39. The selection of instruments. In selecting and devising instruments for the investigation of physical factors, emphasis has first been laid upon accuracy. This is the result of a feeling that it is better to have instruments that read too minutely than those which do not make distinctions that are sufficiently close, particularly until more has been learned about efficient differences. On the other hand, no hesitation has been felt in employing instruments which are not absolutely accurate, when it was clear that the error was less than the efficient difference. Similarly, the margin of error practically eliminates itself in the case of simultaneous comparative readings, when the instruments have been checked to the same standard. Simplicity of construction and operation are of great importance, especially in saving time where a large number of instruments are in operation. Expense is likewise to be carefully considered. It is impossible to have too many instruments, but cost practically determines the number that can be obtained. It is further necessary to secure or invent both simple and automatic instruments for all factors, except such invariable ones as altitude, slope, etc. Simple instruments must be of a kind that can be easily carried, and so constructed that they can be used at a minimum of risk. The sling psychrometer, for example, is very readily broken in field use, and it has been replaced by a protected modification, the rotating form.
In describing the construction and operation of the many factor instruments, there has been no attempt to make the treatment exhaustive. Those instruments which the author has found of greatest value in his own work are given precedence, and the manner of using them is described in detail. Other instruments of value are also considered, though with greater brevity. Some of the most complex and expensive ones have been ignored, as it is altogether improbable that they can come into general use in their present form. While the conviction is felt that the methods described below will enable the most advanced investigators to carry on thorough work, it is hoped that they will be seen to be so fundamental, and so attractive, that they will appeal to all who are planning serious ecological study.
WATER-CONTENT
40. Value of different instruments. The paramount importance of water-content as a direct factor in the modification of plant form and distribution gives a fundamental value to the methods used for its determination. Automatic instruments for ascertaining the water in the soil are costly, in addition to being complicated, and often inaccurate. For these reasons, much attention has been given to developing the simpler but more reliable methods in which a soil borer or geotome is used. The latter is simple, inexpensive, and accurate. It can be carried easily upon daily trips or upon longer reconnaissances, and is always ready for instant use. In the determination of physiological water-content, it is practically indispensable. Indeed, the readiness with which geotome determinations of water-content can be made should hasten the universal recognition of the fact that it is the available, and not the total amount of water in the soil, which determines the effect upon the plant.
Geotome Methods
Fig. 1. Geotomes and soil can.
41. The geotome. In its simplest form, the geotome is merely a stout iron tube with a sharp cutting edge at one end and a firmly attached handle at the other. The length is variable and is primarily determined by the location of the active root surface of the plant. In xerophytic habitats, generally a longer tube is necessary than in mesophytic ones. The bore is largely determined by the character of the soil; for example, a larger one is necessary for gravel than for loam. Tubes of small bore also tend to pack the soil below them, and to give a correspondingly incomplete core. The best results have been obtained with geotomes of ½–1 inch tube. Each geotome has a removable rod, flattened into a disk at one end, and bent at the other, for forcing out the core after it has been cut from the soil. Sets of geotomes have been made in lengths of 5, 10, 12, 15, 20, and 25 inches. The 12– and 15–inch forms have been commonly used for herbaceous formations and layers. They are marked in inches so that a sample of any lesser depth may be readily taken. Such a device is very necessary for gravel soils and in mountain regions, where the subsoil of rock lies close to the surface.
Fig. 2. Fraenkel soil borer.