196 



of humus of fair quality. Crops thus used are called green 

 manures. 



SOIL AIR. 



A proper circulation of air in the soil is just as important as any 

 other factor of plant growth. Nearly half of the volume of ordi- 

 nary soils is occupied by air spaces. The air spaces in the soil 

 wind in and out between the soil particles, just as they do in a heap 

 of larger stones. If the layer of water on the surface of the soil 

 grains becomes so thick as to stop the air passages here and there 

 the soil is then too wet for most crops and needs drainage. Plants 

 have no special breathing organs, the oxygen required in their 

 breathing finding entrance all over the surface of the plant. Plant 

 roots must therefore be supplied with air, and hence the soil must 

 be porous enough to permit of free circulation of air. A good supply 

 of humus and proper tillage will accomplish this result in clay soils. 

 Sandy soils are usually too porous, needing humus to help them to 

 retain water. 



Another reason why air must circulate freely in the soil is that 

 large quantities of oxygen are required to insure proper decay of 

 organic matter to supply plant food. Also, carbonic acid gas is 

 produced iby the decay of organic matter, and this must escape 

 easily to make room for the atmospheric oxygen needed in the soil. 

 The movement of air in the soil is frequently shown by the bub- 

 bles which appear at the surface of the soil just after a heavy rain. 

 As the water soaks into the soil it drives the air out, and bubbles 

 may be seen at the surface if water enough is present to form 

 them. 



One of the most important objects of ploughing is to loosen up 

 the soil and mix fresh air with it. 



SUBSTANCES THROWN OFF IN THE SOIL BY GROWING PLANTS. 



Considerable evidence has been accumulated during recent years 

 to show that the cause of the failure of some soils to produce 

 satisfactory crops may be ascribed to unfavourable conditions pro- 

 duced in the soils by the plants themselves. It is thought that 

 during the growth of the plant certain unknown organic substances 

 are given off which, when they accumulate in the soil to any ex- 

 tent, are harmful to the further growth of plants of the kind that 

 produce them. It is possible that some of the benefits known to 

 arise from systematic crop rotation may be explained on this basis. 

 These harmful substances seem to be disposed of rapidly by cer- 

 tain soils, usually those in which organic matter is readily con- 

 verted into humus. Other soils, usually marked by a lack of the 

 brown carbonized organic matter, do not seem to possess this 

 property of removing harmful plant products to such a degree. 

 This idea is in accord with the common experience that dark- 

 coloured soils, well filled with organic matter, are usually very 

 productive. 



In connection with the study of these poisonous organic pro- 

 ducts, it has been found that they may be destroyed or at least 



