The plcmt food in the soil 



61 



could push the experiment still further and find out more 

 about plant food, but this involves chemical problems 

 and must be left alone for the present We can, how- 

 ever, say that plant remains are an important source of 

 plant food, and since we suppose the black material of 

 the soil to be made of plant remains (see p. 36), it will be 

 quite fair to say also that this black material, the humus, 

 is a source of plant food. We have therefore answered 

 the question we set, and we can explain some at any 

 rate of the differences between the surface soil and the 

 subsoil. The surface soil contains a great deal of the 

 black material, which forms plant food, while the sub- 

 soil does not. Thus plants grow well on the surface soil 

 and starve on the subsoil. We can also explain why 

 gardeners and farmers speak of black soils as rich soils; 

 they contain more than other soils of this black material 

 that makes plant food. Still further, we can explain 

 why the farmer often sows plants like mustard, tares or 

 clover, and then ploughs them into the ground. They 

 are not wasted, but they make food for the next crop 

 that goes in. 



Now let us turn to the results of the subsoil experi- 

 ments. The leaves and stems have increased the crop, 

 but only by 6*4 grams: they have not been nearly so 

 effective as in the surface soil. It is evident that the 

 mustard did not feed directly on the leaves and stems 

 put in ; if it had there should have been an equal gain 

 in both cases. The leaves and stems clearly have to 

 undergo some change before they are made into plant 

 food and the soil has something to do with this change. 

 After the crops are cut the soils should be tipped 

 out and examined. More of the original pieces of leaf 

 and stem are found in the subsoil than in the surface 



4—2 



