FOOTNOTES:
[247] See p. 389.
CHAPTER XXV.
ON THE VALUATION AND ANALYSIS OF MANURES.
Value of Chemical Analysis.
The value of a manure to the farmer depends on the proportion of nitrogen, phosphoric acid, and potash it contains, as well as—and this is hardly less important—the condition in which the ingredients are present. Since these facts can alone be determined by a chemical analysis, it is obvious that manures should always be purchased with a chemical analysis. It is unfortunate, however, that very often a chemical analysis, even when procured, is unintelligible. It may be of advantage, therefore, to say a word or two on the correct interpretation of the significance of the data furnished in the ordinary chemical analysis of manures.
Interpretation of Chemical Analysis.
The first thing that the farmer ought to look for in the analysis of a manure is the amount of nitrogen, phosphoric acid, and potash which the manure contains.
Nitrogen.
The percentage of nitrogen in a manure is generally stated as equal to its equivalent percentage of ammonia. Very often, indeed, in the older analyses, its equivalent of ammonia was alone stated. Now this statement does not necessarily imply that the nitrogen in a manure is actually present in the form of ammonia. Thus, for example, when it is stated in an analysis of bone-meal that it contains 3.5 per cent of nitrogen, equal to 4.20 per cent of ammonia, it is not to be inferred that bone-meal actually contains nitrogen in the form of ammonia. In point of fact the nitrogen is present in an insoluble, slowly available, organic form, which possesses a manurial value very inferior to that possessed by ammonia. This custom is a most unfortunate one, and is much to be regretted, as it is often liable to give rise to serious misunderstanding. It must be remembered, therefore, that an ordinary chemical analysis does not always specify the exact form in which nitrogen is actually present. It is nevertheless of importance for the farmer to know this, of which the nature of the manure analysed is generally a good indication. Unfortunately this is not shown in the case of mixed manures; and this constitutes one of the reasons why mixed manures are sometimes to be regarded with suspicion.
The amount of phosphates present in a manure is usually stated in its analysis as so much phosphoric acid, while in a footnote the quantity of tricalcic (or ordinary bone) phosphate this amount is equivalent to is also given, this being the unit of valuation. When the phosphates are in a soluble condition they are stated as such, and at the same time a statement is made as to the quantity of tricalcic phosphate which would be required to furnish this amount by treatment with sulphuric acid. Thus, for example, in an analysis of a superphosphate of lime, the statement, monocalcic phosphate, 17.3 per cent, equal to tricalcic phosphate rendered "soluble," 27.2 per cent, means that it would require 27.2 per cent of tricalcic phosphate to furnish 17.3 per cent of soluble phosphate. Paradoxically enough, the former amount is called "soluble" phosphate, and such a superphosphate as the above would be described as containing 27.2 per cent of "soluble" phosphate.
Again, there are different forms of the so-called "insoluble" phosphates,[248] although they are often not distinguished in a chemical analysis. As we have already pointed out in the chapter on Basic Slag, phosphoric acid occurs in the slag in the form of tetrabasic phosphate of lime, although it is invariably stated in analysis as so much tricalcic phosphate. Then we have the so-called dibasic phosphate of lime, the form into which soluble phosphate in superphosphate is converted when "reversion" takes place. Hitherto it has not been customary in this country—although the custom is prevalent both on the Continent and in America—to distinguish in the analysis of a superphosphate the "reverted" phosphate from the undissolved phosphate; since the superior value of the former as a manure is not recognised in the manure-trade.[249]
Importance of Mechanical Condition of Phosphate.
A further point to which it is desirable to draw attention is the mechanical condition of the different insoluble phosphates, which has an important influence on their value. A very wide difference, for example, exists between the value of phosphate of lime in such a manure as Malden guano and in the crystalline mineral apatite; although, chemically considered, the form in which the phosphoric acid is present is the same in both substances.
Potash.
Potash ought only to occur in a soluble form in manures. It is generally stated as so much potash, and in a footnote the equivalent amount of muriate or sulphate of potash is given, the former being the more concentrated form of potash.
For purposes of reference a table will be found in the Appendix[250] giving some useful factors for converting different forms of nitrogen, phosphoric acid, and potash into one another.
Other Items in the Chemical Analysis of Manures.
The other items in the analysis of a manure are of comparatively secondary importance compared with those already named. Among them may be mentioned the moisture, the insoluble matter, and the organic matter. The amount of moisture and the amount of sand are two items of importance, since, if these are excessive, they afford presumption that the manure has been adulterated.
Fertilisers and Feeding Stuffs Act.
