The Chemistry of Soils - Including Information on Acidity, Nitrification, Lime Requirements and Many Other Aspects of Soil Chemistry

Various Authors

• Publisher: Hall Press
• Year: 2013
• Print ISBN: 9781446531501
• eBook ISBN: 9781473390331
• Topic: Technology & Engineering
• Subtopic: Horticulture
• Index: Technology & Engineering

"The Chemistry of Soils" is a treatise on the scientific aspects of soil, exploring such subjects as lime requirements, acidity, nitrification, etc. This timeless volume contains a wealth of information that will be of use to the farmer of keen gardener. Contents include: "And Preparation Of Sample", "Plant Nutrients In The Soil", "Absorption Of Substances By Soils", "Flocculation And Deflocculation Of Clay", "Density And Pore Space", "Sticky Point", "Water", "Holding Capacity", "Field Capacity", "Humus", "Ammonification And Nitrification", "Soil", "Sourness, Soil Acidity And Methods For Determining", "The Lime Status Of Soils", etc. Many vintage books such as this are increasingly scarce and expensive. It is with this in mind that we are republishing this volume now in an affordable, modern, high-quality edition complete with a specially-commissioned new introduction on soil science.

SOILS

IN the early days of agricultural chemistry, it appeared reasonable to hope that the simple chemical analysis of soils would unmistakably reveal specific causes of infertility and lead to the growth of larger crops. Although this aim is at present without complete realisation, the intervening years have been fruitful by bringing a broader outlook and a much fuller appreciation of the complexity of soil problems, and in particular of those problems which involve the relationship between soil and plant. Much of a practical nature has, however, been accomplished, for the causes of infertility in large numbers of soils are clearly revealed by laboratory tests. Further progress will follow the correlation of laboratory work with the results of reliable field trials.

The academic worker may be profoundly interested in soil as such, and regardless of its crop potentialities, and the farmer may be solely interested in his soil from the point of view of its crop-producing power, but between the two is the farmer’s scientific adviser, whose soil studies are made with an immediate utilitarian object. There is, moreover, an educational aspect of such soil studies, for in few subjects is it so difficult to obtain the essential view from behind the scenes, to enable the tangled skein of factors which make up soil fertility to be unravelled.

The exercises described in this section illustrate the methods by which the agricultural properties of soils are studied in the laboratory, with sufficient annotation to enable the student to appreciate the educational value of the work.

It is, however, important that knowledge gained in the laboratory should be supplemented by observations on soil structure, variation in texture, reaction, etc. made in the field under the guidance of an experienced person—a form of instruction which written notes can hardly replace and which is impossible to give within the confines of size and scope of this book. Consequently the value of the laboratory work is greatly enhanced if conducted upon soil samples taken during the course of field study.

SAMPLING AND PREPARATION OF SAMPLE

Surface samples of soil are usually taken to a depth of nine inches, unless there is a marked difference between soil and subsoil before that depth is reached. The subsoil is usually taken as the depth 9 to 18 inches or the next nine inches following the change from soil to subsoil. In special cases, e.g. fruit soils, deeper samples may be taken, and in the study of soil profiles, important from the point of view of soil survey work, samples are taken from each level or “horizon” from the surface to the parent rock. It is important that several samples should be taken from the same field or uniform area, and that these be bulked and well mixed before being analysed. In practice, little is to be gained from analysing each sample separately and averaging the results, for the experimental errors are very little larger when only one analysis of the composite sample is carried out.

Various sampling tools are used, the most convenient being a 2-inch auger or a special cylindrical tube made for the purpose. The latter consists of a steel tube 2 inches in diameter and 12 inches long. It has a 3/4-inch slit cut lengthwise and all its edges are sharpened. The tube is fixed to a vertical steel rod bent at the end to a ring 2 inches in diameter, through which a wooden handle is fixed. The core of soil obtained is removed with a pointed iron rod. In many cases a spade or trowel may also be used to take the samples.

In the laboratory, the samples of soil are spread out in shallow trays to dry. When air-dry, the soil is sieved through a 2 mm. sieve to remove larger particles of vegetable matter and stones. The residue from the sieve is rubbed up in a mortar with a wooden pestle, care being taken not to crush stones, and the material is again sieved. The soil passing through the sieve is called “fine earth”; it is well mixed and is used for subsequent analysis. The treatment which it undergoes afterwards will depend upon the purpose for which it is required; e.g. for some of the chemical analyses a sub-sample of the soil is ground as finely as possible in order to obtain a small representative sample for the ultimate analysis.

