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How to test soil

Soil testing

Broadly speaking soil testing includes any kind of examination to which a soil is submitted; for example, when you rub some moist soil between your fingers to assess its texture or dig a hole to see whether it is badly drained, you are carrying out soil tests. But for most gardeners it means soil chemical analysis to find out whether their soil needs lime, and how much, and also whether it needs extra phosphates and potash.

You can test it yourself, using a test kit, or ask your county horticultural adviser to arrange to have it done for you, or send it to a private laboratory. Most soil test kits provide apparatus and chemicals for estimating the acidity, phosphates, potash and the amount of lime needed to correct acidity; some kits include a test for nitrogen, but it is very difficult to make a reliable prediction as to the amount of nitrogen that will be released during the season.

Whatever the method adopted the first thing to do is to get a sample of your soil. Collecting samples is not difficult, but it must be done properly if the tests are to give a reliable and accurate assessment of the nutrient status of your soil. Only about a teaspoonful of soil is required for the actual test, but that spoonful must represent an enormous amount of soil.

Never just take one lump of soil but take a small amount of soil from at least ten different places in a plot, going down to a depth of 16cm (6in) in beds or borders and 8cm (3in) in the lawn. Do not pick places near manure, compost or similar heaps, or bonfire sites and hedges when sampling.

There are special tools for the purpose but most gardeners can make a V-shaped slit in the soil to a depth of 16cm (6in) and take a thin slice of soil for testing. For the lawn a hollow-tine fork will cause the least disturbance.

The samples are then put into a clean plastic bucket and mixed together, saving about 0.5L (0.5pt) of the mixture for the testing. If the sample is to be sent away for testing put it into a strong plastic bag and number each sample.

When the sample reaches the laboratory it will be dried in the air, ground up and sieved through a 2mm sieve in order to remove stones and hard lumps.

The next step is to find out whether the soil is acid, neutral or alkaline. This is expressed in terms of the pH scale, which ranges from 0-14. Values less than 7.0 are acid, values above 7.0 are alkaline.

In the laboratory very accurate pH measurements can be made with a pH meter, which is an expensive instrument and is hardly practicable for most gardeners. There are, however, inexpensive indicator solutions that change color according to the degree of acidity present in the soil.

A rough method of estimating pH in the open consists in placing a small quantity of fresh soil in a white dish and then pouring a little indicator solution on to it. The contact between the indicator and the soil is achieved by slowly rocking the dish to avoid breaking up the soil fragments and the formation of a muddy suspension.

After the soil has soaked up the indicator, the color at the junction of the soil and the indicator should be used to assess the acidity. The color should be checked with a color chart provided with the testing kit.

Most soil test kits provide a more refined method of assessing pH, in which the soil is shaken vigorously with a clarifying agent (usually barium sulphate), distilled water and a soil indicator. On settling, a clear layer is obtained which may be compared with the color chart. With a little practice an accuracy to within half a unit of pH can be obtained.

A pH assessment alone will not give an estimation of the lime requirement.

The amount of phosphates and potash that is readily available for plant use is found by carrying out the appropriate tests with one of the kits. A dilute acid is used for extracting the nutrients from the soil ; the extract is treated with various reagents that produce colors or cloudy suspensions which may then be compared with charts or standard colors in glass tubes.

The estimation of the amount of fertilizer needed is the most difficult part of the operation; it depends on the nutrient content of the soil and the general requirements of the plants to be grown.

Generally autumn is the best time to sample and test. Not only is the soil in a more normal condition after the growing season, but if lime is needed it will have time to act during the winter.

 



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