How can the nitrogen use be improved?

Before drawing up a detailed fertiliser strategy following aspects should be considered:

1. The nutrient need should be calculated on the base of realistic average yields in order to prevent high nitrogen surpluses in unfavourable years. In a good year, when everything matches up, the fertiliser gets used optimally and enough N is available from the organic matter. For example very good fertiliser efficiency can be achieved in high yielding years like 2015 with an optimal rainfall distribution and soil temperatures (Table 1).
2. A good soil rootability up to a depth of 1 meter is the most important requirement for a good nutrient use and water supply. Followed by a deep rooting crop the subsoil is kept porous. In dry conditions unfavourable soil horizons can be broken up. New compaction should be avoided.
3. Harvest residues have to be well spread, chopped and mixed into the soil. Straw mats and large stubble clods are nutrient competitors as well as preventing deep rooting of the subsequent crop.
4. The soil pH-value influences nutrient availability. In clayey soils also the soil structure and cultivability. The fertilisation strategy only works if the soil pH is suited to soil type and humus content. Especially on heterogeneous soils and in the case of site-specific sampling and fertilisation, respectively a correct soil type determination of each subsample is necessary (Table 2).
5. Organic matter improves water and nutrient retention, soil temperature and nutrient supply as well as soil structure and rootabiliy. If the whole crop or straw is removed from the field it should be compensated by applying fermentation residues, slurry, FYM or compost on arable farms. It is not viable to increase the humus content over 3 % in loamy and 3.5 % in clayey soils, respectively as the disadvantages of increasing nutrient fixation and soil hydrophobicity outweigh the advantages.
6. Additionally, catch crops improve the activity of microorganism.However, they have to be cultivated with as much care as the main crop. They also have to be suited to the subsequent crop and the site conditions! Their water usage of 50 to 100 mm/m2 has to be bared in mind. The amount will not be available to the subsequent crop on sites prone to autumn and winter draughtiness. On wet sites catch crops improve soil aeration and trafficability in spring.
7. Only healthy roots and stems are able to take up soluble nutrients. Disease tolerant varieties (e.g. Phoma stem canker and Verticillium wilt in oilseed rape). An early cereal infection by Fusarium, Rhizoctonia or Take-all can be restricted if drilling time is adjusted to soil temperature. Only if the mean daily temperature falls below 12°C the risk of root infection is reduced.

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Do not just keep an eye on nitrogen

Nitrogen has to be dissolved in the soil solution and close to the crop roots when it is required. The nutrients which are needed for the nitrogen use in the plant also have to be available. These are mainly sulphur and phosphorous but also trace elements like copper and molybdenum.

Sulphur is taken up by the plant as negative sulphate ion in the soil solution and is subjected to the same nutrient dynamics as the nitrate ion: Sulphate is leached when it rains onto water saturated soil. If soil temperatures are too low over a longer period sulphate is not released from the organic matter. Plants can only use the starter nitrogen if sufficient sulphur is also fertilized. Cereals need for 7 kg N/ha and oilseed rape for 5 kg N/ha one kilogram of Sulphur per hectare.

Especially in North Germany the last three wet winters had a severe impact on the soil nutrient reserves. Potassium was also leached out of topsoils with a low sorption capacity. The long-term potash trail shows clearly the relation between nitrogen use and potassium and Sulphur supply (Table 1).

Trace elements are important for the reduction of nitrates to amino acids and also improve the N use. Copper is fixed in humus soils. Therefore, it is not even available to the plants if the soil is sufficiently supplied. Manganese deficiency occurs if the pH-value is high and the soil is not reconsolidated enough or too dry. If the pH-value is below 6, molybdenum becomes marginal so that crops with a high molybdenum demand like oilseed rape need a foliar application. In general magnesium should be applied if the soil is undersupplied, wet or has a low pH-value.

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Have a flexible fertiliser strategy, early application of quality fertiliser

Strongly variable weather conditions and a limited application period require a more flexible fertiliser strategy. For example in the spring of 2016 an early vegetation started in combination with cold weather, less fertile soils needed a quick nitrogen and sulphur availability from the starter application. In this case calcium ammonium nitrate (KAS) or ammonium sulphate nitrate (ASN) was more suitable than applying urea. At a soil temperature of 2 to 5 °C urea needs about six weeks until half of the nitrogen is reduced to nitrate, at a temperature of 10 °C at least two weeks. If the soil is water saturated or too acid the reduction is slowed down. The nitrate ratio in KAS and ASN is readily available as soon as the dissolved fertiliser is present in the rooting area.

Often only about 10 % of late applied fertiliser at ear emergence to improve protein content is actually used, although, it can drive a significant grain protein response. However, for a better efficiency it must be applied earlier up until flag leaf emergence in the future. It will depend on yield performance if the necessary protein content is to be achieved. Therefore, with higher yielding crops protein dilution is increased due to higher grain starch levels.

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New techniques improve N usage

Applying nutrients close to the plant roots (strip tillage, mineral fertiliser placement) is interesting on dry sites, for row crops and oilseed rape. If slurry strip tillage is carried out the volatile ammonium ratio is fully available to the crop. A nitrification inhibitor applied to the standing crop improves slurry efficiency. In general, there is still an immense development potential concerning slurry application as still a lot of water (300 l/kgN) has to be transported and applied. Furthermore, problems with sloppy fields are not solved yet.

Summary Only healthy plants in good structured soils are able to use nutrients optimally. The fertilisation strategy should take the interaction between soil physics, chemistry and nutrients into account as well as the interaction of various nutrients with the plants’ metabolism. The fertiliser application technique can also still improve nutrient usage in many cases.

Status: 27.12.16