil fertility rates for your farm.”


If 80 per cent of Manitoba farmers aren’t soil testing their fields, how do they know how much nutrients to apply at the start of every growing season?

That’s anybody’s guess, according to a MAFRI soil fertility specialist.

“The last survey done in 2002 showed that only one in five farmers was actually soil testing,” said John Heard.

“People may be getting their fertilizer recommendations as something that has been passed down from father to son, or maybe they read something somewhere and stuck with it. Maybe your banker sets soil fertility rates for your farm.”

However, the good news is that soil-testing companies have reported more farmers are poking holes in their fields to find out what’s really down there, a trend mainly driven by higher nitrogen prices, tighter manure management regulations, and greater adoption of precision ag equipment.

But interpreting the results that come back from commercial labs can often be confusing, said Heard, in a presentation at the Man-Dak Zero Tillage Association annual workshop in Brandon.

With that in mind, he advised farmers to look for two things in the soil analysis numbers.

First, examine the “need to know” numbers that will directly translate into fertilizer application rates, such as nitrate nitrogen, Olsen phosphorus, exchangeable potassium, sulphate sulphur, pH, and electrical conductivity.

Once that is understood, then make a quick note of the “nice to knows,” such as soil organic matter percentage, chloride, micronutrient levels, calcium, magnesium, and cation exchange capacity or CEC, which is just an indication of clay content in the mix that doesn’t need to be tested often unless the soil is very acidic.

Everything else falls into the “needless to know” category, which tends to clutter up soil test sheets, such as base saturation, Bray, Mehlich 3-P, numbers that may only be relevent to farmers in more acidic regions such as the corn belt.

The information from some soil labs will be in parts per million (PPM), which may be annoying for some farmers who haven’t learned a neat arithmetic trick for translating PPM into the more easily understandable pounds per acre.

“In the zero-to-six-inch depth of soil, there are roughly two million pounds of soil. So, if your results come in parts per million, you simply multiply the number by two to get the result in pounds per acre,” said Heard.

If salinity levels come back very different from government maps, don’t panic, Heard added, because it may just mean that the commercial lab is using a different technique to measure salt levels.

The most accurate picture of soil nitrogen comes from sticking the probe down the full recommended 24-inch depth, because 84 per cent of the nutrients used are taken from that layer. But even if a sample is taken at only the six-inch depth, a rough picture can still be determined by multiplying the number by 1.8, or 1.2 with a 12-inch probe.

The conversion works well as a guideline, he said, judging from surveys taken around the province, particularly in black soil areas.

“Of some concern, is how the phosphorus and potash values change with depth. For example, at Brandon, if you take a zero to 12-inch poke, rather than zero to six, your phosphate and potassium level is half when you include the subsoil.”

Organic matter and zinc levels tend to fall at a greater depth, while pH goes up, he said, so for the most accurate picture, a 24-inch sample is recommended.

Abnormal soil conditions may lead to “goofy” results. Too much moisture in the sample can cause the pH reading to be lower than it is in reality, which may have an impact on the available phosphorus and micronutrient levels in the test report.

Organic matter (OM) percentage is something that farmers may want to take with a grain of salt. Although one per cent OM equates to 1,000 pounds of nitrogen per acre, how much will actually be available to the crop is difficult to estimate.

The old rule of thumb that says 10 pounds per year for every one per cent OM, may only apply to balmier climes such as the corn belt – not in places where the soil is frozen for six months of the year.

“The Holy Grail is to figure out how to estimate how much nitrogen comes out of the organic matter,” said Heard.

“You tell me the weather, how much moisture and heat we’ll have in May, June and July, and then I’ll tell you how much nitrogen we might get out of the organic matter.”

Total phosphorus (P) in the soil is present in levels ranging from 400 to 2,000 pounds per acre, but only about four pounds or two ppm is available in a soluble form that can nourish growing plants at any given time.

Yield response to phosphorus fertilization can vary widely, but a good rule of thumb is that adding 20 to 40 pounds of P above crop removal rates will raise the soil test level by one ppm.

In soils with a medium phosphorus level of 12 to 18 ppm, a farmer may choose to take a “phosphorus holiday” and use just 10 pounds of starter P at seeding time, knowing that there is a good chance that it won’t affect yield.

Although it is less than the 23-pounds-per-acre crop removal rate for 40-bushel-per-acre wheat, the available phosphorus would be drawn down by just 13 pounds, or 0.5 to one ppm on the soil test analysis.

In rebuilding years, or if the price of phosphorus drops, he could put down 50 pounds of P with the crop to gradually restore what was taken out, said Heard.

For hay crops, about 10 pounds of P leaves with every tonne of forage taken off the field, he said. This means that if the “magic of nitrogen” is used to boost production threefold, the resulting higher hay yields can quickly throw the soil into a depleted state, unless phosphorus is put back onto the field either in the form of manure or chemical fertilizer.

daniel. winters

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