Nitrogen fertilisers are fertilisers that contain a lot of nitrogen (N) – often as the sole nutrient element. Many fertilisers are used as mixtures of substances. In addition to nitrogen, they also contain other plant nutrients.
Nitrogen fertilisers play an important role in the garden, because plants cannot exist without nitrogen. Although the air contains plenty of the chemical element, they cannot use this portion directly. They have to take it up from the soil in plant-available form – as nitrate or ammonium ions. Nitrogen is a central component of proteins and chlorophyll. To ensure that the plants are well supplied, the nutrient must be supplied in sufficient quantities and at an appropriate time.
Nitrogen fertiliser: Origin
The different fertilisers can be of mineral, organic or synthetic origin. At the beginning of the 20th century, the Haber-Bosch process made it possible to produce ammonia (NH3) from atmospheric nitrogen and hydrogen in large quantities. This chemical substance is used to produce the nitrogen fertilisers urea, ammonium nitrate, ammonium sulphate and ammonium phosphate.
Among the nitrogen fertilisers, urea has the highest nitrogen content at 46 %.
Ammonium nitrate, which is also often used, has a nitrogen content of 35 %.
Horn meal is a natural nitrogen fertiliser that contains about 10 – 14 % nitrogen.
Nitrogen fertiliser: effect and application
Mineral nitrogen fertilisers contain the nutrient as a water-soluble salt that takes effect quite quickly – nitrate (NO3-) is easily absorbed by plants. The application of nitrogen in the form of nitrate fertilisers, such as calcium ammonium nitrate, therefore provides an immediate supply of nutrients.
Fast-acting nitrogen fertilisers are applied in cases of acute nitrogen deficiency or when certain plants, for example a delphinium, are to develop a second flowering and need a lot of nutrients for this feat.
In the case of nitrogen applied as urea (CH4N2O) or ammonium (NH4+), the fertilising effect also becomes apparent relatively quickly. However, the substances are first largely converted to ammonium (from urea) or nitrate (from ammonium) in the soil before they are taken up by the plants. Ammonium is generally slower acting than nitrate.
In organic fertilisers, the nutrients are often part of carbon and protein complexes, so they are not directly usable by the plants. The fertiliser must therefore first be mineralised by the helpers in the soil (bacteria). Horn meal takes effect after about four weeks. Horn shavings release nitrogen only after two to three months – but the effect then lasts for months.
If the pH value of the soil is to be lowered for bog plants, urea or sulphuric acid ammonia can be used. The latter also provides sulphur for certain vegetables, such as cabbage. Calcium cyanamide, on the other hand, raises the pH value, i.e. it has an alkaline effect. It is only suitable for basic fertilisation of an area without vegetation, as cyanamide is released during application, which has a herbicidal effect.
Burns can occur if mineral fertilisers are not applied properly. Water thoroughly after fertilising and never fertilise on hot days! This is because it is very easy for high salt concentrations to develop in the soil, which damage the plants. By the way, this also applies to foliar fertilisation.
Organic fertiliser should be worked lightly into the top layer of soil when applied – this allows the soil organisms to become properly active.
Danger of overfertilisation
Nitrogen can be present in the soil in different forms. Urea (which can also be supplied via animal excrement) is converted into ammonium ions by soil bacteria.
Nitrogen present as ammonium can be taken up by plants, but is also bound to soil particles. Gradually, soil organisms convert it into the quickly available nitrate – this process is called nitrification.
Nitrate is the substance that becomes a problem when overfertilisation occurs. It can migrate into the groundwater, especially in autumn after the harvest and during heavy downpours. Nitrate should therefore never be present in the soil unused in large quantities. In groundwater – and ultimately in drinking water – it can be converted under certain conditions into nitrite, which poses a health risk. So be careful with fast-acting nitrogen fertilisers!
Too much nitrogen can also impair the vitality of plants. An excess of nutrients causes excessive growth in length and produces soft, spongy shoots and cells. This increases susceptibility to frost and heat, decreases the storability of harvested produce, and plant pests as well as bacterial and fungal diseases set in more easily.
Therefore, you should only fertilise selectively and in the garden it is best to use organic fertilisers or slow-release fertilisers that release nutrients only slowly. On light soils you should divide the total amount into several applications. The application period of all fertilisers should always be limited to the main vegetation period (mid-March to September).
Blue grain is a mineral compound fertiliser or complete fertiliser that contains nitrogen as well as phosphorus and potassium, and is popular with many amateur gardeners. Its use can easily lead to overdosing. For environmental reasons, therefore, the Blaukorn NovaTec®, for example, is recommended, as it contains so-called nitrification inhibitors.
Good for soil life: Organic nitrogen fertilisers
Mineral fertilisers provide little food for useful soil organisms and thus contribute to an impoverishment of the soil fauna. Humus decomposition also often progresses more quickly this way.
Traditional methods in the garden are fertilisation with manure or compost and crop rotations with legumes, all of which also have a positive effect on soil life and soil structure. Compost is a complete fertiliser that usually contains all plant nutrients. A home-made nettle liquid manure supplies the plants with nitrogen, potassium and other important minerals, such as silicic acid.
The legumes form symbioses with certain soil bacteria, the rhizobia. These can process atmospheric nitrogen into ammonium and thus make it available to the plants. In organic farming it is therefore common to cultivate legumes as catch crops and to work them into the soil. This method of green manuring can also be used in the vegetable garden.