Is the Nitrogen from Fall 2021 Still There? – A Spring Soil Sampling Update

Pitura Seed Farm gathers annual soil tests every fall from all fields in order to make fall fertility plans. In the fall of 2021, like many other farms in the prairies, our results came back with very high residual nitrate-N levels in the top 0-18”. Across all fields, we saw ranges of 20 lbs - 64 lbs in the 0-6” samples and 34lbs – 168 lbs in the 0-18” samples.  

On May 12th, I resampled 3 fields – 2 wheat stubble and 1 canola stubble and found that the results had changed since fall 2021 sampling. Here are a few notes:

1.       A redistribution of nitrate-N in the soil profile. In both wheat stubble fields, total nitrate-N levels in the 0-18” sample stayed fairly consistent from fall to spring. However, we saw that most of the N has moved below 6” – meaning that nitrate leaching within the active soil profile is likely the main mechanism we were seeing this year – unusual for heavy clay soils – generally leaching is seen more in coarse textured soils.

2.       A decrease in nitrate-N on the canola stubble field. After harvest in 2021, the canola regrew after receiving a few significant fall rains. The regrowth reached cabbage stage before it was desiccated. The regrowth used roughly 50 lbs of nitrogen from the soil that we were hoping would have been available for the 2022 wheat crop.

Looking forward:

With more moisture, plants won’t root down to depths we saw last year. However, with a starter fertilizer to get them started, they should easily access the nitrogen that has moved lower in the profile. We are preparing to top-dress with UAN if necessary – for example, on the canola stubble field that will be going into wheat.

Additionally, the lower nitrate levels in the top soil may allow peas to establish nodules before the roots reach past the 6” depth. Once the roots access that stash of available N, it is possible that the nodules will shut down as pea nodules become lazy and will take up available N rather than producing their own. 5 lbs of N in the top soil is quite low to get peas started but the pea fields on Pitura Seed Farms will be receiving roughly 48 lbs of MAP as a starter which should supply them with enough nitrogen to get a strong start.

Finally, conditions for denitrification are still present as we receive more rain and the soils warm up to air temperatures. As a reminder – denitrification occurs with saturated soil and is one of the most common avenues of loss for Nitrate N. Nitrate – N is converted into a gaseous form that can be lost to the atmosphere. Since crops are getting a later start, N will remain in the soil through vegetative stages of crops into July when soils are warmer leaving it susceptible to loss. We are accounting for roughly 30-40% losses of N from denitrification and volatilization.