The nuances of managing nitrogen in organic cropping systems are decidedly trickier than conventional systems, which rely heavily on readily available soluble nitrogen fertilizers.
In organic production, nitrogen availability is dependent to a greater extent on complex processes such as mineralization, and reservoir resources such as prior cover crops.
“There are several sources of nitrogen available for crop growth in organic systems,” says Richard Smith, University of California Cooperative Extension (UCCE) Monterey County Farm Advisor.
“They include residual mineral nitrogen, in-season mineralization of nitrogen from soil organic matter and compost, nitrogen from prior crops and cover crops, and applied organic fertilizer.”
It isn't just a matter of quantity, however.
“Generally our soils in this area have somewhere between 1,000 and 3,000 pounds per acre of nitrogen,” Smith says. “Unfortunately, a lot of that nitrogen is present in forms that are unavailable to the plant.”
That's where mineralization comes into play. In-season mineralization of nitrogen from soil organic matter results in the release of NH4+ and nitrate NO3-, which are forms available to plants.
“Approximately 2 percent to 5 percent of soil organic matter decomposes annually,” Smith says.
The vast majority of mineralization takes place in the first foot of soil. “The organic matter in our soils typically runs about 1 percent.”
A soil with 1 percent organic matter has approximately 2,800 pounds of organic nitrogen per acre. Based on that scenario, a grower can expect about 56 pounds of plant-available nitrogen per acre over a two-month period due to mineralization, says Smith.
That calculation is a generalization based on a 2 percent decomposition factor and assuming the other factors such as organic nitrogen content are correct. It is a rather iffy equation, even though the assumptions are fairly consistent in laboratory analysis. In reality, many factors affect mineralization including soil temperature, soil moisture, tillage practices, as well as others.
Mineralization is rather insignificant in soil temperatures below 50 degrees, but increases as the temperature climbs. It is accelerated in moist soils, but only to a certain extent. Overly dry or overly saturated conditions will slow mineralization. Additionally, tillage practices tend to increase mineralization over the short-term due to increased microbial activity, but that effect declines as days and weeks pass.
Even though it is difficult to precisely measure or predict the rate of mineralization or how much it will ultimately contribute to available nitrogen in an organic system, it is a place to start, says Smith.
Another component of the equation is the amount of nitrogen contribution from cover crops.
“Cover crops typically take up or fix between 100 and 200 pounds of nitrogen per acre,” he says. “Again, that is dependent on a number of factors.
“Typically, less than 10 percent to 30 percent of the nitrogen from a cover crop is taken up or utilized by the first subsequent crop. A good deal of the nitrogen remains in the system and is utilized in later years.”
Finally, applied organic fertilizers enter into the equation and should be carefully considered in the overall scheme of nitrogen management. “The real trick is integrating all sources of nitrogen and timing it correctly,” Smith says. “The goal is to achieve effective synchrony between crop uptake and nitrogen supplied by mineralization from soil organic matter, cover crop residues, and fertilizers.”
Typically, nitrogen availability from mineralization and incorporated cover crops peaks before the crop needs it the most. A lot depends on the crop being grown and its corresponding nitrogen requirements.
Supplemental organic fertilizers timed to crop uptake can help fill the gaps between available soil nitrogen and plant needs.