Soils

Most plants grown in horticulture and agriculture are either grown in soil or an

artificial soil-less material known in industry as 'growing media' or in the

home garden as 'potting mix."

A good soil or growing medium should:

• supply plant roots with nutrients, air, and water

• provide space for root growth

• physically support the plant.

Knowing about the type of soil or growing media you are using will help

improve the growing conditions for plants. This can result in:

• higher crop yields

• higher quality crops and plants

• more efficient use of fertilisers and other materials.

Natural soils and soil-less growing media and are usually composed of:

• mineral matter from the breakdown of rocks,

• animal skeletons and shells

• organic matter from dead and decaying plants, animals, animal products (e.g.

manures) and prepared materials (e.g. pine bark),

• water which occupies some of the space between the particles of mineral and

organic matter and contains dissolved nutrient elements for uptake by plants

• air which fills the remaining space between the particles of mineral and organic

matter not occupied by water

• living organisms which vary from small animals (e.g. worms) to tiny microscopic

organisms (e.g. bacteria and viruses).

Differences in soils and growing media are due to the differences in the proportions

and arrangement of each component, having different size ranges and being made

up of different minerals.

Properties of growing media

Properties of growing media fit into three groups:

• Physical properties that can be felt and seen such as colour, texture,

structure or its ability to transmit water and air

• Chemical properties that involve chemical activity and supply of plant

nutrients such as acidity (pH), salt levels and plant nutrients levels

• Biological properties to do with living organisms, that may be visible or

not visible to the unaided eye.

These different soil properties are connected.

For example, chemical properties can have major effects on physical

properties, while biological activity can affect chemical and physical

properties and vice versa.

Soil colour

Humus (organic matter) darkens soil, so that top soils are generally darker

than sub soils. Light grey and whitish coloured soils have usually been heavily

leached, washing-out coloured materials, minerals and organic matter, into

the subsoil, creating a more coloured subsoil.

Colour is useful in indicating how well drained and aerated a soil is. If soils are

poorly drained there is usually a greater amount of organic matter in the

surface layers, giving a very dark colour. The lower soil layers, which contain

very little organic matter, are a lighter colour, indicating that the soil has bad

drainage.

Soil textureSoil texture, how it feels, or the fineness or coarseness, depends on the amount of different soil particles present. Soil texture gives information about the sizes of particles in soil.

A soil is never made up of only one type of particle. Usually at least small quantities of all the types of particles are present. Soils are grouped into a number of textural classes. This enables soils to be described as identifiable textural classes, rather than as loose descriptions of “heavy” or “light” soil.Soil texture varies through a soil profile, a top soil having a different texture to a corresponding sub soil

Soil structure

The arrangement of the soil particle into aggregates or peds, gives a soil its characteristic structure. It indicates how a soil behaves towards air, water and plant roots.A soil with good structure allows good penetration of plant roots, air and water. Good structure can be developed and maintained by good soil management.

Soil permeability

Permeability is the ability of soil to transmit water or air, expressed as cm of

water/hour.

Permeability is the speed of air and water movement in a soil -- this is affected

by texture and structure.

if permeability is high then water moves quickly

if permeability is low then water moves slowly.

The better structured a soil, the more permeable it will be. A soil with good

structure will be more permeable than a soil of the same texture that has poor

structure.

pH (acidity) of soil

pH is a measure of the relative acidity or alkalinity of a growing media.The pH scale ranges from 0 to 14, pH 7 being neutral, less than 7 acidic, and more than 7 alkaline.pH effects: • the availability of nutrients to plants• the amount of nutrients held in soils• toxicities of nutrients• micro-organisms• growth of plants

The acidity of the soil is important in determining whether a plant will grow satisfactorily or not.

Different plants have pH ranges at which they grow best. Some plants prefer a pH that is in the acid range (i.e. below 7) while others prefer a pH in the alkaline range ( above 7). The best pH for most soils and growing media is between 6.0 and 6.5 as the majority of plants will grow within this range.

Safety precautions when soil sampling

Potting mixes, mulches, composts and soils can be harmful to your health

if you don’t take precautions.

Risks include:

• diseases such as Tetanus and Legionnaire’s disease

• exposure to testing chemicals

• injury from tools such as augers and shovels

• trips and falls

• exposure to plants

• exposure to animals, bees, wasps and spiders

• exposure to the weather.

Personal protective equipment and safety facilities • safety gloves• dust masks• hand washing facilities• first aid facilities.

Precautions

• carry out work as instructed by your supervisor• always wear gloves• take care in safe handling of tools and equipment• keep the soil or growing media damp while in use• avoid inhaling dust from the soil or growing media • wash your hands thoroughly after use• be aware of symptoms of possible infections and illnesses.

Ways to improve soils/media

TextureSoil texture can be changed by adding a different type of soil particle, or adding of another soil with a particular texture, e.g. adding sand to heavy soils, or the mixing of a clay loam with a sandy loam to produce a sandy clay.

StructureStructure can be destroyed by cultivating, walking on, driving heavy machinery on a soil when wet, and by excessive cultivating at any time (e.g. with rotary hoes).Structure can be improved by heavy applications of organic matter, especially with sandy soils. Clay soils can be improved by heavy applications of organic matter, and applications of gypsum only for clays with a high sodium content.

Raising the pH of soil

Lime in one of four forms can be used to raise the pH of soils: • burnt lime (calcium oxide)• hydrated lime (calcium hydroxide)• ground limestone (calcium carbonate)• dolomite (calcium magnesium carbonate).

The most common way of raising pH is the addition of calcium carbonate in the form of ground limestone or garden lime.

Lowering the pH of Soils

Mildly alkaline soils can be gradually acidified using acidic composts such as sawdust or fertilisers such as ammonium sulphate. Note that high rates of ammonium can be toxic to plants.

Salinity

The aim of soil salinity control is to prevent soil degradation and reclaim already salty (saline) soils. Saline soils cannot be reclaimed by chemical amendments, conditioners or fertilisers.

Considerations for salinity treatment: • prevention is better than cure• natural saline soils cannot be altered but can be managed according to

seasons and capability• select plant varieties that have a greater tolerance to salt• good soil management maintaining satisfactory fertility levels, pH and

structure of soils• engineering solutions such as subsurface drainage and groundwater

pumping can reduce the water table to below the root zone, allowing leaching of salt and bringing salt-affected land back into production

• changing water supplies as a solution to a water quality problem, if a better quality supply is available

• establishing and maintaining trees and shrubs on the property and public lands.