118 MINNESOTA STATE HORTICULTURAL SOCIETY 



The materials which all agricultural plants obtain from the air are to 

 be had in abundance, and are not the necessary constituents of a perfect 

 manure. But all plants do not have the power of taking nitrogen from 

 the air, and because many soils do not contain it in sufficient quantity 

 for plant growth, it is generally a necessity in perfect manure, and is the 

 only one of the combustible elements that is, commonly speaking, a 

 manure. In other words the addition of the others, would not, in any 

 known instance, be followed by increased crops. 



The material which plants obtain from the land are present in most 

 agricultural soils in abundance. Lime, silica, chlorine, soda, iron and 

 sulphur are seldom if ever necessary in a manure. If elements are lacking 

 in the ash of soils, they are, almost without exception, either potash or 

 phosphoric acid and sometimes magnesia. We find therefore practically, 

 that a complete manure for any soil must contain nitrogen, potash and 

 phosphoric acid and sometimes, though very seldom, magnesia. 



SOILS. 



Soils vary much in composition and of course are made up of the ma- 

 terial from the rocks which by disintegration have formed them. They 

 have most of the necessary elements in abundance. Soils f rom feldspatic 

 rocks are especially rich in potash while those from apatitic rocks are es- 

 pecially rich in phosphoric acid. Again many soils contain much carbon- 

 aceous matter (humus), which of itself is not a plant food, but acts phys- 

 ically in the soil to promote the best condition for plant growth. It re- 

 tains moisture and by its decomposition throws off carbonic acid and 

 starts chemical action in the soil, by which plant food is set free. It is 

 largely present in stable manure. 



The elements of the soil act one upon the other, in connection with the 

 atmosphere, so that ia point of fact the soil is like a great chemical lab- 

 oratory, in which some plant food is being formed and some small 

 amounts are becoming inert all the time. The amount of plant food set 

 loose each year undoubtedly varies greatly. It has been estimated that 

 after a New England pasture once becomes so exhausted that it will only 

 produce green moss, that it will take thirty years of nature's care to re- 

 store it to its original fertility. 



In experiments on a stiff chalk clay in England for thirty years, it has 

 been found possible by extended cultivation and working of the soil to 

 produce an average of 15 bushels of wheat per acre without any manure 

 whatever, which would show that sufficient plant food was set free in an 

 acre each year in that particular soil to produce that much wheat each 

 year. 



CHEMICAL ANALYSIS OF THE SOIL. 



Not many years ago it was claimed by a class of agriculturists that a 

 chemical examination of the soil would show what was lacking and that 

 when it was found out, the thing to do was to add that element only. 

 Without going into the reasons, I will say that this has proved of but 

 little practical value. There may be an immense amount of plant food 

 in the soil, but the plant may starve for it, because it may be in an in- 

 soluble form. A plant may starve for phosphoric acid in a soil composed 

 of ground apatite, or for nitrogen when immense amounts are present in 



