Nitrogen
Nitrogen supply and grain protein content
Nitrogen is a primary constituent of protein, so an adequate soil nitrogen supply is an essential ingredient for producing wheat with a high protein content. Grain protein is modified by the grain yield of the crop - increasing grain yield has a diluting effect on grain protein.
This is why in drier seasons or seasons of low grain yield, a larger proportion of the crop is of a high protein percentage, whereas, in wetter growing seasons, high yields can be produced but may be at a lower protein. This seasonal variation is why paddock grain yield, protein and rainfall records should be kept for a number of years to obtain a true indication of its nitrogen fertility.
Low grain protein, the signal of nitrogen deficiency
Grain protein can be used to indicate if current nitrogen fertiliser or rotation practices are providing adequate nitrogen to the crop to meet the requirements arising from the water supply the crop had access to. Profits may also suffer not only due to lost yield, but potentially downgrading to a lower classification.
Table 1. Using grain protein to signal warnings of nitrogen deficiency.
Nitrogen management should ensure wheat crops consistently produce a grain protein content of 11.5-12.5 per cent to achieve season yield potential. However, applying nitrogen fertiliser to produce grain protein content above 12.5 per cent may not be economical unless high premiums are available for high protein Australian prime hard (APH) wheat.
Soil testing for nitrogen
The approximate amount of nitrogen available in the soil can be determined by soil testing. Soil tests should be taken at various places in each paddock to a depth of at least 60 cm, but preferably 90 or 120 cm, so the quantity of available nitrogen can be calculated. Test results are only an indication, so historical grain yield and protein levels from the paddock should also be used to determine nitrogen requirements. Table 2 indicates the approximate amount of available nitrogen needed at planting for a particular yield and protein.
Table 2. Available soil nitrogen (kg) needed for particular yield and protein.
Factors affecting level of nitrogen required:
- seasonal yield and protein levels - If the expected yield is exceeded due to good climatic conditions, grain
- protein may fall below the protein targeted. If the yield is not achieved, for example due to moisture stress, then grain protein may be above the protein targeted
- planting date, variety and soil moisture - These must be considered in establishing seasonal target yield and protein levels. Early planting and good stored moisture at planting should indicate higher target yields. Using a tool like HowWet is a good approach to estimating plant available water
- SOI and seasonal rainfall prospects - With higher rainfall prospects consider increasing nitrogen rates. Using a cropping simulation tool such as Whopper Cropper can provide further information on yield and protein potentials
- level of soil fertility - Soils farmed consistently for 30 to 50 years (or more) will generally have a higher nitrogen requirement for optimum yield and protein levels than new country
- cropping history - Double cropping will normally require higher nitrogen rates
- zero tillage - Stubble retention combined with zero and reduced tillage has increased the yields and cropping frequency of our farming systems by storing more soil water. Higher nitrogen rates may be required to make efficient use of this extra stored soil water
- protein premiums - High premiums for APH high protein wheat have warranted an increase in nitrogen rates.
Calculation of profile nitrogen levels from soil test values
Most soil nitrogen test results are expressed in milligrams per kilogram (mg/kg) or parts per million (ppm). To make crop recommendations, it is necessary to convert nitrogen test results to kilograms of nitrogen per hectare (kg N/ha).
The formula is:
Soil test value x soil bulk density x sample depth (cm) / 10
Yield
(t/ha) Grain protein (%) 7
Table 3. Bulk densities of some common agricultural soils
N fertiliser application - If paddock records or soil tests indicate that nitrogen fertiliser is required, the calculation below can be used to obtain the quantity of actual fertiliser product required. For example if 40 kg N/ha is required, this rate of nitrogen can be supplied by applying 87 kg/ha of urea.
Fertiliser product required (kg/ha) = Rate of nitrogen required kg N/ha x 100 / % nitrogen in fertiliser product
Applying nitrogen fertilisers High rates of nitrogen fertiliser applied at planting in contact with or close to the seed will severely damage seedling emergence. If high rates of nitrogen are required then it should be applied pre-planting or applied at planting but not in contact with the seed (i.e. banded between sowing rows). Table 4 (below) indicates the maximum rates of fertilisers containing nitrogen that may be applied with the seed at planting using conventional planting equipment. Rates should be reduced by 50 % for very sandy soil and may be increased by 30 % for heavy textured soils or where soil moisture conditions at planting are excellent. Rates should be reduced by 50 % when planting equipment with narrow disc or tine openers are used as the fertiliser concentration is increased around the seed.
Table 4. Safe rates to apply some nitrogen fertilisers with seed at planting (kg/ha)
