EXPERIMENTS IN MANURING IN POTS AND IN THE FIELD. 119 



now the case of a plant growing in a pot kept moist by 

 frequent waterings, we find that if the soil contain "03f° of 

 salts in solution, and also averages about 25f o of water, 

 this water from which the plant is feeding will contain # 12°/° 

 of salts. If then, the pot is allowed to become " dry," that 

 is, to lose about 20°/° of its water (a common occurrence in 

 droughty weather), the soil moisture then becomes a '6 /> 

 solution. We thus see what an enormous alteration in the 

 concentration of the soil water takes place in the ordinary 

 drying of the surface of any soil either in the field or in a 

 pot. 



We will take the case of a culture pot in which it is 

 intended to experiment with the manuring of the plant 

 whose limit of endurance is xf of salts, and assume that 

 when allowed to become "dry," i.e., contain about 5°/° of 

 water, wilting will not occur for a day or so — in other 

 words, that the plant will tolerate a *6°/ u solution. Therefore 

 x is greater than '6 by, say a. Let us now add to the pot 

 nitrate of soda or sulphate of potash or other soluble salt 

 in quantity approximating a. Then the plant will be 

 affected by the drying more or less injuriously according 

 as the quantity of manure is greater or less than a. 



It is thus seen that under field conditions it is desirable 

 that the value of this factor a should be known, as except 

 under irrigation it is manifestly impossible to be sure that 

 the moisture content will not decrease beyond a certain 

 point. We also see that it is not possible to come to a 

 definite conclusion as to the value of a manure unless we 

 know that the moisture conditions at least, are going to be 

 similar in our practical trial, to those of the few experi- 

 mental plants. Here it is that the difference becomes 

 apparent between experiments in the pot and in the plot. 

 It is a very easy matter to prevent drying in the former, 

 not so in the case of the same plants in the same soil treated 



