SOILS AND FERTILIZERS 359 



ring the soil to permit the escape of noxious gases that are perhaps 

 given off by the plants themselves or produced by bacterial action 

 on the remains of excreta of plants. 



The root of a plant is absorbent for water and for mineral mat- 

 ter only at the tip and for a very short distance back from the tip 

 only a small fraction of an inch. It is something like one-tenth 

 of an inch of the root that actually absorbs water and mineral food. 

 This portion of the root is only absorbent for a few days, probably 

 for not more than three or four days. As the tip is extended into 

 new fields of moisture and plant food, the part that was absorbent 

 yesterday ceases to be absorbent today. There is therefore no rea- 

 son why water should move up to the plant from any considerable 

 distance, for the plant itself constantly moves its feeding roots 

 out into new fields. 



Next to temperature, moisture is probably the controlling fac- 

 tor in the growth of plants. The importance of an adequate sup- 

 ply of moisture is most strikingly demonstrated in regions of defi- 

 cient rainfall where irrigation is necessary for the growth of crops 

 (arid regions), but it is no less important in regions where the 

 rainfall is usually considered sufficient for the needs of crops 

 (humid regions). Not only must there be a sufficient supply of 

 moisture, but it must 'be properly distributed throughout the grow- 

 ing season. It is well known that crops may be injured by drought 

 in a season which shows a high total rainfall, because there is a 

 deficiency of rain just at the stage when the plant needs it most. 



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 

 ordinary 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 pile 

 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 admit 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 retain water. 



PERMANENCY OF SOILS. 



The soil is the one indestructible, immutable asset that the 

 Nation possesses. It is the one resource that cannot be exhausted; 

 that cannot be used up. It has been said that dairying is injuri- 

 ous to the soil ; the same statement has 'been made in regard to lands 

 in England. A writer in a recent publication says on that subject: 



It is rather strange at this time of day, after all that has been 

 said and written on the subject, to find anyone harking back to the 

 old fallacy that the keeping of milch cows impoverishes the land. In 

 Cheshire dairying has been carried on for seven centuries, yet a small 

 dressing of bones renovated the pastures within our times. In my 



