130 EXPERIMENTS WITH PLANTS 



How much water should the soil contain to give the best 

 results in growing plants ? Take five tumblers, fill them 

 with soil: add to the first 15 cc. of water each day: to the 

 second, half as much ; to the third, half as much as to 

 the second, etc. Plant the same number of Wheat-grains 

 (or other seed) in each. After two or three weeks a 

 great difference will be noticed : those which receive too 

 much water will not grow, on account of lack of air ; 

 those which receive too little will suffer from drought. 

 Somewhere between will be the happy medium where 

 the plants grow best. What per cent of water does this 

 soil contain ? We may ascertain by weighing a sample 

 of the soil and then drying it in an oven and weighing 

 again. For practical purposes we may turn the ques- 

 tion around and ask, How much air should the soil con- 

 tain ? A simple method of answering this is to insert 

 a small tube to the bottom of the soil, connect it with 

 a funnel and then pour in water (from a receptacle 

 containing a measured quantity of water) until the 

 water stands level with the surface of the soil. Since 

 the water displaces the air in the soil we may consider 

 that the volume of water poured into the soil represents 

 approximately the amount of air it contained : this 

 may be easily compared with the volume of the soil. 



In watering gardens and potted plants, and in irri- 

 gation on a larger scale, it is important to know how 

 much water to apply. The greatest ignorance prevails 

 in this respect. One irrigator will use ten times as 



