Acanthus 'Summer Beauty'

The aim of this page, and it is probably an over ambitious one, is to gather together a collection of terms used in gardening/horticulture and explain what they mean and try to justify there use. The reason of attempting this is two fold. On the one hand it will I hope save me from repeatedly explaining the same words over again in the remainder of the site and on the other provide a general source of reference when the terms are come across elsewhere.

I have deliberately avoided using the word “complete” because lists of the sort can never be complete and some of the explanations may well contradict what you hear elsewhere. If that is the case all I can say in my defence is I will of course endeavour to check my explanations and many will be based on my profession training and experience.

Then again I am no more infallible than the next person and look forward to your feedback.

Nitrogen (N)

One of the main plant nutrients and used by plants to make all proteins, and therefore as well enzymes, chlorophyll and many other essential parts of plants. The amount of nitrogen available to a plant is often the factor which limits it rate of growth and its behaviour within the soil is a very complex one with the amount available to the plant changing constantly. Like all plant nutrients; nitrogen has to be in a suitable form for the plants to take up, as an element it is a gas making up approximately 80% of the air we breathe but in that form is of no use to plants. The important exception to this last point is the legume crops which have evolved a way around this.


This is simply the process of giving things names.


These are chemicals plants use to grow and are often divided into macronutrients and trace elements. These terms are in themselves are of limited use as if the plant needs the chemical to grow, in however small a quality, its absence is going to cause problems. In practice the main ones are nitrogen (N), phosphorus (P), potassium(K), Calcium (Ca), magnesium (Mg) and sulphur (S). Of these the most important are nitrogen, phosphorus and potassium and they are the NPK referred to on packets of fertilizer.


Originally this was something which served as an organ and with time was adopted by chemists to refer to carbon based compounds. As a result of living things being carbon based it extended to which living things and material which resulted from living things; hence organic matter.

The latter half of the 20th century saw the word adopted by the anti-chemical movement to mean something produced without the use of manmade chemicals.

Organic matter

This is the part of the soil which is made up from partly decomposed plant material or material which can beaded to soil to increase the amount of soil organic matter. It is very important as a reserve of nutrients and in the structure of the soil. Typical sources include peat, compost, farmyard manure and leaf mould. Except for peat soils, mixing additional organic matter into a soil will improve it.



In simple terms this is how acid or alkaline something is – only a water based solution can have a pH. A pH of 7 is neutral, below 7 is acidic and above 7 is alkaline. The only way to know the pH of a soil is to measure it, either by adding it to a solution which changes colour according to the pH or using a pH meter. Both have there draw backs, with the solution it can be hard to check the colour as the soil discolours the liquid while on the other hand pH meters; if they are to be reliable they are expensive and need constant recalibration with a buffer solution. I’m very dubious about how reliable the ph meters for the domestic market are and I would say for your own garden the kits of indicator solutions are probably better.


Don’t get too hung up on the absolute accuracy of individual test as the pH of soil is vary variable and you may well find slightly different readings in different parts of your garden The advent of pH metes has lead to people publishing the recommended  for plants down to a tenth. I don’t really see this has any practical value.

Phosphorus (P)

This is the second most important plant nutrient after nitrogen and important in its take up. Phosphorus is an important element in many of the complex compounds in plants which they need. For example it is one of only five different elements in DNA, the others being oxygen, hydrogen, nitrogen and carbon. Even with an adequate supply of nitrogen, plants cannot take up enough nitrogen if there is insufficient phosphorus. A lot of the phosphorus in soils is in an insoluble form and plants can only take up nutrients if they are dissolved in water, in addition there is a tendency for phosphorus which is added to the soil to be converted to an insoluble form in the soil. Phosphorus is more likely to be in short supply in acidic soils.


A common name for potassium.

Potassium (K)

This has a range of vital roles in plants, thought the exact nature and extent of them is still not well understood. It is generally more likely to be in short supply in soils with little clay in them such as peat and sandy soils.

Selective Weed killer

Also called a selective herbicide.

A weed killer that is more poisonous to some types of plants than others. Note that it selective weed killers are first weed killer, i.e. they kill all plants, and then the selective part is just how susceptible different plants are that particular chemical. Or to put it other way if you are not careful to make sure you follow the instructions accurately you will either kill off nothing or everything including the plants you what to keep.

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