German Society of Agronomy

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Whether as animal feed, a protein-rich staple food, or simply a tasty vegetable, the uses of this crop, which was domesticated thousands of years ago, are manifold. Its high content of valuable proteins and starch makes it one of the world's most important legumes after soybeans in terms of area and yield. A total of 14.6 million tons of grain peas were produced on 7.2 million hectares in 2020 (FAOSTAT 2022). The largest producers are Canada, Russia, and China.

In Germany, approximately 97,700 hectares were cultivated with grain peas in 2021 and 4,550 hectares with vegetable peas in 2020 (FAOSTAT 2022, DeStatis 2021). This represents an increase of 18% for grain peas compared to the previous year. This makes peas the most important grain legume in Germany ahead of field beans, but overall they are only cultivated on a good 2% of arable land. One reason for this is the relatively high volatility and often low yields of both grain peas and other grain legumes compared to winter cereals, for example (Reckling et al. 2018). In 2020, the average yield for feed peas was 36 dt per ha, while the average yield from 2014 to 2019 was only 33 dt per ha (DeStatis 2021).

But it is not only the yield that is important: mainly grown as a summer crop, peas can break chains of infection in cereal- or winter crop-dominated crop rotations and counteract one-sided weed infestation. As a humus enhancer, they ensure that the soil is enriched with organic matter, which in turn can improve the infiltration rate and fertility of the soil. These properties are becoming increasingly important in the context of climate change. When choosing crop types, producers must therefore always consider the beneficial effect of peas as a preceding crop. Peas are also becoming increasingly interesting as a winter crop due to advances in breeding. However, the higher yield potential and better utilization of winter moisture are usually accompanied by a higher risk of disease.

Significance in organic farming

Even though they account for only a small proportion of conventionally farmed arable land, legumes play a prominent role in organic farming and are an indispensable part of any crop rotation. The current yields of wheat, barley, and rapeseed in conventional agriculture would be unthinkable without the energy-intensive synthesis of ammonia from atmospheric nitrogen. Organic farming has to do without this and relies on the natural fixation of atmospheric nitrogen by bacteria that live in symbiosis in root nodules with plant species of the legume family (Fabaceae). In peas, these are bacteria of the genus Rhizobium. The nodules form after the bacteria have penetrated the roots of the plants.

The bacteria now produce nitrogenase, the enzyme that converts one molecule of atmospheric nitrogen (N2) and eight hydrogen ions and free electrons into two molecules of ammonia (NH3) and H2. This process requires energy in the form of ATP. Ammonia is quickly converted into ammonium (NH4), which serves as nutrition for the plant. The nitrogen absorbed in this way can now be incorporated into amino acids – the building blocks of proteins. In return, the bacteria are supplied with sugar by the plant. Nitrogenase is sensitive to oxygen. To ensure a low oxygen concentration in the nodules, legumes and nodule bacteria produce leghemoglobin, which binds oxygen much more efficiently than, for example, the hemoglobin in our blood.

The nitrogen fixation capacity of peas

Due to symbiotic fixation, up to 80% of the nitrogen contained in the grain yield of peas can be of atmospheric origin (Matus et al. 1997). With a yield of 30 dt per ha and a protein content of 25%, this amounts to approximately 95 kg per ha. The remaining 25 kg is nitrogen that has been absorbed from the soil by the plants. The harvest residues naturally also contain nitrogen. These usually remain on the field and serve as nitrogen nutrition for the subsequent crop.

According to the literature, the annual nitrogen fixation capacity of peas can be up to 300 kg per hectare. However, this is only the case under optimal conditions; average values are usually only 100 kg, and the proportion of atmospheric nitrogen in the total nitrogen in the yield organ is on average closer to 65%, so that in most cases a more or less balanced nitrogen balance can be assumed (Gollner et al. 2019). This means that the nodule bacteria fix about as much nitrogen as is removed from the field with the yield. In conventional agriculture, this means that peas do not need nitrogen fertilization. This would even have a counterproductive effect on rhizobian symbiosis.

Laut Literatur kann die jährliche Stickstofffixierungskapazität von Erbsen bis zu 300 kg pro Hektar betragen. Dies ist jedoch nur unter optimalen Bedingungen der Fall; die Durchschnittswerte liegen in der Regel nur bei 100 kg, und der Anteil des atmosphärischen Stickstoffs am Gesamtstickstoff im Ertragsorgan liegt im Durchschnitt näher bei 65 %, sodass in den meisten Fällen von einer mehr oder weniger ausgeglichenen Stickstoffbilanz ausgegangen werden kann (Gollner et al. 2019) . Das bedeutet, dass die Knöllchenbakterien etwa so viel Stickstoff binden, wie mit dem Ertrag aus dem Feld entfernt wird. In der konventionellen Landwirtschaft bedeutet dies, dass Erbsen keine Stickstoffdüngung benötigen. Diese würde sich sogar kontraproduktiv auf die Rhizobien-Symbiose auswirken.

Grain peas in mixed cultivation

In conventional agriculture, peas are grown almost exclusively as a monoculture. This is unlikely to change any time soon, as herbicide application is particularly difficult in mixed crops that include legumes.

The situation is different in organic farming, where peas are increasingly being grown in mixed cultivation. Depending on the ripening time of the pea variety used, oats or spring barley are possible partners, and mixed cultivation with rye or triticale is also possible. In these mixed crops, the cereal serves as a support crop for the peas, while the peas fix atmospheric nitrogen. This combination results in good weed suppression and can significantly increase the productivity of the system (Chapagain and Riseman 2014).

The ripe grains of both partners can be harvested at the same time and then separated from each other by sieving. Oats and barley are usually combined with semi-leafless varieties, while rye and triticale are combined with normal-leaved varieties, which are then harvested as whole plants and ensiled. Semi-leafless varieties are those in which the pinnate leaves are transformed into tendrils to increase stability.

Continue with (German only):

• Cultivated plant with tradition
• Use
• Mendel and the Pea
• Different leaf types
• Breeding
• Culutivation of the Pea
• Vegetable Peas

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