Rules of thumb for nitrogen management in crops
Interaction between soil moisture and nitrogen (N) levels plays a major role in determining wheat yield, according to a crop nutrition expert.
Dr Glenn McDonald, Associate Professor at the University of Adelaide’s School of Agriculture, Food and Wine, says having an understanding of the soil’s moisture holding capacity and information about changes in plant-available soil water during the growing season are important aspects of N management.
“The supplies of water and N are the major drivers of growth and yield of crops, and the two interact to determine crop yield,” said Dr McDonald, when addressing a Grains Research and Development Corporation (GRDC) grains research Update in Adelaide (SA) on nitrogen application “rules of thumb”.
“While availability of moisture is recognised widely as an important factor limiting responses to N, it is also important to realise that lack of N can be an important limitation to soil water use and particularly use of subsoil moisture.”
While much of N management focuses on improving post-anthesis growth and grain filling, it is more than likely that the effects of N and its interaction with soil moisture are influencing yield earlier by affecting grain number as well as grain size, according to Dr McDonald.
He said applying N fertiliser at the optimum rate for the anticipated yield or applying it strategically during the growing season could improve yield and grain protein.
“Applications of N up to mid-stem elongation build the foundation for yield and have relatively little effect on protein, while later applications of N can be used to maintain or increase protein, but have little or no effect on yield. Applying N between the flag leaf emergence and flowering stages of crop growth can result in greater increases in grain protein concentration.”
Dr McDonald said the demand for N was driven by crop growth rate and the pattern of N uptake would reflect the changes in growth rates during the growing season.
“Essentially, a cereal crop goes through two stages: uptake of N, which drives crop growth and yield potential; and remobilisation of N to the grain, which influences grain protein. Typically, for a May-sown wheat crop in southern Australia, 20-30 per cent of the total N may be taken up by the crop at the start of stem elongation.
“The greatest demand for N is when the leaf area is expanding rapidly and the crop is growing most rapidly. This corresponds to the period of stem elongation and at this peak time, growth rates can be of the order of 200 kilograms/hectare/day and crops can be accumulating N at 2-3 kg/ha/day.
“The N taken up at this time is not only helping to build the yield potential of the crop but is also providing the N reserves for grain protein as most of the N present in the grain is derived from N remobilised from the leaves and other green tissues during grain filling.”
Dr McDonald said that by flowering, about 80% of the total crop N had accumulated, but this could exceed 90% under dry spring conditions.
“While the majority of N is taken up by flowering, crops can still take up N during grain filling if there is sufficient soil moisture, the roots are active and N is present in the soil.
“This will tend to maintain green leaf area during grain filling and reduce the amount of N remobilised to the grain early in grain filling. These conditions favour starch production in the grain and can lead to low grain protein concentrations.”
Dr McDonald said that at maturity, more than about 75% of the total N taken up by the shoots during the year had been mobilised to the grain, although this could range from 60% up to 80%.
“Under high rates of N, the proportion remobilised to the grain tends to be lower compared to low rates of N.”
If growers do not use decision-making tools such as N calculator or Yield Prophet® to help make decisions about N management in wheat, there are some general recommendations from Dr McDonald that can be followed:
- Target a yield potential at the start of the season, whether it is based on average wheat yield or average plus a bit
- Estimate the volume of N available from the soil, based on soil testing or on guidelines established from a number of years’ paddock histories and local experiences
- Calculate the amount of N required to reach the target
- Apply 70-80% of the N required between mid tillering and mid stem elongation. Adjust the timing of the application depending on initial soil N and factors such as timing of sowing
- The remaining N can be applied later, up to flag leaf emergence, to maintain or boost grain protein depending on seasonal conditions and soil moisture availability. The rates of N can be adjusted in response to seasonal conditions.
Caption: Glenn McDonald, from the University of Adelaide’s School of Agriculture, Food and Wine, speaking about nitrogen decision-making at a recent GRDC grains research Update.
Glenn McDonald, University of Adelaide
08 8313 7358
Sharon Watt, Porter Novelli
Region South, North