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As I have stated elsewhere, these are probably the follow

I. The favouring influences of brilliant sunshine and high temperature upon plant growth.

2. The greater rapidity with which chemical changes probably occur in the soil. This would enable plants to obtain a sufficiency of food from a soil which, under circumstances less favourable to chemical changes, might be too poor in fertilising constituents. Organic matter in our soils very quickly decays, and no doubt the changes which render potash and phosphoric acid available, also proceed more rapidly than in cooler climates.

3. Most of our soils are of open texture and considerable depth, thus allowing of abundant root development.

4. Our rain is richer in combined nitrogen than that of cooler countries, and the rainfall being confined to the growing season, brings almost all its nitric acid and ammonia into contact with the feeding roots. In Pretoria, the total amount of combined nitrogen brought down in the rain amounts to about 7.5 lbs. per acre per annum, as against about half that quantity at Rothamsted, and, whereas in England this addition of nitrogen is distributed almost uniformly over the whole year, and a large portion is thus wasted, in this country it is confined to the summer, when the crops are in a condition to assimilate it. The absence of winter rains is also important in lessening the loss of nitrates from the soil by drainage. In Europe quite a large quantity of nitrogen is washed out of the soil into the drains, especially in autumn and winter, when the land is, for the most part, clear of crops. In this Colony no loss of this kind occurs, as in most districts the winter is absolutely rainless.

5. The dryness and low pressure of our atmosphere leads to rapid evaporation both from the leaves of plants and from the surface of the soil. The rapid transpiration of water from the leaves enables the plant to use up large quantities of the very dilute solution of its food existent in the soil, and thus to obtain a sufficiency from the soil water even when the latter is so weak that it would, in a damper climate, be incapable of providing the needed constituents in sufficient quantity. The evaporation of moisture from the surface of the soil brings up water from below, carrying with it small quantities of dissolved matter from the soil, and concentrates this dilute solution in the upper layers, so as to render it more capable of supplying the roots with plant food. The latter process sometimes goes on to an injurious extent, and sets up a "brak "' condition of the surface soil.

But to return to the character of Transvaal soils as revealed by our analyses.

In a paper read last year before the Chemical Section of the British Association for the Advancement of Science at Johannesburg, I gave in detail the results of the analyses of nearly 100 samples of soils collected from various places in the Colony, and discussed at some length the general manurial treatment necessary to enhance their fertility. Since then some 60 to 70 additional soils have been

examined in our laboratories. I do not propose to give a long array of figures, but will be content to deal with averages as far as possible. In last year's paper, above referred to, will be found details of the methods employed in the analyses, and a discussion of the deductions which may be drawn from the various determinations.

It must suffice on the present occasion to point out that although plants require from the soil a large number of substances, the actual fertility generally depends upon the soil's capability of supplying a sufficiency of four materials, viz., combined nitrogen, potash compounds, phosphates, and lime. Other constituents of plants, e.g., sulphur, chlorides, iron, magnesia, etc., though indispensable, are so widely distributed that it is rare to find soils lacking in them.

For the present, then, only the four first-mentioned constituents will be considered.

COMBINED NITROGEN.

In most soils this is present almost entirely in the form of complex organic compounds, resulting from the decay of vegetable or animal matter. The organic matter, in addition to yielding supplies of nitrogen, has an important influence on the water-holding capacity of the soil.

In this state, nitrogen is not available to plants, but becomes so by processes of decomposition, resulting in the formation of ammonium compounds, nitrites, and, lastly, nitrates. It is in this form that practically all the nitrogen which the plants require is assimilated. The successive changes which lead to the formation of nitrates from complex organic nitrogenous compounds are known as nitrification, and are accomplished under the influence of at least three species of micro-organisms. In order, therefore, for a soil to be well adapted for supplying nitrogen to plants, it is necessary for it to possess a sufficient store of organic, nitrogenous matter, suitable micro-organisms, and the necessary conditions for their activity. Among the latter, some of the most important are sufficient moisture, the presence of some basic material, suitable for the formation of nitrates, and a moderately high temperature. The first and last of these are mainly dependent upon climatic conditions, the only modification which is practicable for man to introduce is by means of irrigation. The basic material most suited for the purpose is carbonate of lime, and the process of nitrification leads to a consumption of this substance.

