“In essence, soil tests work only when soils are severely phosphorus deficient.”
– RIGAS KARAMANOS, VITERRA
Bill Toews saw the price of phosphate fertilizer in the fall of 2007 and decided he’d wait until spring before purchasing what he usually applies to his land near Kane.
However, by spring 2008 prices had soared from $600 to $1,100 per tonne and they were continuing to rise – topping out at around $1,500 in summer before they started to slide.
“I said it was too expensive, so I went cold turkey,” said Toews, a former university instructor who holds a master’s degree in soil science.
Whereas crop advisers base their recommendations on achieving the maximum yields for the farmer, Toews said he has a finite amount to spend on inputs; he looks at a host of input decisions ranging from seed to fungicides to equipment and focuses on where he will get the most bang for his buck.
“I don’t look for maximum yields on my farm, I look for maximum profitability,” said Toews, the sole farmer on a panel assembled to discuss phosphorus fertilizer management in times of high prices and volatile markets.
No yield penalty
Toews does not believe the decision to shun phosphorus for one year cost him much by way of yield, and any yield penalty he did suffer was significantly offset by the money he didn’t spend on fertilizer. That amounts to working capital reserves, which helps stabilize his farm operation.
However, he was careful to point out he has religiously applied phosphorus to his land, a heavy clay soil in Manitoba’s Red River Valley, over the 30 years he has farmed. He plans to go back to that program of replacing the phosphorus removed by the crop he is growing. Because if he doesn’t, he’s mining the soil.
Tom Jensen, an agronomist with the Saskatoon-based International Plant Nutrition Institute, said that with fertilizer prices as high as they are, crop advisers can expect to run into some tough customers this year who will be asking difficult-to-answer questions. Instead of asking how much they should apply, many will ask: how much can they cut back on their fertilizer expenditures without decreasing yield?
And many will place phosphorus on their hit list.
Cash flow issue
For farmers facing cash flow or credit difficulties due to the decline in commodity markets, cutting back will be a necessity, not a choice. As well, only about 20 per cent of the usual volume was applied in fall 2008, which means supplies next spring could be limited by pipeline shortages and the availability of application equipment such as anhydrous tanks.
Jensen said the right answer to the cutback question will differ with each farmer who comes through the door, and potentially, with each field and each crop on his or her farm.
“My biggest concern is we’ll have growers cut back on all their fields; we have some growers with fields they can cut back on and some they shouldn’t,” Jensen said, noting that with the exception of heavily manured soils, most fields in Manitoba are deficient in phosphorus.
Farmers are more apt to maintain their nitrogen applications and trim their phosphorus, potash, sulphur and micronutrient expenditures – which is a valid strategy if there has been a consistent program of nutrient management in the past and soil tests show adequate to excess availability. But if farmers haven’t been replacing those nutrients and soil tests show their soil is deficient, it is a decision that proves more costly than it is worth.
He advises farmers to build their soil phosphorus levels over time to a level that is near sufficient, but not excessive. Then they can switch to a maintenance program based on crop removal.
Jensen also cautioned against some of the money-saving strategies that come to the forefront in times like these. For example, he was approached by a farmer who wanted to switch from using 11-52-0 phosphate fertilizer at $1,150 per tonne to using rock phosphate, which he could obtain for $500 per tonne.
It sounded good in theory. But although rock phosphate does contain phosphorus, it is a form that is less soluble and available to crops than the processed fertilizers. So even though it was applied in the same quantity, the crop was only getting one-third the rate, Jensen said.
That said, rock phosphate can work quite well on soils that are very acidic, such as a pH level of less than five. “If he’d been in the Red River Valley with a pH
of eight, I would have said ‘No way,’” Jensen said.
Another characteristic of phosphorus that often places it on farmers’ lists to cut back is that it’s slow on the uptake. Farmers can apply nitrogen and see a direct yield result, whereas phosphorus uptake is much slower, he noted. In the year of application, phosphorus uptake is often 25 per cent or less. But over a number of years and several crops in a rotation, its uptake efficiency rises to 90 per cent or better.
“Phosphorus is an excellent investment because it is going to maintain long-term benefits,” Jensen said.
That may be so, but it’s hard to demonstrate that to farmers using soil tests, noted Rigas Karamanos, agronomy manager for Viterra. Phosphorus fertilizer recommendations are based on soil tests. But the last time soil tests were calibrated on the Prairies was in the 1970s, when there was more summerfallow and less diversified cropping rotations.
Farming practices and soil conditions have changed since then, Karamanos said. He recently reviewed the results of 223 trials and concluded farmers had a 50-50 chance of seeing a yield response to a fertilizer application in any given year.
“In essence, soil tests work only when soils are severely phosphorus deficient,” he said. However, that is a rarity in today’s context. As well, scientists are noting how phosphorus behaves in the soil is changing.
“Now, systems are more diversified rotations,” Karamanos said. “I think the mycorrhizae in the field are playing an important role they didn’t play before.”
Mycorrhizae are tiny soil micro-organisms that form symbiotic networks with plant roots, helping plants absorb soil phosphorus in exchange for the sugars they need to survive.
Cindy Grant, a soil scientist with Agriculture and Agri-Food Canada in Brandon, raised a similar concern in her presentation on nitrogen fertilizer. Grant said there is evidence to suggest there are higher amounts of nitrogen made available to crops through mineralization than in previous times under different cropping systems. However, that contribution potential is not captured by soil tests. [email protected]