3 i4 GENERAL SCIENCE 



supplying plant food when it decays. It increases the 

 capacity of soils for holding water, which is very important 

 in sandy soils and especially during dry seasons in all soils. 

 The decaying humus is porous and acts somewhat like a 

 sponge in holding water. This can be illustrated by taking 

 equal volumes of sandy soil and soil with a large quantity 

 of leaf mold from the woods; place the two soils under the 

 same conditions, moisten them thoroughly, and see which 

 will hold the more water and which will stay moist the 

 longer. (See page 311.) Humus loosens heavy soil and 

 makes it possible for air to penetrate more deeply and 

 freely; this is particularly important in clay soils, as 

 they are usually heavy and compact. It furnishes food 

 for bacteria which change nitrogen to dilute nitric acid so 

 that plants can use it for food. Decaying humus liber- 

 ates carbon dioxide (carbonic acid gas) which acts on the 

 minerals of the soil and makes them soluble for the use 

 of plants. Humus also makes it more favorable for those 

 bacteria to live which take free nitrogen from the air and 

 leave it in the soil in the form of nitrates which can be 

 used for food by the plants. 



A moderate amount of moisture and air makes the 

 normal condition for the decay of humus at such a rate 

 that plants will receive the proper supply of food. If 

 the soil is too well aerated, the humus will decay too 

 rapidly. If the soil is saturated with water, most of the 

 air will be excluded; then the decomposition of humus 

 practically ceases, organic matter accumulates, and there 

 is no plant food available. Examples of this can be found 

 in swamps where peat and muck are formed. 



Soils that receive a moderate rainfall have about four 

 times as much humus as soils in arid regions. But the 

 humus in soils of arid regions decays much more rapidly 



