3i8 GENERAL SCIENCE 



locked up in the humus and in green manure is trans- 

 formed gradually into nitrate, and is then quite suitable 

 for the building of roots, stems, leaves, and fruits." 

 (From Bulletin of the U. S. Dept. of Agriculture.) 



Bacteria are one-celled plants, and like other plants 

 some are useful and others are harmful and cause disease. 

 There are more than 1000 kinds of bacteria and only 

 about twenty of this number are known to be harmful 

 to man. The bacteria which cause tuberculosis, typhoid 

 fever, lockjaw, and diphtheria are called disease germs 

 and man must learn to keep himself healthy or these 

 germs will decompose his body and death will result. 

 Experiments have shown that certain bacteria in food 

 and in the food-tube of animals are necessary for the 

 existence of the animals. 



219. Chemical Composition of the Soil. All the 

 chemical elements of which plants and animals are com- 

 posed are found in the soil; most of them are in the dis- 

 integrated rock, and the others compose the greater part 

 of humus. Humus itself contains a certain amount of 

 the elements of plant food. Elements which are used by 

 nearly all plants for food are potassium, sodium, phos- 

 phorus, sulphur, magnesium, silicon, calcium, iron, chlo- 

 rine, carbon, hydrogen, oxygen, and nitrogen, and a very 

 small quantity of a few others. These elements are not 

 taken up by the plants in pure form. They are combined 

 into numerous compounds and these compounds must be 

 soluble before the plants can use them. The sulphates 

 of calcium, magnesium, sodium, and potassium are all 

 soluble in water, and they can be absorbed by the roots 

 of plants. The nitrates also are soluble and very im- 

 portant as plant food. 



When humus decays most of the carbon escapes into 



