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.
FOOTNOTES:
[248] The term insoluble phosphates is an unfortunate one, as the word insoluble is purely relative in its significance. Undissolved phosphates would be a better term.