What does 2023’s Weather Tell Us about Anticipated Soil Nitrogen Supply?

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Summary

• Soil nitrogen supply through mineralization and loss varies each spring through different weather patterns.
• In 2023, cool, dry spring weather has dominated the growing season so far.
• Soil N supply is likely to be lower than average.
  • This may require an increase in fertilizer N rate, though determination of optimal N rate requires consideration of realistic yield potential and the price of corn and fertilizer.

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Background

Throughout the growing season, microbes within the soil break down organic nitrogen contained in soil organic matter and release it as plant-available ammonium.  This process is known as nitrogen mineralization.

Nitrogen mineralization is favoured by warm soil temperatures with adequate soil moisture and aeration.  Excess moisture can slow mineralization or increase the potential for losses through denitrification (especially under warm conditions) on heavier-textured soils or leaching in lighter soils.  Droughty conditions may slow mineralization as well, due to lack of a habitable environment for soil microbes.

Soil nitrogen supply in 2023

The spring of 2023 has seen a few short warm stretches, but most of the growing season has been cooler than average.  An unusually dry winter was followed by a few significant rains in late April/early May, but has since remained much drier than normal throughout Ontario’s corn growing area. 

Most regions in Ontario remain well behind 10-year norms in terms of precipitation accumulation since April 1, and Corn Crop Heat Units (CHUs), which start measuring May 1.  See Table 1. below for detailed measurements.

Due to the cool, dry weather thus far in 2023, it is likely that nitrogen mineralization has been slower than average.  This can lead to a lower natural soil nitrogen supply for the corn crop. Fertilizer N applied to corn supplements the mineralized soil N supplied naturally. When nitrogen from the organic N pool in soil is reduced, usually a higher rate of commercial fertilizer N is required to achieve optimal yield potential.

Recommendations

From an agronomic sense, 2023 may not be a year to reduce applied N rates.  Optimal N rates are likely to be somewhat higher this year than on average.  This assumes you are close to the optimum rate based on expected yield and economics, especially with the cost of N fertilizers remaining high.

For those who apply their final nitrogen application later in the season, e.g., Y-drop applications at late vegetative or early reproductive stages, there will be additional time to evaluate corn yield potential and soil N supply and adjust final rates accordingly. While low soil moisture lessens soil N supply, dry conditions from mid-June to mid-July have been shown in Ontario by Dr. Bill Deen to lower corn N demand and optimal N rates.

This article is meant as a relative indication of soil nitrogen supply only, and should not be used as a sole source for adjusting N application rates on any given farm.  Soils vary drastically in their ability to provide N to the crop, and are highly influenced by the local environment and past agronomic practices.

With uncertain weather conditions ahead, identify whether a nitrogen stabilizer makes sense with your in-season N application.

Pre-Sidedress Nitrate Test (PSNT) sampling

PSNT samples are used to gauge how much N that the soil has supplied prior to sidedressing.  To collect PSNT samples, collect several 12″ soil samples across a field using a soil probe.  Sample parts of fields separately if there is reason to suspect differences in N content (past history, soil type, topography etc.).  Make a well-mixed representative sub sample of approximately 1 lb to fill a lab box or bag.  Samples should be chilled to prevent further mineralization and sent to a lab as soon as possible.

OMAFRA PSNT recommendations are only calibrated against natural background soil organic nitrogen mineralization and should not be used where nitrogen fertilizer has already been applied.  A modest amount of N applied with banded or in furrow starter (ie. 30 lb-N/ac) is OK provided sampling can be done mid-row to avoid these bands.  Due to differences in N mineralization timing, nitrogen recommendations in corn fields with manure, red clover or alfalfa in the previous year are more accurate when using nitrogen credits from Pub 811, Agronomy Guide for Field Crops.

		Rain (mm) April 1st	CHU  May 1st
Harrow	2023	93	700
	2022	189	816
	10 YR Norm (11-20)	212	796
Ridgetown	2023	140	647
	2022	170	740
	10 YR Norm (11-20)	187	731
London	2023	180	662
	2022	180	711
	10 YR Norm (11-20)	206	720
Brantford	2023	153	638
	2022	158	684
Welland	2023	144	615
	2022	184	735
	10 YR Norm (11-20)	213	713
Elora	2023	133	574
	2022	163	630
	10 YR Norm (11-20)	206	618
Mount Forest	2023	153	577
	2022	176	656
	10 YR Norm (11-20)	189	612
Peterborough	2023	147	552
	2022	186	659
	10 YR Norm (11-20)	197	632
Kemptville	2023	203	577
	2022	270	720
	10 YR Norm (11-20)	198	669
Earlton	2023	170	546
	2022	140	617
	10 YR Norm (11-20)	160	498
Sudbury	2023	215	537
	2022	183	587
	10 YR Norm (11-20)	183	539
Thunder Bay	2023	130	479
	2022	302	414
	10 YR Norm (11-20)	162	412
Fort Frances	2023	113	686
	2022	298	471
	10 YR Norm (11-20)	154	511