Prepare for soil N and P nutrient depletion

A high yielding wheat grain crop of seven tonne per hectare, an often achieved 2020 yield, at 12 per cent protein, contains 160kg/ha nitrogen for grain alone. Even a modest yielding wheat crop of 4t/ha, at 12pc protein, results in export off a paddock of 92kg/ha nitrogen. A canola crop yielding 3.5t/ha would have removed around 141kg nitrogen, big numbers.

Dual purpose grazing and grain crops, would have even higher soil nutrient losses. A dual purpose wheat crop providing 4t/ha grazing drymatter, plus yielding 6t/ha grain, would have utilised some 266kg/ha of nitrogen and 156kg/ha of this would have been exported off the farm via grain and meat.

While phosphorus removal via high yield isn't as startling as nitrogen, it also is high. A 7t/ha wheat crop removes around 21kg/ha phosphorus and a 4t/ha crop removes 12kg/ha. A 4t/ha canola crop removes 28kg/ha phosphorus and a dual purpose wheat crop of 4t/ha grazing plus 6t/ha grain removes 20kg/ha.

Many long term cropping rotations remove more nitrogen than what is returned via fertiliser input and is a major cropping issue. Losses of nitrogen other than via grain also need to be considered. For example in a wet year via leaching and/or denitrification. However many long term cropping systems are covering well for phosphorus with current fertiliser application rates.

Fertiliser strategy is not just about replacing what nutrients have been removed, although that is an important consideration. Fertiliser use will also be related to how much soil moisture can be conserved in the fallow plus saving what water might have been left after the last crop. Soil testing via Australian accredited laboratory's is a good starting point for assessing likely fertiliser need for the coming winter crop.

Both top 0 to 10cm depth and sub soil (at least to 50cm to 60cm) testing is important for nitrogen assessment. Available nitrogen (mainly nitrate form but also ammonium) can vary considerably not only in total soil nitrogen levels but also on how dry or wet the summer is. Nitrate nitrogen, the main form taken up by crops, tends to be higher following reasonable fallow rains that result in nitrification.

Nitrogen can be mobile, especially on lighter soils. At times available nitrogen is leached down the profile, with a spike at a subsoil layer. Nitrate nitrogen levels in the top 0 to 10cm above 30mg/kg are generally considered good, below 10 low. If an extremely wet period occurs during fallow, especially on lighter and loam soils, nitrate can be low in the top soil but higher at 20cm to 60cm. Leeching of nitrogen is generally minimal on heavier soils.

Estimates of soil nitrogen also needs to take into consideration variable rate of soil nitrate conversion (from the total nitrogen pool) during fallowing and crop development. It can be considerable and depends on aspects like quality and quantity of organic matter, temperature and soil moisture.

Getting a good feel on yield potential depends on many factors like soil water as-well-as likely (predicted?) in-crop rainfall vital. Moisture probes, including the crude but useful push probe, help assess current soil moisture levels. "Soil water" app (http://soilwaterapp.net.au/) is also useful for estimating available soil water, provided fallow management has been good.

Calculating how much nitrogen to apply to crops is helped by more factors other than straight yield potential. For example yields are generally far higher if sowing is timely for a given crop and variety, and disease risk is low. If prospects approaching sowing are indifferent a staged nitrogen application strategy is sensible. Nitrogen can be split between pre-sowing, sowing and post emergence to around the end of tillering.

Phosphorus needs are best generally at sowing. Recent research indicates the usefulness of deep applied phosphorus close to the seed and residual can last for several years.

Next week: Beating grass seed problems with tropical grasses.