36 THE HABITAT 



check, the order is 10:2. It has already been shown, however, that these 

 precautions are not necessary for ordinary purposes. In computing the 

 holard and echard, there is no need to show the figuring, if the process is 

 checked and then proved. Notes upon sky conditions aid in explaining the 

 daily decrease in water-content. The amount of rain and the period during 

 which it falls are of great importance in understanding the fluctuations of 

 the holard. Under community it is highly desirable to have a list of all the 

 species, but it is impossible to include this in the table, and a glance at the 

 formation list will show them. The form indicated above serves for a day- 

 station series, a daily series in one station for any number of check series 

 in one spot, and for scattered readings. In many cases the echard will not 

 be determined, but on account of its primary importance, there should be a 

 space for it, especially since it may be desirable to determine it at some 

 later time. 



56. The permanent record. This should be kept by formation, or if the 

 latter exhibits well-defined associations, the formational record may be 

 divided accordingly. This may seem an unnecessary expenditure of time, 

 but a slight experience in finding the water-content values of a particular 

 habitat, when scattered through a chronological field record, will be con- 

 vincing. The form of permanent record is the same as for the field, except 

 that the column for the formation and that for the society are often un- 

 necessary. 



57. Sums and means. From the great difficulty of determining the abso- 

 lute water-content, and of obtaining a standard of comparison between soils 

 on account of the varying ratio between bulk and weight, water-content sums 

 are impracticable. For the same reasons, means of actual water-content 

 are practically impossible, and the mean water-content must be expressed in 

 per cents. Daily readings are not essential to a satisfactory mean. In fact, 

 a single reading at each extreme enables one to approximate the real mean 

 very closely; thus, the average of 26 readings in the prairie formation is 

 18 per cent. The extremes are 5 per cent and 28 per cent, and their 

 average 16.5 per cent. A few readings properly scattered' through moist 

 and dry periods will give a reliable mean, as will also a series of daily read- 

 ings from one heavy rain through a long dry period. The one difficulty with 

 the last method is that such periods can not well be determined beforehand. 

 Means permit ready comparison between habitats, but in connecting the 

 modifications of a species with water-content as a cause, the extremes are 

 significant as indicating the range of conditions. Furthermore, the ex- 

 tremes, i. e., 5 per cent and 28 per cent, make it possible to approximate 



