46 The plcmt food in the soil 



The rye has taken most of the plant food that was 

 in Pot 1 leaving very little for the second crop. Our 

 soil therefore contained only a little plant food, not 

 more, in fact, than will properly feed one crop. But 

 yet it did not seem to have altered in any way, even in 

 weight, in consequence of the plant food being taken 

 out. In our experiment the soil was dried and weighed 

 before and after the mustard was grown ; the results 

 were : — 



Pot 2 Pot 2a 



lbs. oz. lbs. oz. 

 Weightof dried soil before the experiment... 6 6 6 7 



„ „ after „ „ ... 6 6 6 6 



Difi'erence ... 1 



The experiment is not good enough to tell us exactly 

 how much plant food was present at the beginning. 

 But we can say that the amount of plant food in the 

 soil is too small to be detected by such weighing as 

 we can do. 



Here is an account of a similar experiment made 

 300 years ago by van Helmont in Brussels, and it is 

 interesting because it is one of the first scientific 

 experiments on plant growth: — 



"I took an earthen vessel in which I put 200 pounds 

 of soil dried in an oven, then I moistened with rain 

 water and pressed hard into it a shoot of willow weigh- 

 ing 5 pounds. After exactly five years the tree that 

 had grown up weighed 169 pounds and about 3 ounces. 

 But the vessel had never received anything but rain 

 water or distilled water to moisten the soil (when this 

 was necessary), and it remained full of soil which was 

 still tightly packed, and lest any dust from outside 

 should have got into the soil it was covered with a sheet 



