EXERCISE 112 



TO HOW GREAT AN EXTENT DOES PLANT MATERIAL 

 ACCUMULATE IN SOILS? 



Materials. Samples of soils, including a clay or sandy soil of 

 sterile character, a black loam, a muck soil, and a peaty soil ; sheet- 

 iron soil pans or sheet-iron frying pans. 



Directions for work. Dry all the samples of soil in an oven 

 heated to a point just above 100 C. for several days, or if an 

 oven is not available, spread the samples out thinly on pie tins 

 or other shallow dishes and expose to the air. In either case, 

 decide when the drying has been carried to completion by 

 weighing from time to time. When the samples no longer lose 

 weight, they are ready for the experiment. 



If the samples have been oven-dried, all of the water will 

 have been driven off ; if air-dried, a certain small part of the 

 water will remain in the samples, and the amount will not be 

 the same for the different kinds of soil. It is therefore only by 

 drying in the oven that the soils can be so prepared that exact 

 results will be possible. 



Having dried the soils, place a sample of each in a separate 

 iron pan which has been previously weighed. Weigh the pans 

 with soil in them and determine the net weight of soil in each 

 case. Heat each pan with the contained soil by means of a 

 bunsen burner or blast lamp for at least an hour. Note any 

 evidences that organic material, principally plant material, is 

 being burned or driven off. 



When the organic material appears to be entirely burned, 

 allow the pans to cool and weigh them again. Determine the 

 amount of loss in each case and calculate the percentage of 

 organic material in each sample. In case you have been using 

 air-dry soils, in which direction would your results be influenced 

 by the fact that the soils were not wholly dry at the beginning 

 of the experiment? That is, would the percentage secured be 

 too high or too low? If you did not heat the samples long 

 enough to complete the burning of the organic material, how 

 would that fact affect the results ? 



[138] 



