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we shall see presently Animal manure, however, acts chiefly by the formation of ammonia. The quantity of gluten in wheat, rye, and barley is very different; and they contain nitrogen in varying proportions. Even in samples of the same seed the quantity varies, and why? Evidently because one variety has been better fed with its own appropriate fertilizer than another which has been reared on a soil less accurately adapted by artificial means for its growth. French wheat contains 12 per cent. of gluten; Bavarian 24 per cent. Sir H. Davy obtained 19 per cent. from winter, and 24 from summer wheat; from Sicilian 21, from Barbary wheat 19 per cent. Such great differences must be owing to some cause, and this we find in the different methods of cultivation. An increase of animal manure gives rise not only to an increase in the nu mber of seeds, but also to a remarkable difference in the proportion of gluten which those seeds contain. Among ma nures of animal origin there is great diversity Cow dung contains but a small proportion of nitrogen. One hundred parts of wheat, grown on a soil to which this material was applied, afforded only 11 parts of gluten and 64 of starch; while the same qua ntity of wheat, grown on a soil fertilized with hu man urine, yielded 35 per cent. of gluten, a nd of course a smaller proportion of less valuable ingredients. During the putrefaction of urine, ammonia cal salts are formed in large quantity, it may be said, exclusively; for under the influence of warmth and moisture, the most prominent ingredient of urine is converted into carbonate of a mmonia. Guano.

Guano consists of the excrement's of sea-fowl collected during long periods on certain islands in the South Sea. A soil which is deficient in organic matter is made much more productive by the addition of this manure. lt consists of ammonia, combined with uric, phosphoric, oxalic and carbonic acids, with some earthy salts a nd impurities. The urine of men and animals living upon flesh contains a large quantity of nitrogen, partly in the form of urea. Hu man urine is the most powerful manure for all vegetables which contain nitrogen, that of horses and horned cattle contains less of this element, but much more than the solid excrement's of these animals. ln the face of such facts as these, is it not pitiable to observe how the urine of the stable or cow-shed is often permitted to run off, to sink uselessly into the earth, or to form a pool in the middle of a farm-yard, from which, as it putrefies, the ammonia formed in it rapidly escapes into the atmosphere?

Cultivated plants need more nitrogen than wild ones, being of a higher and more complex organization. The CHAPTER 1. AGRICULTURE

result of forest growth is chiefly the production of carbonaceous woody fibre; of garden or field culture, especially the addition of as much nitrogen as the plant can be made to take up.

Solid Manure.

The solid excrement's of a nimals do not contain as much nitrogen as those which are voided in a liquid form, and do not constitute so powerful a fertilizing material. ln urine, moreover, ammonia loses a good deal of its volatility by being combined and dissolved in the form of salts. ln an analogous manner, one of the uses of sulphate of lime or gypsum, as a manure, is to fix the ammonia of the atmosphere. Charcoal and humus have a similar property

Mineral Matter in Plants.

Besides the substances already mentioned others are needed by plants as part of their food, to form their structure. The firmness of straw for example, is due to the presence in it of silica, the principal constituent of sa nd and flints. Potassa, soda, lime, magnesia, a nd phosphoric acid are contained in pla nts, in different proportions. All of these they must obtain from the soil. The alkalies above named (potassa a nd soda) appear to be essential to the perfect development of the higher vegetable forms. Some plants require them in one mode of combination, and some in another; and thus the soil that is very good for one, may be quite unfit for others. Eirs and pines find enough to support them in barren, sandy soil.

The proportion of silicate of potash (necessary for the firmness of wheat straw) does not vary perceptibly in the soil of grain fields, because what is removed by the reaper, is again replaced in putrefying straw. But this is not the case with meadow-land. Hence we never find a luxuriant crop of grass on sandy and limestone soils which contain little potash, evidently because one of the constituents indispensable to the growth of the plants is wanting. lf a meadow be well manured, we remove, with the increased crop of grass, a greater quantity of potash than can, by a repetition of the same manure, be restored to it. So grass-land ma nured with gypsum soon ceases to feel its agency. But if the meadow be strewed from time to time with wood ashes, or soap-boilers' lye made from wood ashes, then the grass thrives as luxuriantly as before. And why? The ashes are only a means of restoring the necessary potash for the grass stalks. So oats, barley a nd rye may be made for once to grow upon a sandy heath, by mixing with the scanty soil the ashes