Crops may source much of their nitrogen (N) from in-crop mineralisation (ICM) of soil organic matter. Of total crop N at harvest, an estimated average 63% came from ICM in a recent Victorian study. Total crop N includes grain, straw and roots. Root N is about 10% of grain and straw N combined.
But ICM isn’t well accounted for in nutrient budgets. This means fertiliser N may be over applied. Advisors agree predicting ICM is tricky, notably when seasonal conditions are shifting.
How much nitrogen is involved?
Net in-crop N mineralisation is the difference between what’s mineralised and immobilised. A positive ICM value means there is a net mineralisation giving more plant available N. A negative ICM value means N is immobilised, or lost from the system e.g. via denitrification. This study found net ICM ranged from +192 kg N/ha down to -230 kg N/ha.
Ways to predict in crop mineralisation
Many advisors have used the traditional ‘rule of thumb’ calculation:
ICM = 0.15 * organic carbon (OC)% * growing season rainfall (GSR) (mm)
This method is most accurate after the fact when rainfall is known. It is less useful in advance using long-term GSR averages to predict likely rainfall. At the moment there isn’t a more accurate method to predict GSR.
The rule of thumb is a better predictor of ICM than total N. In very wet years N losses can make the formula less useful. Surprisingly, the researchers did not find a clear effect from crop sequence. But adverse seasonal conditions might have had an impact.
Cheap, rapid and reliable ICM predictions are not here yet. Research in Central Victoria looked at Hot KCl, Solvita kits and MIR to predict ICM. So far, all methods are promising. Further research is required before they can be used for Australian soils.
Which tests to use now?
For now, soil tests are the best option to predict ICM. Test OC to estimate potential mineralisation. Deep soil mineral N levels give more information than surface (0–10 cm) samples. Growers may have more soil N than they realise. Through the season growers can expect soils to mineralise more N when soils are moist and where OC levels are high.