ACCORDING TO THE COMPOSITION OF THEIR ASH. 81 



We thus find that there are three great leading 

 classes of ash established : 1. The grains, where 

 phosphoric acid predominates; 2. The roots, where 

 potash and soda abound; 3. The grasses, where lime 

 becomes quite important. 4. The various kinds of 

 straw may perhaps be said to constitute a fourth class, 

 where silica is from one-half to two-thirds of the whole 

 weight. 5. It may be well also to mention a fifth 

 class in trees, the ash from the wood of which contains, 

 in very numerous cases, more of lime than of any other 

 substance. There are particularly large quantities in 

 the apple and other fruit trees. 



SECTION III. ON THE EFFECTS OF CROPPING UPON THE SOIL, 

 IN CONNECTION WITH SPECIAL MANURING. 



In view of these facts, we are now better prepared 

 to consider the effect that cropping has upon the soil. 

 Suppose in the first place that, as is too often the case, 

 wheat or any other grain has been grown upon a new 

 soil crop after crop, and nothing returned in the shape 

 of manure; the yield may be good for a number of 

 years, but then it begins to grow less and less : what 

 is the reason of this? It is, probably, that the phos 

 phates are nearly exhausted; these were not so abun 

 dant as many other substances at the commencement 

 (see Table I.), but more of them than of anything else 

 has been taken away. Second, suppose that the farmer 

 has sold all of his grain, but has been very careful to 

 return the straw as manure : he does not see why the 

 land should run down, and in fact it does not so quick 

 ly now as in the first case; still, after a time, it also 

 begins to show marks of exhaustion. Table III. ex 

 plains this at once : in the straw, he has returned 

 chiefly silica to the soil; it is, however, chiefly phos 

 phoric acid that the grain has taken away, and that 

 he has been selling off. 



