Cover crops – lots of advantages but no silver bullet – Potato Review
Agronomist and Potato Specialist Andrew Goodinson discusses how to get best results from haulm destruction strategies ...
Potato growers could get some welcome help from cover crops in building soil fertility in the battle to keep inputs to a minimum without compromising quality or yield.
Cover crops take up residual N left behind in the soil or released from crop residues after harvesting the preceding cereal crop. When the cover crop is destroyed in the spring, the N is returned to the soil and some of it becomes available to the next crop, and although the amount of N released depends on a range of factors, such as soil, rainfall and species of cover crop used, it still provides a useful input.
“Cover crops are an integral part of good soil management, and the aim should be for no bare stubble fields over winter,” said Andrew, but said they are not “a silver bullet”.
“Cover crops should provide around 20t/ha of biomass above ground with a healthy root biomass, thereby reducing the impact of the rain on the bare soil. The weight of winter rainfall is often referred to as having the same weight as eight bags of potatoes on top of each other, pushing the soil down. Having the ground covered helps reduce the impact of this weight,” he said.
By having roots in the soil, cover crops help to reduce soil erosion and the consequent loss of nutrients such as phosphates. Other benefits include reducing run-off and leaching of all important nutrients, including potash, magnesium and sulphur, which is particularly important when fields are close to river courses.
“By taking up N, there is less risk of nitrate leaching in winter months after rainfall,” said Andrew.
“Moreover, cover crops help improve soil structure and therefore improve organic matter content and water holding capacity. This can improve soil resilience and also cash crops become more resilient to adverse conditions.”
Nonetheless, cover crops have to be cost-effective, he said, noting that planting a radish cover crop with the aim of avoiding the need to subsoil, needs to cost the same, or less, than the cost of cultivation.
A cover crop costs around £40/ha to grow, and it’s about another £30/ha spend on cultivations to get planted. However, a cover crop can supply 35-40kg/N and 15kg/K, which suggests a gross margin of around £50/ha.
“It can be hard to assign a monetary value for the overall benefits of a cover crop to justify their use to a landlord, but cutting and measuring the biomass can help to assess the benefits. All the different species do different things, for example rye is a good scavenger, and it grows well and acts as an effective weed suppressant, while vetches provide N and phosphate for the next crop,” Andrew said.
Decisions on which mix to buy will depend on the needs of the particular field and conditions and the grower’s system, and how it will be destroyed afterwards.
“Some growers flail and chop it, others disc it, and best practice suggests it should be incorporated into the soil before final cultivations.”
Cover crops on heavy land should be destroyed earlier than those on lighter areas because the soil can take longer to dry out before spring cultivations. If the aim of a winter cover crop is a balance between improving soil health and providing better forage for sheep grazing, there are some important considerations, Andrew said.
For example, sheep numbers need to be controlled, and care should be taken to not graze the cover crop down to the earth but leave 5-7cm of above ground biomass.
Addressing concerns about winter cover crops providing a green bridge increasing predator numbers in spring crops, Andrew notes that benefits from cover crops outweigh the threats. However, it can result in higher numbers of slugs and/or wireworm, which may need managing.
He said there have been suggestions that cover crops harbour pests such as rhizoctonia, and explains that weeds belonging to the Solanum species, such as nightshades and chickweed are more susceptible to harbouring the pest.
Aphids, including the peach potato aphid (Myzus persicae) and the Potato buckthorn aphid (Aphis nasturtii), tend to overwinter in hedgerows rather than the middle of the field.
“If you have cereal volunteers in the cover crop, it can provide a green bridge for the grain aphid, the bird cherry-oat aphid and the rose-grain aphid and the subsequent transmission of barley yellow dwarf virus BYDV. However, aphids that will be a problem for potatoes do not overwinter on cereals.
“We also need to remember that a cover crop will also provide a harbour for predators of these pests.”
Andrew explains that when a plant is under stress, it places a lot of N in its tissues which leads to high sugar levels which are attractive to pests such as aphids and slugs. “A cover crop provides soil conditions which promote better root growth and access to nutrients and water ‘from the plate and into the larder’, so the following cash crop is more resistant to adverse weather conditions, harvesting and pests.”
Given the bewildering array of cover crops on the market, there is a real need for clarity, and impressive-sounding claims often have little basis in fact. The choice of cover crop species and seed rate have a great effect on the success of the cover crop.
Andrew said: “However, as with any vegetation, the growth of cover crops is influenced by weather (temperature and moisture) and soil type. The cover crop market can be confusing and expensive, so the team at Hutchinsons got together and we evaluated a number of different crops and ranked them to make it simpler to understand which are the best plant species which are beneficial for the soil.”
