Structure of Soil
The structure of theis just as important as its texture. A careful examination of an undisturbed side of the soil pit will show a very distinct anatomy or structure in many soils. In some soils the particles are clumped together into what resembles the crumbs of a loaf. Other soils may appear to be made up of angular blocks or clods. These predominant shapes of the soil, or aggregates as they are called, are known as the ‘structure’, and structure is as much an essential characteristic of the soil as colour, texture or chemical composition. Although the mechanism of structure formation is not fully understood, it is believed that some kind of natural cement must be present to bind the soil particles together and keep them in these shapes, clay, as well as humus and various oxides of iron, being the chief cements.
The soil can be likened to a city of many inhabitants, and like any city it has its own system of communications. Some of the main routes are the work of earth-worms, of which there may be more than a million in an acre of soil. Their tunnels often penetrate to a depth of 5 or even 6 ft., and allow water and air to circulate through the- soil. They may, ultimately, form the layout of root systems.
Plant roots, too, make their own contribution towards the breaking up of the soil, by running in between the blocks of soil formed by the earthworms’ tunnels and preventing them from sticking together again. This is most noticeable with freshly broken-up grassland. Grass is one of the best agents for producing a good structure, which is one of the reasons for the practice of ley farming whereby fields are sown with grass and grazed or mown for several years before breaking up and recropping. The more fibrous the root system, the greater the effect. Tilth formation may also be in part the handiwork of certain soil microbes, whose gums and sticky sub-stances help to bind the particles together. These microbes feed on, and this explains why organic matter is important.
The individual particles of an ideal garden soil are grouped together into granules or crumbs, and soil scientists consider that the best size of crumb lies anywhere between about one-fifth of an inch to one-twentieth of an inch in dia-meter. Therefore, one aim in cultivating the soil is to try to achieve a good crumb structure when making seed beds. The crumbs, -being porous, soak up the soil water and retain part of it in their tiny internal pores. But there are also large pores between them which allow the rapid penetration and drainage of heavy rains, thus reducing surface runoff and the danger of erosion.
A granular or crumb structure is associated with good tilth — a rather vague and mysterious term used in connection with the fitness of the soil for the growth of a given plant. In other words, a tilth that is ideal foris not necessarily ideal for beans or other larger seeds and mature roots which can live in coarser soil. But it is not sufficient to have a good tilth just at the surface and a cloddy, lumpy soil below. What is really required, but is very difficult to achieve in practice, is a good system of large pores running right down to bedrock or to the water table, through which fresh air can go in and carbon dioxide can come out. An abundance of smaller pores in which water is held is also needed.
To find out whether a soil has a good or bad structure, dig out a spit of undisturbed soil, let it drop on to a hard surface or path, and then notice how it shatters. A moist soil of good structure should come apart in rounded porous crumbs, whereas a poor-structured soil will break into block-like clods with flattened surfaces and sharply angular sides. A cloddy, lumpy soil, with extremely fine and almost invisible pores, is a sure sign of soil compaction or structure deterioration.
MAINTAINING A GOOD TILTH
To produce a good tilth and keep it once it has been produced are two different arts. The principal enemy of soil structure is exposure to heavy rain, when the raindrops are harmful and batter the crumbs into their individual particles, which then clog the pores after muddy water has subsided. When this clod layer dries, a hard crust forms varying anything from a fraction of an inch to several inches thick, and acts as a barrier to the development of. Therefore, where there is poor germination the soil conditions should be examined. Heavy feet walking on wet soil are also powerful agents of destruction, and clay soils which are low in humus soon become massive if stirred or walked upon when wet.
The traditional method for building up and maintaining tilth is to work in bulky organic manures that rot down into humus, which is a valuable soil conditioner. But to do any good a tremendous amount of organic matter is required, for most bulky organic manures consist largely of water. For instance, a ton of farmyard manure consists of 15 cwt. Water and 5 cwt. Dry useful matter. For this reason a good thick coat of such organic manures has to be applied all over the soil for any improvement to be made.
Grassing down is Nature’s restorative, but unfortunately a number of years in grass are needed before the soil fully recovers. The porosity of heavy clay soils can, of course, be increased by mixing in coarse materials such as grit and well-weathered ashes, with a view to opening the soil and improving aeration. The mixing of marl into sandy soils helps to bind the sandy soil particles together into groups and improve their moistureholding capacity. But as the structure of sandy soils is very easily broken down by rainfall, it is probably better to ensure that they are always covered by a crop. Annualgrown during the winter on sandy soils will act as a blanket and take the first impact of the raindrops, and weeds will, of course, absorb nutrients which would otherwise have been lost in drainage. If the weeds are dug in, in the early spring, an improvement in the soil structure will usually be noted.
It is therefore important that the living conditions for plant roots are really good. If they are not good the best results will not be obtained from fertilizers and manures, for if the holes in the soil are too small for roots to enter, they cannot explore every nook and cranny, however rich the soil may be in nutrients. Fertilizers applied to a poor tilth cannot be used to their best advantage.
Chalky soils that are too limy for most plants dry out white in hot weather. Yellow soils often suggest leaching, that is, the washing out of nutrients, as do the grey and white soils in upland areas.
