FLOWER POT COMPOSITION 149 



air can retain at the raised temperature. The ratio obtained bj' comparing 

 the moisture actually present with the greatest amount of moisture the air 

 could possibly contain at a given temperature is called the relative humidity, 

 with 100 per cent as the highest possible value. In the locality of Amherst, 

 Massachusetts, the normal out-of-door relative humidities for the months of 

 June, July, and August are respectively 74, 76, and 78 per cent (5). The 

 driest atmosphere in North America is in Death Valley, California, where an 

 observer has recorded a relative humidity as low as 23 per cent. This humidity 

 is low, and yet the writer on several occasions has found the humidity of 

 rooms in an average building to be as low as 20 per cent. In Canada, where 

 the winter air is much cooler than in Amherst, relative humidity as low as 8 

 per cent has frequently been recorded (12). There has been so little work 

 done in obtaining actual measurements of the relative humidity in homes that 

 little information is available. Daniels (3) states it is probable that our homes 

 iiave a relative humidity of 20 per cent or lower. Such a low humidity indi- 

 cates an atmosphere as dry as desert air. 



The effect of such a low moisture content is an atmosphere with a high 

 evaporating power. This increased evaporating power of the air quickly takes 

 up the moisture from the outside of the porous flower pot. The moisture lost 

 from the pot is replaced by moisture from the soil and the process continues 

 until both pot and soil are dry, if the soil remains in intimate contact with 

 the jjot. Usually, however, this extreme is never reached, as the daily water- 

 ing of the plants provides sufficient moisture to partially replace the daily 

 loss. As a matter of fact, practically one-half of the normal amount of water 

 supplied may be taken up by the pot, if the pot is dry. In Table 1 are given 

 the actual amounts of water adsorbed by dry flower jiots of various sizes and 

 also the amounts of water that constitute a normal watering for these pots. 



Table 1. — The volume of a norjial watering of a clay flower pot that is 



adsorbed by the pot. 



If the evaporation of water from the pot plus the amount used by the 

 plant exceeds the amount applied, there is a deficit of normal moisture in the 

 soil. This deficit may gradually increase until the soil is entirely devoid of 

 moisture in the lower part of the pot. When this occurs the feeding roots 

 die and the above-soil portions of the plant die either gradually or quickly 

 according to the nature of the plant or the rapidity with which this moisture 

 deficit develops. The normal daily water that is applied does not evenly dis- 

 tribute itself throughout the soil. The attraction by the pot itself for the 

 water causes a movement of the capillary water to the pot which usually re- 

 duces the moisture in the upper part of the soil before it has had time to 

 penetrate the lower and drier layer of soil. 



The evaporation of moisture from the outside surface area of the porous 



