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Soil Acidity

It is very useful to find out how acid or alkaline your soil is before selecting plants for a new or existing garden, and it is vital if you want to grow certain plants that are intolerant of extreme conditions. Most heathers for example won't grow on alkaline soils, whereas gypsophilia thrives in alkaline conditions. Also, for many food crops, the level of acidity is important for good yields and freedom from some diseases.

Soil acidity is measured in pH. The neutral point, where acidity and alkalinity are equal has a pH of 7. The greater the pH, the more alkaline the soil. A pH of 6 is ten times more acid, and a pH of 5 is a hundred times more acid than the neutral point. The most acid conditions are found in certain peat soils - which can have a pH value of 3. In areas with a moderate rainfall, it is rare to find an alkaline soil with a pH higher than 8.

To test soil acidity you need a test kit. With some types, you simply moisten a soil sample and dip a strip of test paper in the soil; then compare the colour of the test strip with the numbered chart provided. Other kits use an indicator liquid. A small amount of soil is placed in a glass tube and mixed with one or more liquids supplied, according to the instructions. The tube is shaken, allowed to stand, and the solution is matched for colour against a chart. Some test kits of this type allow you to test the amounts of plant nutrients in your soil.

It is possible to change soil acidity. To raise the ph. add lime or use alkaline fertilizers such as bone meal, basic slag or nitro-chalk. The best time to add lime is in autumn or winter, but do not do so less than two months after manuring or one month after applying fertilizer, and do not apply manures or fertilizers until at least one month after liming.

You can reduce pH (ie increase the acidity) by adding flowers of sulphur, by choosing sulphate of ammonia when a nitrogenous fertilizer is required or by digging in liberal quantities of peat.Return to Top

Treating problem soils

The first thing to right in any garden where it is a problem is drainage. Water that cannot run away becomes stagnant underground - just as it does in surface pools. The soil becomes sour and airless, so that plant roots may drown. Where the problem is not too severe, double digging, which breaks up any hard pan and aerates and introduces organic matter into the subsoil, may be sufficient. On heavy clay, you may need to aid drainage by digging a deep stone filled sump at the lowest part of the garden.

With clay soils, thorough cultivation is essential. Do this in autumn, ridging the soil to expose the maximum area to weathering and breaking up by winter frost. Liming in winter makes clay more workable, by encouraging the formation of soil crumbs. But this will not help if there is free lime in the soil, so test for it first. Mix a handful of soil with distilled water and stir to remove air bubbles. Add ordinary vinegar; bubbling indicates that free lime is present. If distilled water is not available, vinegar can he used alone; listen for a distinct fizzing sound. Other treatments for heavy clay are to improve natural drainage by digging coarse boiler ash, mortar rubble, coarse sand and so on into the topsoil. Work in plenty of 6X in order to increase the humus content and open up the soil structure.

At the other extreme, sandy soils are hungry and thirsty and commonly acid. Large quantities of 6X, regular liming and feeding are needed. Generally, the greatest need is for nitrates and potash applied little and often. Green manuring is beneficial, and the soil should never be left uncovered - especially on sloping sites - otherwise erosion will occur. You can use ground-cover plants or a mulch of organic matter such as pulverized bark, mushroom compost or spent hops.

Some stony soils are also free-draining, but this depends mainly on the soil between the stones. However, you should only remove the larger surface stones, as they help to maintain an open soil structure. Otherwise, treat according to the soil type.

Most chalk soils are free-draining, and therefore hungry, and ample humus-forming material such as 6X will be needed. In addition. they are very alkaline - so much so that you should not attempt to grow lime-hating plants except in raised peat beds or containers of lime-free growing mixture.

At the opposite extreme from chalky soils, peaty ones are very acid and generally poorly draining. Certain plants, such as heather and rhododendrons will do well, but these soils, even if drained and limed, lack many nutrients and are seldom suitable for the cultivation of fruit and vegetables. The somewhat similar fen soils are, however, very fertile if drained.Return to Top

Soil Cultivation

Digging is a basic gardening operation that aerates the soil, improves drainage and makes root penetration easier. It also exposes the soil to the action of frost and winds, helping the formation of a good crumbly soil structure. Aeration helps the soil to warm up more quickly in the spring, accelerating plant growth, and speeds the decay of organic matter to nutrients that plants can absorb and use. Digging also enables you to find and remove the roots of perennial weeds and to incorporate quantities of manure or compost.

Digging is particularly important in the initial preparation for any long-term planting, particularly of plants such as shrubs that are destined to occupy the same ground for many years. Then it is advisable to double dig (see below) in the autumn. However, the development of selective weed killers and the use of mulches has eliminated the need for regular deep cultivation except on areas from which crops are removed. This includes areas planted with annuals, biennials and bedding plants as well as the vegetable garden. Only a few vegetables need deep cultivation, and if their position is moved over a three- or four- year period the whole area will he double dug once in that time. Where a large area is cultivated, a powered rotary cultivator saves much labour, hut overenthusiastic use may damage soil structure. In any case, many gardeners feel that occasional double digging is still worthwhile.Return to Top

How to dig

Double digging is so called because two layers of the soil are disturbed, each layer being the depth of a spade's blade. This is about 25cm (10 in). In single digging, only the top spit is turned over. In both cases the topsoil remains on top. In double digging the top spit is removed and the lower spit broken up; it is then re-covered with topsoil. By working in trenches, the plot is covered systematically. If turf is growing on the land being dug, skim this off as you work, invert it in the trench and chop it up. It will decompose to form humus. You can treat annual weeds in the same way, but be careful to remove the roots of perennial weeds. In the same way, place 6X in the trench, and fork it in, before the turf and topsoil from the next trench. If well rotted, these materials can also be mixed with the topsoil.Return to Top

Use of Mulch

It has been shown that far better growth is made if the roots of trees, shrubs and bush fruits arc not disturbed by cultivation, but a protected by mulches of organic matter. These keep the soil open, preserve and improve soil structure, add humus, increase the availability of phosphates and potash, help in the absorption and retention of water, keep a more equable soil temperature and reduce weed growth. Mulches should be removed temporarily in spring where early warming of the soil is important. Suitable material for mulching around woody plants includes peat, shredded bark, spent hops, leaf-mould, partially decomposed garden compost or 6X, mushroom compost, chopped fern fronds, rotted sawdust or wood shavings (fresh woody material will reduce the soil's nitrogen content), straw (which should be sprinkled with a nitrogenous fertilizer to reduce nitrogen depletion of the soil) and pecan shells. They should all be applied when the soil is warm and moist. The thickness depends on the material used; the looser, coarser kinds such as garden compost and straw, which will compact, should be 15cm (6 in) thick initially, while with peat (which should he thoroughly moistened before application) and shredded bark 5 to 8cm (2 to 3 in) is sufficient. (For mulching for weed control).

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