Colour is a most useful guide to soil drainage. When soils are completely waterlogged their colours become grey or greyish-blue; if they contain much humus they may become black due to the formation of iron sulphides. Poor drainage, which is quite common in many soils of heavy texture or in those with hard layers below, cannot always be detected at the surface. If there is any doubt about drainage, dig some holes about 2 or 3 ft. deep, fill them with water and then wait for about 24 hours. If the water disappears within half an hour or an hour, then the soil is well drained and will be good for all garden plants that require well-drained soils, but if it does not disappear within 24 hours only shallow-rooted plants can be expected to survive. Well-drained soils have a pleasant uniform brown colour, extending to a depth of 3 to 4 ft., but any sign of bluish or rusty mottling is always suspicious. Where the water-logging is continuous there is greyness in the soil. Where the drainage is bad in winter but improves in summer, rust stains and deposits appear.
Colour also provides an idea of the amount of humus in the soil. Brownish-black and dark brown colours usually show a good humus content, but blackness in top-soil in low ground may indicate bad drainage, particularly if peat is present as well. And in low ground, if the soil surface is nearly black-grey or greyish-green then again poor drainage is indicated.
It is important to identify such conditions in advance in order to decide whether they are bad enough to justify artificial drainage or not. If the soil is really badly drained, tile drains for removing the excess water will need to be installed. In cases where water comes primarily from adjoining higher land, an intercepting ditch or drain may be all that is necessary. Where drainage is not feasible, only those plants which can tolerate wet feet can be grown. Or, alternatively,may be used.
It is the winter crops, the deep-rooted crops, and the rapidly growing young plants, that are prone to damage by poor drainage. Some soils that are well drained during the summer and wet only in the spring will grow annual plants, but roots of perennials cannot live over the winter in them.
The soil in an acre may contain anything up to 100 tons of organic matter, which is made up of plant and animal debris, for although plants take a part of their sustenance from the soil they also return to it leaves, decayed stems and roots. And all the numerous organisms living in the soil also play their part in producing this organic matter.
Most soils rarely have enough organic matter unless green crops are regularly grown for digging in. This is why bulky manures are needed to make good the deficiency.
The soil, with its teeming millions of workers, is very much like a factory, and the workers, that is to .say the bacteria and many other types of living organisms, all require the food and energy which organic matter provides. But not only does the organic matter provide nutrients for the soil organisms and for plants, but it also improves the physical condition of the soil. In heavy soil it has an opening effect, and prevents soil particles from settling into a solid mass. Roots can run better in soils which have plenty of organic matter. But more important, in well-drained soils the organisms that decompose organic matter produce substances that lead to granulation and a better structure. Hence the clay soils are made more fluffy and porous, and thus are able to give the roots better circulation.
adds ‘body’ to sandy soils and acts like a sponge in holding moisture. It also holds nutrients. It is no good watering a very sandy soil if there is nothing in it to hold the moisture. Bulky organic manures, such as sawdust, farmyard manure and peat, when applied to the surface of the soil as a mulch, help to control temperature, to reduce losses by evaporation, and to keep down weeds.
Organic matter is also the chief source of nitrogen in the soil. Manure orderived from a wide range of plants give a balanced supply of slowly available nutrients, and in particular the trace elements which are so often absent from fertilizers.
The bulk of organic matter is seldom decomposed completely. Usually a resistant residue of partly decomposed matter remains, which continues to break down very slowly.
This residue is humus; a dark brown or sticky substance, quite different in appearance and properties to the fibrous and bulky material from which it is formed. So, contrary to the belief that it can be bought by the sack, humus can only form in the soil. Not only humus is required to improve the soil but also the strawy or fibrous material remaining in it, as well as the products that are formed in the process of breaking down. Constant replenishment is necessary in order to maintain the turn-over of these valuable products.
Fresh organic materials vary widely in their breakdown rates, and in the amount of plant nutrients, especially nitrogen, that they release.
Some contain fairly large amounts of protein or other nitrogen-bearing matter, and these decompose readily. In fact, the breakdown processes may go on so rapidly and furnish so much nitrogen that the materials are regarded primarily as fertilizers. Castor meal and various other vegetable seed remains have this property. Grass remains decompose very quickly, and so does straw, but the latter is low in protein and so provides very little nitrogen. Most peats decompose slowly and give only minor supplies of nutrients.
In a fertile soil the organic matter contains about 10 times as much carbon as nitrogen. But leaves, straw, old stems and most of the plant materials commonly available are dry, coarse and contain much more carbon — they may have 40 times as much carbon as nitrogen. When added to the soil it is not only hard to mix them in evenly, but they need a large amount of moisture. Their excess carbon provides the bacteria with a great deal of energy food which acts rather like sugar and helps the soil population to increase enormously. But the soil population also needs nitrogen and phosphates, and these it takes from the soil. To compensate for this, add some nitro-gen and phosphatic manures when burying vegetable refuse or very strawy manures, otherwise the plants will be temporarily robbed of the valuable nitrates they need for early growth. Better still, break this refuse or strawy manure down in a compost heap, the purpose of which is to produce an organic matter approximately like that in a fertile soil.
It is a great mistake to bury bulky manures deeply. Always mix them with the top-soil either by placing them against the walls of the trenches when digging or, provided they are well rotted, by incorporating them with a rotary cultivator, or by forking them into the soil where the rows are to be sown or planted.