Planning for Soybeans and Pulses with Soils Testing High in Nitrates

November 15, 2021
  • Agvise Soil Nitrate-N Summaries
  • Delayed and Reduced Nitrogen Fixation
  • More Soybean History, More Resilient?
  • Inoculate Well
  • Plan for IDC
  • Dry Beans: N Fertilization
  • Consider Retesting Soils in the Spring

Drought conditions across Manitoba have resulted in high residual nitrate levels due to less crop uptake and removal from lower yields, early termination of some crops and fewer losses to leaching and denitrification. For high N-use crops this can be an asset in a time of high fertilizer pricing, but for N-fixing legumes this is an added challenge.

How will N-fixing pulses and soybeans respond to soils testing high in nitrates? Below is a summary of what we know so far, based on field observations. Note that the response of legume crops to high nitrate levels has not been well researched.

Agvise Soil Nitrate-N Summaries

Agvise has released early summaries of soil nitrate-N levels in the top two feet as of September 17, 2021 (Table 1).  This average soil test data is not a replacement for actual soil test results from your fields, but rather provides a snapshot for soils across Manitoba. Note that there is considerable variability within each region, and within each zone in individual fields. Soil testing your fields is the best way to know what you may be dealing with. On average, soils in Manitoba are testing above 70 lbs nitrate-N per ac in the top 0-24 inches following wheat and canola, with 17-44% of fields in each region testing above 100 lbs N/ac. This has implications for soybean, pea and faba bean production that rely on biological nitrogen fixation.

Table 1. Residual nitrate trends (average nitrate in each area as well as percentage of samples tested in different ranges) as of September 17, 2021 from soil samples submitted to Agvise following wheat, grain corn, silage corn and canola. Regions with less than 100 soil samples were not included. Source: Agvise

Table 2. Percent of Agvise soil samples testing above 61 lbs N/ac in the top two feet as of Sept 17, 2021.

Delayed and Reduced Nitrogen Fixation

Nitrogen fixation will be delayed in soils testing high in nitrates since plants will become ‘lazy’ and not spend the energy to form nodules in an environment with plenty of readily-available nitrate. Typically, soybeans and pulses require between 3.0 – 4.5 lbs N/bu or cwt for plant uptake throughout the growing season (Table 3). Soybeans fix about 60% of their N requirement, peas 50%, faba beans 85% and dry beans less than 45%. The actual amount of nitrogen fixed by legume crops will depend on inoculation strategies, nodulation, environmental conditions and soil fertility.

The optimum soil test range for soybeans and peas is less than 50 lbs N/ac. They can be grown on fields with higher N levels, but high nitrate levels will generally reduce or delay nodulation, delay maturity, contribute to iron deficiency chlorosis in soybeans and lodging in peas. At excessively high residual N levels (>100 lbs N/ac), it will be more economically worthwhile to pivot those acres to a crop that will better utilize that nitrogen.

Soybeans require some soil nitrate to establish themselves. Approximately 50 lbs N/ac is taken up during vegetative development in the spring prior to N fixation kicking in, which then produces the last 150-200 lbs N/ac needed by the crop. The problem arises when soil nitrate is high enough to inhibit nodulation but is insufficient to meet the whole yield potential demand of the crop.

Table 3. Pulse and soybean nitrogen uptake, removal and approximate amount fixed by nitrogen fixation. Sources: 1Heard 2006, 2Heard and Brolley 2006, 3MB Soil Fertility Guide 2007, 4Klippenstein and Schoenau 2017, 5Schoenau 2017

More Soybean History, More Resilient

As soybeans have been grown more frequently in Manitoba and background soil levels of rhizobia have built up over time, ‘experienced’ soybean fields have become more resilient to high soil nitrate levels. This is likely due to both a great quantity of rhizobia in the soil and that rhizobia existing on soil organic matter, rather than being entirely dependent on the comparatively short-lived and environmentally fragile inoculant-form rhizobia to form nodules.

While nodule numbers were reduced in fields with high nitrate levels, average nodulation was still above the threshold minimum number of 10 nodules per plant if the field had a history of soybeans (Figure 1). Under high nitrate conditions, virgin fields were more likely to drop below the minimum number and experience a nodulation failure. However, at individual sites in the 100 lbs N/ac treatment, nodule numbers fell below 10 at 4/5 (80%) first-time soybean fields and 4/8 (50%) fields with soybean history. Despite lower nodule numbers, yields did not decrease, perhaps due to compensation from the soil nitrate. (Note that these were non-replicated hand samplings).

If your high nitrate field has a history of well nodulated soybean crops, soybeans may be a viable option on that field.

Figure 1. Effect of nitrogen on nodule number at five first-time soybean fields and eight fields with soybean history. Source: Heard, Lee and Tone 2013

Inoculate Well

While N fixation may be delayed or reduced, it is important that the rhizobia present when the crop does try to nodulate are aggressive strains capable of effectively fixing N. Studies in peas grown in dry soils have shown that granular inoculant is the most resilient, then peat, then liquid under dry conditions. Pea rhizobia (Rhizobium leguminosarum) are native to prairie soils, but the strains contained in inoculants are often better at facilitating N-fixation. Soybean rhizobia (Bradyrhizobium japonicum) overwinter in our soils, but there are questions around whether they may lose their efficiency at N fixation over time as they intermingle and reproduce with native populations. Double inoculation is another option to make sure the rhizobia are there when you need them and to provide added resiliency under challenging conditions.

Crown nodules (those located on the main taproot near the seed) are better at fixing nitrogen than lateral nodules (those located on lateral roots). On-seed inoculant is a cheap option to try to maximize the amount of crown nodules that develop in high N soils. Rhizobia infect root hairs where inoculant is placed to form nodules. The closer they are to the roots to begin with, the earlier nodulation can begin.

Plan for IDC

High nitrate levels are one of the risk factors besides salinity and carbonates that can lead to iron deficiency chlorosis (IDC). Choose varieties with good IDC tolerance.

Varietal reactions to IDC are tested in Manitoba each year and are available in the Soybean Variety Guide. Iron chelate products applied in-furrow can reduce IDC; however, susceptible soybean varieties may still see yield loss.


Dry Beans: N Fertilization

Dry beans are fertilized like a non-legume crop since they are poor N-fixers. Inoculants for dry beans are currently being researched. To date, they have not been widely available in Canada, nor effective.

Recent research from the pulse and soybean agronomy lab has found that over five site-years, applied nitrogen rates of 35, 70 and 105 lbs N/ac produced similar yields. The lowest applied N rate to reach a maximum yield was 35 lbs N/ac. Combined with soil residual N at these sites, available N to the crop ranged from 60-90 lbs N/ac. Considering high fertilizer prices, applying N at a rate to reach 70 lbs N/ac (soil residual plus applied) is likely to be sufficient to achieve maximum yield in the coming year.

Figure 2. Dry bean yield (% of non-fertilized control) response to N rate (lbs/ac) at five site-years in Manitoba from 2017-2019. Bars followed by different letters are statistically different at p <0.05. Source: MacMillan 2020

Consider Retesting Soils in the Spring

If we have surplus fall and spring moisture, leaching and denitrification processes will start back up, and we may see nitrate levels decline. Testing soils again in spring will give a more accurate reading of what is in the soil to make more informed, economical management decisions.