NOTES ON THE SOIL. 



483 



Mechanical Analysis of Soil. The amount of 

 humus in soil is easily ascertained by drying the soil, weigh- 

 ing, then burning the soil thoroughly on a clean shovel or in 

 a porcelain crucible and weighing again. The texture of soil 

 is ascertained most readily by the use of sieves. The finest 

 particles (silt-clay) pass through a sieve with 0*05 mm. mesh, 

 and by means of a second sieve with 1 mm. mesh we can 

 separate the residue into sand (passing through) and gravel 

 (remaining on sieve). The structure of the soil is then ex- 

 pressed in percentages, e.g. gravel 10 per cent., sand 20 per 

 cent., silt-clay 70 per cent. 



Dry some soil by spreading it out on paper and exposing it to the 

 sun in a dry place ; weigh out 100 grammes of the dried soil, stir it up 

 in a dish with hot water, transfer to a fine sieve (0'5 mm. mesh), and 

 wash the finer particles through with the help of a camel-hair brush 

 and then with a stream of water. The residue, or "soil skeleton," is 

 then dried and weighed. 



By using a series of sieves we can divide the "skeleton" into (1) 

 coarse gravel, left on sieve with 4 mm. mesh, (2) medium gravel, left 



on 2*5 mm. sieve, (3) fine gravel, left 

 on 1 mm. sieve, (4) coarse sand, the 

 residue which passes through the 

 1 mm. sieve. 



The fine soil, which passed through 

 the 0'5 mm. sieve, is now separated, 

 by washing, into clay-particles and 

 fine sand ; we know the weight of 

 this fine soil, so we need not dry it. 

 Pour it into a washing- cylinder (Fig. 

 18), a glass vessel 30 cms. high and 

 9 cms. in diameter, with a hole near 

 the bottom into which a bent tube is 

 fitted, passing through a cork and 

 having a piece of rubber-tubing at- 

 tached with a clip. The liquid is 

 stirred up, the turbid water run off 

 after about ten minutes, and the 

 vessel filled again with water. These 

 operations are repeated until all the 

 clay seems to have been run off, and 

 the residue of fine sand left in the 

 vessel is dried and weighed. 



The following simple methods do 

 not involve the use of a special washing-cylinder, or of a series of sieves, 

 (a) Get about a pound of garden soil, stir it up in water to form a 

 paste ; place this in a jar, fill the jar with water, cork it, shake well 

 for a few minutes, then allow to stand till the soil settles to the bottom. 



18. Washing Cylinder. 



