Cymraeg

Crop Production
Section 2 - Soil Science

This resource is suitable to support Level 2 Agriculture courses.

Content

This unit on soil science will contain information on the following:

  • Session 1 - Soil components
  • Session 2 - Soil types
  • Session 3 - Soil classification
  • Session 4 - Soil fertility
  • Session 5 - Soil sampling and analysis
    • image of tractor wheel and soil

    Section 2
    Session 1 - Soil Science

    Soil Components

    Soils are complex natural formations that provide a suitable environment for plants to obtain water, nutrients and oxygen and firm anchorage.

  • Soils are formed by the weathering of rocks, followed by the growth and decay of plants, animals and soil micro-organisms.
    • image of soil with rocks in it, a measuring stick is to the right

    The soil profile in Fig 2.1 describes the various soil layers (Ioan Rhys Williams)

    Table showing from top to bottom - Topsoil: (Horizon layer A); Subsoil: (Horizon layer B) Usually finer than topsoil, not fully weathered, receives materials washed down from horizon A; and Parent rock: (Horizon layer C) Rock in early stages of weathering.

    Soil Composition

    Soil composition can be split into four parts:

  • 1. Solids
  • 2. Liquids
  • 3. Gases
  • 4. Living Organisms
    • image showing exposed soil in field

    Solids

    There are two types of solid material in soils:

  • Mineral matter
  • Organic matter
    • image showing someone grabbing loose soil with two hands.

    Mineral matter

    Stones, sand, silt and clay provide structure and texture to soil. The composition of the mineral matter plays an important role in determining the ease of cultivation.

  • The mineral matter provides plants with nutrients but not nitrogen.
    • cracked, dry soil

    Organic matter

    When farm yard manure (FYM) or plant residues are turned into the soil, they are subject to bacterial attack and a lot of it is lost to the air as carbon dioxide but a dark coloured residue is left behind which is called ‘humus’ or ‘soil organic matter’.

  • The soil organic matter is very rich in plant nutrients, it enhances stability in unstable soils and improves the workability of heavy soils.
    • image showing lumps soil on two hands

    Chicken Manure : A Good Source of Organic Matter

    big heap of chicken maure in a field

    Liquids

    Water is an essential part of fertile soils which vary in their capacity to hold water. The spaces between particles in the soil are called ‘pore spaces’ which may be filled with air, water or both.

  • If the pore spaces are only filled with air, there is no water available for the plants.
  • If the pore spaces are completely filled with water, the soil is waterlogged and there is no oxygen for the plants.

    • Air and Water Proportion in Soil

    Ideally there should be equal amounts of air and water in the soil.

  • The amount of water that soils hold is dependent on the size of mineral matter particles in the soil.
  • Sand particles are larger than silt particles which in turn are larger than clay particles.
  • Smaller particles create more pore spaces therefore clay can hold more water than silt which can hold more water than sand.
  • The most fertile soils are those which drain freely but can retain large quantities of water during a drought.

    • Water flow through sandy, silty and clay soils

    Three graphics side by side, from left to right labelled ‘Sand’, ‘Silt’, and ‘Clay’. Sand image shows large circles with three water arrows pointing through, Silt shows smaller circles with water arrows pointing through, and Clay shows even smaller circles with water arrows pointing through.

    Picture from: AHDB Principles of good soil management.

    Gases

    Air competes with water for the pore spaces between soil particles. Plant roots as well as soil animals and micro-organisms require oxygen for respiration and expel carbon dioxide.

  • The air found in soil is atmospheric air that has been changed by these activities, therefore it contains less oxygen and more carbon dioxide.
    • image of soil in hands

    Aeration

    Good aeration is needed for effective plant production, this is the replacement of the used air with fresh air.

