1/4 SOIL CONDITIONS AND PLANT GROWTH 



peated as long as any matter remains in suspension for twenty-four hours. The 

 liquid containing the " clay " is either evaporated in bulk or measured, and, after 

 being well shaken, an aliquot portion taken and evaporated. In either case 

 the dried residue consists of " clay " and " soluble humus ". After ignition 

 the residue gives the "clay,'' and the loss on ignition the "soluble humus". 



Here minimum value of v = cm. per sec., and the minimum dia- 



10,000 



meter of the particles works out to '0013 mm. 



4. The sediment from which the " clay " has been removed is worked up 

 as before in the beaker, which is filled to the i o cm. mark and allowed to stand 

 for 100 seconds. The operation is repeated till the "fine sand" settled in 

 100 seconds is clean, when it is collected, dried and weighed. 



Here minimum value of v per second ; the calculated minimum 



10 



diameter = -037 mm. 



5. The turbid liquid poured off from the " fine sand " is collected in a Win- 

 chester quart, or other suitable vessel, allowed to settle, and the clear liquid 

 syphoned or decanted off. The sediment is then washed into the marked 

 beaker and made up to the 7-5 cm. mark. After stirring, it is allowed to 

 settle for twelve and a half minutes, and the liquid decanted off. The opera- 

 tion is then repeated as before till all the sediment sinks in twelve and a half 

 minutes leaving the liquid quite clear. The sediment obtained is the " silt," 

 which is dried and weighed as usual. The liquid contains the " fine silt," 

 which, when it has settled down, can be separated by decanting off the clear 

 liquid, and dried and weighed. 



For silt minimum value of v = cm. per second, minimum diameter of 



100 



particles = *oi2 mm. For fine silt the diameter obviously lies between this 

 value, and the one found for clay. 



6. Determinations are made of the " moisture " and " loss on ignition " of 

 another 10 grams of the air-dry earth. The sum of the weights of the fractions 

 after ignition + loss on ignition + moisture + material dissolved in weak 

 acid should approximate to 10 grams. 



7. It is advisable to make a control determination of the " fine gravel " in 

 a portion of 50 grams of the air-dry earth. The soil should be treated with 

 acid, as in i, and after that is removed by decantation may be at once treated 

 with dilute ammonia and washed on the sieve with i mm. round holes. The 

 1 fine gravel " left on the sieve is then dried and weighed. 



The American method is somewhat different. The breaking down of the 

 aggregates is brought about by physical means e.g. violent shaking and 

 sedimentation is sometimes hastened by a centrifugal apparatus. Hilgard does 

 not adopt a sedimentation method but proceeds in the converse manner ; he 

 collects and weighs the particles carried off by successive streams of water 

 of varying velocity. Full details are given in Bull. 24, Bureau of Soils, 1904, 

 and in Wiley's Agricultural Analysis, vol. i., where the continental methods 

 are also described. 



