Preface The soil is the most abundant of all the materials on earth with which we are familiar. The land we walk upon, the fields in which we grow crops, the channels in which we run our water, and the dust that blinds our eyes all remind us of the soil and yet we know so little about it. The average man is content to regard it as too familiar an object to be enquired into. If he has a backyard vegetable garden he may sometimes wonder how the soil helps in the growth of such plants and he may be reminded of the need of manures. The lack of water is also very often brought home to him when plants begin to die for want of it. But we all would like to know what mysterious properties lie hidden behind this apparently dirty stuff. This book is primarily written for students of borderline sciences for whom a knowledge of the fundamentals of soil science is absolutely essential These students are, very frequently, confronted with books which are far too foreign in outlook and background, and cannot afford the beginner a picture of the soil that he can view in the light of his own familiarity with objects of everyday life. The intelligent layman who has an interest or stake in the soil will find this book free from technicalities, even an elementary knowledge of chemistry is not assumed. Improvement of soil is the basis of all agriculture and it is hoped that this book besides its text book appeal will help in the awakening of that mass interest in the soil which ultimately must lead to a more intelligent use of nature’s most abundant gift to mankind.

For the intelligent appreciation and understanding of various reactions and processes associated with soils a little basic knowledge of chemistry is helpful. If the reader follows this chapter carefully, the working knowledge of chemistry thus acquired will enable him to follow all the complicated chemical reactions taking place in the soil. What is Chemistry? Chemistry is the science that deals with the chemical changes. What is a chemical change? When two or more substances combine to give a substance entirely different in properties to the individual components, the change is called a chemical action to distinguish it from the purely physical action in which the properties of the product are intermediate between the components. The latter naturally can be separated easily by mechanical means. For instance a mixture of sand and water can be separated by passing through a filter, and a mixture of salt and water can be separated by evaporating the water when the salt is left as residue. But water itself is a compound of two gases: oxygen and hydrogen. Its properties are totally different from its components. We can freeze it, evaporate it and perform a host of other operations on it without changing it in the least. Atoms and molecules: The whole science of chemistry rests on atoms and molecules If these are understood clearly, basic chemistry is reduced to simplicity. Take a familiar object like gold or silver and go on subdividing it into smaller and smaller bits. You will reach a stage that further subdivision will become impossible. You will have reached the primary units, the elementary bricks of which the substance is made up, namely the “Atoms”. An atom in chemistry is the smallest brick from which all material substances in the world are made up. Not that you can see an atom. It is so small that even the most powerful microscope will nut reveal its presence. You could perhaps just see a collection of a million atoms as a tiny spot of light if it is illuminated with a powerful beam of light. Even then you could not say if it was round, square, or oblong. However, you need not look for any such tangible proof to believe in the existence of atoms. Even pure logic would compel you to stop somewhere if you started with an imaginary division and sub-division of a substance. You could not go on ad infinitum. Even the ancients, who knew no modern chemistry, believed in their existence purely from philosophical reasoning.

If the reader has endeavoured to follow the basic chemistry in the first chapter, he would be in a positions to follow up the various reactions that take place in soils and which hitherto had baffled him. Soils contain particles of various sizes ; some so fine that they would remain suspended in water for a long time; others so coarse that they would be sieved out by even a coarse sieve. Soils contain some salts that can be washed out by leaching with water; they contain some water which can be readily evaporated away by exposing to hot sun or by heating slightly. They also con-tain a little organic matter which can be burnt away or dissolved in alkali. When all the extraneous substances have been completely removed we are left with what we might call soil framework. This framework gives mechanical support to the roots of the plants. Without this framework the plant would not stand up even if it could be supplied with all the food material. Let us suppose a soil freed from all the extraneous matter by continuous leaching with water or a dilute acid. In the former case the removal of all the extraneous matter is extremely slow, whereas by leaching with dilute acid everything except the soil framework is removed in a short time. When soils from different localities are subjected to leaching with dilute acid we find that the soil framework in every case, behaves in a similar manner. No matter from what part of the world the soil has been obtained, the properties of its mineral framework only differ in quantity but not in kind.

