Nitrogen Management for your Cereal Crops; Timing and Form of N?

The optimum timing of N application varies from year to year and from location to location.  Soil type, rotations, rainfall events and general weather conditions all influence the crops response to applied N.

 

There are a number of different “N Rate Calculators” available from any number of reputable sources. They all have their strengths and weaknesses; each can be used as a guide to the required rate of N for any given circumstance provided you understand the constraints of each model.

 

N can be applied from sowing right through to anthesis and there is no single right or wrong time. However it is important to know what crop response you will achieve when N is applied at a particular crop stage

To simplify this discussion, we will assume that N is the yield limiting factor.  I don’t believe that you can starve a crop to a point then feed it and achieve potential yield, N deficiency during crop development will limit yield potential.

Areas where wheat yield potential is limited to 3 t/ha, all your seasonal N requirements can be applied at sowing providing you have seed and fertilizer separation to avoid fertilizer toxicity issues. In higher yielding environments there are benefits to splitting your N applications through the season, reducing losses, managing risk, canopy management for disease control and potentially protein management.

 

As rates of N applications increases, the initial crop response is to increase yield potential.  As applied N peaks the rate of yield increase slows and grain protein % starts to increase.

Figure 1. The average yield and protein response of 10 varieties of wheat over several years.

Early N application encourages and facilitates tillering in cereals and grain per head, thus increasing the yield potential of the crop. N deficient crops will limit their tillering to ensure adequate nutrition is available for grain production.

Late tillering to stem elongation applications of N maintain tiller numbers and grains per head to hold yield potential. N deficient crops will abort the youngest tillers to save the older tillers. Late tillers are productive in areas where adequate water is available to support them through to grain fill.

 

N applications from flag leaf emergence to anthesis (flowering) will generally increase grain protein and have limited influence on yield provided the plant has access to the N. That is not to say late N doesn’t increase yield but as a generalisation it doesn’t significantly increase yield.

Figure 2. The timing of applied N in cereal crop and its response to grain yield and protein

Soil applied N must be moved into the root zone before it can be absorbed. This requires rainfall and moist soil. The chemical form of this soil applied N will also influence the speed at which it moves into the soil and how quickly it is available for root uptake. Urea must be converted to ammonium then to nitrate which typically takes about 2 weeks.  This time will vary widely depending upon environmental conditions at the time of application. Ammonium based fertilizers only need to be converted to the nitrate form and obviously the nitrate-based fertilizers are ready for plant uptake. The conversion to nitrate in the soil is facilitated by microbes hence environmental conditions influence the speed of this process.

 

Foliar applied N fertilizers have the advantage that up to 20% can be absorbed into the plant via the leaf, however can burn the crop if applied under the wrong conditions. Foliar N is particularly useful to be applied to a crop recovering form stress – wet or dry as under these conditions the root system has been compromised and is in recovery mode when the stress is removed and not readily absorbing nutrients form the soil. Foliar applied N (coupled with trace elements) will speed up the recovery of the crop due to its ready supply of nutrients. 

 

For further information and advice in relation to this article please contact your Western AG agronomist

Article produced by - Andrew Heinrich, Western AG Naracoorte

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