Fig. 3. American soil borer.
42. Soil borers. There is a large variety of soil borers to choose from, but none have been found as simple and satisfactory for relatively shallow readings as the geotome just described. For deep-rooted plants, many xerophytes, shrubs, and trees, borers of the auger type are necessary. These are large and heavy, and of necessity slow in operation. They can not well be carried in an ordinary outfit of instruments, and the size of the soil sample itself precludes the use of such instruments far from the base station, except on trips made expressly for obtaining samples from deep-seated layers. For depths from two to eight feet, the Fraenkel borer is perhaps the most satisfactory, except for the coarser gravels: it costs $14 or $20 according to the length. For greater depths, or when a larger core is desirable, the Bausch & Lomb borer, number 16536, which costs $5.25, should be made use of. This is a ponderous affair and can be employed only on special occasions. On account of the size of samples obtained by these borers, it is usually most satisfactory to take a small sample from the core at different depths. Frequently, indeed, a hand trowel may be readily used to obtain a good sample at a particular depth.
43. Taking samples of soil. In obtaining soil samples, the usual practice is to remove the air-dried surface, noting its depth, and to sink the geotome with a slow, gentle, boring movement, in order to avoid packing the soil. This difficulty is further obviated by deep notches with sharp, beveled edges which are cut at the lower end. In obtaining a fifteen-inch core, there is also less compression if it be cut five inches at a time. Repeated tests have shown, however, that the single compressed sample is practically as trustworthy as the one made in sections. The water-content of the former constantly fell within .5 per cent of that of the latter, and both varied less than 1 per cent from the dug sample used as a check. As soon as dug, the core is pressed out of the geotome by the plunger directly into an air-tight soil can. Bottles may be used as containers, but tin cans are lighter and more durable. Aluminum cans have been devised for this purpose, but on account of the expense, “Antikamnia” cans have been used instead. These are tested, and those that are not water-tight are rejected, although it has been found that, even in these, ordinary soils do not lose an appreciable amount of water in twenty-four hours. The lid should be screwed on as quickly as possible, and, as an added precaution, the cans are kept in a close case until they have been weighed. The cans are numbered consecutively on both lid and side in such a way that the number may be read at a glance. The numbers are painted, as a label wears off too rapidly, and scratched numbers are not quickly discerned.
Fig. 4. Field balance.
44. Weighing. Although soil samples have been kept in tight cans outside of cases for several days without losing a milligram of moisture, the safest plan is to make it a rule to weigh cans as quickly as possible after bringing them in from the field. Moreover, when delicate balances are available, it is a good practice to weigh to the milligram. At remote bases, however, and particularly in the field, and on reconnaissance, where delicate, expensive instruments are out of place, coarser balances, which weigh accurately to one centigram, give satisfactory results. The study of efficient water-content values has already gone far enough to indicate that differences less than 1 per cent are negligible. Indeed, the soil variation in a single square meter is often as great as this. The greatest difference possible in the third place, i. e., that of 9 milligrams, does not produce a difference of .1 of 1 per cent in the water-content value. In consequence, such strong portable balances as Bausch & Lomb 12308 ($2), which can be carried anywhere, give entirely reliable results. The best procedure is to weigh the soil with the can. Turning the soil out upon the pan or upon paper obviates one weighing, but there is always some slight loss, and the chances of serious mishap are many. After weighing, the sample is dried as rapidly as possible in a water bath or oven. At a temperature of 100° C. this is accomplished ordinarily in twenty-four hours; the most tenacious clays require a longer time, or a higher temperature. High temperatures should be avoided, however, for soils that contain much leaf mould or other organic matter, in order that this may not be destroyed. When it is necessary on trips, soil samples can be dried in the sun or even in the air. This usually takes several days, however, and a test weighing is generally required before one can be certain that the moisture is entirely gone. The weighing of the dried soil is made as before, and the can is carefully brushed out and weighed. The weight of aluminum cans may be determined once for all, but with painted cans it has been the practice to weigh them each time.
45. Computation. The most direct method of expressing the water-content is by per cents figured upon the moist soil as a basis. The ideal way would be to determine the actual amount of water per unit volume, but as this would necessitate weighing one unit volume at least in every habitat studied, as a preliminary step, it is not practicable. The actual process of computation is extremely simple. The weight of the dried sample, w1, is subtracted from the weight of the original sample, w, and the weight of the can, w2, is likewise subtracted from w. The first result is then divided by the second, giving the per cent of water, or the physical water-content. The formula is: (w − w1)/(w − w2) = W. The result is expressed preferably in grams per hundred grams of moist soil; thus ²⁰⁄₁₀₀, from which the per cent of water-content may readily be figured on the basis of dry or moist soil.
46. Time and location of readings. Owing to the daily change in the amount of soil water due to evaporation, gravity, and rainfall, an isolated determination of water-content has very little value. It is a primary requisite that a basis for comparison be established by making (1) a series of readings in the same place, (2) a series at practically the same time in a number of different places or habitats, or (3) by combining the two methods, and following the daily changes of a series of stations throughout an entire season, or at least for a period sufficient to determine the approximate maximum and minimum. The last procedure can hardly be carried out except at a base station, but here it is practically indispensable. It has been followed both at Lincoln and at Minnehaha, resulting in a basal series for each place that is of the greatest importance. When such a base already exists, or, better, while it is being established, scattered readings may be used somewhat profitably. As a practical working rule, however, it is most convenient and satisfactory to make all determinations consecutively, i. e., in a series of stations or of successive days. Under ordinary conditions, the time of day at which a particular sample is taken is of little importance, as the variation during a day is usually slight. This does not hold for exposed wet soils, and especially for soils which have just been wetted by rains. In all comparative series, however, the samples should be taken at the same hour whenever possible. This is particularly necessary when it is desired to ascertain the daily decrease of water-content in the same spot. In the case of a series of stations, these should be read always in the same order, at the same time of day, and as rapidly as possible. When a daily station series is being run, i. e., a series by days and stations both, the daily reading for each place should fall at the same time. While there are certain advantages in making readings either early or late in the day, they may be made at any time if the above precautions are followed.
47. Location of readings. Samples should invariably be taken in spots which are both typical and normal, especially when they are to be used as representative of a particular area or habitat. A slight change in slope, soil composition, in the amount of dead or living cover, etc., will produce considerable change in the amount of water present. Where habitat and formation are uniform, fewer precautions are necessary. This is a rare circumstance, and as a rule determinations must be made wherever appreciable differences are in evidence. The problem is simpler when readings are taken with reference to the structure or modifications of a particular species, but even here, check readings in several places are of great value. The variation of water in a spot apparently uniform has been found to be slight in the prairies and the mountains. In taking three samples in spots a few inches to several feet apart, the difference in the amount of water has rarely exceeded 1 per cent, which is practically negligible. Gardner[[2]] found that 16 samples taken to a depth of 3 inches, in as many different portions of a carefully prepared, denuded soil plot, showed a variation of 7½ per cent. This is partially explained by the shallowness of the samples, but even then the results of the two investigations are in serious conflict and indicate that the question needs especial study. It should be further pointed out that all readings should be made well within a particular area, and not near its edge, and that, in the case of large diversified habitats, it is the consocies and the society which indicate the obvious variations in the structure of the habitat.