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What is Soil?

Soil types

Soils are not just deposited in their present position; they are largely the outcome of the deposits left by the plants and the animals they once supported and of the climate in which they develop and the type of rock that constitutes most of their solid or mineral matter. Not only are there broad differences among soils from different parts of the world as a result of differences in climate and nature of vegetation, but differences are found among neighbouring soils. It was John Evelyn, the diarist, who said in 1675 that he considered that there were no fewer than 179,001,060 different sorts of earth. The basis of this calculation is not recorded, but several hundred soils have already been identified and mapped.

The soils of the world are classified into a comparatively small number of major groups in rather the same way as plants are grouped into families. And, as each plant family consists of many different genera and still more species, so the major soil groups are divided into smaller units, of which the soil series is the most important for survey and mapping work. Most soils come into the following five groups.

Brown earths which give most of the best garden soils in Britain, developed from a wide variety of rocks under fairly dry and warm conditions that favor the growth of deciduous forest; their color is brown, yellow-brown or red. Having no natural lime they are generally slightly to moderately acid in reaction. Since they often have a friable, coarse-textured topsoil and a system of deep fine cracks they are generally well drained. But if you have one with a silty texture you will have to be careful not to cultivate it or water it too heavily for fear of panning the surface which leads to temporary waterlogging.

Gley soils are wet and in contrast to well-drained soils usually have a grey or blue layer somewhere in their profiles or they may have a variegated or mottled appearance.

The reason why plants grow poorly in such soils as these is not so much due to their excess of water but more to their lack of air; the roots of plants must breathe, just as other parts of the plant do, so that, aquatic plants excepted, they must rely upon air in between the particles of soil, if their roots are to survive. After rain these spaces fill with water and until this drains away the roots are unable to get air. The speed at which the water drains away varies greatly with different soils as also does the capacity of different kinds of plants to withstand immersion.

But the reason for the wet condition of a soil must be discovered before you can cure the trouble. Below are given the three main causes of wetness.

Poor penetration The rock underlying the soil may be more or less impermeable and prevent the free passage of water. Roots are swamped in wet periods and when followed by dry periods there is no reserve of water for the crippled root system to draw upon because the impervious rock does not store much water. Waterlogging is therefore followed by drought. This is one of the commonest causes of poor drainage and is found on soils overlying clays and shales.

A water table may be present in soils of low-lying land, resulting in the soil being saturated with water only a little below the soil surface. The surface of this underground lake is known as the water table and it generally rises and falls following wet and dry periods. Provided that the fluctuation is not too great, a water table at about lm (3ft) below the surface of the soil can be an asset since the water will be available to the deeper roots. You can find out whether your soil has a water table near the surface by digging a hole or boring a hole with a post-hole auger about 1m (3ft) deep; leave it open for a day and then see whether there is any water resting on the bottom of the hole. It is not only clay soils that are subject to this kind of wetness; sandy and loamy soils often have high water tables in low-lying areas such as the flood plains of rivers and estuaries.

When the water table rises during the winter the roots of perennial plants are killed and when it falls in the summer your plants are literally left high and dry and then suffer from drought.

Springs are more common than is generally suspected because only the largest are seen at the surface; but large areas of land are waterlogged by subterranean springs.

You can usually tell whether a soil is well-drained or badly-drained by the color of the soil; well-drained soil is an even color of grey brown or brown and free from mottling, whereas defective drainage leaves definite symptoms in the soil. Well-drained soils are normally well aerated and have plenty of oxygen in them, but where the water stagnates the living organisms in the soil and plant roots use the dissolved oxygen as fast as it can be renewed, and some of the iron compounds which impart a yellow, brown or red color to soils, lose part of their oxygen and cause grey or bluish colors to develop. When the water subsides air gets in again and the grey or bluish colors go brown again and so the soil becomes mottled with rust colored stains, or even hard little lumps of rust. Spring water may deposit a heavy, chocolate-brown iron staining.

It is important to note the depth at which these symptoms occur because all the earth above the waterlogged area means an area of good healthy root run.

Podsolised soils are acid and have profiles consisting of very distinct layers. They are best seen on heathland overlying sandy or gravelly geological formations.

The surface layer is often black and peaty, being rich in decaying plant remains. These produce acids which cause the washing out of iron and aluminium compounds from this and the next layer below. Under the dark‑colored surface layer lies one which is very pale brown, often almost white.

This is because the iron, which normally colors a soil brown, has been washed out of it. Underneath this is a dark‑ brown to reddish-brown layer in which clay, iron and aluminium compounds and humus have been deposited. This is often cemented into a hard pan by the iron and organic matter. When you dig these soils the mixture of the bleached layer and the dark-colored subsoil gives it an ashy appearance.

Extreme acidity and low nutrient content often characterize these soils; but by careful liming and the application of manures they can grow plants well, and, of course, many are ideal for rhododendrons and azaleas and other members of the Ericaceae. While their coarse texture makes for very easy working, moisture retention is usually a problem. Deficiency of potash is normal while manganese and boron deficiences may result from over-liming.

Where the garden has been recently reclaimed from heathland, generous manuring with nitrogenous manures is needed to speed up the decay of plant remains and magnesium compounds may have to be added.

Calcareous soils are naturally rich in lime (calcium carbonate) due to their development from chalk and limestone formations in downland areas.