  • This process occurs mainly with the movement of water in and out of the soil e.g. rain water pushes the old air out of the pore spaces, then as the water is used or drained away, fresh air takes its place.
    • image of exposed soil bank

    Good Aeration

    Aeration is assisted by:

  • Changes in temperature.
  • Good drainage.
  • Cultivation.
  • Open soil structure.
    • image of tractor on exposed soil field

    Living Organisms

    There are two types of living material in soils:

  • Micro-organisms.
  • Animals.
    • microscopic picture of micro-organisms
    image of worm in soil

    Soil Micro-organisms

    There are millions of very small micro-organisms in every gram of fertile soil, the most numerous being bacteria. Most bacteria feed on and break down organic matter, they obtain energy from starches in the soil and release carbon dioxide. Some micro-organisms use nitrogen needed by the plants from the soil to build protein but some bacteria have the ability to fix nitrogen from the air which is advantageous to the plants.

    microscopic picture of micro-organisms

    Other Micro-organisms

    Other micro-organisms found in the soil are ‘fungi’, ‘algae’ and ‘protozoa’, the activity of all micro-organisms improve the productivity of the soil.

    Soil Animals

    The most prominent soil animal is the earthworm. It is generally believed that they have a beneficial effect on soil fertility particularly with grassland.. They live in holes in the soil and eat organic matter. They benefit the soil by carrying fallen leaves, straw and similar material down into the soil, make many holes that allow water to enter and drain from the soil, and draw fresh air in.

    image of worms in soil

    The Mole

    The earthworm is the main food source of moles which do so much damage by burrowing and throwing up heaps of soil.

    image of mole in soil

    Other Animals in the Soil

    Other soil living organisms include slugs, snails, millipedes, ants, beetles and larvae, the farmer is only concerned with those which directly damage the crop.

    image of millipedes

    Section 2
    Session 2 - Soil Types

    Soil Types

    The structure and texture of soils differ from one type to another.

    Structure of the Soil

    This is the arrangement of the soil particles. It can be altered by weather conditions, penetration of plant roots and cultivation.

  • It is not possible to measure soil structure but with experience, it’s possible to assess the quality by the appearance and ‘feel’.
    • image of exposed soil bank with measuring stick to the right

    Texture of the Soil

    Soil texture is determined by the amount of clay, silt, sand and organic matter that the soil contains. It’s possible to measure soil texture by a mechanical analysis of a sample which measures the particle sizes. It’s very difficult to alter soil texture.

    Three graphics side by side, from left to right labelled ‘Sand’, ‘Silt’, and ‘Clay’. Sand image shows large circles with three water arrows pointing through, Silt shows smaller circles with water arrows pointing through, and Clay shows even smaller circles with water arrows pointing through.

    Variation of soil on farms

    image of fields near coastline, sea in the background
    image of craggy field with exposed rock

    Clay Soils

    Characteristics

  • They have the smallest particle size.
  • They feel sticky when wet and can be moulded into various shapes.
  • They will swell when wet and shrink when dry.
  • They can hold more water than most soils.
  • Land is wet in winter and slow to warm up in spring.

    • Clay Soils

    Cropping

  • There are cultivating difficulties with clay soils, only minimum tillage is carried out and they are often left as permanent grass and only grazed during growing season.
  • Where arable crops are grown, a 3 – 4 year grass ley is included in the rotation, winter wheat is the most popular arable crop on these soils.
  • Sugar beet and potatoes are grown on some clay soils but there are difficulties in seedbed preparation, weed control and harvesting.
    • image of potatoes growing in clay soil

    Sandy Soils

    Characteristics

  • They have the largest particle size.
  • They have a coarse, gritty feel.
  • They are called ‘light soils’ due to the little power needed to cultivate.
  • They are free draining.
  • They warm up early in spring and crops are susceptible to burn due to the low water holding capacity.
  • They have a low organic matter content (2-3%).
    • image of sandy soils field with crop lines

    Sandy Soils

    Cropping

  • A wide range of crops can be grown on this type of soil but yields are dependent on a good supply of water and fertilizer.
  • Due to the low fertility and water holding capacity levels of these soils, irrigation systems and frequent manure and fertilizer applications are needed, this is made easier due to the fact that the soils are early and easily worked.
  • On the light sandy soils where irrigation isn’t possible, carrots, rye and sugar beet are grown.
  • On the more fertile sandy soils where there is water supply, the main arable crops are barley, peas, potatoes, carrots and sugar beet.
  • Stock can be out wintered on these soils without the risk of poaching.
    • image of carrot sticking out of sandy soils

    Silts

    Characteristics

  • Pure silt has a silky, buttery feel and is difficult to drain and manage.
  • The silts containing fine sand are one of the most fertile soils in the country.
    • image of someone holding wet silty sand

    Silt

    Cropping

  • In the west of the U.K. grass and cereals are the main crops on these types of soil.
  • In the east of the U.K. wheat, potatoes, sugar beet, peas, onions, seed production for root crops and bulbs are grown on these types of soils.

