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Soils: capping and slumping

Course: Soil Management | Last Updates: 7th October 2015

 
Nick Caspell
Biography >>
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What are capping and slumping?

Capping occurs where the surface soil particles bind together creating an impermeable layer. This reduces the ability of soils to absorb water, leading to surface waterlogging and increasing the risk of run-off and erosion, even on very gentle slopes. The impermeable layer can also deprive seed of moisture and oxygen and can impede germinating shoots.

Where slumping occurs the soil may collapse downwards and also, on ridges and raised beds, sideways. Slumped soils may be more prone to erosion and surface runoff.

Where soil collapses into wheelings, roots, tubers or bulbs may be exposed, with resulting quality problems. Both capping and slumping can delay crop emergence and even reduce population density.

How are they caused?

slumping


The action of rainfall or inappropriate irrigation on fine soils causes the individual soil particles to flow together. They bind together tightly and are held by strong capillary forces as the soil surface dries, resulting in a surface cap or crust which may typically be of up to 5mm thick.

Slumping is where the soil profile tightens up, for example under the influence of heavy rain.

What cropping and soil types are vulnerable?

Low organic matter soils such as the fine sands and silts are most at risk. Newly-drilled and planted crops, where the soil has recently been moved and there is little surface strength, will be most vulnerable.

Problems occur most commonly in potatoes, sugar beet and horticultural crops. In these situations a fine seedbed is often needed to encourage germination, and soils are worked into ridged beds.

What can I do to reduce capping and slumping?

Using soil cultivation techniques that leave a coarse surface tilth ensures the structure is less prone to complete collapse and thus a cap forming. Leaving some surface trash will also help reduce the impact on the soil particles of rain droplets.

It also helps create a more open soil surface structure, aiding water infiltration rather than run-off. Such techniques may not be appropriate with all crops, such as small-seeded drilled crops or where there is a risk of disease carry-over.

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Over-cultivation can lead to structural breakdown and puffy seedbeds can be more at risk of capping and slumping. Using ridged rolls, such as Cambridge rolls, rather than flat rolls to consolidate soil prior to drilling or planting can also help.

Other measures include using mulches for a range of vegetable and salad crops, increasing organic matter with manure and home-produced or bought-in compost, subsoiling to break up soil pans and promote drainage, and the use of weed cover, as at Varfell Farm (see below).

Building up soil organic matter levels is a long term job, and sands and silts are hungry soils where it is difficult to achieve significant organic matter increases.

Why has Arthur Andrew’s system worked?

The growers in this part of Cornwall have identified that their soils are particularly vulnerable to capping and slumping. Here it is the combination of sloping fields, a naturally poor soil structure and cultivation of cut flowers and bulbs.

A thorough risk assessment highlights potential problems, and measures are targeted at times of the year, such as early autumn, when soils are most at risk. The weed cover is actively managed to encourage soils to hold their shape.

Aside from the spin-off business benefits, the really admirable aspect here is that growers have worked together with local authorities to ensure their soil management plan meets local objectives, and have received help and co-operation along the way.

The proactive measures and community involvement could easily be applied by many other growers in similar situations.

Case Study

Arthur Andrews – Penzance, Cornwall

arthur-andrews

Like most farmers, a fair proportion of Cornish cut flower and bulb grower Arthur Andrews’ time is spent managing weeds. But unlike most, he is encouraging them to grow among his 450ha of daffodils near Penzance.

Far from holding back the crop, they hold his fragile soils together, keep the mud off the 12-15 million bunches picked each year, and off the feet of the pickers themselves.

"We're seeking to get as much green cover on the ridges as we can – it knits them together and stops the soil from slumping. So managing our weed crop is almost as important as managing the daffs themselves,” explains Mr Andrews. “Although with 450ha in 400 fields, it can be a challenge at times.”

Weeds are rarely a farmer’s friend, but they’re not the only curious allies for the 550ha business Mr Andrews manages for Winchester Bulb Growers. For around seven years, he and other local growers have worked closely with the district council and local community to forge solutions to the particular soil challenges they face.

The Golden Mile Growers Group farms fertile, south-facing soils between Varfell Farm, Mr Andrews’ base of operations, and Penzance. “We’re blessed with a climate that allows growth for 12 months of the year. The soil is light and very free-draining and you can get three crops a year from it.”

But with just ten inches of topsoil overlying granite, often on very steep slopes, the structure of this black soil is poor, and contains a lot of small aggregate. “Once it’s wet, it cuts up and can be mobile, particularly in long periods of heavy rainfall.” This movement can pull apart the ridges in which the daffodils are planted and cause losses from the three-year crop.

It was a series of heavy downpours in 2000-01 that prompted the growers to seek solutions that would keep the soils together without denting productivity. Working with landlords and the farming officer from Penwith district council they introduced strategies such as planting across contours and put in silt traps.

"The council contributed through altering gateways, and stones removed from the field were placed in parking areas or gateways to prevent soil movement on to the road,” recalls Mr Andrews.

This proactive involvement means local objectives meet the farm’s business objectives. And a similar tack is followed with the Soil Management Plan. Before a field is rented, a full soil and risk analysis is carried out that details everything from nutrients to soil pans, gateways to drainage.

“It may seem a costly approach to regulation, but it tells us exactly what we need in terms of fertiliser and cultivations, so we can tailor the strategy to suit.”

Bulbs are planted from beginning of July to end of October, and weeds that develop are left to grow. “From September onwards the soils are vulnerable to movement, so we’ll flick some grass seed around to encourage the green cover.”

Mr Andrews aims for around four to five inches of weed growth on the ridge sides to hold them together. The crop itself takes over in the New Year, and picking takes place until April or May, when the canopy collapses.

weeds

“It’s then a question of managing the weeds – you can’t spray for fear of damaging the bulbs, so you have to do a lot of mowing.”

In late August and September glyphosate is applied, and a residual herbicide in December lasts through the winter. The crop canopy is already flourishing before the weeds begin to recover. But it’s not just their root system that proves useful.

“The dead weed cover means less soil cover on the pickers’ feet when flower-harvesting is taking place, and makes picking easier.”

It’s also a haven for wildlife, and the resulting biodiversity sits neatly with the farm’s environmental policy – sitting in the Marazion Marsh catchment and RSPB reserve, it is involved with several conservation projects and the Cornwall Wildlife Trust and hosts farm walks.

The sustainable outlook is good for business. “Our customers are the high street multiples, who are increasingly proactive in environmental and sustainable farming methods. But business isn’t the only reason.

We’re custodians of the land. If we don’t treat it with respect, we’ll lose it. So if we can change something to help, and it’s cost-effective, it’s worth doing.”

Three Golden Rules to tackle slumping

  • Short row lengths – break up anything longer than 100 metres, or it’s a recipe for large soil movements on light soils.
  • Keep it green – establish a cover as soon as you can after planting and maintain it for as long as you can.
  • Manage your weeds – keeping them under control over summer is the real challenge, then spray them when it’s safe for crop and environment. 

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