The oncoming amendments of the fertiliser regulations will not only be achieved by reducing application rates. Dr. Ute Kropf (FH Kiel), Department of Agriculture, explains that if nitrogen rates are already tightly calculated it is necessary to improve nitrogen use from applied fertiliser as well as soil reserves.
Before drawing up a detailed fertiliser strategy following aspects should be considered:
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.
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.
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.
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.