252 J. F. BIERHUIZEN 



WATER USE AND GROWTH 



The experiments discussed above were carried out under conditions in 

 which the containers were frequently irrigated in such a way that the soil 

 moisture content varied between pF 2-o and pF 2-4. In another series of 

 experiments a constant water-level was applied by means of sub-irrigation 

 with a Mariotte flask. Four ground-water tables of 25, 35, 45 and 55 cm 

 respectively, were used for each treatment with four rephcates. From the 

 Mariotte flask the daily water use could easily be measured. The experi- 

 ments were performed in a clay-loam and in a sandy soil. 



In Fig. 4, dry- and fresh-weight yield are plotted against total water use 

 for both soil types. For dry weight a linear relation occurs, according the 

 formula y — mx, (where y is dry- weight yield and x total water use). The 

 tangent m is approximately the same for the various vegetables treated in 

 these experiments, and for both soil types. It is obvious that water use 

 versus fresh weight shows a larger variation. This might be explained by 

 the fact, that for y = mx, one could write : 



fresh weight times dry weight percent =100 mx 

 or 



fresh weiffht 



X 



ignt . 1 . 1 



— — times dry weight percent = ioo»j 



which relation is a hyperbolic, as is shown in Fig. 5. It is clear that the ratio 

 I — j is low for vegetables with a high dry- weight percentage such 



as beans and carrots, and high for a low dry-weight percentage such as 



lettuce and spinach have. Moreover, a small variation in dry-weight percen- 



, 1 1-1 ■ • • 1 • / fresh weight \ 

 tage at a low value, results in a large variation in the ratio j — I 



The fresh-weight yield varies considerably without a large change in total 

 water use {x). In an experiment with lettuce, for instance, and harvesting 

 under wet soil conditions, the fresh- weight yield and dry- weight per- 

 centage was 656 gm and 4-5% respectively, under dry conditions it was 

 366 gm and 5-5%. In a similar dry treatment, but irrigated the day prior 

 to harvest, it was 546 gm and 4-5% (Bierhuizen and De Vos, 1958). It was 

 evident that with the large change in fresh-weight and dry-weight per- 

 centage, total dry matter and water use did not change to a large extent. 

 A statistical analysis revealed that the various ground-water tables gave 

 no significant difference in dry- and fresh-weight yield. Total water use 

 increased, however, almost linearly with a ground-water table at depths 

 of 55 to 45, 35 and to 25 cm, respectively. This might be due to an increase 



