Photo: Airi Kulmala.
The primary purpose is to adjust the nitrogen supply according to the demand of a growing crop. This can improve nitrogen use efficiency. Split application reduces the risk of losses through leaching. Split application of nitrogen can increase grain yield and grain protein content.
Splitting the dose of N fertiliser between more than one occasion during spring and summer can be good for both the environment and farm finances. It also reduces the amount of fertiliser a farmer needs to handle during seed drilling, as well as the risk of nitrogen leaching if it rains heavily right after spring nitrogen fertilisation.
If other growth conditions are optimal, split N application can result in yield of better quality and higher quantity.
Planning is the key
For the split application of nitrogen to be successful, the farmer has to make many decisions in the planning stage and specify the plans during the growing season. Timing, type and amount of fertiliser given at sowing and later during the growing season have to be decided.
How to estimate the need for additional fertilisers?
More nitrogen is given to plants only if the conditions during the growing season predict high yield, because then the extra nitrogen fertiliser can increase the yield. If the growing conditions during the season are poor, extra fertiliser is not needed because the plants will not be able to utilise them.
Furthermore, complementary nitrogen fertiliser is given to plants only if the growth indicates that there is a need or lack of nitrogen. One way to estimate this is by using a fertiliser plot. This means a plot (10 m x 10 m) that is either over-fertilised with around 30-50 kg N/ha more than the normal amount or under-fertilised with 30 kg N/ha (at least) less than the normal amount.
When the growth around the over-fertilised plot starts to look paler green than the growth in the plot, this means it is starting to suffer from lack of nitrogen. When the growth in the under-fertilised plot is starting to look paler green than the surrounding growth, this means that the plant-available nitrogen for the main crop will soon start to run out.
An over or under-fertilized plot in the middle of the parcel can help to determine the plants need of nitrogen. Photo: Agrimarket.
An over-fertilised and/or under-fertilised plot in the middle of the field can help to determine the crop’s need for nitrogen. Photo: Agrimarket.
Another way of estimating crop nutrient requirements during the growing season is by using a SPAD metering device. It measures the chlorophyll content in the leaves, which indicates the nitrogen content in the plant.
The SPAD metering device can measure differences in plant nitrogen content in a fertiliser plot and surrounding growth, or just plant nitrogen content, which can be compared against table values to identify lack of nitrogen. If the SPAD metering device is not available, so-called KEG colour cards can be used.
The SPAD metering device measures the chlorophyll content in the leaves which indicates the nitrogen content in the plant. Photo: Airi Kulmala.
It is useful to combine colour monitoring with plant sampling. Plants are cut above the soil surface and weighed (5 x 1-m row lengths).
Based on information about colour value, plant mass, seeding date, rainfall and temperature sum, it is possible to predict the yield level and its protein content. Recommendations for complementary N fertilisation can also be calculated.
The simple computer program for this (Laatusadon malli -laskuri in Finnish) can be downloaded from the internet. In Finland, agricultural supply stores also offer colour monitoring and calculation services.
- Choice of fertiliser type for complementary fertilisation
-Appropriate choice if the amount is high (over 10 kg/ha)
-Not good in dry conditions
- Spraying liquid fertiliser in the growing crop
- Appropriate for only smaller amounts (max. 10 kg/ha) because larger amounts can damage the leaves
- Suitable also for dry conditions, although strong sun may burn the leaves
- Applying liquid fertiliser to the soil surface via trailing hoses
- The amount can be as high as with solid fertilisers
- Suitable also for dry conditions
- Requires special equipment (hoses attached to the sprayer)
Complementary nitrogen fertilisation has a different effect on yield depending on its timing. Fertilisation at stem elongation stage or before adds and maintains the number of grains.
Fertilisation during the flag leaf stage or after increases the size and protein content of the grains.
In Finnish field trials, split application of nitrogen fertilisers for spring wheat increased yield and grain protein content (Kauppila, 2006).
Trials were performed in four years (2002-05) and fertilisation amounts were 140 kg N/ha at sowing or 105 kg N/ha at sowing plus 35 kg N/ha later during the growing season. On average, the split fertilisation increased the yield by 100 kg/ha and its protein content by 0.5%.
A new trial on spring wheat was carried out in 2009 (Kauppila, 2009).
The nitrogen fertilisation amounts were:
- 130 kg N/ha at sowing
- 87 kg N/ha before sowing plus 43 kg N/ha at sowing
- 43 kg N/ha before sowing plus 43 kg N/ha at sowing plus 44 kg N/ha at the tillering stage
- 43 kg N/ha at sowing plus 44 kg N/ha at the tillering stage plus 43 kg N/ha immediately before the earing stage.
The yield was best when all fertilisation was given at sowing (option 1) and second best with option 4, followed by option 3.
The yield was lowest, nearly 100 kg/ha lower than in the best option, in option 4 2. However, the grain protein content was highest in option 4 and second highest in option 3 (2% lower than in the best option).
Kerminen (2010) analysed FarmitNetWisu research data (information about crop yields on participating Finnish farms) for 2009 and found that on farms using split application of nitrogen for spring wheat the yield level was 720 kg/ha higher than on farms with one application. With the split application 16 kg/ha more nitrogen was used. This means that 44 kg wheat was produced per kg extra nitrogen.
Grain protein content clearly rises when additional nitrogen is given during the milk ripening stage. The realistic target is for protein content to be increased by 0.5-0.7% units with 10 kg nitrogen.
On the other hand, Salo (2009) has reported that a complementary N fertilisation during the growing season of 2008 only had small effects on the yield of three spring wheat varieties (Marble, Epos, Trappe) in studies carried out by MTT Agrifood Research Finland and Yara. The N fertilisation levels were 130 and 150 kg N/ha.
Additional nitrogen (0 or 20 kg N/ha) was applied during the heading and the milk ripening stage. The yield was highest when 130 kg N were applied at sowing (Marble, Trappe). The yield of Epos increased slightly in most cases when additional N was applied.
The effect of complementary nitrogen on the protein content of the yields was significant. The protein content of Marble was highest when nitrogen was applied during the milk ripening stage. In the case of Epos, the protein content was highest when additional sulphur was applied together with nitrogen and the effect of timing was negligible.
The yield level of Trappe was probably so high that all nitrogen was needed to produce grain and protein content was unaffected (Salo 2009).
Splitting the nitrogen dose reduces the risk of plant uptake being sub-optimal and of leaching of nitrogen. Leaching of nitrogen is a high cost for the farm, as more efficient use of the applied nitrogen will lower the amount needed and therefore lower the costs of buying nitrogen. Split application can also lead to yield with better quality, which is very important for farm finances.