Big yields, big nitrogen requirements

Estimating crop nitrogen requirements is closely related to likely potential crop yield. For example, a 7.0 tonne a hectare wheat crop at 11 per cent protein, will take around 133kg/ha nitrogen. Available soil nitrogen through the growing period will need to be at least double and often three times this amount.

Research indicates it is important firstly to estimate, as far as possible, likely crop available soil nitrogen levels at time of sowing and what extra amount is likely to become available through the growing season (from the soil organic nitrogen pool). Soil testing, in the top 0-10 centimetre layer, as well as the subsoil layer, all helps with estimating current and likely soil nitrogen pool for the crop.

Typical non-legume crops in much of the Australian cropping region receive around 50-60kg/ha nitrogen via fertiliser with the most common being via 10-20kg/ha from a phosphorus based starter fertiliser, plus from around 40-50kg/ha from a nitrogen fertiliser like urea.

In a rotation not involving legumes (crop and or pasture phases) high yielding crops are commonly contributing to a gradual decline in soil nitrogen levels, because fertiliser added nitrogen is not keeping pace with export via grain.

Recently I have seen good media coverage of examples of farmers successfully using higher fertiliser nitrogen rates. For example the winner of the Premer Shield applied 170kg/ha nitrogen to a grain sorghum crop that yielded 9.65t/ha. That yield would result in nitrogen export of almost the same amount, around 155kg/ha.

Another example was a wheat crop, in a high yielding part of the grain belt, yielding 9.3t/ha. Nitrogen applied to the crop was 150kg/ha with removal via grain of around 170kg/ha. Another recently published example was a 6.3t/ha wheat crop with 11.5pc protein receiving 154kg/ha nitrogen. In this example the crop removed 116kg/ha of nitrogen. I am sure many growers can provide similar examples.

A key aspect is calculating applied nitrogen and removed nitrogen. Many soils with good clay content, commonly don't experience much loss of nitrogen other than by grain removal. Left over nitrogen can contribute to residue soil nitrogen for use in future crops. Very little nitrogen is fixed in the soil via free living nitrogen fixing bacteria and little falls from the sky via lightning into crop available nitrogen.

Hence the role of legumes, either or both pastures and crops. A lot of research has studied this aspect with enormous variation in nitrogen build-up via them. Nitrogen build-up can be as high as 400kg/ha and as low as nothing or even a loss via a legume crop that uses soil nitrogen rather than building it up. A general guide is that pastures with good legume content that is well nodulated builds soils by around 40kg/ha per year. Pulse or legume crops commonly leave around the same amount post harvest.

For pulse crops to leave good nitrogen for future crops a few provisos are required. Effective nodulation with the most efficient rhizobia for that crop is essential. Pulses sown into stubble retained paddocks are likely to fix more nitrogen. A key aspect is that dense vegetative pulses can fix more nitrogen than less vegetative stands. Crops like faba beans, which are more vegetative than chickpeas, and especially lentils, commonly, but not always, fix and leave more nitrogen. Sowing earlier in a crop's sowing window can also help greater nitrogen fixation.

Nitrogen budgeting is to a degree subjective. There are many variables, especially climate, rate of in-crop conversion of nitrogen to crop available forms (mainly nitrate), in the fallow and growing period. Contribution from pulse crops and pasture legumes can involve gradual nitrogen release as their herbage and roots break down. But budgeting is worth pursuing and helps avoid long term soil nitrogen run down.