101 Questions answered
Nitrogen from Organic Matter: How Much Is Released and When
The amount of nitrogen (N) released from soil organic matter (SOM) depends on:
- the amount of organic matter in the soil
- how long the soil has been cropped
- whether the soil is cultivated or no-till
- what type of previous crop residues e.g. legume, oilseed, cereal, droughted/failed crop, green/brown manured crop, pasture)
- whether, where and when fertiliser has been applied
- soil moisture, and
- soil temperature.
Even during dry weather, SOM is continually decomposed by soil microbes, releasing N that will be available to plants when roots are active. In drier environments, the rough estimate is that the rate of turnover is about 2% per year, but it will vary considerably with seasonal and management conditions.
How much is released?
A general rule of thumb is that for every tonne of carbon (C) in SOM, about 100 kilograms (kg) of N, 15 kg of phosphorus (P) and 15 kg of sulphur (S) become available as the SOM is broken down. We can do a rough calculation with some assumptions. Let’s take a soil with a bulk density of 1.5 g/cm in the top 10 cm and a soil organic carbon content of 1.3%. This equates to 19.5 tC/ha to 10 cm depth and 1.95 tN/ha. If 2% of this organic N turns over, then 39 kg of N will be released, along with 5.85 kg of P and S.
Note that not all of these nutrients released from SOM breakdown are available to plants. Microbes take their share (sometimes around half), and some is lost from the system. There is also often mineralisation when the crops aren’t able to use the nutrients, such as in summer and early autumn after rain.
How fast does it become available?
This is hard to put a timeline to, other than to say that mineralisation slows down in colder weather and with less soil moisture, and is faster in warmer, wetter conditions. So, the warmer and wetter the climate is, the faster the N becomes available.
