CHEMISTRY OF THE SOIL
Sixteen chemical elements are known to be necessary for the growth of plants. Carbon, hydrogen and oxygen find their way into the plants from the atmosphere or fromwater.
Those that occur in compounds in the soil are nitrogen, phosphorus, potassium, calcium, magnesium and sulphur, manganese, iron, boron, zinc, copper, molybdenum and chlorine. Although all these elements are essential for healthy growth, some are needed in greater amounts than others. Boron, manganese and some others are regarded as trace elements, because plants need very small amounts of them, but they are just as essential as nitrogen, phosphorus and potassium, which the plants require in much larger amounts.
The soil is like a bank with large reserves of nutrients, but these reserves are not available for immediate use. They are gradually released by weathering processes, and are then dissolved in the soil solution, from which plants obtain most of their food.
Although the exact method whereby plants take up their nutrients is still a mystery, it is known that they drink, rather than eat, by means of root hairs growing a short distance behind the root tips. These root hairs make very intimate contact with soil crumbs — a fact readily seen when a seedling is lifted from a seed bed.
The elements most often lacking of seriously deficient in the soil are nitrogen, phosphorus and potassium — the so-called fertilizer elements. The other elements are usually present in sufficient quantities, and it is only in special conditions that they have to be provided.
Before using fertilizers it is best to have soil tested to determine its nutritional level. A soil test can be arranged with the county horticultural adviser.
In order to provide a sample of soil from the garden, take a small amount from 10 to 12 different places, going down to a depth of 6 in., using a trowel. Put the sample into a bucket, mix the whole-mass together and then take enough of the mixture to fill a pint carton. If any part of the garden or lawn differs from the rest, that part should be sampled separately.
When posting a sample of soil to the adviser, give as much information as possible about the previous growth and soil treatment. He will then be better able to give an accurate recommendation for manuring the next crop. It is helpful, too, to say what plants it is intended to grow next.
The autumn is generally considered the best time for sampling, since the soil is then in a more normal condition after cropping, and if an application of lime is recommended it will have all the winter to act in the ground.
SWEET OR SOUR SOIL
At the time the soil is tested for nutrients, tests are also made to find out how sweet or sour it may be.
A sour soil is one that is acid; a sweet soil is alkaline. The degree of acidity or alkalinity is expressed by the scale of pH values, ranging in the case of soils in Great Britain from about -4.0 to 8.5. The neutral point is pH 7.0, that is, the soil is neither acid nor alkaline. All figures lower than 7.0 denote acidity, and all figures higher than 7.0 denote alkalinity.
There is probably a best pH value for each plant species, but most plants will grow well in soils with fairly wide ranges of pH values. There are exceptions, however. The low pH which suitswould be disastrous for , beans and lettuces. Scabious and anemones like plenty of lime, and prefer a pH value of 7.5. They would fail in acid conditions.
Just why crops grow poorly in acid soils is not fully understood. Phosphates become locked up and potassium and magnesium leach more easily from acid soils. Iron and aluminium are released in too large amounts, thus causing toxicity or poisoning, and this, together with the inactivation of most of the beneficial micro-organisms, may be the cause. But soil acidity need be no problem, since liming materials are a simple cure and easy to use.
It is much more difficult to accomplish the acidification of an alkaline soil. Flowers of sulphur will help to make the soil acid, but do not use them without first taking expert advice.
There are many kinds of liming materials, but ground limestone or chalk are probably the best forms to use; they last longest and do not burn growing plants. Hydrated lime is also very popular, being fine and more concentrated than ground limestone.
The rates of application of lime depend upon the soil types. A clay soil needs ij times as much lime to correct the same degree of acidity as would a sandy soil. So it is always advisable to have soil tested first and not just to apply lime because a neighbour is doing so. He may be making a great mistake, since over-liming can lead to the locking up of nutrients and can cause deficiency diseases.
Apply lime to the surface of the soil after digging, and then rake in. It can be applied at any time of the year, but is likely to wash into the soil quickest under the autumn rains. To obtain an even application delay raking in until the soil is on the dry side.