— 13 —

mineral salts, with much the same final results as if it had been placed in a

furnace.

Tillage and Aeration.—The primary object of all tillage and drainage is the

aeration of the soil, and fertility is the direct result of the processes of oxidation

already described. The presence of oxygen is a first essential to the activity of

the beneficial bacteria, and in the natural or uncultivated soil aeration is

effected by the permeation of a mass of fine roots and microscopic plants, which

are constantly growing and dying, thus creating a spongy or porous layer, of

which the open pores represent the spaces formerly occupied by vegetable

matter, since decomposed. This is the natural soil, which is juSt sufficiently

aerated to support native grasses, &c. To increase aeration and consequent

fertility, recourse is taken to ploughing and surface cultivation, which has the

eilect of bringing a larger surface into contact with the air, and of facilitating

the work of the oxidising bacteria. Lack of aeration may be due either to the

compactness of the soil particles or to saturation by moisture. An example of

the former condition is frequently found in our heavy cotton soils, and of the

latter only too frequently in irrigated and undrained areas. In the former case,

micro-organisms are conspicuously absent ; in the latter bacteria are present,

but their action is of an injurious, rather than beneficial, character. In lieu of

oxidation, or decomposition, a process of reduction is set up ; oxygen is withdrawn

from, and not added to, decaying organic matter, and loss of fertility is involved.

The beneficial, or aerobic, bacteria rely upon atmospheric oxygen, and by their

activity add this to the substances with which they come into contact ; the

injurious, or anaerobic, bacteria, in the absence of air, withdraw oxygen from

the medium in which they exist. Drainage can almost be measured by the rate

of nitrification.

A Bacteriological Problem.—With more than ample supplies, in most cases,

of the mineral constituents of plant food, the problem of fertility in desert soils is

resolved into the provision of conditions necessary to the vital activity of the

different groups of beneficial bacteria, and the ascertaining how such conditions may be secured by systems of irrigation, drainage, cultivation and manuring.

The results can be most easily read in terms of crop reports which reflect

the relative activity of the soil bacteria acting as intermediaries between the

soil particles and the absorbent plant roots, and as purveyors of soluble nitrates.

The Biological Factor.—Having ensured a general amelioration of the

unfavourable desert environment, it remains to examine the idiosyncracies of

the tropical and subtropical crops which may find in the new conditions a not

unfavourable medium of growth. Crops grown on the same land will vary in

composition, and consequently in the character of the demand they make on the

mineral ingredients of the soil. Thus, the principle of " dominant ingredients "