54. Chresard values of different soils. The following table gives the water-content values of six representative soils. The per cents of holard (at saturation) and of echard are those determined by Hedgcock[[3]] with six mesophytes as test plants for each soil. The chresard has been computed directly from these.
| HOLARD | ECHARD | CHRESARD | ||||
|---|---|---|---|---|---|---|
| Sand | 14.3 | 12.6 | .3 | .25 | 14 | 12.3 |
| Clay | 47.4 | 32.5 | 9.3 | 6.3 | 38.1 | 26.2 |
| Loess | 59.3 | 37.1 | 10.1 | 6.4 | 49.2 | 30.7 |
| Loam | 64.1 | 39.1 | 10.9 | 6.6 | 53.2 | 32.5 |
| Humus | 65.3 | 39.6 | 11.9 | 7.2 | 53.4 | 32.4 |
| Saline | 68.5 | 40.8 | 16.2 | 9.6 | 52.3 | 31.2 |
The first column indicates the per cent based upon the dry weight, the second upon the weight of the moist soil.
While these can not be considered absolute for a particular soil other than the ones investigated, they are found to correspond somewhat closely to the results obtained for other soils of the respective groups. For accurate research, the chresard must of course be ascertained for each formation with respect to its peculiar plants and soil. The influence of the ecad in more or less determining the echard is also shown by Hedgcock, who found that floating plants wilt at 25 per cent, amphibious ones at 15–20 per cent, mesophytes at 6–12 per cent, and mesophytic xerophytes at 3–6 per cent. The echard is also somewhat higher for shade plants than for heliophytes.
Records and Results
55. The field record. It is superfluous to point out that a definite form for field records saves much time and prevents many mistakes. The exact form may be left to personal taste, but there are certain features which are essential. Many of these are evident, while others may seem unnecessary; all, however, have been proved by experience to have some value in saving time or in preventing confusion. The two fundamental maxims of field work are that nothing is too trivial to be of importance, and that no detail should be entrusted to the memory. The field record should contain in unmistakable terms all that the field has yielded. These statements apply with especial force to water-content, in many senses the most important of physical factors. The precise character of the record depends upon the way in which the readings are made, whether scattered or in series. As the day-station series is of the greatest importance, the record is adapted for it especially, but it will also serve for all readings. The record is chronological, since this is the only convenient method for the field. A proper form for a field record of water-content is the following:
| Can No. | Date | Formation | Station | Community | Soil | Sample | HOLARD | ECHARD | Chresard | NOTES | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Weighings | % | Weighings | % | Sky | Rainfall | ||||||||||||
| 1st | 2d | Can | 1st | 2d | Can | ||||||||||||
| 10 | 2/8/04 | Spruce forest | Jack Brook | Mertensiare | Loam | 10 | 58.7 | 50.1 | 25.52 | 2 | 58.7 | 53.41 | 25.52 | 10 | 16 | Cloudy | 0 |
| 17 | „ | Spruce forest | Milky Way | Gentianare | ¼ mold ¾ gravel | 10:2 | 64.25 | 57.5 | 21.35 | 16 | 64.25 | 59.6 | 21.35 | 5 | 11 | Cloudy | 0 |
| 40 | „ | Gravel slide. | Hiawatha | Asterare | Gravel | 2:10 | 78.55 | 74.3 | 22.85 | 8 | 78.55 | 74.85 | 22.85 | 1 | 7 | Cloudy | 0 |
A general designation of the soil composition is a material aid, especially where there is a difference in the core. For example, in a mountain forest or meadow, the upper layer will usually be mold, the lower sand or gravel. A careful estimate of the relation between the two throws much light upon the chresard. Under “sample” the number taken to reach the desired depth, if more than one, is indicated by placing the number before the depth, thus 2:10. When two or more full cores are included in the same sample for a 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 convincing. 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 unnecessary.
57. Sums and means. From the great difficulty of determining the absolute 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 readings 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 extremes, i. e., 5 per cent and 28 per cent, make it possible to approximate the mean, 18 per cent, while the latter gives little or no clue to the extremes. It is hardly necessary to state that means and extremes should be determined for a certain habitat, or particular area of it, and that the results may be expressed with reference to holard and chresard.