31 october 2017
Nitrogen (N) is an important nutritional element for plants in order to grow properly. Of all the nutritional elements in plants nitrogen often has the highest levels. Rightly so this month the first in a series ‘Focus on nutritional elements’: nitrogen.
Under normal circumstances nitrogen (N) is absorbed by the roots as nitrate (NO3-) or ammonium (NH4+). About 70 percent of this is stored in the leaf green (chlorophyll) of the plant. The nutritional element plays an important role in the metabolism and many other processes in the plant. A sufficient nitrogen supply improves the growth of leaves. The nitrogen level in plants varies from 0,3 to 6 percent of the dry matter.
Nitrogen is in mineral and organic fertilisers. In mineral base fertilisers for growing media the rate ammonium (NH4+)/nitrate (NO3-) is usually around 40/60. This proportion is important when it comes to influencing the pH. In organic fertilisers nitrogen is freed as ammonium because of the decomposition of micro-organisms. Ammonium is then converted to nitrate by other bacteria. This process is called nitrification. This process requires oxygen. Substrates with a low air percentage or substrates in wet cultures, have a very low oxygen supply, which slows the nitrification process. The nitrification bacteria are also not naturally present in all substrates, which means that the process doesn’t always occur.
Too little or too much nitrogen, both are not good for the crop. In bio assays RHP usually clearly perceives the effect of a nitrogen deficit because of which the crop won’t grow (see graph below). Also because of the deficit the colour of the leaves can change uniformly lighter or even completely white (in Chamaedoria), in tomato plants the leaf veins can turn light purple and in crops of deciduous trees the leaves fall earlier in the season. When there is a serious lack of nitrogen, the crop is more susceptible to diseases and plagues. Nitrogen excess expresses itself in a dark green colour of the crop and also stunts the growth. However, excess of nitrogen doesn’t occur quickly.
The results of bio assays with violets and lettuce. The effect of the nitrogen level (N in mmol/l) is clearly visible in the growth and the plant weight.
Effect on pH
The effect of fertilisers with a higher level of ammonium (NH4+) is a (usually unwanted) decrease of the pH in the growing medium. This is because plant roots react by giving off H+-ions. With a higher nitrate level (NO3-), the pH will increase. Growers can use this in their advantage to create a desired pH balance in their culture.
Another disadvantage of ammonium is that at high pH it will disintegrate into ammonia (NH3) and H+. The toxic effects of ammonia on the plant’s metabolism will cause damage to the crop. A few plants are capable of quickly attaching ammonia to amino acids, so that a negative effect is avoided. Ammonia can also evaporate, which can lead to leaf burning in closed greenhouses.
Bio assay with lettuce. The difference in growth is clearly visible. The stunted growth is caused by a lower nitrogen level.
Fertilisers that are used in RHP substrates are always checked on the levels specified by the producer. Substrates with the RHP quality mark need to comply with product specific, chemical requirements with a minimum and a maximum level of nitrogen. Consumers who choose potting soil with the RHP Consumer quality mark can count with this on a good start for their plants. After 4 to 6 weeks extra fertilising is usually always necessary. For substrates with the RHP Horticulture quality mark the substrate producer and the grower decide together what the level of nitrogen needs to be, suitable for the culture.