Soils Soil is one of the world's most important natural resources. Together with air and water it is the basis for life on planet earth. Most of our food depends on soil - it is where we find the plants and many of the animals which make up our food, and it is home for billions of organisms. Not only does it play the major part in allowing us to feed the world's population, but it also plays a major role in the recycling of air, water and nutrients, and maintains a number of natural cycles, ensuring that life in generations to come. Soil also gives support for many of our buildings and structures. Without soil, the world's population neither would, nor could survive. Soils form very slowly, maybe as little as 1 cm of thickness in 500 years, so we can't just replace them in our own lifetime. Many of our soils are becoming damaged and are at risk, so it is important that we understand our soil and make sure it is there for future generations. What is Soil? Soil consists of a mixture of mineral grains that come from the rock deposits and sediments beneath them. This mixture gives the overall texture of the soil, namely whether the soil is mainly sandy, loamy or clayey. Importantly, soil also contains organic matter, mainly in the top 20 cm. Organic matter comes from rotted and decomposed vegetation, broken down by soil organisms. Soil also contains varying amounts of water depending on the climate and the water holding capacity of the soil. The remaining important ingredient is air. The amounts of each of these components can vary in time and across the landscape. This is a magical mixture of ingredients that allows the soil to perform many vital needs of living beings. Just as we humans have parents, so all soils are formed from materials called parent materials. Most soils are formed from hard rock or sediments, many of which date back millions of years. The main exception to this is the peat soils which are derived mainly from buried plant remains. Those soils that have formed from the breakdown of the underlying rock, mineral sediments laid down perhaps by wind and water, or the extensive deposits of the last Ice Age are dominated by mineral particles which have become free after the breakdown of larger rock fragments over many centuries. The mineral particles in soils come in a large variety of shapes and sizes.

These particles have been classified into four main size fractions: Rock = fragments greater than 2mm diameter which under the microscope and often with the naked eye still bear the appearance of the rock material from which they are formed. Sand-sized particles = normally 0.05-2 mm in size Silt-sized particles = normally 0.002-0.05 mm in size, and cannot be seen with the naked eye, i.e. some form of magnifier is needed. Clay-sized particles = less than 0.002 mm in size. Cannot be seen with the naked eye. In the case of humans, our bones form our skeleton. In the case of soils this mixture of mineral grains of different sizes forms the skeleton of the soil. Sandy soils have at least 70 percent of sand-sized grains whereas clay soils are predominantly (>60 per cent) made of clay particles. Because of the size and nature of the particles that make up these soils they behave very differently. Sandy soils have a coarse feel to them, clay soils have a smooth, velvety feel, particularly when moist. Sandy soils tend to hold little moisture and tend to be droughty whereas clay soils hold lots of water some of which is available to plants. There are three other components that make up the soil. We must not forget the living organisms that inhabit the soil. These are so important that we have a special topic feature on them (The Living Soil). They are extremely important because of their numbers (billions in just a tea spoon full of soil), the fact that they break down the organic remains, the dying vegetation, deposited on and in soils, to release nutrients for the next generation of plants, and they also play a big part in creating the structure of soils. The final important part of the soil is the water that it contains. The soil acts as a reservoir for water and moderates the flow of water to lakes and rivers. The water that is held in the pores of the soil and around mineral grains is vital in sustaining plant growth and the living soil organisms. At times it can fill the pores making the soil waterlogged but often it will drain through the soil and the pores will be only partially filled. The water that occurs in the pores will contain various nutrients in solution and as plants take up water so they will take in nutrients which are a major source of food for the plants that grow in the soil. This balance between the soil mineral particles, organic matter, the living organisms in the soil, and the pores containing air and water is so important in maintaining the human population. It is important that all users of soils understand this relationship.

Organic Matter in Soils Organic matter is the other main non-living solid component in soils. It is formed from the decay of plant and animals remains. There are a few notable world soils that are totally composed of organic matter throughout most of their depth but for most soils in the world the organic matter is mainly in the top 20 cm. This is because the source of the organic matter is generally surface related, i.e. much of the organic matter comes from falling leaves and other plant debris and from decaying organisms that have occupied the top layers of the soil in huge numbers. This organic matter is probably the most important of the components of soil because it is a source of valuable nutrients and also helps the soil to hold water and develop a good structure. In soils under cultivation, as in agriculture and horticulture, ploughing and other working of the soil means that the organic material becomes thoroughly mixed with the mineral soil over a much larger depth than in non-agricultural situations. The organic matter and the mineral fraction become closely mixed together. The term topsoil is often used by farmers to describe this upper layer that is so important to them.

