Summary:
- Nitrogen supply from the soil to the corn crop may be exceptionally variable throughout Ontario.
- In 2024, early warmth in the late winter has led to warm, but wet soil conditions in most of the province.
- Net soil N supply is likely to be slightly low to around normal.
- N mineralization is likely in a normal range or slightly above average, but
- N losses through leaching and denitrification are likely higher than normal.
- Soil N supply is likely to be highly site-specific, as areas with less rainfall through the spring may have less N loss to account for, and higher net N supply.
- Some of the recently emerged crop is showing deficiency symptoms, but this may be a result of cool, wet conditions and should green up rapidly with warmer weather upcoming.
Background:
Throughout the growing season, microbes within the soil break down organic nitrogen contained in soil organic matter and crop residues 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, due to lack of a habitable environment for soil microbes. Soil compaction, which is common this spring, can also reduce mineralization rate.
Some corn may have been planted in less-than-ideal conditions this spring, and adverse weather following planting has exacerbated the stress that the crop is dealing with. Nutrient deficiencies are appearing in fields where we might not normally expect, as a result of reduced nutrient uptake caused by impeded root growth, as shown in Figure 1. In many cases, a return to warm, dry weather should rapidly improve these symptoms as roots expand and nutrients taken up by the plants.
Soil Nitrogen Supply in 2024:
A mild winter through 2023/24 gave way to a spring with warmer than normal temperatures. Although there has been some cooler weather as of late, this warm trend has continued through the growing season.
In addition to warm soils, precipitation has been above average throughout the growing season, and in some areas double the normal precipitation from April 1st, as shown in Figure 2. Fields have remained heavily saturated in some areas as crop planting has been delayed significantly.
It is likely that due to warm conditions and adequate moisture early in the season, nitrogen began mineralizing somewhat more rapidly than normal. This would normally result in increased soil N supply. However, due to above average precipitation received thus far, microbial mineralization rates may be decreased in soils that have remained wet below the top couple of inches. N availability from fall-applied manures and organic amendments is likely normal to slightly lower as well, as saturated soil conditions have slowed mineralization of the organic pool of N.
It is likely that N losses – including from fertilizer, manures, and soil nitrate – are higher than normal in most of the province, especially on heavy clay soils where denitrification loss is prevalent.
Reports of Pre-Sidedress Nitrate Test (PSNT) analyses from soil laboratories are generally showing average to below average results. However, there is quite a variance depending on fertilization practices, rainfall and field conditions.
Recommendations:
Optimal N rates for corn are likely to be around normal to slightly higher on average in 2024. This assumes that your planned N rate is based on current expected yields and economics.
It’s important to note that extra N fertilizer on late-planted corn will not make up for the loss of growing season. If expected yield has decreased from a month’s delay in planting, N rate adjustments should be considered to maintain optimal profitability of corn.
Consider the weather forecast for the next few weeks ahead when making decisions on whether to utilize a nitrogen stabilizer with your in-season N application. In general, urease inhibitors show value when applying nitrogen with little rain in the immediate forecast, or when N is surface applied or side-dressed without proper closure of the injection trench. Nitrification inhibitors provide greatest return when used in wet conditions on heavy clay soils with potential for ponding, or on light sands with leaching potential and heavy rains forecast.
Please note that 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.
Pre-Sidedress Nitrate Test Sampling:
PSNT samples are used to gauge how much N that the soil has supplied prior to sidedressing. To collect PSNT samples, collect several 30cm (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 500 g or 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.