    • Loams

    Characteristics

  • They have an intermediate texture between clay and sandy soils.
  • They have most of the advantages and a few of the disadvantages of the clay and sandy soils.
  • They warm up reasonably early in spring and are fairly resistant to drought.
    • image of crops growing in Loamy soil

    Loams

    Cropping

  • These soils are regarded as the best all-round soils because they are naturally fertile and can be used to grow any crop provided the soil depth is sufficient.
  • These types of soils are ideal for mixed farming.
  • Cereals, potatoes and sugar beet are the main arable crops while grass lays provide grazing and winter bulk food for dairy and beef cattle and sheep.
    • close up of some wheat crops

    Calcareous Soils

    Characteristics

  • Derived from chalk and limestone and contain various amounts of calcium carbonate.
  • Ease of working depends on the amount of clay and chalk or limestone present.
  • Free draining and shallow soil, can be prone to drought Sharp stones and chalk formations can effect sowing and harvesting and hard wearing on machinery.
  • Contain low levels of some major nutrients, the alkaline conditions can also cause trace elements to be unavailable.
  • pH usually > 7.0

    • Calcareous Soils

    Cropping

  • Cereals are a suitable crop for this type of soil.
  • Root crops on the deeper calcareous soils.
    • image of exposed calcarous soil in field

    Peaty soils

    image of hands holding wet peaty soil
    image of exposed peaty soils

    Peaty Soils

    Characteristics

  • Pure peat contain 50 – 90% organic matter but the peaty soils contain 20 – 25 % organic matter.
  • These have been formed in waterlogged areas where mosses, heather and rushes grew and the dead material was only partly broken down.
  • These are rich in Nitrogen but have low levels of phosphates and potash.
  • pH range from 4-6.
    • image of peaty field

    Peaty soils on moorland

    image of landscape showing flooded peaty soils in front of hills

    Peaty Soils

    Cropping

  • Good grass and clover lays can be established on these types of soils but they must not be overgrazed or poached in wet weather or rushes and weed grasses will quickly re-establish.
  • With good cultivation, most arable crops can be grown but potatoes and oats are the most suitable.
    • image of crops

    Section 2 - Session 3


    Soil Classification

    Soil Classification

    Soils can be classified into different types according to the amount of clay and sand present.

    image of someone pouring soil from one hand to another

    Soil Texture Classification (Ioan Rhys Williams)

    Section 2 - Session 4


    Soil Fertility

    Soil Fertility

    A fertile soil provides a plant with all the nutrients and water it requires for growth.

  • At any one point in time soil fertility is dependent on the natural make-up and condition of the soil.
  • Farmers will always aim to maintain soil fertility through appropriate husbandry techniques.
    • cross section of crop in soil showing roots, crop, and sprouting

    Natural Make Up of Soil

    Factors that influence natural makeup are:

  • Texture and chemical composition of the soil.
  • The topography (slope), this affects drainage, temperature and workability of the soil.
  • Climate and local weather.

    • These have a significant influence on the rental and sale value of land.

    image of churned up soil

    Maintaining Soil Fertility

    Many management factors contribute to the maintenance of soil fertility:

  • Maintaining or increasing organic matter.
  • Ploughing in straw and similar crop residues, FYM and composts.
  • The roots and stubble of crops are sufficient in maintaining organic matter (humus) levels in some cases e.g. growing cereals continuously.
  • Increasing the organic matter content of a soil is the best way of increasing its water holding capacity.
    • image of compost on top of field

    Drainage

    Measures to assist soil drainage are frequently practiced by farmers particularly with heavy clay soils susceptible to waterlogging.

    image of drainage pipe draining water

    Benefits of Drainage

    The benefits of drainage include:

  • Minimizing excess water.

    • Waterlogged conditions will deprive plants of oxygen and stop plant respiration resulting in stunted roots that prevent plants from absorbing water during drier periods.

  • Improving soil workability.