Soil Layers Soil is the thin mantle that covers the surface of the earth. To most people, soil is what you see in a freshly dug garden or a freshly ploughed farmer's field. This uppermost layer of the soil is usually some shade of brown or black. If you have the opportunity to dig deeper, or even better, have a hole dug, or find an exposed depth of soil you will see there is much more to most soils than just this topsoil. The soil below the dark surface layer is usually composed of a series of layers. Most soils have formed from rocks and sediments. Some have developed over many thousands of years by the gradual breakdown of rocks that occur beneath them and on which they lie. This is a slow process but if the soils remain reasonably undisturbed significant depths of soil can form. In many parts of the world (including Scotland), soil development has been disturbed by various geological events. The Ice Age in particular scoured away existing soils, and left behind a mixture of rocks, sediments and soil in which new soil formation has taken place. Throughout the world new sediments are being laid down, in for example deserts and floodplains, and soils will develop in them in years to come. Each soil consists of a series of layers, termed soil horizons. The soil profile is the term used for the whole depth of soil including all the layers (horizons), from the surface of the ground down as far as the rock or sediment below. Each soil type has at least one horizon, but usually three or four different horizons can be seen. You can see these distinctive layers whenever you are able to see a cutting down through an undisturbed soil. The topmost horizon is the 'A' horizon and is usually the darkest layer of the soil because it contains most of the organic matter. Here, remnants of plants and even animals are incorporated into the soil. The numerous organisms in this soil horizon are responsible for breaking down these additions of plants and animals and releasing the nutrients they contain to become available for the next generation of plants and animals. In the A horizon the most visible plant remains, e.g. leaves, roots, are nearest the surface. Where the breakdown of plant remains is slow, perhaps because of acid conditions, there may be distinct layers of recognisable organic remains lying at or near the surface. This is often the case, for instance in a pine forest. Where soils are cultivated, as

in the garden or the farmer s ploughed field, the organic matter is all mixed up in the topsoil. Which has a more or less uniform brown colour. The B horizon is the layer beneath the A horizon, and is part of the subsoil. The B horizon is usually quite different to the A horizon in colour and in structure (the architecture of the horizon), and often has a coarser structure. The C horizon lies below the B horizon. It is the intermediate horizon between the more or less unaltered sediment or rock below and the B horizon above. The C horizon contains fewer organisms than the horizons above and it contains very little organic matter, except where roots have penetrated the horizon. Soil Texture & Soil Structure Two of the most important properties of soils are their texture and structure. By texture, we mean what soils are composed of and how this affects the way they feel and their cultivation. The main components of soil texture are: sand, silt and clay particles and organic matter. For example, soils that are dominantly composed of sand-sized particles feel gritty They contain lots of pores because of the way the grains stack together. This also means that rainwater entering the soil can easily drain away through the soil which leads to the soils being quite droughty. Clay soils by comparison contain many smaller pores which means that water does not pass through the soil as freely and means they can become waterlogged in wet periods and are sometimes difficult to cultivate. Most soils benefit also from the presence of organic matter which comes from decaying plant remains. Organic matter plays an important part in topsoil structures and is the major 'glue' holding particles together It can improve the workability of most soils whatever their texture. This is why farmers often add farmyard manure to the soil to increase the amount of organic matter. This then leads to the development of well structured soils, easy to work, and ideal for farming. Gardeners also maintain good levels of organic matter in their soil for the same reasons. The components of soil described above rarely occur as separate particles. There is, for example a strong bond between decomposed organic matter and the clay fraction of soils. In most soils, the particles are moulded together into aggregates to form the architecture of the soil. Most soils have a structure which takes the form of aggregates of soil particles separated by pores (holes). Roots pass through these pores in search of water. Depending on the soil texture, amounts of organic matter and the way the soils are managed, the soil structure will vary with depth in the soil and from one soil type to another. The main types of soil structure are crumb, granular, blocky, platy and prismatic.

