38 Follow us on @PrairieFarming Better Farming | January 2025 Agronomy Insider C:N or carbon-to-nitrogen ratio of the volunteer growth. Nitrogen immobilization is the temporary loss of available soil nitrogen resulting from the decomposition of volunteer residue. Nitrogen immobilization is also referred to as nitrogen tie-up or nitrogen penalty. It could possibly result in additional fertilizer costs for farmers planting nitrogen-hungry crops like corn, wheat or canola. When discussing the challenge of nitrogen immobilization, it is essential to understand an alternative outcome called nitrogen mineralization, which can also result from residue decomposition. With mineralization, plant-available nitrogen is released by microbes as they break down the volunteer growth, whether it’s incorporated as a green manure or terminated ahead and left on the surface or incorporated. C:N (carbon to nitrogen) ratio The C:N ratio or carbon-to-nitrogen ratio is how many carbon units there are to every nitrogen unit in organic material (plant tissue and residue). For example, a C:N ratio of 40:1 means there are 40 units of carbon to every one unit of nitrogen in that plant material. This ratio will influence whether nitrogen is immobilized or mineralized, and to what extent. Nitrogen uptake is most rapid through the stem elongation stage, and by anthesis fully 83 per cent of total nitrogen uptake has occurred. Research done in Western Australia revealed C:N ratios in wheat during various growth stages. As expected, it is low earlier in the year as the N content is high, then drops off as N concentration decreases. They report a ratio of 5:0 at tillering and a ratio of 27:1 by flowering. Nitrogen mineralization Nitrogen mineralization is essentially the opposite of immobilization. In this scenario, the plant residue feeding Jason Voogt photo, Mark Huso photo The photo on the left shows volunteer oats from regrowth in fall 2021, where N 80 lbs/ac, P 15 lbs/ac, and K 85 lbs/ac were removed by the seventh leaf stage. The photo on the right is from crop consultant Mark Huso in North Dakota from the fall of 2021. The above wheat crop yielded 45 bu/ ac. When the soil was sampled right after harvest, there was 140 lbs/ac of residual nitrate N. The above volunteer growth grew to the flag leaf stage, and when sampled again, the residual nitrate N was now 30 lbs/ac, showing just how much nitrogen was taken by the volunteer wheat. As the wild oats progress through their growth stages, the C:N ratio also increases. The soil microbes' ideal food has a C:N ratio of 20:1. When their food source has a higher C:N ratio, insufficient nitrogen is available for their biological functions. This means the microbes must use the nitrogen available in the soil to meet their needs. The amount of nitrogen microbes must consume from the soil is the nitrogen penalty, since it is temporarily unavailable for plants like cash-crops. Eventually, this nitrogen will be released by the microbes and made plant-available, but it is often too late for next year's crops. Uptake and release of nutrients from wild oats Schoenau, 2009
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