26 CONCEPT AND HISTORY. 



three rotation pastures to permit grazing during one-third of the growing 

 season — spring, summer, and fall respectively. The behavior of the community 

 under the different degrees and kinds of grazing is measured by means of an 

 unusually complete system of chart- and cut-quadrats. The details of the 

 method are discussed in Chapter VI. 



CHRESARD AND WATER REQUIREMENT STUDIES. 



Significance. — While practically all studies of the chresard or available 

 water in soils have been made without definite reference to indicator plants, 

 it is clear that they have a direct bearing upon the latter. This is likewise 

 true of researches upon water requirements, especially those that relate to 

 controlling physical factors. Since the value of an indicator depends upon the 

 exactness of its correlation with direct factors, and especially water, it is often 

 totally misleading to relate it to obvious or superficial facts. For this reason 

 a scientific system of indicators has but recently become possible. It was a 

 distinct step in advance to connect species with the total water-content or 

 holard. But this gives trustworthy results only for the same soil. To obtain 

 exact results it has become necessary to determine the water-withholding power 

 of different soils and the water-using capacity of different plants. It has like- 

 wise proved imperative to take into account the salt-content and air-content 

 of the soil solution. In the further analysis of indicators, it proves desirable 

 to utilize their form, growth, and abundance for more minute and exact values. 

 Hence a knowledge of the growth requirements, which are largely water 

 requirements, has come to be highly significant. 



Much work has been done upon the chresard of different soils and plants, 

 and a still larger amount upon water requirements. Most of the former is 

 American, and has been done in the West. As a result, it has a direct bearing 

 upon the problem under consideration here. Of the great mass of water 

 requirement data only a few deal with native or non-cultivated species, and 

 are pertinent to the present discussion. For these reasons a concise account 

 is given of the progress of the chresard concept. 



The chresard. — The earliest studies of the water-content non-available to 

 plants were incidental and failed to recognize the fundamental importance of 

 the distinction. 



Sachs (1859, 1865 : 173) found that a young tobacco plant began to wilt in 

 a mixture of sand and beech mold at 12.3 per cent and that the chresard for 

 this soil was 33.7 per cent. A second plant in clay wilted at 8 per cent, with 

 a chresard of 44.1 per cent, while for a third the echard in sand was 1.5 per 

 cent and the chresard 19.3 per cent. Heinrich (1874) determined the echard 

 of barley in peat as 47.7 per cent and of rye as 53.4 per cent. In calcareous 

 soil corn wilted at 8.6 per cent and broad beans at 12.7 per cent. Mayer 

 (1875) observed that pea plants wilted at 33.3 per cent in sawdust; 4.7 per 

 cent in marl, and 1.3 per cent in sand, while Liebenberg found that beans 

 wilted in loam a£ 10 per cent, in marl at 6.9 per cent, and in coarse sand at 

 1.2 per cent. 



Gain, 1895. — Gain (1895 : 73) has studied the behavior of three mesophytes 

 in six different soils, with the results indicated in the table below. The echard 

 varies less than 50 per cent for these species in any one of the first three soils, 