An Act was passed, and came into operation in January 1894, for the purpose of compelling every vendor of manure manufactured in this country or imported from abroad to give to the purchaser "an invoice stating the name of the article, and whether it is an artificially compounded article or not, and what is at least the percentage of the nitrogen, soluble and insoluble phosphates, and potash, if any, contained in the article, and this invoice shall have the same effect as a warranty by the seller of the statements contained therein."
Different Methods of Valuing Manures.
The monetary value of a manure depends upon a number of more or less complicated commercial considerations, such as the questions of supply and demand, &c., which need not here be discussed, and which similarly regulate the monetary value of any other article of commerce.
"Unit" Value of Manurial Ingredients.
For the purpose of affording data for ascertaining the approximate value of a manure, tables have been drawn up giving what is called the "unit" value of the different manurial ingredients in various manures. This is obtained by dividing the market value of a manure per ton by the percentage of nitrogen, phosphoric acid, and potash it contains. Thus, for example, sulphate of ammonia of 97 per cent purity contains 25 per cent of ammonia, and at present (Dec. 1893) is valued at £13, 15s. per ton. In order to obtain the unit value of ammonia in sulphate of ammonia, we have only to divide £13, 15s. by 25, which gives us 11s. The value of such tables depends on the competence of those drawing them up, and they require to be subjected to constant revision. In the Appendix will be found two of these tables, taken from the 'Transactions of the Highland and Agricultural Society of Scotland.'[251]
Intrinsic Value of Manures.
But there is another way of valuing manures, and that is by attempting to ascertain what their intrinsic worth is in producing an increase in the returns of the crops. Of course it may be said that the intrinsic worth of manure affects directly its market value. This is doubtless true, but it is not the only factor in determining the market value of a manure.
Again, the intrinsic worth of a manure may be said to vary according to the soil to which it is applied and the climatic conditions. This being so, it is important for every farmer to try and ascertain for himself what the intrinsic value of different manures is on the soil of his farm; and this can only be done by carrying out manuring experiments for himself. This leads us to say a word or two on the important subject of
Field Experiments.
It is impossible that every farm should be able to support an experiment station for the purpose of carrying out elaborate experiments on the effect of different manures on different crops. Nevertheless it is possible and highly desirable for every farmer who is engaged in arable farming on any scale to carry out simple experiments for the purpose of ascertaining the characteristic manurial requirements of his soil. This can be done at the expenditure of a little time and trouble, and should be carried out in the following way. The field on which it is desired to carry out the experiments should be divided into the requisite number of experiment plots. These, which may be the tenth, twentieth, or fortieth of an acre in extent, should be, if possible, on a level piece of ground—all of them equally free from the shelter of hedge or tree, and otherwise subjected to the same conditions. The nature of the soil of the different plots, as well as its past treatment, should be similar. It is desirable, in order to minimise experimental error as much as possible, to carry out the experiments in duplicate, or even triplicate. In the first place, there should be what is called a nothing plot—i.e., a plot receiving no manure. The produce obtained from this plot, compared with the produce obtained from the other manured plots, will thus furnish data for estimating the respective amounts of increase obtained by different manures. One very simple kind of experiment is what is called the "seven-plot" test. It consists in testing the results obtained by using nitrogenous, phosphatic, and potash manures alone and in different combinations. Thus the plots would be manured respectively as follows:—
| No. |
| 1. Nothing plot. |
| 2. Nitrogen. |
| 3. Phosphates. |
| 4. Potash. |
| 5. Nitrogen and phosphates. |
| 6. Nitrogen and potash. |
| 7. Phosphates and potash. |
The subjects of other experiments might be such as the respective values of nitrogen in the different forms of sulphate of ammonia and nitrate of soda; phosphoric acid as superphosphate, and in an undissolved form as Thomas-slag; the relative importance of artificial and farmyard manure; the effect of manures applied at different times, as well as the effect of different quantities of the same manure; the most economical manures for different kinds of crops; and numerous other interesting problems connected with the practical application of manures.
In carrying out these experiments, care should be taken not to have the experimental plots immediately adjoining one another, as the manure applied to the one plot may, by soaking through the soil, affect the result on the adjoining plot. Especial note ought to be taken of the weather during the progress of the experiment. In order to make such experiments as valuable as possible, they ought to be continued year after year. At the conclusion of the experiment the produce obtained from each plot should be carefully weighed.
Educational Value of Field Experiments.
The educative value of such experiments is very great, and in this connection the remarks made by Mr F. J. Cooke, in a recent lecture delivered to the London Farmers' Club, are worthy of most careful consideration.
