12 PL-\XT PHYSIOLOGY -\XD ECOLOGY 



15. Time of water content readings. Since conditions in a 

 plant-house are nearly constant, a single reading of water con- 

 tent made at any time is fairly representative. This is not the 

 case in the field, owing to the influence of rainfall, evaporation, 

 and gra%'itv in changing the amount of soil water. An isolated 

 reading has very slight value, and it is therefore necessan." to 

 obtain a basis for comparison by making either a series of read- 

 ings in one place at different times or in different places at the 

 same time. WTienever the proper amount of time can be given 

 to field work, the best method is to select a series of stations or 

 habitats, and to take readings throughout the entire series at 

 different times. Under ordinary conditions the time of day at 

 which a particular sample is taken is of httle importance, since 

 the variation during a day is usually slight. This does not apply 

 to exposed wet soils or to soils which have just been wetted by 

 rain. When a series of readings is made in different places, how- 

 ever, it is better that the readings be made in rapid succession. 

 Simultaneous readings are necessary 011I3- when it is desired to 

 determine whether there is a difference in the rate of loss in the 

 various habitats. 



16. Location of readings. In determining the location of 

 readings in the field, it is desirable to obtain as great a range as 

 possible. Where the topography is much broken, as in mountain 

 regions, a series of stations a mile long will iaclude a number 

 of different habitats. In general, fewer habitats are accessible, 

 and it is then necessary to locate a station in each of the more 

 or less diverse areas of each habitat. Grassland, woodland, and 

 marsh show striking differences of water content as well as of 

 other factors. A hiUy prairie that drops through meadowland 

 into forest offers an unusually good opportunity for a series of 

 stations that will show the effect of soil, slope, cover, etc., upon 

 the water content. On accotmt of the small differences always 

 present, each station shotild be definitely located where differ- 

 ences of soil, slope, etc., are the most striking. For the sake 

 of future readings, the exact location of each station is perma- 

 nently marked and recorded. Successive readings are made as 

 near to the preceding ones as possible, though new samples 

 should not be taken too near the old holes. A difference of a 

 few feet produces no appreciable error, if the station is uniform 

 IQ character. 



