soil. 95 



soil, hence the classes — gravelly, sandy, and clay soils. 

 The operation is performed more quickly and with more 

 certainty by using a set of three sieves. One with holes, 

 2 millimetres in diameter, keeps back the coarse gravel while 

 it permits the fine gravel, sand and clay to wash through ; 

 the second, of 1 mm. diameter, separates the fine gravel, the 

 third of J mm. permits only the clay, silt and finest sand to 

 pass, and the last may be separated by sedimentation. If the 

 samples of gravel and sand be dried and weighed, and their 

 weight subtracted from the total it gives the quantity of 

 clay. 



Humus. — There is a fourth class of soils called vegetable or 

 peaty soils, which, when mixed with gravel, sand or clay, make 

 respectively gravelly, sandy or clay loams. Children do not 

 find it difficult to discover the cause of the dark color of the 

 surface soil. In talking about it with the teacher they learn 

 to use the terms humus, leaf-mould, vegetable-mould and loam. 

 If comparison is employed and children are asked to bring 

 samples dug in various situations, one taken from the surface 

 in the woods will be found to be almost wholly vegetable in 

 its origin, and even less mineral matter will be found in one 

 taken from the surface of a bog. The proportion of humus is 

 determined by comparing the weight of the sample of dry soil 

 with the weight of ash left when it is burned on a fire-shovel 

 over a bed of coals. It is easy to set up experiments to 

 demonstrate that humus aids sandy soils in retaining moisture, 

 and that it contributes to water-drainage and air-circulation 

 through clay soils. It renders all soils more fertile than they 

 would be without it. This fact opens the way to introduce 

 fertilization. Why is it better for the gardener to bury the 

 stems and prunings of plants in it than to burn them and 

 scatter the ashes ? What kind of soil— light or loose sandy 

 soil or close-grained clay — receives greater benefit from having 

 straw or .chaff worked into it ? Humus, rough straw-manure, 



