Gardeners claim that nitrogen levels in the soil increase after rain. Water floating down through the air picks up nitrogen and deposits it on and near plants. Lighting adds even more nitrogen to the process. They base their conclusion on the perceived greening of the garden after a rain.
In agriculture, it is well understood that heavy rains reduce plant available nitrogen and can lead to nitrogen deficiencies in crops. That is the opposite opinion of gardeners.
Who’s right – gardeners or farmers?
Key Takeaways
- Mineralization and leaching have a bigger effect than nitrogen from rain.
- A rain event reduces the net amount of nitrogen in the soil.
- Farmers are right, in this case.
Nitrogen Levels After a Rain
Four factors affect nitrogen levels in the soil during and after rain.
- Leaching effect on nitrogen in the soil.
- Addition from rain.
- Addition from lighting.
- Improved soil mineralization due to an increase in water.
Water runoff during rain can also be significant, but I’ll ignore that effect in this discussion because its effect depends very much on the local topography.
An estimate of these effects is as follows:
- Leaching: – 1.2 g/m2/rain event
- Rain: + 0.5 g/m2/y
- Lightning: + 0.2 g/m2/y
- Mineralization: + 1.5 g/m2/y
Leaching and mineralization have the greatest effect. The nitrate levels in rain and from lightning are insignificant compared to the other factors.
Compare that to the amount of nitrogen added by a single application of fertilizer to your lawn: 4.9 g/m2.
The supporting data for each of these factors is reviewed in the following sections.
Leaching Effect On Soil Nitrogen
The forms of nitrogen used by most plants, known as PAN, are nitrate, nitrite, and ammonium. These molecules are very soluble in water and the first two don’t stick well to soil or organic matter. When it rains, the water washes these nutrients to lower soil levels, reducing the amount available to plant roots.
A heavy rain can have a dramatic effect on crops.
Nitrate levels in agricultural soil range from 10 to 30 mg/kg or 2 g/m2 to 10 g/m2 in the top 15 cm of soil (calculated by ChatGPT).
Leaching on well-structured clay or loam soil with actively growing crops and careful fertilizing can be in the range of 10 to 30 % or even less under moderate rainfall. An average loss of 20 % with an average nitrate level of 6 g/m2 means that a single rain event would reduce nitrate levels by 1.2 g/m2.
Although rain significantly reduces the nitrogen level in soil, it is difficult to estimate the actual amount which depends on several parameters.
Addition of Nitrogen From Rain and Snow
The most common form of nitrogen in rain is nitrate which comes from two sources: lightning and pollution. lightning is discussed in the next section.
References in this post to rain, include snow, which is just frozen rain.
A study in Okaloma reports the total nitrogen from rain is 0.77 g/m2/year (6.9 lb/acre/year). If we subtract the effect of lightning, it is 0.57 g/m2/year (5.1 lb/acre/year). Another review for Benton County, Oregon, and Garrett County, Maryland suggests a nitrogen value from rain of 0.1 to 0.3 g/m2. The actual amount does depend on location and the amount of pollution in the air.
According to Fertilizer for Dummies, a single application of fertilizer for lawns should be 4.9 g/m2 (0.15 oz/sq yd), or 10 times the amount of nitrogen added by a full year’s worth of rain.
Addition of Nitrogen from Lightning
The common belief is that rain adds a lot of nitrogen to the soil during a thunderstorm. I have had a close look at this and concluded that although lightning does add nitrogen, the amount is quite small compared to the other factors. A rough estimate is 0.2 g/m2/yr.
Improved Mineralization Due to Water
Mineralization of nitrogen is the process of converting organic nitrogen found in organic matter into inorganic forms like ammonium and nitrate. The amount of nitrogen added by this process depends on the amount of organic matter in the soil, the temperature, and the amount of water in the soil. The number is quite variable.
As a rough estimate, with 1.6% organic matter and soil water at 50% of field capacity, the mineralized nitrogen released would be in the range of 1 to 3 g/m2/y. That is about 10 times the amount added by nitrogen in rain.
Rain would increase the amount of water in the soil. It is estimated that an increase in the field capacity from 50% to 80% would increase mineralization by 73%. This would result in a nitrogen increase of 1.5 g/m2/y, significantly more than the amount added by rain or lightning.
