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Soils: compaction

Course: Soil Management | Last Updates: 30th October 2015

 
Fraser Milne
Independent Consultant
Siguna Consultancy
Biography >>
FACTS2FADS2

What is compaction?

Compaction is over-consolidation of soils resulting in a layer of reduced soil porosity. This restricts water and air movement in the soil profile.

It affects seed germination, impedes root growth, restricting nutrient and water uptake, leading to a reduction in crop canopy area which then affects yield and quality.

Increased fertiliser application and irrigation will partially overcome the effects of soil compaction but increases input costs. Plants in compacted soils are also less resistant to pests and disease.

Soil in hands

How is it caused?

The use of larger and heavy machinery in agriculture over the years has increased the loads exerted on soils, often resulting in compaction. In the topsoil, this is created mainly by high ground pressure.

In ploughed fields a hard pan (10-20cm band) is frequently formed below the topsoil, at just below plough depth. Soil compaction is more persistent and harder to alleviate the deeper it gets.

Mineral movement in soils can also cause compaction. Calcium carbonate and the oxides of silicon, iron and aluminium can bind soil particles together in a hard brittle mass, making a naturally-occurring indurated (cemented, impermeable) layer.

What soil types are vulnerable?

Poorly-drained soils are more at risk of compaction, which in turn increases the risk of water-logging, erosion and run-off. Fine soil particles, such as clays, will bind together easily, causing smearing and compaction.

Wet soils are far more prone to compaction when worked, while conversely working dry soils will help to open up fissures.

What are the visible signs?

soil-compaction

Tell-tale signs include areas of a field with suppressed yield and thin crops. These are often seen in high traffic areas, such as gateways and headlands.

Investigation of the soil profile to identify areas of compaction is fundamental and remedial actions should be based on the findings. A spade is an old-fashioned but very useful tool.

What can I do to reduce compaction?

Restrict traffic movements within fields, especially when soils are moist. Stick to the same wheelings, such as tramlines, where possible, and keep non-essential traffic, such as grain carts, on the headlands. Use low ground-pressure tyres. Subsoil in dry conditions to open up fissures and lift pans.

Why has Nick Baird's system worked?

These fine, alluvial soils are typical of those that are very prone to smearing and compaction. Potato enterprises can be a particular problem because soils are worked and travelled on in late autumn and early spring when they are most vulnerable.

Timing has been the big winner here – moving the soil when it is dry in the summer helps shatter any pans that have developed and opens up fissures to increase rooting and nutrient uptake.

Attention is also paid to lifting rather than turning soils. This increases porosity but retains structure and does not expose fresh soil to the surface, where it could cap or bake, causing further problems.

The emphasis on precision has many soil-related benefits. The correct bed size ensures wheels stay clear of the ridge sides. This means less smearing and damage on the bed shoulders, which can expose tubers and roots.

When irrigating, an even bed ensures the correct volume of water is applied, so less risk of slumping or wash-off. The improved soil structure close to the surface means capping from irrigation or heavy rain is less of a problem.

The combination of measures Nick Baird has adopted, that he has tailored carefully to suit his soils and farming set-up, has ensured a more uniform crop with better rooting. The business benefits these accrue are apparent.

Case Study

Nick Baird – Funtington, Chichester

Nick Baird

The traditional view of a move to minimum tillage is that you're likely to have to compromise your view of what a perfect seed-bed should look like.

Not so for no-plough pioneer Nick Baird. You won't find an uneven bed among the 344ha (850 acres) of potatoes he grows on mostly rented land around Funtington near Chichester. And yes – that's potatoes without the plough.

"I'll guarantee every bed is bullet-straight and parallel to the next one. There's nothing worse than narrow beds or wide beds – you get green potatoes and you waste land, fertiliser, chemical and seed."

The alluvial, Grade one Chichester Plain silty, clay loam is typically taken on for potatoes shortly after harvest. And it was about five years ago it became apparent the plough could be holding back progress.

"Ploughing is slow, expensive and requires skilled labour. These soils are also prone to smearing and compaction. We found we were getting a hard layer under the potatoes that we suspected was being caused by plough pans, through working the soil in wet conditions," explains Mr Baird.

The first step was to bring the cultivation window earlier. "Moving the soil when it is at its driest aids shattering and produces natural soil fissures," notes Mr Baird.

But this increased the summer workload, so the obvious answer was to dispense with the plough altogether. A five-leg flat-lift subsoiler comes through first, crossways to the intended path of the ridges and as soon as possible after the previous crop.

grimme-3bed-ridger

Then follows a Grimme three-bed ridger. With the beds marked out so early, the emphasis on precision starts here, so a GPS self-steer unit ensures they are accurate to within one inch.

Following autumn ridging, the beds – still cloddy – are left to weather down over the winter. A couple of applications of glyphosate keep weeds at bay and nutrients are precision-applied according to a variable-rate sampling map.

In the spring, once the soil is ready, a cultivator with 14in tines is drawn through and then the next day the ridges are reformed, with two extra subsoiler legs attached to the ridger passing directly in line with where the seed will be placed.

"Our spring cultivations lift the whole bed up without losing the structure. If we turn it upside down it goes like concrete."

The business is now reaping the benefits. "We can subsoil twice the acreage in a day than we used to plough, and there's less wearing parts," points out Mr Baird.

Bringing the cultivation window into the summer brings a myriad of benefits – less smearing and compaction means better rooting, which helps yield and reduces the need for irrigation. With the beds formed early, inputs can be applied at the optimum time and the improved soil structure means you can get on the land earlier in the spring.

The business' 17,000t potato crop is destined for the quality pre-pack market. Some is sold washed and packed to the local Waitrose, and the farm has just launched its own, premium Kingsley Vale brand.

Golden Rules

  • The right timing is essential Cultivate and ridge up in the summer when soils are dry. Bringing your cultivation window ten days earlier can make a world of difference.
  • Precision brings perfection The perfect bed will yield a top quality crop. On a large acreage, a GPS self-steer is a very worthwhile investment.
  • Experiment to suit your soils Challenge conventional beliefs and don't be afraid to give it a go. Use the experience and knowledge of your soils to perfect your cultivation policy.
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