18 Agricultural Chemistry. 



in the same way produced in the fiftieth year 9% bushels of grain 

 per acre, the average for the last eight years being 111/2 bushels. 

 The soil seems capable of keeping up the yield indefinitely, but 

 the amount of crop produced ceases to be profitable. 



It is evident that the virgin soil must have contained large 

 amounts of some substances that were necessary for vigorous 

 plant growth arid that these were removed by the successive crops 

 when harvested. The rapid decrease in fertility finds its most 

 rational explanation on this basis. Changes in climate and phy- 

 sical condition of the soil are inadequate as explanations for this 

 decreased productive power. 



A description of the elements important to agriculture has al- 

 ready been given and the very reason for their importance to the 

 farmer lies in the fact that they are the elements which constitute 

 the compounds of plants and are removed from the soil when the 

 crop is harvested. 



Source of elements. However, not all of the elements de- 

 scribed have come from the soil. Plants obtain the elements of 

 which they are built up partly from the soil and partly from the 

 atmosphere. From the soil they obtain by means of their roots 

 all their ash constituents, all their sulphur and phosphorus, and 

 in most cases, nearly the whole of their nitrogen and water. From 

 the atmosphere they obtain, through the instrumentality of their 

 leaves, the whole or nearly the whole, of their carbon. There are 

 exceptions, especially in regard to nitrogen, which is obtained 

 from the atmosphere by certain plants, such as alfalfa, clover, 

 vetch, pea and bean, under certain conditions to be described 

 later. 



Composition of the plant. The most abundant ingredient of 

 a living plant is water. Many succulent vegetables, as the turnip 

 and lettuce contain more than ninety per cent of water. The 

 green corn plant contains eighty-five to ninety per cent of water. 



Combustible part of plants. If a stalk of corn is dried and 

 burned the greater part is consumed and passes away in the form 

 of gas. But there is always left behind a small quantity of white 