"Local experiments," he says, "teach the simple principles which should determine the selection of manures, as well as scientific accuracy and method in their use. The value of experiments is thus brought home to men who would not go far to discover it; and the practice of a few simple trials upon a correct system, each on his own farm, is encouraged. That such trials may be conducted with very little expense to the farmer, or other difficult qualifications, and yet to his great practical advantage, I will venture to assert on the ground of my own personal experience. For some twenty years I have annually conducted private experiments on a very humble scale, and am not aware of any other separate practice which has been so useful to me. It has been pursued upon two light-land farms in different parts of the same county. Yet, in respect of manurial requirements, the proper treatment for one of them has differed so essentially from the other that a common practice upon both would have been simply ruinous."
Value of Manures deduced from Experiments.
Tables have been constructed for the purpose of showing the comparative value of different kinds of manures as deduced from such experiments, and may be fittingly compared with the tables giving the trade prices. We have already quoted some of these tables in the Appendix to the chapter on Mineral Phosphates. These tables show the relative intrinsic value of different forms of phosphatic manures. In the Appendix[252] to this chapter tables showing the relative value of different kinds of nitrogenous and potash manures will be found.
Value of Unexhausted Manures.
A subject which has had much attention devoted to it of late years is the question of the value of unexhausted manures in the soil. In the Agricultural Holdings Act special provision is made for giving compensation to the out-going tenant of a farm for unexhausted manures in the soil. The Act has given rise to endless disputes between landlord and tenant, owing to the extreme difficulty of arriving at a satisfactory estimate of what the value of the unexhausted manures in reality is. The difficulty arises from the fact that we have not sufficient data available for guiding us in estimating this value, which further varies under different conditions. The fertilising ingredients of a soil are present in the soil for the most part in an inert condition, from which they are only slowly converted into an available form.
Potential Fertility of a Soil.
As indicating the total amount of the more important mineral ingredients present in a soil, it may be mentioned that it has been calculated, in the case of a poor sandy soil, that the amount of potash it contains (provided it were in an available condition) would be sufficient to yield three or four average crops of potatoes; of phosphates, nineteen average crops; and of lime, seventy-three. But then only a very small amount of this fertilising matter is in a readily available form.
It is for this reason that artificial manures, although added in such small amounts, exercise so striking an influence in increasing plants' growth. Their effect, however, is to a large extent only of a temporary nature; and in attempting to assess the unexhausted value of a manure a year or two after its application, we must remember this fact.
Some manures are very speedily taken up by plants, and some are very easily washed out of the soil. Others, again, it would seem highly probable, have a tendency to become converted into a more or less inert condition after a while. This remark may be especially applied to the fertilising constituents (chiefly nitrogen) in farmyard manure.[253] The whole question, however, is little understood. One or two points may be drawn attention to. In the first place, it may be safely affirmed that little direct effect can be expected from such quickly available and easily soluble forms of nitrogenous manures as nitrate of soda and sulphate of ammonia a year after application. Potash and phosphates, on the other hand, may exercise an effect for a considerably longer period; and what the length of this period may be will depend on their amount and condition. Thus it is not likely that superphosphate will have much effect more than two years after application. On the other hand, such manures as bones, basic slag, and farmyard manure may exert an appreciable influence for a number of years. How long exactly, it is wellnigh impossible to say, the rate at which they are applied and the nature of the soil having an important influence.
Tables of Value of Unexhausted Manures.
Numerous tables have been drawn up for the purpose of guiding farmers in estimating this unexhausted value at different periods after application, and in the case of different manures. Such tables, as a rule, furnish only very rough approximations, and are little better than mere guess-work. Still more complicated is the attempt to assess the manurial value of foods consumed by the stock of the farm. Lawes and Gilbert have devoted much attention to the elucidation of this difficult question, and have drawn up most elaborate and valuable tables, furnishing data for calculating unexhausted manure value in the case of commonly used foods. These tables are given in the Appendix.[254] In them will be found the manurial value of different cattle-foods, calculated on the basis of numerous experiments carried out at Rothamsted.
Thus these experiments have demonstrated that, on an average, probably not more than one-tenth of the nitrogen, phosphoric acid, and potash a food contains is removed from the food in its passage through the animal system. The exact amount will obviously depend on a variety of conditions, referred to already in a previous chapter.[255]
In explanation of these tables, it may be pointed out that Table I. gives the total quantities of the three fertilising ingredients in various foods; while Table II. shows the proportion retained in the animal body and the proportion voided in the manure, as well as the manurial value of the food, assuming that it exercises its full theoretical effect. As this, however, is never fully realised, it is necessary to make some deduction. The deduction suggested by the Rothamsted experimenters, on the basis of their wide experience, is 50 per cent for food consumed within the last year. That is to say, the manurial value of food consumed during the last year is only one-half its theoretical value. For food consumed within the last year but one, they suggest a deduction of one-third of the allowance for last year; while for food consumed three years back, a deduction of one-third from this latter sum should be made; and so on for whatever number of years, down to eight, may be taken.