PLANT NUTRIENTS IN THE SOIL

Most soils contain adequate amounts of the plant nutrients, but not necessarily in a form in which they are available for plants. The presence of these constituents in a soil may be tested for qualitatively.

Nitrogen. (a) Organic. Heat about 2 gm. of soil with an equal weight of soda lime in a test tube. Note that ammonia is given off.

(b) Nitrates. To about 20 gm. of soil add 100 ml. of distilled water and shake for 5 minutes. Allow the soil to settle, filter off the clear liquid and evaporate to a small bulk. To the residue when cool add a little pure strong sulphuric acid and pour into a test tube containing a little diphenylamine reagent. The formation of a blue colour indicates the presence of nitrates.

Phosphate. Ignite a few gm. of soil in a basin, cool, and then boil with 10 ml. of strong nitric acid. Cool, add an equal volume of water and filter. To the filtrate add ammonium molybdate solution and warm. A yellow precipitate indicates the presence of phosphate.

Potassium. Boil about 10 gm. of soil with 25 ml. of dilute hydrochloric acid for 5 minutes. Filter, evaporate the filtrate to dryness and ignite the residue. Cool, and extract the residue with hot water and again filter. To the filtrate add sodium cobaltinitrite solution. A yellow precipitate indicates the presence of potassium.

Iron. Boil about 10 gm. of soil for 5 minutes with 20 ml. of strong hydrochloric acid. Dilute with an equal volume of water and filter. Test a portion of the filtrate for iron by the addition of potassium ferrocyanide or potassium thiocyanate solution. A dark blue or blood red coloration respectively indicates the presence of iron.

Calcium. Take the remainder of the filtrate from the iron test, boil with a little strong nitric acid, and add ammonium chloride and excess of strong ammonia to precipitate iron, etc. Filter, concentrate the filtrate and add ammonium oxalate solution. A white precipitate indicates the presence of calcium.

ABSORPTION OF SUBSTANCES BY SOILS

Certain substances are absorbed and retained by soils, and they can be conveniently divided into three classes: (1) cations (bases), (2) anions (acid radicles), and (3) colloidal matter, especially organic matter.

Absorption of Cations. This is now regarded simply as an exchange of cations, e.g. if K or NH₄ ions are absorbed by the colloidal soil particles, equivalent amounts of Ca or Mg or Na ions are replaced.

Absorption of Anions. This is most probably due to chemical precipitation, e.g. phosphates form insoluble salts. The anions that are not absorbed, HCO₃, SO₄, NO₃ and Cl, do not form insoluble salts with the soil bases at their concentrations in the soil.

Absorption of Organic Matter. This is mainly due to precipitation of the organic matter in a manner similar to that which occurs in the flocculation of clay. It is of great importance in soil fertility for practically all of the organic matter added to soil by plant residues and as dung remains near the surface. Again this property is made use of in the purification of sewage on sewage farms.

Absorption of Salts (Qualitative). Cover the bottom of a lampglass with a piece of muslin, and fill with a clay soil to a depth of about 8 inches.

Prepare a solution containing 0·1 per cent. each of ammonium sulphate, potassium chloride, sodium phosphate and sodium nitrate. Pour the solution on to the soil, allow to drain through and collect the filtrate. Carry out comparative tests on the original solution and on the solution after passing through the soil, care being taken to use the same volumes of solution and of reagents in each case.

(a) Ammonia. Mix 1 ml. of each solution with a little oxalic acid solution, make up to 100 ml. and allow the precipitated calcium oxalate to settle. By means of a pipette transfer 50 ml. of the clear liquid to a Nessler cylinder, mix with 2 ml. of Nessler’s reagent, and allow to stand for 5 minutes. Note the depth of colour produced in each case.

(b) Nitrates. Take 5 ml. of each solution and evaporate nearly to dryness. Cool, mix with pure strong sulphuric acid and a drop of diphenylamine reagent. Note the depth of the blue coloration in each case.

(c) Potassium. Acidify equal quantities of the two solutions with acetic acid, and test with a fresh solution of sodium cobaltinitrite. Note the extent of the yellow precipitate in each case.

(d) Phosphate. Acidify equal quantities of both solutions with a few drops of nitric acid. Add ammonium molybdate solution, warm, and compare the amount of precipitate produced in each case.

FLOCCULATION AND DEFLOCCULATION OF CLAY

The finest particles of a soil—that is those of diameter 0·002 mm. or less—make up the clay fraction of a soil. Most of these particles have colloidal properties, and under certain conditions may be aggregated or coagulated to form larger ones, which themselves act as separate soil particles. This phenomenon, which is termed flocculation, is most important from...

Table of contents

• Cover
• Title
• Contents
• Soils