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Making sure your soil conditions are set up to beat black-grass

Poor soil health is contributing to the development of serious black-grass problems on many farms. But there are  cultivation and crop residue strategies which can help, says soil and tillage expert Steve Townsend. Abby Kellett reports.

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Making sure your soil conditions are set up to beat #black-grass #arable

Groundswell’s first No-till Show and Conference, held in Hertfordshire, attracted growers practising no-till as well as those keen to observe any benefits which can be gained from the system.


One of the most important benefits identified was the reduction in black-grass germination as a result of minimal soil disturbance. However, long-term soil improvement also has a part to play in discouraging the weed, according to soil and tillage expert Steve Townsend.


He identified a lack of carbon in UK soils as [one of] ‘the only problem with agriculture’ and claimed no-till, along with crop residue cover and a diverse crop rotation, were important in rebuilding carbon levels to improve soils and reduce black-grass abundancy.


See also: Five pointers to stack the odds against black-grass


He said: “If you want long-term control, you need to change the environment you have created, because at the moment, we have an environment which is fantastic for growing black-grass.


“Fundamentally, you should think of weeds as an indicator and to me black-grass is an indication of cultivated, wet, anaerobic, sour soils, so we have to change these characteristics.”


Which soil conditions allow black-grass to thrive?

Which soil conditions allow black-grass to thrive?


Cultivation of soils leads to oxidation, which is largely responsible for soil carbon loss, according to Mr Townsend.


“By ploughing deep, you basically lose the same amount of carbon as you would typically gain by chopping straw.”


He said opting for a no-till system helps maintain carbon levels, which in turn improves drainage.


“If you improve carbon, you improve drainage which puts pressure on water-loving weeds such as black-grass.”


Equally, he argued an increased worm number, which is seen as being an advantage of no-till systems, provides a form of natural drainage, making the soil dryer and less favourable to black-grass.


While some farmers cope with black-grass by burying the seed further down the soil profile, Mr Townsend said knowing where the weed seed is allows for best control.


steve townsend pic

Steve Townsend

“Black-grass is a ‘cultivation junkie’. In no-till, you leave any seeds which might return on the soil surface, which is not good news for a black-grass plant because it wants to be mixed in with soil.


See also: Ultra-low disturbance methods help cut costs and increase black-grass control


“When it is mixed, residual herbicides cannot do their job. However, if you keep the seed in the right place, roots have to grow down through the residual herbicide.”


Do not mix systems

It can take between three and 10 years before the full benefits of no-till can be recognised, but Steve Townsend warned when growers mix systems, they are not allowing these beneficial factors to accumulate.


Instead, he advised growers to stick to their chosen system, whether this be no-till, min-till or ploughing. He made it clear while raking and light disc cultivation is only light tillage, it is still minimum tillage and so weeds should be managed accordingly.


“Min-till needs consolidation and means frequently spraying off flush weeds. Not doing so would be a recipe for disaster.”


Mr Townsend suggested using crop residue as a form of mulching and shading, which he said are both good forms of weed control.


“Black-grass needs a light flush to start its germination process, if it is in the dark, it doesn’t get the encouragement to grow.”


He said establishing cover crops either pre- or post-harvest provides further residue to reduce black-grass germination, but also improves soil drainage and soil organic matter.


See also: Cover crops: How are they best used?

More recent research suggested decomposing cover crops create an allelopathic effect, which can reduce black-grass germination, but there is a danger cash crop germination may also be affected.


Mr Townsend said: “There are six acids which are excreted from biology when they digest cover crop residue. All those have an effect on germinating seeds, whether they be weeds or your crop.


“This is why it is important to drill underneath this residue and why we need to leave black-grass seeds on top.”


Soil nutrients and pH

Soil nutrients and pH

The assumption that calcium is the only nutrient which largely influences pH was described as ‘the biggest lie in agriculture’ by Steve Townsend.


“Magnesium and potassium have more of an influence on pH kilo for kilo than calcium does. I do not think we have been putting on enough calcium over the past 40 years.


“Calcium gets leached out of the soil profile, but magnesium does not and since magnesium tends to make soils wet, high magnesium soils can encourage the growth of black-grass.


“If you have a soil with a pH of lower than 7 and a magnesium index of 3 or above, you might be short of calcium.”


By rectifying this, growers can help dry out their soils while still maintaining a favourable pH, according to Mr Townsend.


Whether it be cover crop residue or previous cash crop residue, he urged farmers to avoid incorporation when possible.


“Residue needs to be kept on the surface – you should not have to incorporate it, it should be done by worms and fungi, not with steel.


“By incorporating straw, you are preserving it. Have you ever cultivated straw you buried last year? If you have, it is telling you you have cultivated your straw too deep for your soil type.


“If you want to make your soil anaerobic, which is what black-grass loves, bury residue too deep because this is the quickest way to do it.”


Take home messages

    • Aim to put more carbon into the soil than you take out
    • Chop straw and establish cover crops where possible for mulching and shading
    • Avoid incorporating straw mechanically to reduce carbon loss
    • Leave black-grass seeds on the surface to aid control and to maximise allelopathic effects
    • Consider applying calcium if soil pH is more than 7 and the magnesium index is more than 3
    • Stick to one cultivation system – do not switch between them
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