76 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



ordinates and abscissae are equal. It need not be drawn at all, 

 except for illustrative purposes. For each of the native soils three 

 or more points may be plotted. The capillarity point has as its 

 ordinate the actual capillarity of this soil; but the abscissa has the 

 value of the capillarity for the well soil, similarly, for the moisture 

 equivalent at 100 gravity (which is still a capillarity measure) and 

 the wilting coefficient, or any other point at which osmotic pressure 

 of the several soils would be in equilibrium. (See also diagrams 8, 

 9, and 10 and discussion, p. 116.) 



These curves may then be used to transpose moisture values for 

 the soil well directly into moisture values for the native soil, realiz- 

 ing the probability which has been mentioned that at any moment 

 the well and native soil may be far from a state of actual equilibrium. 



TECHNIQUE OF PERIODIC SAMPLING. 



The field work of soil sampling is essentially the same where soil 

 wells are used as where it is feasible to sample the native soil. Soil 

 samples should be taken at permanent stations at least weekly during 

 the open season. Definite depths of 1, 2, and 3 feet, and more if 

 necessary, will recommend themselves in preference to long cores, 

 which show less definitely the location of the moisture. The 1-foot 

 sample may be obtained by a core extending from 10 to 14 inches; 

 the 2-foot, at 22 to 26 inches ; and the 3-foot, at 34 to 38 inches. If 

 intermediate values are desired, they may be obtained by interpola- 

 tion. 



Each sample as obtained should be placed in a soil can, the num- 

 ber of which may immediately be entered on a convenient field 

 form, 10 together with the number of the station and the depth. 

 There is an infinite variety of soil cans, but perhaps the most gen- ' 

 erally serviceable form is a rather heavy, stamped aluminum, screw- 

 top can, about 2^ inches in diameter and 2| inches high. 



Soil cans containing moist soil must be shielded from the sun and 

 from excessive heat, and should be weighed at the earliest oppor- 

 tunity, the weight being ordinarily determined to the nearest centi- 

 gram. (Fig. 2.) 



Soil samples of any ordinary texture, and of weight not exceed- 

 ing 100 grams, should be dried for at least eight hours in an oven 

 having the temperature of boiling water. Unusually moist samples, 

 or those of very fine texture, should be given a longer period. Dry- 

 ing for 24 hours is not too long to make good results certain. Espe- 

 cial care must be used with humus soils of low conductivity. Only 

 trial weighing will show when a sample is as dry as possible for the 

 conditions of the oven. 



10 Forest Service Form 486, fitting notebook cover 874-C. 