Next to the mineral frame work the salts in the soil deserve consideration. In fact these salts constitute the main food material of plants besides what the root hairs can get out of the mineral framework in the form of basic materials already dealt with. We are, however, not concerned with the plant food materials, but salts that are in excess and which alter the properties of the soil to such an extent as to influence its behaviour profoundly.

Soil texture and mechanical analysis: The chemistry of the mineral framework was explained in Chapter II. Its physical properties are quite as important- When all the extraneous matter is removed from the soil we are left with a collection of particles of various sizes. These particles are not spherical in shape, but most of them are nearly so. At any rate the discussion is very much simplified if we suppose that they are spherical and the arguments advanced are equally applicable if the shape deviates from the spherical.

Moisture plays by far the most important role in soils as regards vegetation. It is through the medium of water that the plants obtain their quota of mineral food vital for their growth. If water is gradually withheld from the soil, at a certain stage, the plant will begin to wilt and soon dry up. The soil may still contain some moisture but the plant is unable to make use of it. The minimum moisture in the soil below which wilting will take place is called the Wilting Coefficient and at one time great importance was attached to it. The determination of this Coefficient, however, is tedius. One must grow a number of plants and slowly watch them wilt and determine the moisture content at that point. Moisture content of the soil is determined by heating a weighed quantity of the sample in an oven kept at 100-110 oC. The moisture is expressed as percentage on the oven-dry soil.

Soil mechanics as a distinct branch of soil science is comparatively of recent growth. Its need arose from the fact that the requirements of the engineer are very often quite different from those of the agriculturist. Whereas the former would as a rule require hard, compact, impervious soil, the latter would have no use for it for the growth of his plants which require soft, friable and porous soil. The agriculturist generally has to look up to nature to help him out of his difficulties, the engineer has to depend on his scientific skill to modify the soil to suit his requirements. In this chapter an attempt will be made to explain in a simple language the requirements of the building, road, and irrigation engineer for particular types of soils and how they are met by modifying the soil if the required properties already do not exist.

Soil fertility is the resultant of several factors, not the least important of which is the soil itself. Theoretically, the soil only plays the part of a support and could be very easily dispensed with and replaced by sand and even that can be replaced by a wire framework to support the plant mechanically and it will grow quite well if the right amount of nutrient materials have been added to water. ‘Where does the soil come in?” you will ask. The importance of the soil is two-fold: (1) It provides vast reserves of plant food which is slowly made available. (2) It provides limiting factors which restrict plant growth; these limiting factors must be neutralized. We have already discussed these reserve food materials and limiting factors in Chapter II. It has been shown that if we treat a soil with dilute acid exhaustively we can remove everything from it except the mineral framework. If we try to grow anything in this mineral framework nothing will grow. Yet it is not like sand, for whereas in sand by adding appropriate amounts of nutrient materials in water solution you could grow a crop, nothing will grow if you added the same solution of nutrient materials to the soil. This mineral framework, therefore, is not inert like sand, but it is active and can react with the nutrient salts in such a way that either they are rendered unavailable or something else which is toxic is produced by the interaction between the soil and the nutrient salts. This mineral framework is not acted upon by dilute acids, but it reacts with alkalies in such a way that the alkalies are neutralized. In this respect, therefore, the mineral frame work behaves like an acid. With alkalies it forms salts and different alkalies are required in equivalent amounts for the formation of these salts. Thus we can have salts of a number of bases and if we use sodium hydroxide for neutralizing we shall have sodium soil, similarly we can have a calcium, magnesium or potassium soil. Now in nature, all the various bases principally, calcium, magnesium, sodium, and potassium, are associated with it in different proportions. The nature and amount of these bases determines the physico-chemical properties of the soil affecting crop yield and leading to soil infertility. We have already indicated the method of bringing the soiil to the normal state when it suffers from any of the defects associated with the soil framework.

Index Alkali, definition, 7 Alkalization, degree of, 24 Alkali soils, 33 Aluminium, 3 Agrobiology, 109 Agronomy, 109 Ammonium chloride, in soil, 39 Artificial manures, 123 Atoms and molecules, 1 Capillary water, 105