A portrait of catch crops

Humus loss, erosion, nitrates, pesti­cides in flowing waters and ground­water, as well as the desire for greening and the onset of climate change are giving broad support to inter­crop­ping. We present a portrait of twelve impor­tant types of catch crops.

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.