292 GENERAL SCIENCE 



increase the crop. Suppose that the average crop of corn 

 in a certain kind of soil is 80 bushels to the acre, but that 

 the supply of nitrogen is so small as to limit the crop to 60 

 bushels to the acre, and that the supply of potash limits 

 the crop to 50 bushels to the acre. Then of course the 

 crop will be 50 bushels, if there are no other deficiencies. 

 In such a case the problem is to determine what the soil 

 lacks and then add these materials. 



A soil may decrease in productivity from a number of 

 causes. 



1. It may lose its power to hold water. This may be 

 remedied by tile drainage and the addition of humus. 



2. The supply of available plant food may be exhausted 

 by raising the same crops year after year. This may be 

 remedied by the addition of fertilizers and by drainage. 



3. The soil may become acid and unfavorable for the 

 growth of soil organisms. In such cases an application of 

 lime is beneficial. 



The fertility of soils depends largely on the humus they 

 contain. Humus is simply decayed organic matter. 

 Humus in soil increases its capacity for holding water, 

 furnishes nitrogen by its decomposition, and gives an 

 acidity to water which makes it a better solvent for other 

 plant foods. The farmer should never lose an opportunity 

 to add humus to his soil, since it not only contains plant 

 food elements but renders available much of the food 

 that is already in the soil. Decayed vegetable matter and 

 manures are excellent sources of humus. 



Fertilizers. It is quite a common idea that soils may 

 be analyzed to determine what fertilizers should be used 

 on them. In general this is true ; however, the chemical 

 analysis only shows the amounts of different substances 

 present and not the amounts of available foods. Experi- 



