66 MECHANICAL IMPROVEMEMENT OF THE SOIL. 



by means not connected immediately with alteration 

 of its chemical composition. They bring on chemical 

 changes, it is true, but still the operations themselves 

 are purely mechanical. Some soils, for instance, are 

 too light, and others too stiff and heavy. There are 

 various ways of removing these defects. 



a. In situations where clay can be obtained, it is 

 found to be the most valuable possible application for 

 light soils; it consolidates them, causes them to retain 

 water and manure, and for the objects of permanent 

 improvement is worth more, load for load, than manure. 



b. Upon very heavy clay lands, on the contrary, 

 sand is laid in large quantities with equal success. 

 Here the effect is the reverse of that desired on light 

 sands. The clay is mellowed, made less retentive, 

 dries sooner in spring, and does not bake so hard in 

 summer. Such operations as these, in favorable si- 

 tuations, are very profitable; and although expensive 

 at first, are in the end far cheaper than manuring in 

 the ordinary way. 



SECTION II. THE EFFECTS OF TOO MUCH MOISTURE IN THE 

 SOIL. 



I come now to mention a defect in soils which is of 

 very great importance, and which has not as yet been 

 fully appreciated in this country. This is the presence 

 of too much moisture. Wherever water is so abun- 

 dant in the soil as to completely saturate it, various 

 evil effects take place. 



a. The necessary decomposition of organic sub- 

 stances is arrested, and certain vegetable acids are 

 formed, called by chemists humic, ulmic, geic acids, 

 etc. In swamps and low grounds generally, these 

 accumulate to a large extent, and form the deep black 

 soil or muck of such situations. 



b. So long as these acids are present in such ex- 

 CP cclve quantity, valuable plants refuse to grow; but, 



