California growers have taken on an important role in their state’s broader efforts on climate change, including responding to regulatory requirements to improve their nitrogen (N) use efficiency and reduce their environmental impact. To meet these regulations, while maintaining crop yield and quality, growers must consider all potential N sources, including organic amendments and irrigation water. It is essential that growers gain a better understanding of how these sources contribute to crop-available N, so they can confidently adjust fertilizer programs.
What is organic nitrogen? Organic N refers to nitrogen compounds that originate in living material and contain carbon (C). Organic soil amendments, both plant and animal based, contain variable amounts of organic N that can be released when applied to the soil. Some examples of organic amendments include crop residues, cover crops, composted green waste, composted animal manure, and animal byproducts like feather meal.
What is mineralization? Mineralization is a microbial process that converts organic N to ammonium (NH4+), a mineral form of N. Mineralization releases nutrients from organic matter, making it available to plants for uptake.
How does mineralization work? The mineralization rate and nutrient content of organic material is dependent on the material’s composition, age, and C to N ratio (C:N). The C:N ratio is the mass of C in relation to the mass of N in a substance.
Soil microorganisms, responsible for mineralization, thrive on material with a C:N ratio close to 20:1. Generally, if organic material with a C:N ratio of 20:1, such as rotted manure, is added to the soil, microbes will consume it quickly, incorporating the C and N into their biomass. If organic matter with a higher C:N ratio, such as wheat straw, is added to the soil, the microbes will require additional N to balance the excess carbon in the straw. The additional N consumed by microbes is removed from the pool of plant-available N and causes soil N deficiencies. Conversely, when organic material with a lower C:N ratio is incorporated in the soil, such as broccoli residue, microbes will consume the material and release NH4+ into the soil. This mineral N is available for plant uptake.
In summary, materials added to the soil with a C:N ratio greater than 20:1 can result in a temporary N deficit (immobilization) and materials added with a C:N less than 20:1 can result in a temporary N surplus (mineralization).
| Organic Material | % C | % N | C:N |
|---|---|---|---|
| Sawdust | 50 | 0.05 | 1000 |
| Wheat Straw | 38 | 0.5 | 76 |
| Corn residue | 40 | 0.7 | 57 |
| Rotted Manure | 41 | 2.1 | 20 |
| Broccoli Residues | 35 | 1.9 | 18 |
| Young Alfalfa | 40 | 3.0 | 13 |
| Broccoli Residue | 11 | ||
| Vetch Cover Crop | 40 | 3.5 | 11 |
| Soil Fungi | 50 | 5 | 10 |
| Soil Bacteria | 50 | 10 | 5 |
What other factors affect mineralization? The rate of mineralization is also dependent on environmental factors including temperature, soil moisture, soil texture, and tillage practices. In general, as the temperature of the soil increases, mineralization increases. The same is true with soil moisture until saturation is reached, which can lead to anoxic (oxygen-free) conditions, that will in turn impact the microbial community.
What are some of the benefits of incorporating organic amendments? Aside from the benefits of added nutrients, incorporating organic amendments can also influence several soil characteristics.
Microorganisms that degrade organic matter produce a by-product called glomalin that can bind clay particles together, increasing soil aggregates, or the soil’s ability to resist erosion. Organic soil amendments have also been shown to improve the soil’s capacity to retain water, thus increasing the time water is available in the root zone for crops. Additionally, studies have found that the incorporating organic soil amendments can decrease bulk density, increasing pore space available to hold water or oxygen.
What are some potential impacts of mineralization for growers? Organic amendments tend to vary widely in nutrient composition. And with the added variability of environmental factors, mineralization rates can be tricky to predict and make nutrient management planning and reporting especially difficult. However, mineralization of N from organic amendments can make significant contributions to crop nutrient requirements.
Learn More
Now that you know the basics of N mineralization, please refer to the following articles at your leisure for more detailed information and research results.
- FREP Research Update: https://blogs.cdfa.ca.gov/FREP/index.php/research-update-nitrogen-management-of-organic-leafy-green-vegetables-on-the-central-coast/
- Salinas Valley Agriculture Blog: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=26970
- SJC and Delta Field Crops Blog: https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=23911