Agronomic Insights

Quantifying nutrient removal this harvest

Many growers are now harvesting summer crops in northern New South Wales and Queensland, after another tough summer cropping season.

While it is not traditionally a time to consider nutrition plans, harvest time does present an easy opportunity to get some valuable data. Not by soil testing, which is unlikely to provide insights yet for the next crop when soil moisture levels are low, but by grain nutrient testing.

Grain nutrient testing helps growers extract information from their crop that may be used to better quantify nutrient removal from the paddock, assess the performance of their summer crop nutrient program and start to plan fertiliser programs for the coming season.

Nutrient testing is available on all grains and is a simple way to get a head start on future seasons.

Growers and advisers have been using yield results for many years to estimate nutrient removal, but these rely on average removals. However, it is difficult for published averages for grain nutrient removal numbers to be meaningful for crop nutrition programs, especially when the seasons have been anything but average.

Consider the differences in yield, grain quality and nutrient levels seen in these grain tests after three different years in crops on trials sites at the Darling Downs, western Downs and in northern New South Wales (Table 1). The nitrogen, phosphorus, potassium and zinc values have varied due to the seasonal conditions and protein, while the sulphur levels have been fairly consistent over the years.

 

Table 1: Grain nutrient test results in sorghum

	Trial data	2017-18	2016-17	2015-16	Published data
	Mean grain yield (t/ha)	4.56	7.52	3.15	Average removal*	Range
	Grain protein %	13.9	11.6	14.5
N	(kg N / t grain)	36.6	18.6	23.2	18.0	9 - 26
P	(kg P / t grain)	4.0	5.6	2.9	3.4	1.4 - 4.0
K	(kg K / t grain)	10.3	6.0	7.3	3.3	2.6 - 4.1
S	(kg S / t grain)	2.1	2.2	2.0	2.4	0.9 - 3.2
Zn	(grams Zn / t grain)	53.0	49.5	47.0	Up to 72	13 - 24

Source: Data from Incitec Pivot Fertilisers’ trial sites. Grain tests conducted by Nutrient Advantage laboratory.

* Sourced from Pacific Seeds Nutrition Booklets (2015), More Profit from Crop Nutrition (2015), Australian Soil Fertility Manual (2000), Incitec Pivot Fertilisers’ data (unpublished).

Grain test results from the last three years of summer crops revealed wide variations in nutrient removal levels depending on the season.

It might be easy to just use an average figure and assume that 18 kilograms of nitrogen is being removed per tonne of sorghum grain, but grain test results (Table 1) show the actual nitrogen removal figure could be up to 100% higher.

Grain nutrient tests have also revealed that the rate of potassium removal from the sorghum trials has been much higher than the recognised published average of 3.3 kilograms of potassium per tonne of grain, at between 6 and 10 kilograms of potassium per tonne of grain.

The phosphorus results show instances of both higher and lower phosphorus removal than the recognised average of 3.4 kg P/tonne grain.

 

Start grain testing this season

Put your best foot forward and grain test at harvest before embarking boots and all into the next fertiliser application season. Grain testing is as simple as collecting 400 grams of harvested grain from the header, truck or silo and submitting it to the Nutrient Advantage^® laboratory for analysis.

You may wish to take samples from different summer crop species, or from numerous varieties or hybrids of the same species. Even comparing short and long fallow paddocks is likely to unearth some useful differences. Local data is always the best.

By combining this information with yield results you can calculate actual nutrient export data by crop species, paddock or hybrid/variety. This provides a very useful base for the next crop’s fertiliser planning.

Consider running nutrient budgets based on previous soil tests, overlaying paddock history and fertiliser applications to estimate how much is left.

 

Role of soil testing

Of course, a robust crop nutrition program should still include regular soil testing. This is best undertaken after some moisture recharges the soil profile. Consider soil sampling midway through the fallow to see where nutrient levels of individual paddocks are sitting in relation to critical values for the intended crop.

Full profile samples for nitrogen and sulphur can be taken again later in the fallow, when moisture has built up. This provides a more accurate picture of both the quantity of nitrogen and sulphur in the profile and its position. Soil testing paddocks with 50-60% water filled pore space allows nitrogen budgets to be more reliant on fact, and less reliant on mineralisation assumptions.  Application strategies can be fine-tuned from previous pre-plant applications.

Soil samples should be taken to the anticipated crop’s rooting depth, with three to four segments such as 0-10 cm, 10-30 cm, 30-60 cm and 60-90 cm. This will show which nutrients are present and where they are in the soil profile. It can also reveal whether there are any subsoil constraints, such as salinity or chloride.

Problems with subsoil constraints can vary from time to time due to soil wetting and drying cycles. Damaging levels of chloride and salinity in the subsoil have been prevalent and problematic recently due to the dry weather.

Some species are more susceptible to salts than others, so it is worth knowing if or how much the intended crop may be affected. The value of soil testing in these situations is priceless.

For more information on grain nutrient testing or soil testing, call me on 0417 896 377 or email bede.omara@incitecpivot.com.au.

This is a guide only, which we hope you find useful as a general tool. While Incitec Pivot Fertilisers has taken all reasonable care in the preparation of this guide, it should not be relied on as a substitute for tailored professional advice and Incitec Pivot Fertilisers accepts no liability in connection with this guide.