THE SOIL AS A HABITAT FOR LIFE — RUSSELL 453 



SOIL WATER SUPPLY 



Pore spaces become more or less filled with water after heavy rain. 

 Much of it drains away under the action of gravity when the rain 

 ceases, but surface forces which are far more powerful than gravity 

 retain a considerable quantity depending on the sizes of the pores: 

 only those above 30 microns equivalent diameter lose water by drain- 

 age. After drainage ceases water may still occupy two-thirds or more 

 of the pore space — roughly equivalent to some 30 to 40 percent of the 

 total volume of the soil. It coats the walls of the spaces with a thin 

 layer which allows the development on them of bacterial and fungal 

 colonies, and is deep enough to allow bacteria, protozoa, nematodes, 

 and rotifers to move about in it. Plant roots can suck out much of this 

 water : those of our common plants can exert pulls of about 10 atmos- 

 pheres or 300 feet of water. But some water still remains; K. K. 

 Schofield has shown that the last of it requires for its removal a suc- 

 tion of the order of 10,000 atmospheres — equal to a column of water 

 60 miles high. 



GASES IN THE SOIL 



Air occupies the soil pore space not taken up by water. Measure- 

 ments in progress by J. C. Hawkins indicate that under a mature 

 potato crop the oxygen consumption may be of the order of 6 to 13 

 liters per square meter per day, of which the soil population took be- 

 tween a third and three-quarters, the lower amount on a sandy soil 

 and the higher on a fen soil. Air diffuses so rapidly into and out of 

 normal soil that quantities of this order are readily supplied ; the air 

 6 inches down in the soil of a Kothamsted wheatfield differs little in 

 oxygen content from the atmosphere, though it contains somewhat 

 more carbon dioxide. There were greater differences on the grass- 

 land, but apparently insufficient to affect the soil organisms; these are 

 more numerous than on arable land. 



HAZARDS TO THE SOIL POPULATION 



These are many and drastic. The acidity consequent on the absence 

 of calcium carbonate profoundly alters the soil population, cutting out 

 many members and favoring others. Earthworms disappear, but 

 whether owing to the acid or to lack of calcium is not clear. Severe 

 drought is harmful, but its effect is negligible deep in the soil ; bacteria 

 and protozoa can escape by passing into a resting condition. 



The greatest hazard to the soil propulation appears to be the wide- 

 spread predacity of its members and the constant and varied warfare 

 that prevails. The larger prey upon the smaller: protozoa, certain 

 nematodes, mites, and springtails all prey upon the bacteria, while the 

 protozoa and nematodes are consumed by mites, which iii turn are con- 

 sumed by other anthropods — and so the tale goes on. The predacity 



