GROWING PLANTS WITHOUT SOIL. — JOHNSTON 383 



the relative amounts of these essential elements that would give the 

 best growth. Years of practical agricultural experience had taught 

 man that plants needed nitrates, phosphates, and potash in the soil. 

 By many trials they also found that some proportions were better 

 than others. Science is now showing by means of water culture 

 studies why some of the earlier practices worked out so well. By this 

 method man is endeavoring to find out the exact uses made of the 

 different elements by plants. 



A natural question which arises is, why should investigators pre- 

 fer to grow plants in water containmg dissolved chemicals instead 

 of putting the same chemicals in the soil where plants grow nor- 

 mally ? A brief discussion of some of the difficulties involved in the 

 use of soil will show why water cultures are preferable in experi- 

 ments where all the conditions influencing plant growth arc rigidly 

 controlled. Physically, the soil is made up of rock particles vary- 

 ing in size and shape which are usually mixed with organic material. 

 The amount of water that soils hold dejDends on these properties. 

 Water easily runs through a loose soil. In a closely packed soil 

 water movement is slow. In experiments where several plants are 

 to be grown with their roots in exactly similar media, it is at once 

 realized how difficult the problem is of potting a number of plants 

 in soil so that all of them may be rooted among particles of the 

 same size and shape, and similarly packed. When the packing, or 

 arrangement of particles, is different, it is found that the capillary 

 water films in the soil as well as in the air spaces are altered. All 

 these different physical factors would be possible sources of error in 

 an otherwise accurately controlled experiment. 



The problem of obtaining two soils that are exactly alike in their 

 chemical composition is even more difficult than that of securing two 

 that are similar physically. Even a slight difference in the chemi- 

 cal composition of two soils may bring about a marked difference in 

 plant growth. A single illustration will serve to show the importance 

 of this consideration where quantities as small as one part of a certain 

 element in 2,000,000 parts of the soil water makes enormous differ- 

 ences in plant growth. Boron, a substance found in boric acid and 

 borax, and until recently, not seriously considered important in plant 

 growth was the element involved in this work. Pure quartz sand 

 was used instead of ordinary garden soil as the medium for grow- 

 ing a number of potato plants. The sand was watered with a solu- 

 tion containing all the elements then considered necessary for growth. 

 New glazed earthenware crocks were used to hold the sand. In the 

 first two series of experiments the plants grew very well, but in later 

 work they became unhealthy and usually showed early death of the 

 stem tips. Later when the importance of small traces of boron was 

 recognized, the real explanation of the poor growth was obvious. 



