How much nitrogen in the soil after legumes?
Legumes, thanks to symbiosis with bacteria that fix atmospheric nitrogen, are largely self-sufficient in meeting the demand for this nutrient. They obtain nitrogen "for free" and use it for their own needs, but after the end of vegetation, they leave crop residues rich in this "free" nitrogen in the field. The amount of this element, delivered to the soil with the residues after the legume harvest, depends of course on the species, the amount of biomass produced and the efficiency of nodulation. Determining the actual fertilizing value of plant residues of legume species is therefore not an easy task.
For the purposes of preparing nitrogen balances, the so-called nitrate program includes average amounts of active nitrogen remaining after the cultivation of legumes. In the most common variant of cultivation of large-seeded legumes, i.e. their harvest for seeds and mixing crop residues with the soil, we can count on providing 25 kg of nitrogen for the successive plant in this way. When cultivating legumes in a catch crop mixture, 15 kg of nitrogen will go to the soil as green manure (table).
Type of pre-crop
Legumes in pure sowing
Fabaceae
in mixtures
with grass or cereals main crop catch crop main crop catch crop ploughed in crop residue 25 15 15 10 ploughed in whole
plants for green
fertilizer 50 25 30 15
Source: Annex No. 8 to the Programme (table 13) (Journal of Laws 2023, item 244)
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When will nitrogen from legumes reach the soil?
We know that legume crop residues enrich the soil with a certain amount of nitrogen. However, the question arises – whether and when this component can be used by the following plant? As Prof. UPP Dr. Hab. Witold Szczepaniak explains in an interview with farmer.pl, in order to "release" this nitrogen into the soil, legume crop residues must undergo mineralization. In turn, the rate of mineralization, and thus the rate of release of components from residues, depends, among other things, on the C:N ratio in the biomass and the course of weather conditions. Is it therefore possible that a winter plant grown after a legume forecrop will already use this pool of nitrogen?
– This nitrogen will be available and will be used by the successor plants, although not immediately and not in its entirety. The pre-harvest value of legumes is based primarily on nitrogen-rich crop residues. These residues must be mineralized, and since nitrogen promotes mineralization, it will be faster than in the case of cereal or corn straw. A lot depends on the weather factor. In favorable conditions in the autumn – warm and humid – the successor winter plant, such as rape, will partially use the nitrogen from the current mineralization in the autumn. The remaining part will decompose in the spring and will also be used – explains Prof. Szczepaniak.
In practice, therefore, in the case of a legume pre-crop, we consider the site to be rich in nitrogen and most often we give up the pre-sowing dose of this component for winter crops. Nitrogen from the current mineralization of legume plant residues is most often sufficient to cover the demand of winter cereals in the autumn. In the case of winter rape, whose nutritional needs for nitrogen in autumn are slightly higher – whether the amount of nitrogen released from crop residues will be sufficient depends on the rate of mineralization. If the conditions for this process are not favorable, it may be necessary to correct and apply nitrogen emergency after rape emergence.
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What about nitrogen in intercropping with legumes?
Farmers are looking for additional sources of nitrogen in various ways, such as by growing rapeseed with leguminous companion plants or undersown legumes in corn and other cereals. As Professor Szczepaniak points out, in these cases nitrogen will also be released into the soil, but it will happen later.
– If companion plants freeze during the winter and their remains begin to mineralize, some nitrogen will return to the soil in the second part of the spring vegetation – in warm weather in May and June, i.e. during the rapeseed flowering period or after it, and remember that 70-80 percent of nitrogen should be accumulated by the plants by the beginning of flowering. By growing rapeseed with companion plants, we therefore influence the balance of the component in the entire rotation, with the successor plants in mind – comments Prof. Witold Szczepaniak.
As Prof. Szczepaniak adds, in the case of legume undersown in corn, the matter becomes even more complicated. The legume plant is not destroyed in this case (as is the case with catch crops or accompanying plants), so its remains will not undergo mineralization. The scientist adds, however, that small amounts of nitrogen are released into the soil solution during the vegetation of corn in the legume undersown from the ongoing mineralization of root nodules, fragments of the root system or falling, older leaves of the legume plant that die during vegetation.
