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By F. B. GUTHRIE.
Tue analysis of soils constitutes a large part of the routine work of the chemical branch of the New South Wales Department of Agriculture, the number of complete analyses of different soils made during the four years of its existence being about 350, exclusive of a large number of which only a partial examination was made.
Concerning the value of soil analysis to farmers, I am aware that there is considerable difference of opinion, some excellent authorities denying its value altogether, whilst there are not wanting those who go to the other extreme, and expect a chemical analysis to indicate both the nature and the exact quantity of fertiliser which is required to make the soil productive.
In this, as in most other debatable matters, I believe that the truth lies somewhere between the two extremes, and that a great deal can be learnt as to the proper treatment required from a rational system of analysis, which sball take into account the nature of the operations going on within the soil as well as its percentage composition.
That soil analysis, rationally conducted, has a considerable economic value I am convinced, and this conviction is strengthened by the continually increasing number of soils sent in for report from all parts of the Colony, by the number already done, and, unfortunately, also by the arrears which accumulate.
Those who deny any value to soil analysis found their objections upon the means at present at our disposal in the laboratory of reproducing the natural condition of affairs going on within the soil; in other words, they argue that we cannot say what quantity of any given ingredient is in a condition in which it can be assimilated by the plant.
Let us hear what M. Ville says on the subject>" Chemistry is powerless to throw light upon the agricultural qualities of the soil, its resources and its needs, because it confounds in its indications the active assimilable agents with the assimilable agents in reserve, the active with the inert and neutral principles.”
This is the conclusion he arrives at from the discussion of analyses which give the percentage composition of the soil together with the so-called mechanical analysis, the proportions of sand, clay, gravel, &c. M. Ville further points out that extraction with water yields results no less unsatisfactory, since the plant is able to utilise soil material which is insoluble in water.
*A piper read before the Brisbane meeting of the Australian Association for the Adrance.
ment of Science, January, 1895.
In order to remedy this evil, the existence of which I suppose no one will be hardy enough to deny, various methods have been suggested and tried with the object of attacking the soil in a manner representing as nearly as possible the actual conditions which prevail in a field under cultivation. A few such reagents may be mentioned ; they include water saturated with carbonic acid, oxygenated water, acetic acid, citric acid, and different salts, such as ammonium citrate.
In & recent series of researches Dr. Bernard Dyer* has experimented with a l per cent. solution of citric acid, which appears to approach closely, in its action upon the soil, the solvent power exerted by the acids secreted by the roots of certain plants. I venture to think that, notwithstanding the great scientific value of such a line of investigation, and of the light it may be expected to throw upon many obscure functions of plant-life, it leaves us pretty much where we were if we attempt to base upon its use any practical advice to the farmer as to the nature of the manures or other treatment his soil requires.
I am prepared to go a step further than M. Ville, and to say not only that we are unable to reproduce the agents at work within the soil in supplying the plant with food, but that we should gain very little from an economic point of view if we were possessed of them.
For, let us assume that the universal solvent” has been found, that we are possessed of a reagent which exercises the same solvent action on the
soil the same amount of mineral and nitrogenous matter as the wheat crop will extract during the period of its growth. We are met with the following difficulties:
Our wheat crop, though it contains less nitrogen (say, one-third less) than a crop of turnips, will nevertheless benefit very much more than the latter by an application of nitrogenous manure; that is to say, the wheat crop cannot make the same use of the nitrogen in the soil as the turnip does exercises, in fact, a different solvent action upon the nitrogenous constituents.
Or, since the nitrogen in the soil is continually changing its condition, and there are external sources of nitrogen which may have some bearing in the above instance, we may take a case which is even less ambiguous.
The mangel crop removes from the soil nearly double as much phosphoric acid as the turnip crop does ; nevertheless, manuring with superphosphate is of less benefit in the case of mangels than with turnips, the recognised reason being that mangels are able to utilise the phosphoric acid, as it exists in the soil, to a greater extent than turnips. So that it will be necessary for us to derise one solvent for turnips and another for mangels, one for phosphoric acid and one for potash-a separate set of solvents for every crop; and such a scheme, if it were feasible, would be far too cumbersome for practical purposes.
A second objection lies in the fact that the agencies at work within the soil are unceasing, and, as a consequence, the combinations in which the nitrogen and mineral matter exist are also constantly changing. What is trne of the chemical constitution of the soil to-day is no guide as to its constitution a week hence.
The determination, especially of the quantities of nitrates, of ammonium compounds, and of"organic" nitrogen, provides us with no information to the purpose, for these, of all soil constituents, are most rapid in their changes.
* Journal of the Chemical Society, March, 1894.
Further difficulties present themselves in the large quantities of soil which it is necessary to employ in the determination of the substances soluble in water and weak acids, and the consequent length of time required for each determination, and also in the initial difficulty which presents itself in all soil analysis of ensuring the proper selection of a sample which shall represent anything but itself.
