WHEAT yielding five tonnes to the hectare at 11 per cent protein results in export off-paddock of about 100 kilograms per hectare of nitrogen. Really big crops like 8.0 t/ha wheat cause 150 kg/ha nitrogen to exit the paddock.
Dual purpose crops require nitrogen for good grain yield but it is equally important for grazing. A crop providing 5/ha drymatter for grazing, in excess of grain needs, will use a further 170kg/ha nitrogen. Much comes back via dung, urine and trampled material, but is required for growth.
A generally high yielding year such as 2016 has seen big exports off paddock of nitrogen in cereal and canola crops. Therefore many current crops are likely to be potentially fairly deficient in nitrogen if the season turns again good to high yielding.
Jim Laycock, Incitec Pivot Fertilisers’ agronomist and a highly regarded crop nutritionist, reports widespread low soil nitrogen levels. Recent analysis of more than 700 deep nitrogen soil tests in NSW noted that 43 per cent showed less than 30kg/ha of plant available nitrogen in the main root zone. Equally critical was that 75 per cent of tests showed less than 60kg/ha of nitrogen.
Jim Laycock said unless adequate levels of nitrogen fertiliser are added to crops where soil nitrogen is 30kg/ha or lower, cereal yields would be capped at around 1.6t/ha of APW grade wheat. And this estimate is based on assuming a 40kg/ha in-crop contribution over the growing season from nitrogen mineralisation.
Clearly how much nitrogen fertiliser a crop will require depends on many factors with water being especially critical. A good estimate of current in-soil water reserves is therefore valuable. Soil moisture probing or use of modelling such as the app “Soil water” can also assist in estimating crop-available soil water levels provided one accurately factors in soil type and has efficiently stored soil moisture over the fallow.
It would be good if medium- and longer-term rainfall forecasts could be relied upon but, at least for many areas, that is asking too much. One has to largely be realistic and not be over optimistic about likely future in-crop rainfall. One also needs to be realistic about what contributes to soil nitrogen, other than fertiliser. What falls out of the sky is generally regarded as low. Contribution from free-living nitrogen fixing bacteria is generally believed to be low, although some research suggests it may be higher in a zero-till environment than cultivation.
One also needs to be carefully budgeting soil nitrogen based on legumes in the rotation. Pasture legumes, can contribute around 30 kg/ha nitrogen per t/ha drymatter production. A four-year lucerne stand can build soil nitrogen by several hundred kg/ha over its rotation sequence. Even a one or two-year annual legume phase can build nitrogen by a few hundred kg/ha if it’s a good, dense stand.
Legume crop nitrogen benefit to following crops can range from quite high (more than 100kg/ha) to little, depending on crop bulk minus grain harvest. It is important to assess post legume contribution to nitrogen availability for a following crop. Especially for legume pastures sometimes while total soil nitrogen can be high, availability can take some time to convert to available soil forms.
If rain uncertainty is high and if soil water levels are moderate, a staged approach to amending nitrogen shortages is a good idea. Using our dual-purpose crop as an example, with soil water relatively low to moderate (according to app “SoilWater”), we will add 40kg/ha nitrogen ahead of the next rain and if the season turns good more will be added ahead of the event after that.
Next week. Organise now for next year’s winter legume pasture sowing program.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.