WATER-CULTURE METHOD — HOAGLAND AND ARNON 469 



because of unfavorable light or temperature, failure may also be 

 expected under water-culture conditions. Sunlight and suitable tem- 

 peratures are essential for green plants, in order that they may carry 

 on one of the fundamental processes of plant growth, known as 

 photosynthesis. In this process, the element carbon, which forms so 

 large a part of all organic matter, is fixed by plants from the carbon 

 dioxide of the atmosphere. This reaction requires a large amount of 

 energy, which is obtained from sunlight. 



Plants depend on photosynthesis for their food, that is, organic 

 substances, such as carbohydrates, fats, and proteins, which provide 

 them with energy and enter into the composition of plant substance. 

 The mineral nutrients absorbed by roots are indispensable for plant 

 growth, but they do not supply energy, and in that sense, cannot be 

 regarded as "plant food." Animal life is also absolutely dependent 

 on the ability of the green plant to fix the energy of sunlight. 



TEMPERATURE RELATIONS 



An earlier report of a preliminary experiment by other investigators 

 suggested that under greenhouse conditions heating the nutrient 

 solution would produce large increases in the yield of tomatoes. 8 

 Experiments that we have carried on with tomatoes in a Berkeley 

 greenhouse (unheated except on a few occasions to prevent tem- 

 peratures from falling below 50°-55° F.) have now given evidence 

 that under the climatic conditions studied, the beneficial effects of 

 heating the nutrient solution (to 70°-75° F.) are not of significance. 

 If favorable air temperatures are maintained, there seems to be no 

 need to heat the solution. Attempts should not be made to guard 

 against frost injury or unfavorable low air temperatures merely by 

 heating the nutrient solution. Proper provision should be made for 

 direct heating of the greenhouse. This may be found desirable even 

 when danger from low temperatures is absent, in order to control 

 humidity and certain plant diseases. 



These experiments on tomatoes suggest that if greenhouse tempera- 

 tures are properly controlled, the solution temperature will take care 

 of itself. Certainly no expense, either in a greenhouse or outdoors, 

 should be incurred for equipment for heating solutions until experi- 

 mentation has shown that such heating is profitable. There is no 

 one best solution temperature. The physiological effects of the 

 temperature of the solution are interrelated with those of air tem- 

 perature and of light conditions. 



Most amateurs who try the water-culture method will grow plants 

 in warm seasons and probably will not wish to complicate their 



« Gericke, W. P., and Tavernetti, J. R., Heating of liquid culture media for tomato production, Agr. 

 Eng., vol. 17, pp. 141-142, 184, 1938. 



