AIR FILLED POROSITY (AFP) - The proportion of the volume of a growing medium which contains air, after it has been saturated and allowed to drain.
AMMONIUM NITRATE (NH4NO3) A water-soluble fertilizer containing 35% nitrogen, equally divided between ammonium and nitrate forms. A useful material for adding to bark-based media when supplementary nitrogen is required.
AVAILABLE NUTRIENT - Plant nutrients which are in a form suitable for uptake by plant roots.
BUFFER CAPACITY - This refers to the ability of a medium to resist changes to the pH and is closely linked to its cation exchange capacity. The term is often also used to describe the capacity of some materials, notably bark, to reversibly absorb nutrients into their structure.
BULK DENSITY - The mass per unit volume of moist material, expressed either in grams per litre (g/l) or kilograms per cubic metre (kg/m3). Loamless growing media commonly fall into the range 250-350 g/l.
CATION - A positively charged ion.
CATION EXCHANGE CAPACITY (CEC) - The CEC value gives an indication of the ability of certain nutrients within a growing medium to resist the effects of leaching. It is defined as the total amount of exchangeable cations, e.g. potassium, calcium and magnesium ions, that a medium can absorb per unit weight or volume, expressed as milliequivalents per 100 grams (meq/100g) or milliequivalents per 100 cubic centimetres (meq/100cm3). The latter units are conventionally used in reference to loamless growing media.
Materials such as peat and bark have moderate CEC's, whereas relatively inert products such as perlite, have virtually none. Lignite-based products such as Melcourt Adheart have high CEC's, and can be added to growing media for the specific purpose of decreasing the likelihood of nutrient leaching.
COMPOST - A term commonly used to describe both growing media and the decomposed organic material resulting from the process of composting.
COMPOSTED - A term used to describe organic materials after they have been through the process of composting. Often used in conjunction with the processing of bark. See MATURING.
COMPOSTING - A term used to describe the decomposition of organic matter by the heat-generating action of a mixed population of micro-organisms.
CONTROLLED RELEASE FERTILIZER (CRF) - Fertilizers which are coated in materials such as resins or polymers. The coating allows only a gradual release of nutrient into the growing medium, giving two main advantages. Firstly, a reduction in leaching of the fertilizer. Secondly, it allows more fertilizer to be added to the growing medium at the outset without the risk of harming the plant. This can obviate the need for liquid feeding. Controlled release fertilizers are available in different longevities, ranging from a few weeks to two years.
DIRECT STICKING - This term describes the process of placing an unrooted cutting directly into either a liner or final pot. The method can save time as it cuts out the need for potting on from the cutting tray. However, as it takes up considerably more space per cutting than traditional methods, it is only suitable for subjects which give high percentage success rates. Using more than one cutting per pot can improve both success rate and general appearance of the finished plant.
ELECTRICAL CONDUCTIVITY - A measure of the soluble salt or ion content of a material, expressed in various units, most commonly microSiemens per centimetre (µS/cm). Soluble salts or ions, carry an electrical charge. The higher the quantity of charges, the greater will be the electrical conductance of the medium. A measure of the electrical conductance therefore, gives an indication of the concentration of soluble salts within the medium.
Although a measure of electrical conductivity does not give any indication of which nutrient ions are present, nor any information about undissolved nutrients, it gives a useful measure of the overall available nutrient levels. Typical levels for ordinary potting media would be 300-600 µS/cm.
ERICACEOUS - A term describing plants belonging to the family Ericaceae, many of which are lime-haters, i.e. intolerant of alkaline conditions. They are often also intolerant of high soluble salt levels in either the soil or growing media. The term is also used to describe growing media which are suitable for growing plants, such as the Ericaceae, which require a low pH and low nutrient levels.
FINES - A term referring to the fine particle fraction of a growing medium or mulch.
HUMIC ACID - A variable mixture of organic substances which can be precipitated from organic matter under certain conditions. Their presence can add to the chemical activity of growing media.
