What is ENR?

ENR stands for Estimated Nitrogen Release. This is a calculated estimation of how much potential nitrogen may be released from soil organic matter (SOM) in one year. The actual amount and time of nitrogen release is dependent on the composition of the organic matter, soil moisture, and weather.

ENR is a calculation from the soil organic matter value on a soil test. Soil organic matter is reported as the percent of organic matter by weight. ENR is a calculation that is based on a couple of basic concepts regarding soil and the composition of soil organic matter. While these concepts are rooted in scientific research, they can vary.

The first concept explains that an acre of soil weighs approximately 2 million pounds. This is a rough estimation of the soil weight that is tilled/turned when an acre is plowed to the depth of a “standard” moldboard plow, which is assumed to be 6 2/3 inches. Using this standard value of 2,000,000 pounds per acre, we can approximate the amount of soil organic matter in an acre of soil, based on the percent of organic matter found through a soil test. For example, if a soil has an organic matter level of 3%, we can use the 2,000,000 pounds per acre value for the weight of the soil to calculate the total amount of soil organic matter per acre:

2,000,000 x (3/100) = 60,000 pounds of soil organic matter

The second concept states OM is approximately 5% nitrogen by weight. This may vary slightly based on several factors including soil type, management, and composition of the soil organic matter. From our previous example using soil with a 3% organic matter level:

2,000,000 x (3/100) = 60,000 pounds of soil organic matter (SOM)

60,000 pounds SOM x (5/100) =  3,000 pounds of N/acre

While this seems like an impressive value, unfortunately not all the nitrogen is available to the growing crop in a given year. The release of nitrogen from soil organic matter, a process referred to as mineralization, is a biological process that is facilitated by microorganisms within the soil. These microorganisms break down soil organic matter and, in the process, release nitrogen (along with other nutrients) into the soil solution where they can be utilized by the crop. However, the rate of mineralization is not particularly fast, and is governed by many factors. This makes it quite variable year to year. Therefore, it is assumed that only 2 to 4% of the nitrogen in OM will become available in any given year. From our previous example:

2,000,000 x (3/100) = 60,000 pounds of soil organic matter (SOM)

60,000 pounds SOM x (5/100) =  3,000 pounds of N/acre

3,000 pounds N/acre x (2/100) = 60 pounds available N

3,000 pounds N/acre x (4/100) = 120 pounds available N

60-120 pounds available N / acre

 Weather conditions that promote strong plant growth, such as warm temperatures and adequate soil moisture, are also beneficial in the conversion of SOM to plant available nitrogen. Therefore, in those cropping years where weather conditions favor strong yields, they also tend to favor higher mineralization rates. These factors cause greater releases of N from soil organic matter. This greater rate of N release can therefore serve as a kind of buffer, supplying more N to a crop that could essentially benefit from higher nitrogen rates.

Determining a nitrogen application rate that is economically and agronomically optimum can be challenging when the soil has a higher OM content. For example, the nitrogen release from organic matter in a field with 6% OM can range from 120 pounds/acre to 240 pounds/acre. The variation of the nitrogen released is often weather dependent during the growing season and causes challenge when determining nitrogen application rates.

Note - Organic soils, those with greater than 20% SOM, were able to develop over time due to reduced SOM decomposition. Saturated soil conditions for a portion of the year slows SOM decomposition in organic soils, thus reducing the mineralization of nitrogen. Those organic soils that have artificial drainage may experience a higher ENR than those that are not drained, but at a level less than determined by the ENR calculation.