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© 2001 Paul Barlow


 To the uninitiated the soil under our feet is a lifeless, uninteresting dirty media, which sticks to their boots and soils their clothes.  To us gardeners, there beneath us lies a vast biological factory and nutrient reserve, which must be nurtured, restocked and cultivated.  Apart from the obvious soil fauna, i.e. insects and earthworms of which there are ten common species in Britain, depositing an extra 5mm. of topsoil per hectare per year, in the form of worm casts. There are also vast numbers of bacteria, estimated at 1,000 million in each gram of fertile soil and despite their microscopic size the top 150mm. of topsoil carries about one tonne of bacteria per hectare.


Soil is formed by a number of evolutionary changes taking place, the most important of which is weathering. From the moment rocks are formed and exposed to the elements they are subjected to erosion and the movement of rock fragments in the soil.  Weathering can be further split into two main causes:

  • PHYSICAL WEATHERING: Which is the action of rain, snow, sleet, wind and flowing water, also alternate freezing and thawing.
  • CHEMICAL WEATHERING: Rainwater is a weak acid (Carbonic Acid), which dissolves Limestone. Plant roots also produce weak acids. Oxygen reacts with many rocks to make them crumble, (a similar process to rusting). The decomposition of organic matter produces a weak acid.

All these various factors are instrumental in the fragmentation of rock, which is the first stage of soil formation. Plant roots then explore the small fissures in the rock and prize them wide open. The plants die and then bacteria work on the remains, and after thousands of years a living soil is developed.


Organic soils, which are made up of partly decayed plants, are called ‘PEATS’. Soils made up of largely inorganic substances, i.e. different size rock fragments, with some organic matter present are called ‘MINERAL SOILS’.

SOIL TEXTURE: This is usefully defined as the relative proportions of the sand, silt and clay particles in the soil.
The Agricultural Development and Advisory Service (ADAS.), classify soils as follows: Sandy - Loamy Sands - Sandy Loams - Sandy Silt Loams - Silt Loams -Sandy Clay Loams.  Clay Loams - Silty Clay Loams - Sandy Clay - Clay - Silty Clay.  Any of the above soil groups may be formed on chalk or limestone, so they are given the prefix "Calcareous" e.g. Calcareous Clay Loam.

HUMUS: This is essential to maintaining soil structure. This vital ingredient is a sticky black liquid which remains in the soil when plant remains (organic matter) are decayed or broken down by bacteria. It is responsible for binding together the mineral particles and making the desirable ‘crumb’ structure.
Organic matter — Used by Earth worms and Bacteria — Bacteria — Sticky layer of Humus coats soil particles — Soil Crumbs — Over cropping, Over cultivating, Failure to add O/M, Water logging, Erosion, Compaction — Loss of Crumb Structure.

SOIL STRUCTURE: This is the process of the arrangement of particles in the soil. The structure formation turns mineral particles into crumbs.
Once a soil has ‘Structure’, plant roots can obtain oxygen from large pores and water from the smaller pores. They also obtain most of their Mineral Foods from the breakdown of Humus.


In a natural plant community, when plants die, they decompose and nutrients are recycled and become available to the plants again.  When, however, a crop is harvested and removed from the soil, the nutrients are lost, and unless returned in some other form, the soil becomes poor and infertile. Nutrients are also leached out of the soil by rain.
Growers can take some steps to help prevent depletion of nutrients by crop rotation etc. but these measures on their own are not sufficient to meet the demands of intensive growing methods. The grower therefore has turned to the application of fertilisers.  In order to grow satisfactorily, plants require SIXTEEN different mineral elements. The ones needed in the largest amounts are called the Major or Macro Nutrients.