Most of our Transvaal soils are low in lime, and the process of nitrification is probably limited in many cases by the deficiency of basic material. On the other hand, other conditions for rapid nitrification, e.g., high temperature, porosity, and, in the summer, moisture, are favourable in this country.

The amount of combined nitrogen in most of our soils is very low, compared with ordinary English soils. There are, of course, many exceptions, for soils from marshes, or "vleis," are often rich in organic matter, and also in nitrogen.

In an ordinary English arable soil one usually finds about 0.2 to 0.3 per cent. of nitrogen. The average of the 168 samples of Transvaal soils, examined in our Laboratories, gives o.126 per cent. But this alone would be misleading, for, leaving out 17 soils which contained more than 0.20 per cent., the remaining 151 soils only contained an average of 0.093 per cent. of nitrogen. It is not infrequent to find 0.05, 0.03, or even 0.02 per cent. of nitrogen in samples of our soils. Soils containing so little nitrogen as, say, 0.1 per cent, would be regarded in England as hopeless unless heavily manured with nitrogenous material, but yet many such soils in this Colony are found to yield fair crops. It is probable that this is mainly due to greater rapidity of circulation of nitrogen in the warmer climate.

POTASH.

In this constituent our soils are, as a rule, fairly rich, the average amount in all the 168 samples being about 0.23 per cent. Few samples are below 0.10 per cent., while in a few cases about 1.0 per cent. was found. It is probably rare that potash manures are needed on our Transvaal soils, though a few samples have yielded results indicating that they would be benefited by additions of potash. In about 50 samples out of the 168 examined, the proportion of potash extracted by treatment with 1 per cent. solution of citric acid was less than 0.005 per cent., which has been proposed for English soils as the limit below which potash manures are probably needed.

PHOSPHATES.

These substances are markedly deficient in most of our soils. In England the proportion of phosphoric acid present varies greatly, but is usually about 0.10 or 0.15 per cent. In our Transvaal soils the amount rarely reaches o.1 per cent., and is often less than 0.04, or even 0.03 per cent. The mean of all the 168 samples gives 0.06 per cent. of phosphoric acid, but this includes some exceptionally rich. specimens, containing from 0.15 to 0.20 per cent. of phosphoric acid. Phosphates are as much needed on most of our soils as nitrogen and lime. In only 32 out of the whole 168 samples did the amount of "available" phosphoric acid, i.e., the quantity extracted by 7 days' treatment with 1 per cent. solution of citric acid, exceed 0.01 per cent., which, in England, is taken as the limit, below which phosphatic manuring is considered advisable.

LIME.

The presence of lime in the form of carbonate is of great importance, not only for the effect upon nitrification already alluded to, but also for the important action it has upon the physical properties of the soil. Soils containing clay in sufficient quantity to be heavy" and tenacious, become much more friable and porous if

carbonate of lime be present.

The amount of lime present in the samples examined has varied greatly from the merest trace to as high as 30 per cent. in some limestone soils. But, except on limestone outcrops, the soils are, as a rule, very low in this constituent. The average of all the 168 samples gives 0.81 per cent. of lime, but, as already stated, this includes several samples, consisting largely of powdered limestone. Taking 0.25 as a standard, there were 70 samples containing more than this, and 98 containing less.

The average of the 70 is 1.86, that of the 98 only 0.093 per cent. In many cases less than 0.05 per cent. of total lime was found. There can be little doubt that the application of lime in small dressings to many of our soils would be attended with a great increase in fertility.

