Understanding nitrogen

Nitrate Soil Test

Any soil test measuring nitrogen only gives you a "snapshot in time". Nitrogen levels are constantly changing due to the influence of many factors, including:

• soil temperatures (for microbial activity)
• soil moisture
• the amount and timing of nitrogen applied in the previous season (what's leftover after last season's crop less losses to leaching, atmosphere) and the current season
• cover crop (takes up N in fall, releases it in spring as it breaks down)
• previous crop or cover crop (legume or not)
• organic matter levels
• soil properties

OMAFRA's Agronomy Guide (Pub. 811) gives more information on nitrate soil tests and their limitations. Nitrogen recommendations based on the nitrate soil test have only been developed for corn in Ontario. We are not yet sure how to interpret nitrate levels in determining the fertilizer requirements of other crops. However, by looking at nitrate levels at different times of the season and by learning more about your field's nitrogen dynamics (see boxes below for ideas), you will become better at making nitrogen decisions for your crops. For the 12" sampling depth, you can convert the soil nitrate ppm to kilograms of nitrate-nitrogen per hectare by multiplying by 4. Don't start thinking you can subtract that number from your fertilizer applications though! The OMAFRA nitrogen recommendations take some of this soil nitrogen into account, as the research on which they are based was conducted on actual Ontario soils. Again, this test only shows what was there the day the sample was taken. If you want to learn more about nitrogen in your own fields, there is a simple way. Before applying your broadcast nitrogen, anchor a sheet of plastic or a tarp in your field. It must be large enough that tillage will not drag fertilizer across the whole area when the tarp is removed. Spread your fertilizer, stake the area, and remove the tarp. When side-dressing nitrogen, lift up and skip the staked area. Compare how your crop performs in the zero nitrogen area. Take 12" nitrate soil samples in the centre of the zero nitrogen area and in the adjacent fertilized area. Over time, you will learn how your soil releases nitrogen and can start to account for its contribution.

The Dynamics of Soil Nitrogen

In soil, most of the nitrogen (besides what you apply as fertilizer) is present in the organic matter. This soil nitrogen becomes available to plants through mineralization, the microbial breakdown of the organic materials to release the mineral forms of nitrogen. Initially, the nitrogen is released as ammonium, which is rapidly converted to nitrate by bacteria in the soil. This is called nitrification. Since soil micro-organisms are responsible for these processes, they occur once soils are warm enough for microbial activity. At the same time, nitrogen is being taken up again by soil micro-organisms (immobilization) and losses of nitrogen may be occurring though losses to the atmosphere (denitrification) or leaching. If a cover crop was present, it will also release nitrogen as it breaks down.

How Nitrogen Can Be Lost

The amount of nitrogen lost after heavy rains depends upon the amount that was in the nitrate form. Ammonium is held by the cation exchange complex and not lost. The conversion of ammonium to nitrate (a microbial process) is interrupted when the soil is saturated. Nitrate however can be lost through leaching and denitrification. Nitrate leaching is the downward movement of nitrate ions. Nitrate is a soluble anion that is not held or retained by the soil. As more rainwater enters the soil it displaces water already in the soil downward, carrying any associated ions like nitrate. So how far does the nitrate leach? Well, did the rain actually move through the soil, or run off? There may be minimal leaching if the rain came in intense blasts, with significant runoff (especially if the soil was already near saturation).Also, work from the Cornbelt suggests that on silt loam or silty clay loam soils, an inch of water entering the soil (remember, this is not an inch of rain since a portion of the rain will run off, especially when the soil is already wet) will only move nitrate down about 6 inches, due to the high water holding capacity of these soils. However, on a sandy soil an inch of water can move down a foot. Deeper-rooted crops like tomatoes and corn have the ability to recapture this nitrate if growing conditions are good. Of perhaps more concern is denitrification, when the field experiences extended wet soil conditions and slow drying. Denitrification refers to the microbial process where a group of soil bacteria (facultative anaerobes) use nitrate as an oxygen source when the soil is saturated and has low levels of oxygen. When oxygen is taken from the nitrate in soil, nitrogen gas or/and nitrous oxide is produced and released to the atmosphere. The rate of denitrification is influenced by temperature, soil pH, energy sources (carbon etc), etc. There has been some research done to quantify the amount of denitrification that can happen, however the work was done primarily again in the Cornbelt so it is a very rough guide. Keep in mind that this refers to clay loam and silt loam soils that are saturated.

If denitrification is a concern, consider carefully the crop condition, field topography, amount and type of fertilizer applied and the rainfall pattern before deciding to add more nitrogen. By its very nature, flooding or ponding is spatial in its variability. About ten years ago sampling was done in Ontario to track the nitrogen losses from heavy rainfall and water ponding in late June. The nitrate analysis suggested that there was very little impact or loss of nitrogen from the study areas. The ponded areas tended to have higher organic matter from years of erosion and were able to release more nitrogen. The key in that case was removing the excess water and getting oxygen back into the soil to reduce root damage. Often, nitrogen losses from heavy rainfall are not as high as you might expect. When making a decision on adding supplemental nitrogen, get all the information you can. Take the crop condition into account. Plants that have been sitting in saturated soils have probably lost some yield potential and will not use as much nitrogen as a healthier crop. Also consider the rainfall amount and intensity. Did the rainfall run off or soak in? When you suspect that you have lost nitrogen after application, if you do go back in with extra nitrogen, leave a few strips without it. What you learn from that will help you out the next time you have to make this decision.