Soil Structure and Testing

Written by 
Rate this item
(5 votes)
Recently Dug Soil Bed Recently Dug Soil Bed

When taking on a new plot or garden it’s important to establish what sort of soil you have before you start planting. Different soils need to be treated differently, and since this is your most valuable asset on the plot its worth spending a few moments of your time establishing what sort of soil you have and what work you will need to do to improve it.

Soil Structure

The first task is to ascertain whether you have a light sandy soil, a loam soil (ideal) or a heavy soil. This is very easy to do and extremely worthwhile.  Work through the following tests and you should have a very good idea of the type of soil you have.

First of all we need to check both the top soil and the subsoil.

  1. Skim off any weeds so you have a clear patch of soil.
  2. Dig a square hole about 50cm deep and 50cm across.

Once you have done this take a look at the hole. You should see the topsoil and the subsoil –the subsoil will most likely look heavier and will be a lighter colour. If the hole holds its shape easily then you are more likely to have a heavier soil, if it’s all crumbling into the hole then it is much likely to be a lighter soil. The structure of the topsoil and the subsoil will be slightly different.

As a second test, take a handful of moist topsoil and squeeze it into a ball. If the ball holds its shape easy and can be polished with your thumb then it is likely to be a heavy soil – if it crumbles and won’t form a ball at all then it is a lighter soil.

Clay Soils

A clay soil is heavy and will hold its shape and feel sticky to the touch.  Its main disadvantages are that it does not drain well, is slower to warm up in the spring, and its dense structure can reduce root penetration and the oxygen levels in the soil.

It is however very rich in nutrients and provides great support for plants once established – holding your winter brassicas firmly in the soils and preventing them from blowing over.

The best treatment for clay soils is to dig it in autumn, and then the frost will break up the clumps of soil over winter – doing the hard work for you. Add plenty of organic matter to lighten the soil, and if it’s very heavy then dig in some sharp sand or grit to improve the structure.

Loam Soils

A loam soil is ideal. It consists of a good mixture of sand and clay, is free draining, holds nutrients well and is easy to work. This dark brown soil really is a pleasure to work with.

Light Sandy Soils

The main disadvantages of a sandy soil is that it is too light to hold nutrients and will dry out quickly in summer making it necessary to water your plants more often. It is however easy to work, very quick to warm up in spring, and is the ideal soil to start sowing seeds directly into the ground.

Light soils can be improved by adding humus. Also avoid digging light soils until spring so the rain doesn’t drain all the nutrients over winter.


Next we need to check the drainage. Empty a couple of cans of water into the hole you have already dug. If the water takes hours to drain then you may have a problem with waterlogged soil, and may have to install some form of drainage on the plot. If it drains quickly then no problem.

So with these simple tests you will have an idea if you have heavy or light soil – of more likely something in between, and you will also know if your soil is free draining or not.

Free Draining Soils

The ideal soil will be free draining. Most plants grown on allotment don’t want to be in waterlogged soil.

Waterlogged Soils

If the ground becomes waterlogged for even part of the year then certain problems can arise for the gardener. The water will expel air from the ground, which can cause nutrient deficiencies. Crops will rot if their roots are sat in too much water, and certain crops won’t grow at all.

There are a few different courses of action that can be taken to remedy the situation depending on how bad it is. If it is very mild then double dig the plot and add plenty of organic matter to lighten the soil.

For slightly worse cases then the addition of raised beds to the plot  an go a long way to sorting the problem – and the higher the bed then the more free-draining topsoil can be added and plants can be grown directly in this top-soil.

In the very worst cases – and these are likely to be very rare on allotment sites, especially those that have been worked over the years, you may need to install some form of rubble drain. The idea is simple dig a rubble drain 60cm deep across the site – gently sloping towards a soak-away about 1.5m square and filled with rubble and  gravel in the corner of the plot – You could always top this off with decorative gravel or turf and use as a seating area)

Soil Acidity

The second task is to work out if your soil is an acid soil or an alkaline soil. This is best done using a soil testing kit available quite cheaply from most garden centres. The acidity will both affect what type of plants will grow in the soil, and also affect how it should be treated.

A soil testing kit will give you the pH of the soil on a scale of 0-14, and the reading usually ranges from pH5 to pH8. The lower the figure the more acid the soil is, and the higher the figure the more alkaline it is.

Most plants prefer a slightly acid soil and so a pH6 to pH6.5 is ideal.

Acid Soils

If the soil is too acid then dig in lime each year – The lime will only affect the soil acidity for a relatively small period.  The more acid the soil is then the more lime you will need to dig in. You should avoid doing this at the same time as adding manure as the two will react with each other, and so at least a couple of months should be left between adding manure and liming soil.

Alkaline Soils

Soil in Britain tends to be on the acidic side, so a very alkaline soil is unusual. If it’s higher than pH6.5 however this can be rectified quite easily by adding plenty of organic matter to the soil to re-address the balance – this can include leaf-mould, compost, and manure and so on.

Phyl @ The Allotment Gardener

Lifelong gardener and allotment enthusiast. Now have 3 allotments!


Related items

Login to post comments

NOTE! This site uses cookies and similar technologies.

Unless you change browser settings you agreeing to the use of cookies.

I understand