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Fertilizer rates and mineralization

Mineralization can be unpredictable and difficult to measure, but it should 
be taken into account when you’re calculating fertilizer rates

Mineralization is difficult to measure, but it accounts for a significant portion of available nitrogen and other nutrients. Taking into account both residual nitrogen and nitrogen mineralization helps calculate accurate fertilizer rates.

Mineralization 101

More than 90 per cent of the nitrogen and sulfur in the surface soil is in the organic form, usually in the form of humus, says Dr. Jeff Schoenau. Schoenau is a professor at the University of Saskatchewan and the strategic research chair with the Saskatchewan Agriculture.

“So in a typical prairie soil, you may have 6,000 pounds of total nitrogen per acre in that top six inches. And almost all that nitrogen is in the form of organic matter that has to be broken down or released, converted, into plant available inorganic forms. And that’s the process that we call mineralization.”

Schoenau says the top six inches of Prairie soils can release anywhere from 10 to 80 pounds of nitrogen per acre, depending on the circumstances.

Farmers looking at renting or purchasing new land will want to keep mineralization in mind, Schoenau says.

“I think a good measure of that land would be a measure of its organic matter content, because this will give you a very good idea of how much you might expect to become available through that mineralization process over the short term and the longer term as well.”

Mineralization varies between soil zones. For example, black soils, which contain more organic matter, have higher sulfur and nitrogen mineralization than the brown soils.

Grey soils have very low mineralization rates because the organic matter doesn’t decompose easily, says Schoenau. Sulfur is especially resistant to mineralization in grey soils.

Mineralization can also vary within fields. Erosion lowers the organic matter and reduces the soil’s ability to mineralize sulfur and nitrogen. Generally soil in low spots mineralizes more quickly than knolls. But if low spots are flooded, mineralization slows and nitrogen is lost.

During warm and moist falls, mineralization will continue until freeze-up.

“Which means if you’re taking soil samples very early in the fall, those available nutrient levels may have changed by the time you get around to the spring when your crop is seeded and growing and using it.”

Because mineralization involves bacteria, protozoa, fungi, and earthworms, environmental conditions play a big role. Flooded or cold soils shut down mineralization. Warm, moist conditions, and wet-dry cycles boost mineralization.

“Those are the kind of years where you get this continued mineralization of available nitrogen. And oftentimes what it shows up as is unexpectedly high protein in your cereal crops.”

Soil organisms also convert organic sulfur into plant-available sulfate in the mineralization process. Schoenau says soil scientists estimate about one to three per cent of the organic sulfur in the soil mineralizes into sulfate.

“So in a prairie soil, when you do the math on this, it works out to anywhere between maybe five to 15 pounds of sulfur per acre are made available to the crop through this mineralization process.”

Management and mineralization

Farming practices, such as tillage and crop rotations, also affect mineralization.

Researchers studying the effects of tillage and crop rotation in black soils around Indian Head, Saskatchewan, found that both practices affect mineralization. Fields with pea stubble had higher rates of nitrogen released through mineralization than wheat stubble. Land under no till for 25 years had more organic matter, and more nitrogen available for mineralization, compared to land under no till for only five years.

Another long-term study at Indian Head showed that rotations that included clover as a green manure had higher rates of nitrogen mineralization compared to fallow-wheat or continuous cropping rotations.

Fertilization influences mineralization, too. Agriculture and Agri-Food Canada researchers at Swift Current studied the long-term effects of adding nitrogen and phosphorus, versus the effects of adding phosphorus only, to a continuous wheat rotation. Adding nitrogen increased the organic matter, the mineralizable nitrogen and the available nitrogen.

Manure and mineralization

All manures are not created equal.

Adding organic amendments such as manure helps to build soils. But micro-organisms can’t release all the nitrogen in the manure the first year after application.

When liquid swine manure is applied to soil, only about 20 to 30 per cent of the organic nitrogen is mineralized the year it’s applied. Cattle manure’s mineralization rates are much more variable. Estimates for how much nitrogen will be released in the year of application range from 10 to 50 per cent.

The carbon to nitrogen ratio partially determines how much nitrogen will be released the year the manure is applied. Cattle manure that includes bedding will mineralize more slowly because micro-organisms need to chew through more carbon before hitting the nitrogen.

“Generally, for manures and composts, if the carbon to nitrogen ratio is greater than about 15 or 20 to one, you’re probably not going to get much available nitrogen released from that manure or compost in the year of application. Same with sulfur if the carbon to sulfur ratio is greater than 200 to one.”

It may not be immediate, but over the years, soil organisms will release the nitrogen from carbon-rich manure.

“And so that’s why I think when it comes to managing manure, in some cases to compensate for the low mineralization rate of some cattle manures — penning manures with a lot of straw or wood chips — we may need to be adding some supplemental fertilizer nitrogen in the first one or two years to compensate for that.”

Cattle fed distillers grains excrete manure with more nitrogen than cattle fed barley. Schoenau says the reason is distillers grains have more nutrients.

“So what goes into the animal — higher nutrient content — ends up coming out the other end as manure with higher nutrient content as well.”

Measuring mineralization

Predicting how much of a nutrient will be mineralized is tricky. No scientific models and indices used to predict mineralization are perfect.

Combining scientific models or indices with local knowledge about soils is probably the best way to measure mineralization, Schoenau says.

“Some agencies that make fertilizer recommendations do make a prediction of nitrogen mineralization based on an assessment of the organic matter content of the soil sample that you send in.”

Predictions can also be adjusted based on the crop grown in the previous year. For example, roots and nodules from last year’s peas can give a 20 to 40 pound nitrogen credit per acre, if the pea yield was high. Agriculture and Agri-Food Canada research set in the grey soil zone shows that as much nitrogen can be released the second year after a pea crop as the first year, Schoenau says.

Farmers with disappointing canola yields in 2012 may wonder how much leftover nitrogen, phosphorus and sulfur they’ll get back this summer.

“You can expect to get some of that nitrogen in the straw and especially in those canola seeds that are laying on the ground. You can expect to get some of that back. How much? Difficult to predict.”

Schoenau says seeds will release nitrogen first, but because straw has more carbon, farmers won’t get all the nitrogen back in 2013.

But farmers will get back much of the sulfur and phosphorus from the canola residue in 2013.

“And that’s because a portion — typically about half — of the sulfur and phosphorus in crop residues is in a water-soluble form, meaning that it really leaches out of that straw.”

“And in fact with potassium, you’re probably going to get it all out right away because potassium isn’t bound in the organic matter.”

Flooded fields will have lost nutrients to leaching, and mineralization will have slowed. Crop roots may be able to reach nutrients, depending how far down the nutrients have leached. Cereals that access leached nutrients might see a protein boost rather than any yield benefits.

Schoenau farms, and has had land that’s dried out in the spring and been kept weed-free. The previously flooded patch had high levels of available nitrogen by the fall, partly because of the humus.

If spring 2013 proves to be cold, mineralization rates will drop, and farmers will likely have to boost fertilizer rates a little to compensate, Schoenau says.

“It’s difficult to measure, not easy to predict, but I think (it’s) something you always want to be thinking about. And especially when you’re thinking about the quality of land, the ability of that land to naturally supply nutrients and how your management practices are liable to influence that over time.”

“And hopefully you’ll prove that contribution and build a more fertile soil.” †

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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