Pores in Soils All soils contain pores. These are vital for the movement of roots, water, air and organisms. The pores are present because all soils contain a mixture of particle sizes and these never fit together exactly so there will always will be tiny gaps or pores between the particles. Soil is a bit like a sponge, with lots of holes in which the water can sit and through which air can circulate. Pore space can vary from a few percent to over 40 percent of the volume of the soil. All soils contain pores. Pore space is also a vital component in developing the architectural structure of the soil. Good soils need plenty of pores and the best soil structures tend to be in the topsoils. This is mainly because there is usually organic matter present and this attracts huge numbers of organisms which work to decompose it. The organisms eat the soil and create numerous burrows in it. Crumb and granular structures formed in this way are ideal topsoil structures for farmers and gardeners because they contain plenty of pores through which air and water can circulate readily, and through which plant roots can readily move in search of air, water and nutrients. Water & drainage in Soils Water is a very important component of soils and plays a huge part in the way soils behave and in the ease with which they can be worked for agriculture and other uses. Soils vary in their water holding capacity. Water is held in the soil around particles and the edges of pores. It is held in these positions with various degrees of strength. Sandy soils tend to hold little water whereas clay soils can hold much water though some of it will be unavailable because it is held strongly in tiny pores. The drier the soil becomes, generally the stronger the soil will hold onto its water. Soil water is essential for the survival and growth of plants and for maintaining soil organisms.

Soil Chemistry The chemistry of the soil is also very important property as this will determine what will grow and how well it will grow. One of the most important chemical properties of a soil is its acidity or alkalinity, often stated as the ph of the soil. The ph of the soil ranges from about 3 to 8. Below 5.5 the soil is quite acid. Above ph 7 the soil is alkaline. Soil with a ph in the range of 5.5 to 7 tends to be the most flexible and a wide range of plants can thrive within this ph range. Once the ph drops below 5.5, firmly into the acidic range, there is only a limited range of plants that like this level of acidity and can tolerate these acidic conditions. Once the ph is above 7.0, the soil tends to be colonised by a limited range of lime-loving plants. Soils in wet climates and certainly those developed on acidic rocks, such as granite, will tend to be acid. Soils in high rainfall areas tend to be acid because the rainfall leaches the soil of many of its nutrients which otherwise help to keep the ph higher. Acid soils can be improved by adding lime to soil. This is a common agricultural practice where farm soils need to be maintained at a ph from 5.5 to 7 in order to grow a wide range of crops. Soils formed in dry climates are often alkaline, i.e, with a ph above 7. Here there is a lack of rainfall to flush the nutrients out of the soil and they stay within the soil. In some dry conditions also evaporation may lead to deep-lying nutrients being brought to the surface. This can lead in some dry-climate soils to excessive amounts of salts being brought into the root zone. In extreme cases this can lead to salinization, in which the soils contain too many salts, which can prevent the growth of many crops.

Soil Nutrients Soils contain about 18 nutrients that are important and even vital to plants. Some of these, termed macronutrients, are needed in fairly large amounts, nutrients such as calcium, magnesium, potassium, similar to what we humans need for growth and health. Other nutrients, called micronutrients, are needed in much smaller amounts but are also essential for a healthy plant. These include iron, zinc, copper, boron. All of the 18 nutrients are required by plants. Many of these nutrients are present in the rocks from which soils are derived and have been introduced into soil as rocks are broken down to form soil. Others reach the soil through the rotting and breakdown of dead organic matter. As roots grow through soil and take up water, they also take in nutrients. In addition to this route there is another very beneficial one, that between fungi, termed mycorrhizae, and roots. Mycorrhizae take up nutrients, particularly phosphorus, and pass them into the roots of plants. In natural situations, such as woodlands, nutrients are constantly recycling between soils and plants. Dead plants fall to the ground and are broken down to release nutrients, which then take part in the next cycle of growth. With agriculture, when the crop is harvested, all the nutrients in the plants are also removed from the field and this important recycling of nutrients is not possible. The farmer must then add extra nutrients to the soil - that is what fertilizer or farmyard manure does by maintaining soil fertility and productivity for the next crop. Humans receive their minerals, including vegetables (the meat we eat is raised on plants so in fact all food depends on soil for its mineral content). IN some parts of the world soisl can be deficient in certain minerals which leads to food containing less of these minerals. In some parts of China and Russia soils are very low in selenium. Selenium deficiency is often reported in those regions because most food in those areas is grown and eaten locally. In the future proper management of the soils is required to ensure that our food continues to provide us with the appropriate mineral intake.