Agronomy research scientist Sheri Strydhorst has some tips for getting the biggest nitrogen bang from pulse crops
Having legumes in your rotation not only adds nitrogen, but also improves soil water availability, interrupts pest life cycles, and increases soil organic matter, phosphorus, potassium and sulphur.
But you’ve got to treat them right, Alberta Agriculture agronomy research scientist Sheri Strydhorst said at a recent event at the University of Alberta’s long-term research Breton Plots.
There are several things a producer can do to maximize the nitrogen potential of legumes, she said.
“When a plant fixes nitrogen, it’s a very energy-intensive process, similar to the (industrial) process of making nitrogen fertilizer,” said Strydhorst.
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About 1.5 kilograms of fossil fuels are burned to make one kilogram of nitrogen fertilizer. Plants also have to spend a huge amount of their energy to make their own nitrogen. Increasing legume growth increases legume nitrogen demands.
“The more vigorous your crop is, the more nitrogen it will need and the more it will fix,” said Strydhorst.
Crops without adequate amounts of phosphorus will not be able to fix as much nitrogen. To increase the percentage of nitrogen in the crop derived from fixation, the effectiveness of rhizobia bacteria needs to be maximized by inoculation.
“Inoculating your rhizobia is a very important thing. The rhizobia sold in these inoculants are selected because they are very efficient converters,” she said.
While native rhizobia bacteria can fix nitrogen, they don’t provide as much nitrogen for the energy consumed as the inoculants.
“I think it’s an excellent practice for producers to inoculate their legumes every year,” said Strydhorst.
Reducing soil nitrates also helps increase nitrogen fixation. Producers won’t get any nitrogen benefits if they seed grain legumes on a field that has been treated the previous fall with anhydrous ammonia. Putting grain legumes on manured soils isn’t a good idea either, and weed pressure will also lower nitrogen fixation. Seeding cereals into weedy pulse crops reduces the nitrogen benefits and yields the following years, while seeding into clean pulse stubble results in a more vigorous cereal crop.
But increasing the seeding density of pulse crops to double the recommended rate can result in an increase of 35 per cent of nitrogen fixation per acre, likely due to a more vigorous plant stand. “Grain legumes are excellent phosphorus scavengers, so their roots can be high in phosphorus,” said Strydhorst.
“When these roots break down, that phosphorus is made available to the subsequent crop.”
