The Science of the Soil 119 



flowers, for instance, required 5619 tons of water (equivalent to 56 in. of 

 rain) to mature, it is obvious that they could not be grown in many 

 parts of eastern Great Britain, where the total annual rainfall only averages 

 24 or 25 in., or 2400 to 2500 tons per annum. As the Sunflower crop 

 would require only about 150 days at the most to mature, from start to 

 finish, something like 38 tons of water (or '38 in. of rain) would have to 

 fall on an acre of ground each day. Assuming that 10,000 Sunflowers 

 were grown to the acre, this would mean that each plant would absorb 

 and transpire about 8J Ib. of water per day. 



Professor Bentley, in his Manual of Botany, states that "a common 

 Sunflower, 3^ ft. high and weighing 3 Ib., gives off on an average 20 oz. 

 of water; and a Cabbage plant about 19 oz. of fluid in a single day". 



It may be remarked that a Sunflower 3| ft. high and weighing 3 Ib. 

 is a poor specimen. Taking an average specimen, 6 ft. high and about 

 6 Ib. in weight, it bears about 30 leaves, each with a superficial area of 

 about 45 sq. in. The total transpiration leaf surface for one Sunflower is 

 therefore about 1350 sq. in. Assuming that a plant of this size will trans- 

 pire only 24 oz. of water per day for 150 days, each plant will transpire 

 in the season 225 Ib. of water from its leaves, or over 1000 tons for a crop 

 of 10,000 plants to the acre. Assuming also that each plant, when fully 

 grown, contains 4 Ib. of water, that would give 40,000 Ib., or about 18 tons, 

 more moisture taken from the soil. It would therefore appear that an 

 acre of Sunflowers would require about 1020 tons of water (equal to 10 in. 

 of rain) in the course of the year. 



The figures on p. 120 give an idea as to the approximate quantity of 

 water taken out of the soil during the growing season by various crops. 



It will thus be seen that such crops as Sunflowers, Jerusalem Arti- 

 chokes, and Cabbage crops require at least 10 in. of rain in 150 days to 

 enable them to flourish, while Beetroot and Lettuces require over 23 in. 

 of rain, and Runner Beans require about 26 in. in the course of about 

 100 days. If all the rain that falls is not absorbed by the soil, it is evident 

 that the crops must suffer, unless moisture can be kept round the roots in 

 some way. 



Mr. A. D. Hall, in his book on The Soil, calculates that 300 Ib. of 

 water transpired is equivalent to 1 Ib. of dry matter. It is obvious that 

 the amount of water given off will depend largely upon the season, whether 

 wet or dry, hot or cold, and also upon the way the crops are cultivated, 

 and whether they are planted at proper distances apart, and are in a 

 free-growing healthy condition, free from insect pests and fungoid dis- 

 eases. The nature of the crop itself must also be taken into account. 

 Some plants containing very little dry material (e.g. Lettuces, Turnips) 

 would give off more moisture than others of a more woody nature. And 

 again, some very fleshy plants, like most of the Cactaceae, many of the 

 Euphorbiaceae, and of the Asclepidese (like Stapelias, Haworthias), and 

 such plants as Stonecrops, are specially adapted to conserve their moisture 

 even in the hottest weather, owing to the very few stomata on their 