This led to putting together some cover crop mixes with brassicas, legumes and forbs (eg linseed) in different ratios which offer varying degrees of biomass, roots and nutrients.
Vigorous rooting helps improve soil structure friability, while ground cover provides soil armour and weather protection, particularly over the winter months, he said. Mixes can also be made up from home saved seed from legume crops such as peas or beans or even volunteer peas or rape and make a cheap cover crop.
“One of the challenges growers face with cover crops is that the different-sized seeds in mixes can separate in the hopper, so we have replaced the percentage of seeds in the mix to the percentage of seeds per metre for the different species.”
While some in the industry are promoting cover crops as a panacea for problems with soils, they are just one of a number of elements which form part of a programme for soil health, says Andrew.
“Cover crops are not a quick fix and in heavy soils it can take more than one cover crop in the rotation before the benefits can really be assessed.”
A biofumigant is also a cover crop, but with some differences. They can help suppress some soil-borne pests (such as PCN and wireworm) because they have biocidal effects from glucosinolates.
There are a number of plants from the Brassicaceae family (which includes oilseed rape, cabbage, and radish) which produce these glucosinolates, and some of these have a higher concentration.
The most effective biofumigants for PCN suppression are Indian and brown mustards and they should be planted as soon as possible after harvest prior to the next spring’s potato crop, says Andrew. They are moderately winter hardy but a cold winter will remove the biomass.
However, the challenge with all biological solutions is getting everything right, from planting and emergence, through biomass building to maceration. Each step is crucial to success. Although, ideally biofumigant crops are generally grown in summer and chopped and incorporated before soil temps drop below 10 deg. C in autumn, in rented fields it is sometimes not a practical option because of late signed contracts and/or PCN testing.
“In that case you should be looking at planting before September 12th but there are some exceptions, like this year when the soils are still warm and they have some moisture, so you can delay for a few days.
“If your field conditions are not suitable, for example in very dry or very wet soils, you are likely to struggle to get establishment. Therefore, before taking the decision, you will need to consider whether it is advantageous to plant a biofumigant.”
Andrew emphasises that biofumigants need to be treated as cash crops with a difference and he recommends using either N or chicken manure so the crop grows as big and lush as possible, with a large canopy which will come into flower in the spring.
Destruction should take place at the onset of flowering and not be done later than 5 per cent flowering.
“This is the time when it has the highest level of glucosinolates, and so it will provide the most effective biofumigant properties. But you need to treat it like you would a chemical application.”
When it comes to destruction and incorporation, timing is key to making sure the compounds toxic to potato cyst nematodes are incorporated into the soil before they volatilise, which happens in as little as two hours.
“Flailing and incorporation should be done as with one pass, using a flail on the front of the tractor with a plough/rotavator/spader behind. Of course, maceration technique and timing are key to ensuring you get the maximum damage possible to the crop through both bruising and chopping which stimulate the release of gas.”
“Biofumigants are useful for getting on top of a PCN problem, but it is only one part of the picture, and is not a one-pronged solution.”
Maceration and bruising a brassica crop, such as Indian mustard, releases products from the plant’s metabolism, which are known as glucosinolates. These are then broken down by enzymes in the presence of water, to produce isothiocyanates (ITC) and volatile sulphur compounds, which have a biocidal effect.
Prior to renting, soil samples should be done well ahead – about 10 to 11 months – to give you time to put a cover crop in before the potatoes are planted, says Andrew, adding that this should not be a last-minute task.
Andrew also warns growers to avoid skimping on soil sampling when testing for PCN, particularly if they are planning on renting a field for growing potatoes or you suspect a field may be infested, warns Andrew.
This is because the pest occurs in hotspots, so it can easily be missed, and although in the West, PCN levels tend to be lower than in the Eastern counties of England, the pest is present in some fields.
If it is found to be present, integrated control, which will include the application of nematicides plus an Indian mustard biofumigant high in glucosinolates, will be necessary, and this implies costs of getting the field into conditions for growing a potato crop.
“There are only two nematicides, Nemathorin (fosthiazate) and Velum Prime (fluopyram), which have approval for use, so we may end up using all of these options,” said Andrew.
- Helping to suppress weeds with their biomass
- Improving drainage and reducing runoff
- Mopping up excess N and slowly releasing nutrients ready for the next crop
- Helping build soil organic matter and creating better conditions for beneficial soil micro-organisms
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