SOIL ANALYSIS. 
291 
water. At the end of twelve or twenty-four hours the height to 
which the water has visibly risen in the tube is read off. The 
determination presents no special difficulty, and I will not waste your 
time with long descriptions of this or other methods mentioned here. 
They are all capable of being rapidly and accurately performed. 
The capacity of a soil for water is also of special interest, and 
depends partly upon its porosity, and partly on its content of organic 
matter. Peaty and humus soils, other things being equal, have the 
highest capacity for water, followed in order by marls, clays, loams, 
and sand. 
The hygroscopic power — that is, the power of attracting water- 
vapour — is of practical importance in that it prevents undue evapora- 
tion, and prevents the soil from becoming parched up. It also serves 
as a guide to the absorptive power for other gases. This property, 
like capillarity, is due entirely to the fineness of texture, and the 
order is the same — humus, clay, loam, marl, sand, and coarse sand. 
The absorptive power of the soil for salts is a factor of very great 
importance in determining the fertility of a soil. 
This power which soils possess of removing saline matter from 
solution, and retaining it within their pores, is due partly to the 
chemical nature of the soil, resulting in a chemical interchange of 
basic constituents, and partly to its mechanical structure, the fineness 
of its texture, substances such as humus and clay possessing the 
power in a remarkable degree. 
This property is determined by a method elaborated by Knop. 
The absolute iveight of the soil , though it has no bearing upon its 
fertility, is a point that should always be taken into account, since a 
heavy sandy soil, though it may contain a smaller percentage of 
fertilising material than a light clay soil, presents a larger mass to the 
plant in the same space. 
We now come to the most important property possessed by soils, 
as affecting their fertility, and, at the same time, the most obscure, 
namely, their — 
Power of Nitrification. — This property depends upon a number of 
points, on some of which our information is not very clear. 
From what we know of the process of nitrification, we can lay 
down with tolerable certainty the following conditions as being 
favourable to the process : — 
We must have free access of air and moisture, a certain degree of 
warmth, the presence of nitrogenous organic matter prone to oxidation 
(represented by humus) . The presence of reducible mineral matter, 
such as sesquioxide of iron or metallic sulphates, is also favourable, 
A sufficiency of basic substances to combine with the nitric acid appears 
also to be advantageous to its nitrification. 
Putting on one side the bacteriological aspect of the phenomena 
involved, we shall find that the formation of nitrates within the soil is 
due to oxidation, and that within certain limits the power of oxidation 
which a soil possesses is also the measure of its nitrifying power. 
