Soil and Compost Analysis

Soil and Compost Analysis

Most soil analysis procedures are based on the extraction of nutrients with an extractant which attempts to estimate the quantity of nutrient that the plant itself can extract from the soil or growing media. Whether the results of the analysis truly reflect the quantities of nutrients actually available to the plants is a matter of some conjecture.

This does not mean that soil analysis procedures can be set aside lightly, as they do have considerable value if interpreted correctly and linked to practical growing experience.



Testing ‘kits’

Several different types of ‘kits’ are available: (a) litmus papers which indicate alkalinity or acidity when dipped into wet soil; (b) a series of litmus papers which indicate the approximate pH of the soil; (c) indicator fluids which, when used either separately with a solution of soil and water or with barium sulphate as a precipitant, give a remarkably accurate pH figure by comparison with a colour chart; (d) complete testing outfits which, by use of indicator fluids and colour comparison give, in addition to the pH figure, the relative availability or percentage deficiency of the three main elements — nitrogen, phosphorus and potash; in some cases magnesium, too, is indicated; (e) pH and ‘salt’ meters.

soil and compost analysis - soil testing kit The value of these kits and meters is that they give a fairly accurate indication of the fertility of a soil or growing medium without the inevitable wait associated with laboratory analysis. It should be pointed out, however, that the kits and meters do not work entirely on the extraction principle and therefore do not indicate the available levels of nutrients in the soil but rather the total amounts of soluble nutrients.


Laboratory analysis

The availability of free laboratory analysis services varies greatly, but it is unlikely that there are any areas in Britain where laboratory analysis is not available on payment of a fee. The range of services carried out also differs according to the body concerned, but the following are the most important issues:

Organic matter

Where a soil-based media for growing is involved, the percentage of organic matter is a useful figure, especially for border cultivation. Figures of 8-10% (including moisture) are average, although some soils may have figures as high as 15-16%, or indeed much higher. The significance of the organic matter content of soilless growing media will be discussed later.

Any gardener with a small crucible and accurate scientific-type scales can note the weight difference in a soil sample before and after combustion over a gas jet, and, by calculation, find out the percentage of organic matter content, which will include the moisture content unless the soil is oven-dried beforehand.

Lime requirement

Exchangeable calcium is determined by several means, but usually by a pH meter, when lime requirement is then calculated by reference to a chart prepared by a series of buffering experiments. Most bodies report not only the pH figure, but the amount of calcium carbonate (ground limestone) necessary to achieve the required pH which will vary according to the plant species concerned. Small meters give an immediate reading of pH.

Available nitrogen

This is not a test carried out by every laboratory unless specially requested, as it varies so considerably according to temperature, season, whether soil has been heat sterilized, and other related issues. The analyst usually states whether his test indicates high, medium or low availability and either (mg/litre) p.p.m. Or index factors, e.g. 51-150 mg/ litre (ppm.) = Index 2.

Available phosphorous (P2O5 or P)

This is reported as mg/litre (ppm) P, or index factor; also stated as low, medium or high. Index factor P:4-5 is standard for most greenhouse crops; below this, phosphorous is needed; above 4-5 little phosphorous required.

Available potassium:

(K2O or K) This is reported in similar fashion to phosphorus mg/litre (ppm) K, or index factor; also stated as low, medium or high.

Index factor K: 4-5 is standard for greenhouse crops.

NOTE: Exact conversions from P2O5 to P and K2O to K and vice versa are complicated by differing extraction methods. Magnesium Some bodies now report magnesium, usually as an index factor, levels of around 3-5 being normal. Soluble salt This is a very important issue in greenhouse culture and is usually stated as low, normal or high. Some laboratories quote the figure as pC while others quote CF, the respective values of which are as follows, the index factor also being stated to give their true significance. Index 3 is the safe level for most greenhouse plants, up to 4 for tomatoes and chrysanthemums, but above this level there is danger of damage. Most analytical bodies now report conductivity figures in µmhos. Index 3-5 = µmhos 2610-3000.

These figures are equal to approximately pC 2.8-3.00, or CF 15-10. It cannot be over-emphasized how important these figures are; spot checks should be carried out on a regular basis before and after planting on a large scale. Salt concentration meters are considered a good investment by commercial growers, especially for checking the salt concentration of liquid feeds, and also of growing media to ensure even distribution of the fertilizers. It is vital that all laboratory analysis be interpreted by the analysing body who will generally give advice on the nutrient needs of the particular crop or crops or purpose concerned.


Spectrochemical analysis

This is used for determining the levels of trace elements in both soil and plant tissue. The elements mainly concerned are zinc, manganese, copper and iron. Spectrochemical analysis is generally used as a further check following normal soil analysis, more so with plant tissue than with soils.

Tissue analysis

In addition to spectrochemical analysis for the trace elements, tissue analysis ensures that the plant is assimilating the correct amount of nutrient. It is generally necessary to carry out a comparison between normal and abnormal tissue. Tissue analysis is becoming very much a part of commercial growing in countries such as the USA and increasingly in the UK.

Observation of cultures

Fungal and other cultures are developed under laboratory conditions in specimens of soil. It is claimed that these methods give a much more accurate account of the nutrient levels actually available to the plant, as compared with those determined chemically. Visual assessment of actual plant response still remains a vital part of plant nutrition, and can be learned only by practical experience.

Eelworm determination

Soil or growing media infested with the cysts of the potato cyst eelworm Globodera (Heterodera) rostochiensis are unsuitable for the culture of tomatoes, potatoes and other solanaceous plants, which crops it attacks with equal severity. A determination of the number of cysts per gramme (or 100g) of soil or growing media can be made by most advisory bodies, along with a similar statement of the number containing live larvae.


28. March 2011 by Dave Pinkney
Categories: Greenhouse Gardening, Manures and Fertilisers, Organic Gardening, Soil Cultivation | Tags: | Comments Off on Soil and Compost Analysis


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