The first step in deciding on nitrogen (N) fertilizer rates is to estimate how much N will be available to a crop over the growing season or ENR (estimated nitrogen release). You can be leaving both yield and quality on the table if this is not part of your N calculations.
Glyn Evans, working with Rolla Ag out of Rolla B.C., asked this question, I have fields of HRS wheat with yields ranging from 50 to 90 bushels, so there has to be significant differences in amounts of N tied up. Fields range from minimum till to zero till, which also should have an effect on the amount of tie-up. I was wondering is there a formula or rule of thumb for estimating how much N will be tied up by cereal straw?
That question sparked an early morning soliloquy from my fellow senior agri-coach, Elston Solberg. It s all about the immobilization and mineralization portions of the N cycle. This dynamic process is cycling pretty much all of the time. My advice is to start by simplifying this calculation because there are so many variables to consider.
Oats tend to have a wider C: N ratio and the ratio narrows for wheat, followed by barley, then canola, then peas and pulses, generally. There are other factors that impact this of course, but in general, count on a range of 50 to 80:1 for cereals, which is why there is so much immobilization. Canola sits around 30-35:1. This the tipping point for immobilization/ mineralization. I usually say canola is a wash unless straw is not spread well, then canola straw will immobilize N. At 20-25: 1 for peas, the net effect is mineralization plus credit some additional N for protein as well for the N released past the yield window.
Knowing all this, we can take our soil test N levels and decide whether the straw will result in a nitrogen credit or debit and adjust our N rates accordingly. When I said simplify, I meant initially because over time if we knew the yield and protein of the grain, we could fine tune things considerably because the whole process is governed by C: N ratios. A high protein grain will by default produce a higher protein straw within the same variety. If we knew the straw to grain ratios of the various varieties grown, we could fine tune things more because a variety that grows more straw will immobilize more N than a variety that grows less with all other factors similar. If we know the degree of straw incorporation, we could make a better estimate because the more the straw is incorporated (and evenly), the more and faster the N will be immobilized but also the quicker it will be re-mineralized. Each farm and even field could have a different ENR number and depending on the information available, the more accurate that ENR number.
Agri-coach Jim McComb, out of Marmora, Ont., adds When the Ontario farmers who have removed the straw residue for years decide to switch (to incorporating), we need to get the N cycle into momentum, like getting a full truck load of grain rolling down the road. It takes a lot of diesel and horsepower to get things rolling, but once you are up to speed, it takes less to keep it there. The momentum of the N cycle comes from building the healthy life in the soil and the N is returned to be available. Now we are using split N applications to better manage nitrogen. The soil testing, tissue testing and visual observations all provide us with clues that enable us to fine tune our ENR numbers.
I was impressed by the insight shown in this discussion. Jim s analogy of the full truck load of grain fit well with early research on immobilization in the Prairie region. High amounts of immobilization were observed after a big wheat crop on fallow (a big truck load of high C: N ratio residue following a year of none).
If you use your local knowledge and the guidelines Elston has given, you won t be far off. Remember the first word in ENR is estimated.
Which crop stubble will tie-up or release N in the first year? How much?