  • Nitrogen: The presence of N. in the soil is the result of bacterial action Nitrogen fixation, or the addition of artificial fertilisers. It is almost entirely absent from the bedrock from which soils are made.
    Nitrogen is of extreme importance - found in proteins, chlorophyll, enzymes and nucleic acids (genetic material).
  • Phosphorus: Most soils contain large quantities of P. but only a small proportion is available to plants. P. is released from soil organic matter by micro-organisms but most of it is quickly converted to insoluble forms by a process called phosphate fixation, insoluble aluminium, iron and manganese phosphates are formed at low pH and insoluble calcium phosphate at high pH. Phosphorus is important in the production of nucleic acid and large amounts are found in the meristems (growing points). It plays an important role in respiration and large amounts are found in roots, fruit and seed.
  • Potassium: It is usually present in sufficient amounts in clay soils where it is firmly bound; it is readily leached from light sandy soils. It has no structural role in plants, but is important in the metabolic processes. It activates enzymes including those that make proteins. It is important in flowering and fruiting.
  • Magnesium: Mg. is less abundant than calcium and deficiency may occur on sandy or acid soils. It is required in large amounts by most plants and the use of magnesium limestone is widespread. It is a constituent of chlorophyll, and is involved in the activation of some enzymes and in the movement of phosphorus.
  • Calcium: Present in most soils, and plants seldom suffer from deficiency. It is important in the formation of plant cell walls, especially in the meristematic regions where rapid cell division takes place.
  • Sulphur: Present as the sulphates in many soils and also as insoluble sulphides of iron that are unavailable to plants. Sulphur is present in proteins and enzymes.
TRACE ELEMENTS: (Minor or Micro nutrients)

Boron, Zinc, Copper, Iron, Sodium, Manganese, Aluminium, and Molybdenum. These are present in plants in very small amounts but are just as essential for healthy growth as the major nutrients. THEY CAN BE TOXIC if present in too high a concentration. Most mineral soils have adequate reserves of trace elements.


Nitrogen, Phosphorus, Potassium, Calcium and Magnesium become more available as pH increases (more alkaline) though Phosphorus is less available above pH 7. Iron, Manganese, Boron Copper & Zinc become more available as pH falls (more acid), though Manganese, Boron, Copper & Zinc become less available below pH 5.

Calcium and Magnesium are nutrients which not only become more available at higher pHs, but are the main components of LIME which is used to adjust soil pH to a level just below neutral (pH 7) pH 6.5 is found to be most favourable for the availability of all plant nutrients and is the pH to which most soils and soil based composts are adjusted, Nitrifying bacteria work best above pH 6 therefore Nitrogen is more available.

As the reader can now appreciate the importance of correct pH is vital to healthy plant growth, and the importance of having your soil pH accurately measured and obtaining expert advice on it’s correction is first priority at the start of the growing season.


Growers can as well as having the pH of their soil tested, have a major nutrient analysis.

We at HORTIBOTANIC UK will be happy to carry out this work. All the gardener has to do is to send a soil sample to us, (instructions for sample collection etc. is contained in our free fact sheet, S.A.E. please, ALTERNATIVELY ON OUR WEBSITE) telling us which plants are to be grown, and leave the rest to us.

You will receive, (within 7 working days from receipt of sample) a detailed test report, containing details of nutrient levels, actual soil pH, how to adjust if necessary, how to correct any nutrient deficiencies, with what fertiliser, and in what amounts.  Also included is an assessment of soil composition, soil type, and workability and how to improve if necessary, we will even tell you how much organic matter it contains at a small extra charge if you require it.

We test hundreds of samples from all over the U.K. every year from professional growers to major exhibitors to weekend gardeners, so you can be assured of a professional service at a reasonable price with expert advice.


OPTION 1: Standard Analysis.  includes pH test - tests for N, P, K. and a full assessment of Soil Composition etc

OPTION 2: Exhibition Analysis.  includes pH test - tests for N, P, K. and a full assessment of Soil Composition etc., PLUS Test for Magnesium and Soil Conductivity, (Soil Conductivity is measured electrically and gives a measure of the salt concentrations in the soil. If salt concentrations are too high, symptoms of water stress become apparent. Salt concentrations can occur from over zealous application of fertilisers or in protected culture, greenhouse or poly-tunnel, which are not exposed to rainfall etc.)

Also available: ORGANIC MATTER DETERMINATION. The results are expressed as a percentage.

For current price list and additional information use any of the following contact methods:

Barry Goss,
Hortibotanic UK,
Soil Analysis Lab
14 Allard Close, Rosedale,
Cheshunt, HERTS. EN7 6JW.

Telephone: 01992 625380

web site:

If you would like further information or wish to comment on this publication please send your e-mail to:

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Last updated on 16 April, 2002