Growers can reduce fertilizer costs by using a forage rotation or adding manure to the field, says soil scientist
Part three of the Fertilizer Return on Investment series
EDMONTON — Producers looking for top yields might find that previous good crops and traditional fertilizer strategies have plundered their soil bank accounts.
“It’s amounts in, amounts out, and that is very important to look at over the long haul,” said University of Saskatchewan soil scientist Jeff Schoenau.
Big crops are one cause of nutrient depletion in the soil, but low commodity prices and high fertilizer costs can also play a role in reducing nutrient availability.
Rigas Karamanos, a fertilizer researcher and agronomist with Koch Industries said producers must remember that those large crops have removed a lot of nutrients from the soil and it took time to get some of them there in the first place.
“You take a 60, 65 bushel canola crop, and this isn’t unheard of any more, and you took 60 pounds of phosphorus from the field. Did you replace that? How do you replace that? Producers need to consider these issues.”
He said rotation choices can also cause serious problems.
“Canola, canola, canola or canola, canola, wheat, canola. These can pull tremendous amounts of nutrients like phosphorus from the soil,” Karamanos said.
“Rotations can be critical to making your soil deliver for your crops.”
Jeff Schoenau said more farmers should consider their crop rotations as a way to deliver nutrients.
“There is a lot of merit in looking at your rotation to help reduce your fertilizer costs,” he said.
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“Even, for example, forages in a rotation, even in the short term, a couple of years of alfalfa or clover.”
Recent work that Schoenau was involved in at the Western Beef Development Centre has found that an alfalfa crop, especially where moisture wasn’t limiting, such as Melfort, Sask., provided a nitrogen fertilizer value to subsequent crops of 200 to 300 lb. per acre.
“Having that legume in the rotation, even though that forage takes more phosphorus out of the soil, in fact the available phosphorus levels were maintained and the soil was able to supply phosphorus to the following crops, even without any phosphorus fertilizer in a reasonable proportion,” said Schoenau.
He said the alfalfa crop was likely recycling phosphorus from deeper in the soil’s profile, where annual crops can’t normally reach.
In 2008, North American farmers were confronted with phosphorus prices that exceeded $1,000 per tonne. It caused many growers who had regularly applied it at 30 to 35 lb. per acre to stop applications. Many pulled rates back to 10 lb. as a starter kick for the crop and then reduced levels for a year or two until prices returned to sustainable levels.
Karamanos said most crops usually receive only half the nitrogen in a given year, “maybe see between 15, 35, 40 percent of their phosphorus, maybe half of their sulfur fertilizer. It depends on the crop and the year’s conditions. But when you fail to apply it, you develop other issues over time.”
Schoenau said he also reduced his phosphorus applications when the price spiked.
“A few years (later), you have to put that back,” he said.
“Lots paid the price for that later on. You don’t want to have that crash a few years down the road. (Set) up a plan that takes into account big crops, years when you didn’t put in all that you should.”
Fields with a history of reliable nutrient application actually have a greater capacity to pull nutrients from the soil through the breakdown of soil organic matter and crop residue.
Keeping crop residue on fields and tillage to a minimum can deliver more than just improved water holding capacity.
Some producers can derive good value from straw, but it can be a balancing act.
Tom Jensen from the International Plant Nutrition Institute in Saskatoon said livestock producers can replace nutrients and other soil improving features that are lost to straw removal by spreading manure on those fields.
“That is recycling the material,” he said. “In those areas where we are removing the straw and not putting back those nutrients, we can short the soil, especially in potassium.”
Schoenau said the seed portion of the crop takes with it high percentages of nitrogen and phosphorus. The crop’s residue contains higher levels of potassium and about half the sulfur.
