How nutrient fixation affects (or doesn’t affect) soil
April 17, 2023 by ROBYN ROSTE
When it comes to soil management, whether you’re a grower applying your own manure or someone hired to spread manure on someone else’s land, there’s a lot to consider.
Finding and maintaining optimal biological, chemical and physical conditions while also minimizing environmental risks is a careful balance in the best of times. And with the ever-rising prices, not to mention market volatility, supply-chain breakdowns and extreme weather fluctuations, there’s a lot of pressure to manage production costs and timelines without impacting yields.
“With what the fertilizer prices are right now, soil and manure tests will help identify where it might be the best target for those acres, if you have flexibility for application,” says Daniel Kaiser, associate professor at the University of Minnesota.
Kaiser has written extensively on crops and nutrient management, and focuses on potassium (K) and phosphorus (P) fixation in the soil.
He says both potassium and phosphorus follow a “diminishing return” for each additional pound of fertilizer applied, so one of the best ways to manage costs is to perform regular soil and manure tests. That way, growers can focus on applying manure to the acres that are deficient, or where they’ll see the best return.
When deciding which manure to apply, the government of Saskatchewan points out that solid and semi-solid manures have higher organic content than liquid, and both solid and liquid manures provide a lower concentration of nutrients compared to commercial fertilizer.
“When using manure as a fertilizer, it is important to understand that only a portion of the manure nutrients are immediately available,” the government says on its website.
The agriculture department of the government of Manitoba released a fact sheet recommending the best way to estimate the nutrient content of manure is by testing before each application.
“This should be based on well-mixed, representative sample (which can be difficult). Sometimes more than one sample is required to estimate the nutrient concentration because the characteristics of the manure change,” it reads.
The fact sheet also suggested that while field test kits provide immediate estimates, laboratory analysis should also be consulted to indicate total nitrogen, ammonium N, total P, total K and dry matter content.
Soil testing is now widely accepted in agri-business, although Kaiser cautions growers to look at the big picture when analyzing soil and manure data to make manure application decisions, rather than at one individual test.
“Growers tend to switch sampling time and get different results, then panic seeing the soil tests change—particularly when it drops,” he says. “Try to sample at the same relative time during a calendar year.”
A term that often comes up with discussing nutrient management is “fixation.” In an article written for Minnesota Crop News entitled P and K “fixation” in the soil: What you need to know, Kaiser explained how fixation is often misunderstood to mean a nutrient is lost forever once it is fixed, rather than retained by the soil for a period of time before becoming available to the plant once again.
Fixation is normally applied to potassium and ammonium. Both nutrients can fit into pockets of clay and are then released as the soil shrinks and swells. However, phosphorus in the soil forms compounds rather than sitting in air pockets.
“P is not ‘fixed’ in the same sense as K and ammonium. Phosphorus forms compounds in the soil with calcium, iron and aluminum that can be very insoluble, making P in the soil less available for crop uptake,” Kaiser said.
In the article, he added that research from Iowa State University found instances where potassium fertilizer didn’t increase in the soil test, yet there was a yield increase.
“P is retained in a less available form maybe, but you’ll see cycling back and forth,” says Kaiser. “It’s not a one-way door; it’s a dynamic process.” He suggests using the term “retention” instead of “fixation,” for phosphorus, since it can be chemically reactivated in the soil and used by plants at a later date.
At present, the research is showing a natural process of fixation and release for nutrients retained in the soil. There is not a lot a grower can do to access retained nutrients, other than gaining a better understanding of the chemical reactions happening, and keeping track of the annual cycling process through regular soil samples.
“It comes down to soil testing; how things behave,” says Kaiser. “P tests are more stable over time while K has a seasonal variation. Monthly samples over the growing season can be vastly different fixation—less available forms that tend to impact the soil test itself.”
Kaiser’s research aims to develop better options to determine available phosphorus and potassium for crops, and a stronger understanding of the nutrients’ dynamics in the soil.
“I have been asked about whether biological products can ‘unlock’ fixed nutrients. The simple answer is that there is nothing out there that can be considered a silver bullet that would allow us to not apply fertilizer. Soils that are low in available nutrients will need fertilizer or manure application to produce a profitable corn crop,” he says.
While most growers don’t need to have deep knowledge of the chemical reactions nutrients have within the soil, it’s important to understand that it’s possible for soil to retain nutrients and then re-release them for plant uptake in future months or years.
That said, Kaiser still recommends using tests to determine how much P and K to add with manure.
“Research has not shown that accounting for some of the larger pools of nutrients in the soil is better than the routine soil tests suggested for use in determining nutrient availability for crops. Routine soil tests are still the best way to determine how much P and K are available to the crop at a given point in time,” he says. •