1 Soil Details

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Soil Details
As plant growth is dependent on the condition of the soil, a careful analysis is necessary to ascertain its condition. Your checklist should include the following areas of investigation.

General Inspection
Soil is a medium for plants to grow in and is basically made up of two layers – topsoil and subsoil. These vary greatly, but in general the topsoil is the more fertile layer, being darker in colour, and containing the larger amount of organic matter. Most of the plant’s roots are in this layer, along with most of the soil organisms, micro-organisms and plant nutrients. A good topsoil will, therefore, be teeming with life. The subsoil is generally lighter in colour, with less organic matter and fewer soil organisms, micro-organisms and nutrients. Only the deeper roots reach this level. Although plants do not rely on subsoil for growth, the condition of the subsoil is crucial to plant welfare. Subsoils should be able to drain excess water away. Sometimes, impermeable layers exist in the subsoil, known as hard pans. These can exist naturally, or can be caused by compaction due to heavy machinery. The latter is, therefore, common around new construction sites. These pans make cultivation almost impossible and prevent drainage. If plants are to succeed, remedial action is crucial. This may be achieved by double, or deep digging, breaking up the compacted layer. In more extreme circumstances, use of a mechanical sub-soiler may be necessary. In either case, care must be taken not to bring the subsoil to the surface, and bury the fertile topsoil. It is important to appreciate the value of topsoil and put it to one side at the beginning of any construction work if it is likely to get damaged by compaction, or covered with sub-soil. The depth of topsoil can vary, from just a thin layer over rock to many metres deep. The depth of topsoil can be checked by digging a hole 60cm deep. The cut sides will reveal the soil profile and the darker layer of topsoil. This may be a very distinct layer, or may gradually merge into the lighter subsoil. A depth of 30–45cm is ideal for vegetables and flower borders but lawns will only need around 7.5cm. Additional topsoil can be bought but it must be similar in texture and pH to the soil which exists and it must be sourced from a reputable supplier.

Soil Texture
This refers to the varying proportions of the different sized particles, clay, silt and sand which make up the soil. As their names suggest, clay soils have a high proportion of clay particles; silty soils, a high proportion of silt particles; and sandy soils, a high proportion of sand particles. Loamy soils occur where the particles are mixed. When we talk about a sandy loam or a clay soil, a silty soil or a sandy soil, we are talking about the soil texture.




Each type has advantages and disadvantages and a variety of plants which are suited to them.

Sandy Soils
Sandy soils have large particles and gaps which allow water, and with it nutrients, to drain away freely. Unable to hold water or nutrients, sandy soils tend to be less fertile and dry, especially in summer. On the positive side, without water to cool the soil, it warms up early in spring. Its large particles, gaps and inability to hold water also mean that the particles do not bind together easily, making it very easy to dig.

Clay Soils
Clay soils have very small particles, so small that water drains out slowly and the soil becomes easily waterlogged and hard to work. Its ability to retain water results in a soil that retains nutrients but warms up slowly in spring. When it does dry out, it forms an impenetrable mass that dries with a hard crust which water cannot permeate, so in summer it can be very dry and is liable to bake and crack. When well managed, clay soils can be very fertile, as the clay particles have valuable plant nutrients.

Silty Soils
Silty soils also have fine, closely packed particles which bind together easily, making them difficult to cultivate unless combined with organic matter.

Loamy Soils
Loamy soils with a balanced mix of different particles, combine the best qualities of sand and clay. They retain water without waterlogging and are fertile and easy to work. Soil texture can be surmised by checking it in dry and wet conditions. If it’s hard like rock when dry and sticky when wet, it’s very likely to be a clay soil. If it’s light, easily drained and easy to dig, it’s probably a sandy soil or possibly a loamy sand. To be more precise, check the soil texture by taking a small amount of soil, wetting it and kneading it in your hand to a smooth paste. How it feels and behaves when kneaded and rolled will determine its texture: • Loam – if it feels sticky but also gritty, it’s loam. If it’s mostly sticky, roll it into a worm shape and if it breaks up when rolled, it’s also loam • Clay loam – if it rolls into a worm shape easily but feels rough, it’s a clay loam • Sandy clay – if the worm shape becomes shiny when rubbed but still feels gritty, it’s sandy clay • Clay – if the worm shape becomes shiny and does not feel gritty, then it’s clay • Sand – if it feels gritty, roll it into a ball. If it won’t form into a ball, it’s sand • Loamy sand – if it forms into a ball but falls apart easily, it’s loamy sand • Sandy loam – if it forms into a ball and doesn’t fall apart, it’s sandy loam • Silty loam – if it feels more slippery or silky, it’s silty loam.



Some soils have very little topsoil and are almost wholly influenced by the subsoil. A good example is a chalky subsoil which soaks up and holds large quantities of water but is too hard for roots to penetrate through to the water. The roots are restricted to the shallow surface layer where they form into dense mats, using up the surface water supply. Chalk contains a lot of lime, which makes it very alkaline.

Plants, such as Viburnum, tolerate chalky soils The essential nature of the soil texture is very hard to alter, but it is possible to take advantage of the beneficial aspects whilst lessening the impact of the problems associated with different types of soils. It is, therefore, important to know what type of soil is present in the garden since it will dictate the choice of plants and the cultivation techniques.

Soil Structure
This describes the condition of the soil. More specifically, it describes the arrangement and mix of solid particles and organic matter which bind together to form soil crumbs, and the size and proportions of pore spaces for air and water. For plants to thrive, a good balance of air and water is vital. This is achieved by a good balance of pore spaces, which in turn, is influenced by the arrangement, or structure, of the particles. Good soil structure provides the ideal medium for plant growth and can be achieved by correct cultivation methods and the addition of organic matter. Organic matter, such as well-rotted farmyard manure, leafmould or compost, is a soil conditioner which is beneficial to all soils. It breaks up clay soil and improves drainage, as well as binding together a sandy soil to improve water retention. Generally speaking, if the soil is dark, it’s likely to have a high organic matter content which results in good soil structure. Environmental factors can affect soil structure. In the garden, a healthy eco-system of plants and wildlife has appreciable benefits on the structure of the soil. The seasonal lifecycle of leaf fall is broken down by insects and soil bacteria into organic matter, and worms aerate the soil. In woodlands, the soil structure is naturally improved by leaf fall



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