Agronomist says alfalfa used in rotations can cut nitrogen fertilizer requirements by half the following year
Alfalfa and timothy are ideal forages to put into an irrigated crop rotation, even with their high nutrient and water demands.
Ross McKenzie, former Alberta Agriculture crop researcher and current consultant, said both crops need 100 to 150 millimetres of water for every dry ton they produce.
That’s a high water need, but their benefits are also high, McKenzie told a Jan. 19 Irrigated Crop Production Update.
“We’ve been talking about the benefits of forages in rotation for soil health and soil quality for 40 years, long before it was ever sexy, but they do really provide tremendous improvement to soil quality and go a long way to improve soil organic matter and organic carbon levels,” he said.
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“They help to improve the physical structure of the soils, which then could lead to improved water infiltration, which is very important when we’re irrigating with pivots.”
Other benefits include higher organic matter content in the soil, which leads to better fertility and nutrient cycling. Then there’s the nitrogen benefit that alfalfa has in the field when it is replaced by another crop.
“Typically after we take out alfalfa, we probably reduce nitrogen fertilizer requirements by about 50 percent and about a 30 percent reduction in nitrogen fertilizer requirement two years out, after alfalfa,” McKenzie said.
“If we grow something like hard red spring wheat, we can often see up to a two percent increase in protein … when we grow wheat after alfalfa.”
A six-ton, three-cut alfalfa program will need 300 to 350 pounds per acre of nitrogen to meet its potential.
“That sounds bad, but the good news is alfalfa can pretty much fix most of that nitrogen,” McKenzie said.
It also needs 60 to 70 lb. of phosphate, upward of 250 lb. of potash and 30 lb. of sulfur, as well as micronutrients.
A good crop of timothy, yielding five tons and two cuts, will also need more than 200 lb. of nitrogen, 50 to 60 lb. of phosphorus, 200 lb. of potash, 25 lb. of sulfur and micronutrient amounts similar to alfalfa.
McKenzie said soil testing is critical before establishing a forage crop. It can provide a basis for fertilizing the crop for the next four or five years without the need to sample again.
He is a proponent of sampling at three depths: zero to six inches, six to 12 inches and 12 to 24 inches. Those samples should be sent to a lab that uses the modified Kelowna method of analysis, rather than the Bray or Olsen methods.
“Tests that use the Bray are absolutely stupid. Don’t forget that I said that,” he said.
McKenzie said most forage crops in Alberta’s irrigated region are low in plant available phosphorus.
He recommended applying 150 lb. per acre if a pre-planting soil test showed less than 80 to 100 lb. of actual phosphorus.
Alfalfa and timothy have high potassium requirements, so they should be started in fields with more than 200 lb. per acre of the nutrient as indicated by a soil test.
Irrigated forage crops rarely need additional sulfur, which they gain through irrigation. Every 12 inches of irrigation water adds about 30 lb. of sulfur, in sulfate form, to the crop.