    • Reduction in water content improves soil strength.

    Low water content in the topsoil increases cultivating opportunities.

    Benefits of Drainage

  • Increasing soil temperature.

    • This can lead to more rapid seed germination.

  • More efficient fertilizer use.

    • More aerobic, warmer soil conditions make more efficient use of fertilizer, especially nitrogen.

    image of tractor spraying fertiliser

    Benefits of Drainage

  • Beneficial to grassland.

    • Drainage will extend the growing season and alter the composition of the sward, increasing the proportion of productive grasses at the expense of weeds.

  • Beneficial to livestock.

    • Drainage will increase the number of days that livestock can graze in the year without damaging the soil structure by poaching.

  • Animal liveweight gain is improved.
  • Reduces the incidence of liver fluke and foot problems.

    • Drainage Methods

    Ditches and Open Drains

  • These are normally located along boundaries, they may be adequate to drain areas by themselves but are usually outlets for underground drains.
    • image of open drain in between fields

    Drainage Methods

    Pipe Drainage

  • This is where plastic pipes are permanently installed underneath ploughing depth to carry water away.
  • Distance between pipes depends on soil texture e.g. pipes need to be closer together in heavy, clay soils.
    • close up of drainage pipe

    Drainage Methods

    Mole Drainage

  • In heavy soils where a lot of drains are needed, it becomes uneconomical to lay permanent drains at regular intervals, therefore a mole plough will create temporary channels underneath ploughing depth 2 – 3 meters apart, these will lead into a ditch or over a permanent drain pipe.
    • image of mole plough in field

    Mole Drainage

    image of mole plough in field

    Crop Rotation for Maintaining Soil Fertility

    A rotation is a cropping system in which two or more crops are grown in a fixed sequence.

    An example of a six year rotation on an arable farm would be:

  • Wheat
  • Wheat
  • Beans
  • Wheat
  • Barley
  • Oilseed Rape
    • image of wheat being harvested

    Mixed Farm Rotation

    An example of a rotation on a mixed farm would be a 2-3 year grass ley, then a fodder crop such as forage maize.

    image of sheep in field

    Main Reason for Rotating Crops

    The main reasons for rotating crops on a farm are:

  • Nutrient needs of different crops are different therefore there isn’t a strain on that particular nutrient each year.
  • Manure can be applied to the soil, providing nutrients and organic matter.
  • Effective cultivations improve fertility.

    • Other Reasons for Rotating Crops

    Other reasons for rotating crops include:

  • Weed control.
  • Encourage biodiversity.
  • Make good use of natural resources.

    • Section 2 - Session 5


    Soil Sampling and Analysis

    Soil Sampling and Analysis

    A sample of soil should be analysed from a particular field every 3-4 years.

  • This is an important procedure that should be carried out with care and precision because the productivity of the land is dependent on the farmer ensuring that the land has the correct nutrient level.
  • Being aware of the soil chemical composition will allow the farmer to adjust the composition to meet the production potential.
    • image of spade in soil

    Taking a Soil Sample

    It is important to ensure that the sample taken is representative of the whole field as the chemical properties of the soil can vary from point to point in the same field.

  • This can be achieved by taking a number of sub-samples from different areas and combining them to create one sample.
    • imge of soil samples being taken in a field

    Techniques in Soil Sampling

    If there are obvious differences in the soil composition within the same field e.g. a badly drained area, then a separate sample should be taken for each area.

  • Soil sub-samples dug out via a hollow circular corer to avoid sample contamination should be taken from the topsoil at a depth of about 15cm for arable land and about 10cm for grassland.
  • A large number of sub-samples will reduce variability and provide a more reliable representation of the field as a whole.
    • image of hollow circular corer taking soil from field.

    Figure shows an example of how a field should be sampled.

    Analysing the Soil Sample

    A sample is taken to a laboratory to be analysed.

    It is air-dried, crushed (not breaking the stones) and then passed through a 2mm sieve.

    The analysis is carried out on the ‘fine earth’ fraction to determine the following:

  • Concentration of nutrients available to plants.
  • Organic matter content.
  • pH.
  • Soil texture.
    • image showing petri dish of soil, above a hand to the right shows a sample being taken and on the left a test tube is ready for the soil to be inserted
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