Back when crop rotations of rapeseed, wheat and barley were considered best practice, there were only a few farms that focussed on cultivating catch crops. Why should they, given that the soils, fertiliser and weather supposedly worked well despite the close crop rotation? So why cultivate something ‘in between’? Instead, high crop yields strengthened the tendency for monotonous cultivation methods of the time.
But those days are over. Humus loss, erosion, nitrates and pesticides in surface and ground water, as well as the desire for greening and, finally, the onset of climate change, are now providing broad support for intercropping. Cover crops are now thriving all over: Oil flax, lupins, field beans, vetch, various clovers, phacelia, oil radish or gold-of-pleasure – to name but a few.
Christoph Felgentreu, who has long been responsible for catch crops at the German seed breeder DSV, continues to emphasise the importance of mixtures. “The greater the variety, the less complicated the cultivation,” emphasises the 67-year-old agricultural engineer. “This also reduces the farmers’ concern of doing something wrong. Since the catch crops pioneer’s recent retirement, he has been even more active in promoting the holistic advantages of using gold-of-pleasure, radish and others in the ranks of the Interessengemeinschaft Gesunder Boden (Community of Interest for Healthy Soil). “The interaction between the individual species is of great importance here; they can both co-operate and compete.” It’s best to do both, says Mr Felgentreu: “That’s when the most power develops in the soil.”
Before these desired activities can begin, however, it’s important to avoid a pitfall in agricultural practice; namely, when things get tight in the summer in terms of labour economics or availability for cultivation – it can’t be managed on the side. This can take its toll, because farmers who neglect the relationships between intercrop, main crop and soil nutrients – as well as carbon nitrogen (C-N) ratios – can quickly experience counterproductive effects.
However, those farmers who work knowledgeably and carefully with the complex material will quickly notice the lasting positive effects on cultivation, fertilisation, and soil fertility, and it is not only Mr Felgentreu who is deeply convinced of this. After three to five years, for example, soil fatigue will be overcome again, adds the nationally known expert. “To a large extent this is the result of the immensely important work done by the roots of the catch crop and their exudates.”
To ultimately select the right mixture for your own location and crop rotation, it is therefore even more important to know which species they consist of. In a discussion with several experts, although certainly not all could be considered, 12 important types of intercrops emerged, which are presented below:
1. The field bean (Vicia faba)
The taproot is a protein power plant which within just a few months can produce more than 500kg N/ha. In addition, the deep root system and the associated exudates revitalise the soil right down to deep layers. The field bean is also cultivated as a companion plant for sunflowers and rapeseed, as well as maize and cereals. Field beans require a lot of moisture to germinate, which is why a significantly deeper sowing depth is necessary compared to all other intercrops.
2. Buckweat (Agopyrum esculentum)
The protein-rich grains resemble beech nuts. It suits best as a short intercrop before an autumn crop because it can regerminate when sown too early. Buckwheat can fix large amounts of phosphorus and calcium via its roots together with mycorrhizae, its special talent. Disadvantages: Very sensitive to frost, hardly any nitrogen-fixing properties.
3. Clover: Incarnate (Trifolium incarnatum),
Alexandrine (Trifolium alexandrinum),
Persian (Trifolium resupinatum)
Valued legumes, of which the garnet-coloured incarnate clover is said to be the hardiest. Nitrogen bound by incarnate clover is only released slowly and therefore not fully available to the following crop. Alexandrine is characterised by rapid growth – provided it gets sufficient warmth and water. It is more suitable as a plant cover between two main cereal crops. Persian clover is not much different from Alexandrine, but is considered more robust to cold, wet, and calcareous soils.
4. Gold-of-pleasure (Camelina sativa)
A vegetation period of only 100 days is sufficient for seed formation to occur. The crop is characterised by a high displacement performance against weeds and proves to be well tolerated in mixtures. The relatively short oil plant with its upright growth fits well, for example, with vetch or soy. Mr Felgentreu considers gold-of-pleasure to be an indispensable mix component in water conservation areas.
5. Lupins (Lupinus albus)
A member of the legume family, sometimes also called the wolf bean. Breeding has almost eliminated the poisonous bitter substance lupinin; the lupins usually have long-stalked and soft, green to grey-green leaves that are often densely covered with silvery hairs. It is a deep-rooted legume. Its very close relationship to peas and beans requires a careful look at the crop rotation, otherwise serious problems can arise in the subsequent cultivation.
6. Oil flax (Linum usitatissimum)
The German seed producer PH Petersen has it in a mixture before maize, and there are several other suppliers of catch crop mixtures who integrate oil flax, not least to serve as a support crop for legumes. Overall, the oil flax is rather ascetic, does not make great demands on nutrients and water, and with its pale blue, white or even red flowers, it adds a refined touch of colour to the landscape. On the other hand, biomass formation is rather weak compared to others.
7. Oil radish (Raphanus sativus sp)
This cruciferous plant fits all soils, is easy to sow, germinates quickly and grows well in both dry and wet conditions. But advisor Nicolai Hilbert-Pack warns: “When it reaches seed maturity, it brings problems,” and urges timely termination. Despite this risk of sprouting, Mr Hilbert-Pack is a fan of the crucifer because even with a late sowing date, it has a favourably narrow C-N ratio and thus potentially provides good amounts of nitrogen to the following main crop.
8. Phacelia (Phacelia tanacetifolia)
This purple beauty is hard to miss and can be admired in many locations. Bees like the flower head. “It requires a good seedbed, and chaff and straw must be well distributed,” says Mr Hilbert-Pack. “Phacelia seed should not be placed deeper than two centimetres.” Phacelia has a high organic phosphate absorption capacity. In combination with clover, phacelia can provide around 60-70kg/ha of nitrogen for the main crop.
9. Ramtilla (Guizota abyssinica)
Belongs to the asteraceae family. An annual mass producer, it is related to the sunflower. Ramtilla has also rightly come into focus, as Mr Hilbert-Pack says, because it freezes off safely. Ramtilla is also susceptible to slugs, which is why it is also valued as a companion plant in rape. According to many connoisseurs, ramtilla and phacelia go well together. As a composite flower, the lush intercrop is also not a carrier of diseases for subsequent main crops.
10. Black oats (Avena strigosa)
The level of awareness of this sweet grass (Poaceae) has been rising unwaveringly in recent years. It suppresses nematodes, closes the plant population well, successfully suppresses the remaining weeds and impresses with its root performance. In addition, Avena strigosa has a strong mycorrhizal effect with a growth height of about 1.50m. Black oats should be mixed with legumes if not in water conservation areas, so that they do not cause nitrogen deficiency in the following crop due to their wide C/N ratio.
11. Mustard (Sinapis alba)
It is a favoured pasture for bees, bumblebees and hoverflies. Yellow mustard is one of the fast-growing plants; it takes just 30 to 45 days from sowing to the closing of the stands. But caution is needed as it has a negative effect on the N-bacteria and acidifies the soil; if the pH value in the rhizosphere falls below five, this is very bad for earthworms and the entire soil structure.
12. Summer vetch/winter vetch (Vicia sativa)
The comparatively expensive seed of spring vetch quickly forms a dense mass; it binds a relatively large amount of nitrogen and complements well with oats and clover to form a whole-crop silage (WCS). Vetches – similar to incarnate clover – are excellent at nutrient digestion and, due to their protein quality, indispensable in promoting humus dynamics.