GARDEN SOIL

Wairarapa Times-Age, 21 April 1939, Page 2

GARDEN SOIL

WAYS TO IMPROVE IT. Soil is sometimes called the raw material from which plants are formed; but it is not the only source, for quite a large part comes from the an. Still it is most important, and anyone who gardens should make themselves familiar with the various kinds and their correct treatment. Soils can be conveniently divided into three sections—those which have been formed where they are found by the disintegration and weathering of the rocks and the remains of plants and animals, which form a substance called humus. Those found usually in level tracts which have been conveyed from higher up by means of rivers and other causes, these generally having some relation to the soils in the districts from which they came; peaty soil usually found in swamps or semi-swamps and are composed almost entirely o decaying vegetation. The first soil is characteristic of the upland country in and around Dunedin; the second is characteristic of the flats round the north end; and the third is found mainly on the flat where the subsoil is a puggy clay which does not drain readwe take a section through the soil on the uplands «e will M that the top layer is darker, this being due to the humus or organic matter m it, and it may be up to a foot in depth. Below we have a layer of soil with less humus, and lower still there may be no humus at all. Below this, at varying depths, is rotten rock, and lower still hard rock which has not been weathered at all. The difference between the raw stuff in the lower layers andthe top soil is due to the presence of humus and if we could expose the of soil to the weather and add humus in time, it would become good garden S °Humus is soil organic matter It has been formed by the change and decaj of the remains of plants. Leafmould ; s a familiar form of humus, so also is peat, farmyard manure e°™P° s r heap. Though humus is usually foun in the top soil, the gardener who wants good crops of flowers or vegetables has to add more and this can be done by burying in farmyard manure, which is not only humus, but also a valuable source of plant food, also green dressings, which are quick growing crops) of "lupines or mustard, which when fully grown, but before they f° im seeds, are dug into the soil If we examine the subsoil we vMll find that it consists of particles and grains 01 varying sizes too small to be distinguished by the naked eye. The finest materials of all are called silt, the particles of which are so small that we can hardly distinguish them under the microscope. Now in the soils of our gardens the clay fraction and humus occur in intimate mixture forming what we call the clay-humus complex. The general nature of the soil depends on the amount of soil colloid present and the relative proportions of clay and humus. For a garden soil there should be about 25 per cent of clay colloid pre-

sent, made up of about 17 per cent of the clay fraction and 8 per cent of humus. Of course, many good garden soils, do not show this composition, but there should be at least 7 per cent of humus and there should not be more than 25 per cent of clay. The raw materials in the soil consists of particles of grains of varying sizes, but these particles are not free from one another like grains in a heap of sand. They are held together in what is known as soil crumb. A soil crumb consists of a number of particles held together by the soil colloid, and it is this arrangement of the soil particle in crumbs which gives the soil its texture or tilth. This is the soft spongy structure which we find in good cultivated soils. A soil in good tilth readily absorbs the rain which falls upon it, holding the moisture in its crumbs, for the use of plants and allowing the surplus to drain away downward through the spaces between the crumbs. An adequate supply of clay and humus makes good tilth possible, but if it is tramped over, especially in wet weather, or if it remains uncultivated, the spongy structure is beaten down and the soil becomes heavy and sodden. I should explain here that what we mean by a heavy and light soil is not their relative weight, but the ease with which they can be cultivated. One composed largely of sand is light because it is easily worked, whereas one mainly clay is heavy because it is hard or heavy to work. Tilth is restored by good cultivation and exposure to the air and frost and in this respect it is helpful to do the trenching or digging as early in the autumn as possible and to leave the surface loose and open to admit air and expose as much as possible to the action of frost and alternate wetting and drying. The best way of improving a heavy soil is to add liberal dressings of farmyard or stable manure with plenty of straw, compost heap made up of decayed or partly decayed vegetables, and all manner of material which will rot. Also by digging in leafmould, peat, green dressings, or spent hops. It is also improved by the addition of coke breeze, lime rubble, sand or lime. To increase the texture of a sandy or gravelly soil, dressings of well rotted farmyard manure, compost heap, friable clay or in fact any kind of organic matter can be used.

Another way of building up the humus content of soils is to lay them down in grass. A soil in grass steadily increases the humus so that when old turf is brought into cultivation it has the fertility of virgin soil, and for this reason old turf is always recommended for growing plants in pots or tubs. An ideal garden soil would be a sandy loam, rich in organic matter, deeply cultivated and well drained. This is important for wet soils are cold and late and are difficult to cultivate in wet weather.