HA RD WICKE ' S SCIENCE- G OSS I P. 



209 



potash from the soil as wheat. Oats require about 

 five times as much lime as wheat, and barley takes 

 twenty-six times the amount of silica from the land 

 as an equivalent crop of wheat." If, therefore, the 

 same crop be grown for several years in succession, 

 the whole of certain constituents would be extracted 

 from the soil, whilst others would remain com- 

 paratively untouched. This may be remedied by the 

 use of manures. 



By the analysis of p'ants we can tell what their 

 composition is, and what elements they take from the 

 soil for their nutrition. If a plant be dried and 

 incinerated, the organic matters will be driven off as 

 gas, and the mineral elements left as an ash, very 

 small in quantity, and varying in composition for 

 different plants. Liebig, one of the earliest of 

 agricultural chemists, carefully analysed these ashes, 

 and thought that by adding to the soil mineral 

 manures containing the same elements its fertility 

 would be restored. He, therefore, advised the use 

 of mineral manures, and formulated the so-called 

 mineral theory that the crops on a field diminish 

 or increase in exact proportion to the diminution or 

 increase of the mineral substances conveyed to it in 

 manure ; and that the ammonia in the atmosphere 

 more than supplied the nitrogen needed by the plant. 

 Messrs. Lawes & Gilbert, however, found that this 

 was not so, but that the fertility of soils depended 

 also on the presence of nitrogen from the decom- 

 position of nitrogenous organic matter or supplied as 

 manure. This is seen from one of their experiments : 

 having rendered the ground on their farms at 

 Rothamsted as uniform as possible, they divided it 

 into plots, with which they obtained the following 

 results : — 



Kind of Manure. 



Dressed 



Corn. 



Bushels 



per acre. 



Unmanured 



Fourteen tons farmyard manure ...... 



The ashes of 14 tons farmyard manure .... 



Mineral manure, consisting of 350 lbs. phosphate 

 of lime, 364 lbs. phosphate of potash . . . . 



Mineral manure as above plus 65 lbs. ammonium 

 sulphate 



The chief constituents of manures are nitrogen, 

 phosphorus, potash, iron, lime, magnesia, chlorine, 

 soda, sulphur, and silica, which we will now consider 

 more in detail. 



Nitrogen is by far the most important of these, not 

 only as one of the constituents of protoplasm, and 

 therefore essential to plant-life, but because of its 

 scarcity. Soils have been formed by the dis- 

 integration and oxidation of rocks, and naturally con- 

 tain all, or most, of the other elements plants require ; 

 but not nitrogen, for any nitrates which may have 

 been formed have long since been washed away, 

 owing to their extreme solubility. Plants, too, are 

 entirely incapable of assimilating nitrogen in a free 



state Irom the atmosphere, but only in the form of 

 nitrates from the soil. Unless supplied by manures, 

 the little nitrogen that is in the soil is derived from 

 two sources: — (1) from the atmosphere in the form 

 of ammonia, and (2) from the decomposition of 

 organic matter, such as dead roots and leaves. The 

 soil itself absorbs a minute quantity of ammonia from the 

 air ; but more is carried into it by rain. Messrs. Lawes 

 & Gilbert found that in three years at Rothamsted an 

 average of 6"i lbs. per acre of nitrogen was carried to 

 the soil in the form of ammonia, and 074 lbs. as 

 nitric acid, making a total of 6-84 lbs. per acre. 

 This is, however, more than counterbalanced by the 

 loss of nitrates by drainage. The ammonia is 

 rapidly oxidised in the soil into nitric acid, which at 

 once combines with the surrounding bases to form 

 nitrates. The decomposition of organic matter is 

 chiefly effected by earthworms and bacteria. The 

 former live on the organic matter and leave their 

 castings, which contain about C35 per cent, of 

 nitrogen, on the surface of the soil. These are 

 broken up and spread over the fields by rain, and it 

 has been reckoned that in one year worms bring to 

 the surface 10 tons per acre of castings, which is 

 equivalent to a manuring of from 70 to 80 lbs. of 

 nitrogen per acre. The bacteria, too, live on 

 decaying organic matter, keeping all they require for 

 sustenance, and ejecting, among other substances, 

 the nitrogen probably in the form of ammonia, which 

 is rapidly oxidised into nitric acid. But bacteria 

 will only grow in warm and moist weather, hence 

 most nitrates are formed in summer. This action, 

 called nitrification, takes place when nitrogenous 

 manures are added to the soil, the nitrogenous 

 substances being converted into nitrates. But some 

 nitrogenous substances are more easily decomposed 

 than others, and so act much quicker as a manure ; 

 thus a ready-formed nitrate acts quicker than 

 ammonium-salts, and the latter than dried blood, 

 and so on. Nitrates are very expensive, and being 

 so very soluble that they are soon washed away, it is 

 advisable to supply the nitrogen in another form, 

 which will require to be decomposed in the soil 

 before available for plant-food, and so be used by the 

 plant on formation. Manures used for the nitrogen 

 they contain are nitrates, ammonium-salts, farmyard 

 manure, guanos, soot, dried blood, bones, oil-cakes, 

 woollen rags, etc. 



Phosphonts is also a constituent of protoplasm, 

 and as phosphoric acid forms a large proportion of 

 the ashes of plants, those of wheat and barley 

 containing about 12, of turnips 9, and of potatoes 14 

 per cent., most soils contain phosphates, but only 

 in small quantities, which render them only second 

 in importance in manures to nitrogen. Manures 

 containing phosphoric acid are mineral phosphates, 

 bones, animal-charcoal, guano, farmyard manure, 

 and oil-cakes. 



Potash is necessary for the production of starch, 



