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T. Lyttleton Lyon and James A. Bizzell 



purpose because of the large proportion of plants that were winterkilled. 

 The table contains also a statement of the quantity of water that passed 

 into the air by transpiration and evaporation (this being the difference 

 between rainfall and percolation expressed in acre inches), a record of 

 the dry matter contained in each crop expressed in pounds per acre, 

 and the percentage of the annual rainfall that percolated thru the soil. 

 The annual rainfall as here given dates from May 1 of each year, which 

 is about the time when crops are planted and growth of Derennials begins. 

 The period between harvest and the following May gives an opportunity 

 for the moisture to readjust itself in the soil, and thus the next crop starts 

 off with about the same moisture content (which is the capillary water 

 capacity of the soil) as did its predecessor. 



The minimum transpiration ratios shown in table 12 should be narrower 

 than the transpiration ratios obtained by carefully controlled experi- 



TABLE 13. Unit Water Requirements of Plants as Determined by Various 



Investigators 



* Pots holding 42 pounds of field soil were used. Evaporation from the soil was reduced to a very 

 low degree by perforated glass covers cemented on the pots. The figures quoted are from unfertilized soil. 



t Wollny grew plants jn humous sand in quantities ranging from 5 to 12 kilograms. ISvaporation 

 was reduced to a very low degree by perforated covers. Actual evaporation from uncropped cans was 

 observed. 



t Hellriegel grew plants in 4 kilograms of clean quartz sand and supplied them with nutrient solutions. 

 The loss by evaporation from uncropped pots was used in determining losses by transpiration. In later 

 experiments covers were used in order to cut down evaporation. 



II King used cans holding about 400 pounds of soil. Some of these cans were set down into the earth, 

 while others were not. Part of the work was carried on in the field; the remainder was run in vegetative 

 houses. Normal soils were used. Evaporation from soil was very low, water being added from beneath. 

 The data quoted are the average of a large number of tests. 



§ Jars containing from 12 to 48 kilograms of soil were used. Loss by evaporation was determined 

 on bare pots. The plants were grown in culture houses or in screened inclosures. 



1 Plants were grown in cans holding 250 pounds of soil. Evaporation from the soil was prevented by 

 means of a paraffin covering. Work was conducted in screened inclosures. The data are the average 

 for several years. 



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