Spotlight on nitrogen

11 May 2020 - Articles

Nutrients are very important to plant growth. Suboptimal (too much and too little) fertilization can make a big difference in yield. A good reason to take a closer look at the role of the various nutrients in the plant. After all, if there is a deficiency in a nutrient, one will pay for it in less production and poorer quality. Let’s begin with the element Nitrogen (N).

Nutritional elements can be classified according to the function they have in the plant. The different functions can be divided into four groups. Nitrogen is quantitatively the most significant nutritional element for plants. This element is not only a building block for the proteins in the plant; it also plays an important role in cell division. Moreover, nitrogen is a component of enzymes, chlorophyll, DNA and vitamins.

Nitrogen is quantitatively the most significant
nutritional element for plants

If there is a nitrogen deficiency, the plant looks weak and grows slowly. The young leaf gets smaller, the old leaf turns yellow and dies prematurely. Nitrogen is a mobile element; a deficiency is therefore first visible in older foliage.

Nitrogen excess, however, is also harmfull. Too much nitrate-nitrogen (NO3) causes symptoms of salt damage and also hinders the absorption of another important nutritional element: phosphate (P). When the concentration of ammonium nitrogen (NH4+) is too high, it causes root burn and obstructs  the absorption of other positively charged nutritional elements: potassium (K+), calcium (Ca2+), magnesium (Mg2+) and iron (Fe2+).

Follow up feeding with nitrogen

Nitrogen can be given in various simple and composed fertilizers. Not only is the total nitrogen content of the fertilizer important, but so are the different forms of nitrogen. For example, calcium ammonium nitrate contains both nitrate and ammonium; ammonium sulphate contains only ammonium; and calcium nitrate mainly contains nitrate and a small part of ammonium.

In addition to nitrate, ammonium and urea as fast-acting N-forms, slowly releasing fertilizers may also contain various long-chain urea compounds that must first be converted into ammonium in the soil in order to be absorbed by the roots.

Organic or organo-mineral fertilizers contain organically bound N that is only released for absorption via microbial conversions in the soil as well. In addition, rapidly absorbed forms of nitrogen can be delayed by using coatings or nitrification inhibitors, which are added to a fertilizer.

The N content can vary greatly
after administering urea

Urea - unlike other nitrogen forms that are absorbed through the plant roots - is one of the few forms of nitrogen that can be absorbed easily by the leaves. This makes urea suitable for quickly adjusting a deficiency of nitrogen via the foliage. 

However, even when spraying, a large portion of the fertilizer drips from the leaves, and ultimately most of the nitrogen is still absorbed via the root. However, the small amount absorbed through the leaf already results in  a (temporary) colour effect (greener leaf). 

Urea passively penetrates the plant root, where it is converted into ammonium by the enzyme urease. This reaction also takes place outside the roots, in the soil, in the substrate moisture or in stagnant (irrigation) water. The rate at which urea is converted depends on the temperature, the EC and the pH. Temperature is the most significant factor in this. At 32°C, the conversion of urea is twice as fast as at 18°C. Due to conversion, irrigation water that is stagnant in a silo or pipe will contain less urea and more ammonium than fresh irrigation water direct from the fertilizer tank. The sodium concentration in a fertilizer tank is often too high for the enzyme, so no conversion takes place.

The nitrogen level can vary greatly after administering urea, due to the decomposition processes. If 50% of the total nitrogen is dosed as urea, it is not sufficient to measure only the nitrate content and the ammonium. Therefore, it is important to determine the urea content before calculating a recipe.

 At Eurofins Agro it is possible to measure supplemental urea in both drain water and organic and inorganic substrates (in addition to the usual pH, EC and major and trace elements).

Analysis provides an insight into the
available nitrogen

Understanding the amount of nitrogen available, and the form in which it is available, is important for establishing a feeding scheme. The analysis by Eurofins Agro provides this information. The following nitrogen analyses are available:

- Analysis of soil, water and substrate
The analysis reports of the root environment (water, organic substrate) list the different forms of nitrogen: nitrate, ammonium and, on request, supplemented urea. The Greenhouse Soil Check reports nitrate and ammonium; the Greenhouse Soil Stock reports the total N stock, C/N ratio and the nitrogen supplying capacity.

- Crop analysis
The crop analysis (dry matter analysis) reports the N-total, and the plant sap analysis of the PlansapCheck reports the ammonium and nitrate content.

- Fertilizer analysis
The analysis reports on synthetic and organic fertilizers list nitrate, ammonium, and, on request, additional urea.

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