Soybean School: Unused soil nitrate could affect soybean nodulation in ’22

Drought in 2021 has caused some crops to not use all of the available nutrients in soil, and soil tests suggest that some fields have elevated soil nitrate levels.

High soil nitrate levels can pose a problem for next year’s soybean crop, as they can prevent nodulation from happening which could prove a problem later in the season. Plus, it could result in efficient use residual nutrients.

John Heard, soil fertility specialist with Manitoba Agriculture and Resource Development, joins us for this Soybean School episode to explain the risks of surplus nitrate to a soybean crop, and what can be done about it.

Heard says that first of all, there are two time frames to think about when it comes to high nitrate levels: when soybeans were just being introduced on the Prairies and the present day.

“In those [early] years, we saw nodulation failures that seemed to be connected to high nitrate levels in the soil,” notes Heard, adding that early on, Manitoba Ag developed some general rules to direct farmers away from soybean crops dependent on applied inoculant.

Since that time, and about 15 years later, that same ground will have seen five or more soybean crops, and the risk of poor nodulation is lower because of background levels of rhizobium bacteria in the soil.

The real risk of high nitrates posed to a soybean crop actually comes later in the season. Some soil test levels suggest crops will have access to 110, 140, and up to 200 pounds of residual N per acre without factoring in mineralization during the season. If the crop uses the available soil nitrate early on instead of initiating nodulation and creating that symbiotic relationship, the crop could run out of N supply later in the season when it needs N the most — during pod-fill.

Heard says that nodule numbers are certainly thrown for a loop when there are excess nitrates, but provided there are 10 nodules per plant, nodulation is still considered successful.

Iron deficiency chlorosis is also more likely to happen when high levels of nitrates are present, warns Heard, and if a rescue N application is made too early, IDC will set in.

“It’s a risk factor the same way as calcium carbonate or free-lime in the soil, and salinity — high nitrate levels are also a risk factor, and for that we would suggest just with variety selection, make sure you choose those varieties with the best tolerance,” says Heard.

Heard’s advice for high nitrate fields? Find another field to seed to soybeans. Current fertilizer prices are high, and any field that has high nutrient carryover can equal a lot of fertilizer dollars.

“I’m hoping that within their rotation, growers can find fields or crops that will exploit that. Wheat, canola, corn, crops that will exploit that so you can reduce some of the fertilizer bill for those crops, and I’m hoping that through soil testing, there are some fields that will be appropriate for their soybeans,” says Heard.

Heard adds that herbicide carryover is a major concern for 2022, after the drought year and will also need to be factored into rotational planning.

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