LOOSEN UP Is subsoiling one of those self-inflicted chores - or is it a routine operation that needs to be carried out on certain soil types? Andy Collings takes a look at the subsoiling scene and discovers there is perhaps more to be achieved with subsoilers than one might first imagine.

Subsoiling - its one of those subterranean activities which needs a lot of horsepower yet, visually does not appear to be achieving much for the effort involved.

For many farmers, it has to be said the decision to subsoil is frequently made on a whim or a "gut" feeling rather than any demonstrable reason. Either the weather seems favourable, or there is a little spare time, or it seems to have been a long time since it was done last time - there is rarely that much in the way of sound reasoning behind such decisions.

But there is clearly some benefit in moving ground at depths below normal cultivation depths - it's just evaluating the cost of such operations, and the return that can be expected on this investment.

The other way of looking at it is to ask how much yields are going to be reduced if ground is not subsoiled.

What is certain is that subsoiling is not one of farming's cheaper operations. As such, it is an operation, which perhaps deserves a little more consideration than it currently receives.

So, what should be the reasons for subsoiling? Is it that it's been a particularly wet time and heavy traffic has created deep ruts across a field?

Harvest time can impose a great deal of compaction with combines, trailers, balers, loaders all doing their thing and at other times of the year there is the sprayer running up and down tramlines that get deeper with every pass.

But more than that, it is generally recognised that some soil types - particularly those with high clay content - benefit from a regular loosening to help with drainage, aeration, increase worm populations and generally improve soil structure.

It helps to create a structure in which plant roots can descend and find moisture and nutrition during dry times and, in wet times, enable adequate draining to be achieved without the roots being totally immersed in water.

Traditionally, of course, the subsoiler was used to bust up plough pans - those smeared, impermeable areas of soil created by ploughs and by heavy tractor wheels that ran in the furrow.

So the object of the exercise is to create an element of movement within the soil at a depth of say 30cm (12in) or more. Worth recalling at this stage that the subsoiler is not a mole drainer and has never been designed as one but, needless to say, there is a certain bravado in running it as deep as possible with the beam on the deck. OK if the shear bolt supply is plentiful but heavy on the fuel.

Most subsoilers have some form of wing on their base which is designed to lift the soil like some mobile earthquake and introduce fissures within it. There is generally no attempt to create a soil mixing effect or, indeed, to produce a surface tilth.

Having said that there are those implements which have legs that are designed to operate at slightly less depth and at greater speeds and, as such, are able to introduce to the surface a degree of soil that is the starting point for successive tines and discs to know down into a seed bed.

But for the bona-fide subsoiler, it is the deep, lifting action that achieves the intended task. It could be argued, and many do, that the best conditions to subsoil is when the ground is dry and brittle so the maximum amount of shatter can be achieved. These conditions are, of course, most likely to be encountered just post harvest - a point not lost on stubble cultivator manufacturers many of which offer implements with subsoiler legs as primary cultivators.

There is also a strong argument for taking a spade out in the field and digging down to observe just what sort of job the subsoiler is making - if is simply cutting two grooves through a sticky wet clay it could be worth asking if the cost is going to be worth the effort.

But if there is evidence of lifting and cracking which will encourage drainage and better root development, then there is a case for proceeding.

Subsoilers then, clearly have a part to play in the current min-till cultivation regime now practiced by many farmers as they look to reduce crop establishment costs.

But for oilseed rape, the use of the subsoiler looks as if it may take on an even greater importance as a system for plant establishment which offers a higher degree of success, at reasonable cost, than most other systems.

Oilseed rape has, more than most other crops, been subjected to a wide number of crop establishment systems. At one extreme it is simply broadcast onto the stubble of the preceding crop and at the other, it is drilled into ground that has been ploughed and cultivated to produce an appropriate seed bed.

The key to it is moisture and both these systems can suffer from a lack of it - seed broadcast directly before or after the combine harvester has done its work may have very dry surface conditions to cope with and, the land ploughed and cultivated may have lost a lot of moisture during the cultivation process.

Enter then the subsoiler/seeder system which requires a multi-leg subsoiler to be used to work at, say 30cm (12in) depth and, at the same time to have seed broadcast into the bands of soil disturbed by the legs. A press then consolidates the ground and helps to ensure seed is in contact with the moist soil.

Visually, the freshly subsoiled land - particularly on dry heavier land - looks to be far from being a seed bed for such a small seed. But on reflection, it will be realised that the seed is presented with warm moist soil, there is plenty of drainage and the soil is now able to accept deep root growth. A pass with a Cambridge roll helps to level out the field and provide further seed/soil contact.

With this system now being advocated by a number of subsoiler manufacturers - and fast becoming accepted by growers - it is worth noting that, when compared with ploughing, cultivating and drilling, it is significantly less costly and quicker overall.

Subsoiling, used prudently with regard to soil conditions, clearly has an important part to play in establishing crops that can germinate successfully and perform to their best ability.