HUMUS - A complex and resistant mixture of dark brown amorphous and colloidal substances of organic origin, which have been modified from the original organic matter or synthesised by various soil organisms.
ION - An electrically charged particle into which the atoms or molecules of some chemicals dissociate when dissolved in water. The water into which they dissolve becomes a conductor of electricity.
LEACHING - The loss of nutrients from growing medium caused by the downward movement of water. Nutrients will be more easily leached from some materials than others.
LIVERWORT - A leafless, stemless plant of the class Hepaticae. Liverwort can develop on the surface of growing media if they are poorly drained, which is unsightly and which in severe cases can inhibit the flow of the plant roots.
MATURING - A term used to describe the ageing of organic materials such as bark often carried out in large bulk piles or windrows. Does not necessarily refer to the more extensive breakdown of fibrous structure, caused by the process of composting.
MOISTURE CONTENT - The quantity of the water in a material, conventionally measured in growing media determining the loss in weight after heating to 105ºC. Usually expressed as a percentage by weight of the original sample.
PARTICLE SIZE DISTRIBUTION - The quantity of particles within specified size fractions, which can be used to physically characterize a growing medium or its ingredients.
PATHOGEN - An organism capable of causing disease. For example, Botrytis cinerea, Pythium ultimum and Phytophthora cinnamoni.
pH - A measure of acidity within a material, defined as the negative logarithm of the hydrogen ion concentration. The pH scale measures from 1 to 14, 7 being neutral, below 7 being acid and above 7 being alkaline. Because it is logarithmic scale, an increase in 1 unit of pH represents a ten fold decrease in the level of acidity.
pH is an important feature of soils and growing media as it has a strong influence on the availability of nutrients. In loamless growing media a pH of 5.5 is considered to be the level at which the maximum availability of nutrients exists.
POROSITY - The volume of a medium not occupied by solid particles.
POT MULCHING - The application of a layer of material such as bark or grit, to the growing media surface, for the purpose of suppressing the growth of moss, liverwort and weeds. The mulch will also reduce surface evaporation from the pot.
SALT-SENSITIVE - A term used to describe plants which are intolerant of high levels of available fertilizer within a growing medium. Examples are Azalea, Pieris, Primula, Begonia and Lobelia.
SOIL ASSOCIATION - The Soil Association was founded in 1946 by a group of farmers, scientists and nutritionists who observed a direct connection between farming practice and plant, animal, human and environmental health. Today the Soil Association is the UK's leading organic organisation, their symbol being found on over 70% of Britain's organic produce - a guarantee that it has been grown or produced to the highest standards of organic integrity.
SOLUBLE SALT - A term used to describe the soluble fraction of fertilizers and soil nutrients. The total soluble salt level in a growing medium is measured by electrical conductivity.
Soluble salt levels are an important indication of available nutrient levels. If the level is too low the growth of the plant will be reduced. However, plant growth can also be inhibited if the levels of soluble salt are too high, due to toxicity caused by a specific ion, or as a result of the interference to water uptake that high soluble salt levels can cause.
TOXIN - Poisonous substance of natural origin. In the case of bark, the waxes, tannins and resins naturally present can remain toxic to plants on badly processed material. It is also possible to generate toxins by incorrect maturing procedures. Obtaining supplies from reputable suppliers is essential if such problems are to be avoided.
TRACE ELEMENT - The term used to describe nutrients essential for successful plant growth but which are only required in very small quantities.
Expressed in terms of percentage, AFP is a useful indicator of compost quality. A compost with a low air-filled porosity, e.g. 5%, will hold a high proportion of water after draining, and can make plants susceptible to waterlogging. Conversely, a compost with a high AFP, e.g. 25%, will provide plant roots with plenty of air, but will require frequent watering. For most purposes, AFP levels of between 10 and 20% are desirable.
Soil Association Certification Ltd enforces these standards through certification and regular inspections of producers, processors and suppliers.
Most of the products in Melcourt’s Growers range are certified by the Soil Association. See individual product information for details