You can generally see fragments or sometimes fairly large lumps of calcium carbonate in these soils and the presence of this substance makes them lkaline (the opposite to acid). This accounts for the fizzing (effervescence) when a small amount is treated with dilute hydrochloric acid.

When newly broken up they are almost always neutral to the surface and do not require liming. They are so alkaline that elements like iron, boron, agnesium and manganese are less readily available to plants and symptoms of their deficiency can often be seen in many species.

Since many calcareous soils are not very thick they lose their reserves of moisture in a very short time during dry weather, unless very large amounts of humus-forming manures are either dug in or added as a mulch.

The deeper calcareous soils have brown or reddish-brown sub-surface layers and make quite good garden soils, provided that you do not want to grow rhododendrons or most other members of the Ericaceae in them. They will also grow plums, apples and most of the soft fruits.

If the clay content is high you have a real problem soil to contend with and great skill is needed to cultivate them just at the right time, when they often break down into an excellent tilth. But when wet, they are plastic and if cultivated in this condition the structure is destroyed and they are then liable to dry out into large blocks. Peat or well-rotted manure will help to prevent them from becoming like concrete, but it should always be worked into the surface and not buried deeply.

Organic soils are rich in humus and have a very dark brown or black color. If they have over 50 percent organic matter in a layer which is more than 40cm (15in) thick they are classed as peat soils of which there are two main types, bog peat and fen peat. Bog peat consists of the residues of heather and mosses, particularly sphagnum moss. Bog peat is usually formed on an impervious rock formation which prevents water from draining away. So the natural vegetation decays very slowly in these badly aerated wet soils and peat is the result.

Given shelter, many garden plants will survive if the soil is limed, but it is not generally necessary to lime them to the point of neutrality, because many plants will grow well at pH values lower than would be possible in a mineral soil ; in fact heavy liming may result in boron and manganese deficiences.

The other main type of peat soil is fen peat, which is composed of residues of reeds, rushes and sedges and other water plants. This type of soil is usually formed at the edges of slow-moving or blocked rivers and streams. Such accumulations are generally much less acid and have pH values of about 6: the areas where they are formed may have been drained by rivers that have previously passed through limestone and chalk formations.

Some of the fen peat soils are among the richest in England, being deep and very easily worked. Plants often produce rank, luxuriant, vegetative growth, owing to their naturally high content of nitrogen.

Phosphate content is often low and extra potash is generally required, but nitrogenous fertilzers are seldom needed in the richest fen peats

Sandy, loamy and clay soils The more usual way of classifying soils is on the basis of their texture. This is a property that depends on the relative amounts of the different sized particles that they contain. For example sand is the name given to coarse gritty particles that you can see quite plainly, whereas silt particles can be seen only with a microscope and a clay particle which is even smaller can be seen only with an electron microscope. All soils contain sand, silt and clay in varying ratios.

Texture is important because it affects the handling, drainage, aeration and nutrient content of the soil. Lime and fertilizer requirements are also keyed to texture. It can be assessed by hand. To do this take a handful of moist soil and rub a portion between your finger and thumb. Sand can be detected by the sensation of grittiness or roughness; the finer the sand the less the grittiness. Silt has a floury or talcum powder-like feel when dry and is only slightly plastic when wet. Clay may feel smooth but the surface becomes polished when rubbed between the fingers and clay is sticky when wet. A true loam is smooth and not gritty, silty or sticky when moist.

Sandy soils These are warm and are most suitable for early vegetable crops. If less than 40cm (15in) deep they cannot be recommended for fruits and shrubs.

Soft fruits can be successfully grown in these soils provided they are mulched or irrigated. The only way to improve the soil is to add as much bulky humus-forming material as possible, and cow and pig manure are much favored.

Fertilizers produce the best effect when they are applied in small quantities at frequent intervals during the growing season, but neither these nor the manure should be dug in too deeply. Sandy soils soon become acid and generally need frequent but small applications of lime.

Loamy soils A true loam has a well-balanced proportion of sand, silt and clay and is really the ideal texture as it is the easiest soil to look after. With good management most loams readily acquire and retain a good crumb structure and almost all plants grow well in them provided they are deep and in a good position.

Clay soils These are very retentive of moisture and are sticky when wet. On drying they form hard clods, which are impossible to break down until they are moistened again. After long spells of hot dry weather they form deep, wide cracks which cause root rupture and loss of moisture. It is necessary to hoe regularly in order to fill in the cracks to provide a surface soil mulch. There is no quick way of improving clay soil. Artificial drainage may well be the first essential before any other form of improvement can be attempted.

Very thorough digging, or better still ridging in the autumn, will produce a frost tilth in the spring. Humus-forming materials are necessary to preserve the crumb structure so formed. Strawy stable manure is best, but a fine grade of peat when applied regularly can transform a heavy clay into quite workable soil in a few years. Gypsum can improve the structure of some clay soils when applied at the rate of 110-220g (4-8oz) per sq m (sq yd). Clay soils are naturally rich in plant nutrients. These nutrients are not always readily available since roots tend to follow the cracks and fail to tap the nutrients in the soil lumps. Hence the proper use of fertilizers will produce still better results.

Clays are not always acid as is commonly supposed but when they are, large applications of lime are needed, at rates depending on the results of the soil analysis.

 



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