The needs of our Transvaal soils, generally, then, are organic matter (humus), combined nitrogen, phosphates, and lime. It is of interest and importance to consider how these needs may be supplied by the natural resources of the country. Before doing so, however, it will be advisable to briefly discuss the form in which these constituents may be applied to the soil. Organic matter, present in the very complex and indefinite form known as humus, is in the great majority of our soils, very deficient in amount. Its amount may be increased by the addition of bulky animal or vegetable matter. Stable or kraal manure, if available, is undoubtedly one of the cheapest and best manures for this purpose, especially as it also supplies small quantities of all the constituents needed by a soil. But in this Colony the quantity available in any one locality is very limited, and the cost of transport is too high to allow of its being brought from a distance. The amount of organic matter in such substances as guano and bats' guano is too small to allow of these substances having much influence directly upon the amount of humus in the soil, though they have great value as true manures. organic matter may, however, be very greatly increased by the practice of green manuring. By growing any crop on land and plowing it in, before it seeds, a large addition of carbonaceous matter obtained by the crop from the air is added to the soil, and may greatly improve it by increasing its water-retaining powers, and in other ways.

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If the crop chosen be a leguminous one, an additional advantage is gained by the addition to the soil of the nitrogenous matter contained in the crop, which has been obtained mainly from the nitrogen of the air. In many of our soils, the micro-organisms necessary to effect this absorption of free nitrogen in the root tubercles of many leguminosa are already present, but, in any case, their presence may be insured by inoculation of the seed before sowing, with cultures of the specific organisms.

This is a plan which might with great advantage be adopted on many of our soils, but it must be remembered that the other necessary constituents, phosphates, potash, and lime, must already be present in sufficient amount to allow of a luxurious growth of the leguminous crops. or the advantage gained will be small.

NITROGEN.

may be applied in the form of complex organic compounds, such as are present in animal and vegetable matter, as already described; in the form of ammonium compounds, or, lastly, in the form of nitrates. Organic nitrogenous substances are contained in kraal. manure, guano, various oil cakes, human excreta, animal matter of all kinds, blood, offal from butchers, bones, and similar refuse. Before such substances can be utilised by plants, it is necessary, as already stated, that they undergo the process of nitrification, and therefore that the soil contain some carbonate of lime. Ammonium salts, e.g., sulphate of ammonia, have also to be nitrified before they can be absorbed into the plant. Moreover, the acid of the ammonium compound has to be combined with some base from the soil, so that with these manures, the presence of much carbonate of lime in the soil

is essential.

Nitrates, on the other hand, are directly assimilated, and have not to undergo any previous change. They, however, are not retained by the soil, as are ammonium salts and most other manures, and therefore should not be applied until the plant is well rooted and can absorb them.

PHOSPHORIC ACID.

This may be applied in three forms.

I.

As tricalcium phosphate, e.g., in bones.

2. As acid calcium phosphate, e.g., in superphosphate.

3. As tetra-calcium phosphate, e.g., in basic slag.

The first has the disadvantage of being insoluble in water, therefore difficult to distribute through the soil. It is slow in its action, as it is only absorbed by the plant after it has come into solution in the soil water by action of carbonic and other acids produced by chemical changes in the soil or plant. The second form of phosphatic manure has the advantage of being easily soluble in water, and therefore quickly distributes itself throughout the soil. It is, however, soon converted by the calcium carbonate into the first form. Nevertheless it is readily available to the plant, because of its fine state of sub-division and good distribution throughout the soil. This form of phosphatic manure can only be used successfully on soils containing a fair amount of carbonate of lime. The third form of phosphatic manure, though insoluble in water, is much more readily soluble in saline solution than the first, and, if applied in a sufficiently fine state of sub-division, is rapidly assimilated by plants. Basic slag contains free lime, in addition to its tetra-calcium phosphate, and is especially suited to soils poor in lime.

It gives excellent results on soils rich in organic matter, probably because the free lime in it promotes nitrification, while the vegetable acids resulting from the decay of the organic matter aid in the solution of the phosphoric acid.

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