It’s well known that soil salts contribute to hard pan problems, but the question of whether commercial fertilizers are to blame has circulated for years.
It’s a question University of Manitoba soil scientist Don Flaten has dealt with many times.
He said there are two concerns when it comes to salt in the soil: excess salinity and excess sodium, which creates sodic soils.
“Sodium is the real problem when it comes to soil structure. You get those large massive solid aggregates, soil crusting and the hardpan layer,” said Flaten.
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“Those are all symptoms of excess sodium, but it doesn’t come from synthetic fertilizer. I don’t know of any synthetic fertilizers that contain sodium.”
He said livestock manure contains sodium so when manure is applied to fields, farmers could experience sodium buildup and soil structure problems.
Flaten added that most synthetic fertilizers are actually salts, or they convert into salts in the soil. These water-soluble nutrients are designed to quickly dissolve so they can feed plants. They don’t hang around and cause problems.
Flaten described the three main synthetic fertilizers used on the Prairies and explained how their salts function.
Potassium chloride
Potassium chloride, or potash, has two nutrients. Both the potassium and chloride are beneficial to crop production.
“This is a clear example of a simple salt. When they separate, the potassium and chloride ions act like salts. They dissolve into the moist soil and are taken up by the plant roots.
“The question is whether synthetic fertilizers leave behind undesirable ions. Most prairie soils already have low concentrations of chloride, which is taken up by plants. Additional chloride from potash fertilizer is also taken up by plants and is beneficial to crop nutrition.”
The potassium is not a risky salt. It’s basically the same as the potassium salt many people buy for the water softeners in their homes.
Monomonium phosphate
Commonly known as 11-52, monomonium phosphate consists of two water-soluble salts: ammonium ions and phosphate ions.
“When these salts dissolve in the soil, the ammonium becomes the nitrogen source for the plants and the dissolved phosphate becomes readily available for the plants.
“Both dissolve into inorganic forms that can be quickly taken up by the plant when they’re needed. And both, of course, are highly beneficial to the crop.”
Urea
Urea is the most popular source of nitrogen in Western Canada. It’s technically not a salt.
From a chemical point of view, it’s a soluble organic molecule that converts to ammonia and carbon dioxide when it dissolves in the soil. The CO2 goes off into the atmosphere and the ammonia converts over to ammonium, which is the nitrogen that’s used by the plants.
“We want all these synthetic fertilizers to dissolve and release nutrients in the form of inorganic ions that the plants will use.
“I don’t see that synthetic fertilizers contribute to excess salts if they’re applied according to crop requirements.”
When it comes to salt loading, Flaten said that the main concern should be the livestock manure applied to fields.
Flaten said he is currently helping to revise the Manitoba Manure Management Training Manual. The new version will deal with sodium-rich, sodicity, and saline hazards. It is expected to be available later in 2011.
He said the best source of information right now on sodium in manure is the Alberta manure management manual.
“As for a product that will break up large aggregates and hardpan, I’m not aware of anything that can cure those soil structure problems.”
