There is 60 kilograms/hectare of nitrogen that marches out from the farm every time a 12 per cent three tonnes/ha wheat crop is harvested. There is 120 kg/ha of nitrogen that leaves the paddock from a six t/ha crop.
Even more nitrogen is used by a dual-purpose crop because of nitrogen used in the grazing component. Four t/ha grazing (drymatter) utilises another 140 kg/ha, although eventually much of this returns to the soil nitrogen pool (but not the current season).
Nitrogen is the largest used element in cropping and the largest to address as average and expected crop yields increase. Some research also indicates that although pulse crops are capable of fixing (via rhizobia bacteria) large amounts of nitrogen, they may respond positively to topping up with fertiliser for very high yielding situations.
Measuring as accurately as possible paddock levels of available soil nitrogen, acknowledging that they can be extremely variable, is an important aspect of assessing likely crop nitrogen needs. Soil tests for nitrate and ammonia nitrogen is useful for assessing current soil nitrogen status. Generally agronomists adjust likely crop requirements by adding likely nitrification rates (conversion of non-available to available nitrogen) to available levels.
Starting nitrogen point, organic matter levels, likely (commonly using average) late fallow and in-crop rainfall and temperatures, and organic matter quality (for example legume pasture or crop residue including their roots) all impact on likely conversion of crop unavailable nitrogen to available nitrogen.
While standard soil testing generally is undertaken assessing the top 10cm, where the majority of nutrients commonly are, available soil nitrogen is mobile and can leech down the profile if excessive wet in-fallow and in-crop rains occur. Agronomists generally get a feel for where nitrogen is placed in the profile for a given year once deep soil test data comes in from several typical paddock situations. Deep soil testing is slow and expensive and minimal data well interpreted can be extremely valuable.
Leeching is more likely to be an issue with lighter textured soils than heavy clay ones. Soils with light top soils but heavy sub soils can commonly have a band of higher available nitrogen around the top of the clay layer.
A second important aspect is to assess is what amount of nitrogen will have and will leave the field for a past crop and for a targeted crop and base fertiliser needs on replacement basis. Legumes, crops or pastures, in the rotation require assessment of soil nitrogen build-up for following cereal or other non-legume crops (e.g. canola).
Research largely discounts likely major contribution to available soil nitrogen from natural causes such as lighting. Free living nitrogen fixing soil micro-organisms (for example fungi or bacteria) are generally believed to not contribute much soil nitrogen.
Legumes can build soil nitrogen, although sometime its availability to a following crop can be delayed. Pasture legume mainly returned to the soil by grazing and desiccation can build total soil nitrogen by around 30kg/ha/t legume drymatter. Legume crops vary in their soil nitrogen build up from nil to over 100kg/ha per crop.
Good sub soil moisture timely sowing and low risk of major weed or diseases points to feasibility of adding most of a crops likely nitrogen fertiliser needs before or at sowing. Provided opportunities occur for in-crop application (ahead of rain) similar responses can occur and can be tailored to developing seasonal conditions.
Next week: Boggabri field day focused on planning essential for top pastures.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact (0428) 752 149.
