SOILS. 211 



and pulling them apart. Sand does not change its bulk by wetting or dry- 

 ing. Likewise a sandy soil can be improved mechanically by the addition 

 of clay, lime or vegetable matter. Lime has the peculiar power of lighten- 

 ing heavy soils and also of making light soils hold together better, bring- 

 ing the two extremes to a happy mean as it were. 



The general chemical composition of soils is ex'tremely similar, owing 

 to the general mixing of the soil ingredients that has been going on since 

 soil first began to form; through the action of water, dissolving and carry- 

 ing material from place to place, through the action of streams, floods and 

 glaciers of burrowing animals, worms, etc., of the wind and even plants. 

 Silica or quartz, because it is so hard and insoluble, is the chief ingredient 

 both by volume and weight of all soils. It is a combination of silica and 

 oxygen. Aluminum probably comes next in abundance, being a funda- 

 mental constituent of true clay, feldspar and ice mica. Some of the other 

 elements in the soil are oxygen, which occurs free and in combination with 

 nearly all the other elements. Carbon occurs as part of the humus, also 

 united with calcium and magnesium in the form of carbonates, also as car- 

 bonic acid gas, which plays such an important part in the solution of 

 plant food. Sulphur occurs as sulfates. Hydrogen is united with oxygen 

 in the water. Chlorine occurs in limited quantities, generally in the form 

 of common salt sodium chloride. It seems to be in some way essential to 

 plant life. Phosphorus is never found in nature in a free state but always 

 combined with some other substance. It is very generally distributed 

 through the soil, but in small quantities, and is very essential to plant life. 

 Nitrogen is found in the soil in a combined form in the humus and the 

 vegetable and animal matter which, upon decaying, gives us the nitrogen 

 in the form of ammonia, which is turned into nitric acid by bacteria. The 

 nitric acid unites with potash, soda or other soil ingredients and is taken 

 up by the plant as a nitrate. Nitrates are extremely soluble and easily 

 washed out of the soil. Calcium and magnesium in the form of carbonates 

 compose the limestone beds of the earth. Both calcium carbonate or lime 

 and magnesia are necessary plant foods, and both are generally present in 

 the soil in sufficient quantities to supply the plant with the required amount. 

 Potassium is another element found in soils which is "very necessary. It 

 is widely distributed as a constituent of some feldspars and micas. Sodium, 

 which is the base of common salt, is also widely distributed; it very much 

 resembles potassium as a chemical element, but can in no sense take its 

 place in plant life. Iron is always present in the soil in sufficient quan- 

 tities for the plant. 



It would be supposed that to find what foods were necessary for soils 

 all that would be necessary would be a chemical analysis of the particular 

 soil. Then, if any element was found to be lacking in sufficient quantity, 

 the addition of this or these elements would give the desired results. But 

 chemical analyses of the soil, as they have been made, unfortunately can 

 and do throw but a very dim and uncertain light upon either the condition 

 or the amount of plant food a soil may contain. 



It is true that the results of these analyses show a marked difference 

 in soils but, from the data at hand, these variations may reasonably be 