This difficulty, which is felt in all attempts to judge of the character of a soil from a giren sample, applies more particularly to a chemical analysis, and increases in proportion as the quantities of the estimated substances diminish.
A chemical analysis alone, therefore, is of little value in guiding the farmer as to the requirements of his soil, and it is not in the refinement of chemical methods that we may look for help in this direction. We shall, I believe, obtain much more valuable information if we can ascertain the conditions under which the fertility of the soil is maintained.
The fertility of a soil depends in the first place upon the presence of a sufficiency of plant food, and secondly upon certain properties, possessed more or less by all soils, which effect the splitting up of the mineral ingre. dients in such a manner as to render them available to plants, as well as regulating the supply of water, air, warmth, &c.
We shall discuss the most important of these properties, and shall find, I think, that they are capable of identification in the laboratory. A large number of those properties conducive to fertility are dependent upon the porosity of the soil-in other words, its fineness of texture.
By the porosity of a soil is meant the fineness and number of its pores. We must distinguish between this and permeability to water; a coarse sand, for example, being permeable to water, but possessing properties exactly opposed to those of a porous soil. Humus soils are especially porous. On the fineness of texture depend the following characteristics:
The capillary power, by which is understood the power of imbibing water. This property maintains a continual circulation of water within the soil, and consequent aeration. It is, moreover, largely through the agency of this circulating water, which is charged with carbonic acid and different salts, that the mineral, and in a less degree the organic matter, of the soil is rendered available for plant food and presented in solution to the plant.
The capillary power of a soil depends very largely upon the fineness of its texture. The nearer the texture approaches that of a sponge the greater will be its capillarity.
Humus has a very high capillary power, which is not possessed to any extent by either coarse sand or clay.
This property is determined by filling a tube of known length with the finely powdered air-dried soil; the tube is open at both ends, the lower end being closed by a piece of fine muslin, and stands in 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 wateris 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 evaporation, 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 weight 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 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 the soil possesses is also the measure of its nitrifying power.
We are, therefore, I believe, justified in assuming that a soil will be most favourable to the development of the nitric ferment which combines the fol. lowing characteristics :
1st. A fair proportion of humus. 2nd. A warm climate. 3rd. Provision for free access of air and of moisture (these depend upon
its porosity, and are determined by its capillary power). 4th. Good drainage to prevent stagnant water accumulating. 5th. A certain proportion of basic substances.
It will be seen that, beyond the presence of certain mineral and organic matier, the conditions favourable to nitrification are those whose presence otherwise indicates fertility-panely, fineness of texture and absence of excessive water. If the capillary power of a soil is low, it indicates an unfavourable condition for nitrification.
It has recently been stated by a French writer that the presence of nitrates in the soil assists in rendering soluble the potash in such insoluble combina
Provided, then, that the condition of the soil, as indicated by the physical properties above enumerated, is favourable to what I may call the metabolism
of plant food, its fertility will depend upon the amount of that plant food, and it is immaterial whether that food be now in a soluble state or not. If the mineral and nitrogenous matter are present in sufficient quantity, and the soil possesses high absorptive capacity, high capillary powers-in short, is of good texture, and possesses the conditions conducive to nitrificationit may, I think, be fairly expected to prove a fertile soil ; and in cases where one or more of the conditions conducive to fertility are absent, we may look to improved methods of cultivation to attain that fertility.
The tabulated result of such an analysis as I have indicated would be as follows:
Reaction of soil.
chloric acid. (Phosphoric acid. The quantity of organic matter (which is the volatile matter after deducting water and carbonic acid) affords a sufficiently close indication of the amount of humus present.
The nitrogen determined is total nitrogen. If nitrates are present, the modification of Kjeldahl's method is the most suitable.
I believe the above represents the fewest determinations on which an accurate judgment can be based.
I also believe that, with the aid of the above data, practical experience, and a modicum of mother-wit, thoroughly reliable and useful advice may be given as to the means to be adopted for ameliorating the soil.
The manures to be used and their quantities will to some extent depend upon the nature of the soil, and to a much less degree upon the quantities of fertilising ingredient found to be present, but principally upon the nature of the crop.
Soil analysis in the past has been too much occupied with the notion that the amount of fertiliser required depends upon the quantity already in the soil, and that nothing is necessary but to add so much of the particular ingredient in an available form as, together with what is already present, will produce a sufficiency for all requirements. I believe the principle is a sound one, which tells us to manure the crop and not the ground, and that the soil is to be improved, not by chemicals, but by proper cultivation, by deep-ploughing, draining, liming, green-manuring, and other means of improving the texture, without which it is impossible to maintain the conditions essential to fertility.