54 Department of Soil Investigations. 



large closely to resemble field conditions, which is not true of the quantity 

 contained in pots. No covering is to be placed over the tanks, but in 

 every way natural conditions are to be permitted. The top soil and 

 subsoil will be placed in their relative positions. It is expected that the 

 rainfall will be sufficient to meet the needs of the crops, but if the 

 plants suffer during- periods of drought they can be watered arti- 

 ficially. 



Any desired type of soil may be used, which is not possible with 

 ordinary field experiments. It is also possible to make a comparison of 

 different soil types under any desired condition, which may be very 

 serviceable in ascertaining the eff"ect of those properties differentiating 

 these types upon certain factors in soil productiveness. 



The chief feature of the plan is that of keeping accurate records of 

 the factors affecting plant-growth without producing artificial conditions. 



The tube leading from the bottom of the tank is designed to carry off 

 the drainage water into a receptacle which will permit the quantity to 

 be measured and its constituents to be determined. This will permit of 

 an estimation of the amount of moisture used by the crop, and will make 

 it possible to trace more closely the disposition of plant-food. 



The accompanying diagram shows the plan and cross-section of 

 these tanks. 



There are certain experiments involving fundamental problems in soil 

 productiveness that can be conducted only where it is possible to control 

 and measure the conditions affecting plant growth, and maintain the 

 experiments through a long period of years, as is the case with these 

 soil tanks. Some of these problems are as follows : 



Effect of the continuous use of large amounts of mineral fetilizers on 

 the physical and chemical properties of the soil and on the bacterial 

 flora and bacterial activity. 



The conditions under which lime is lost in the drainage water. 



Changes that occur in a series of years when soils gradually deteriorate 

 or improve. 



Extent to which soils under field conditions are renewed by accession 

 of lower soil to the plowed part through the wearing away of the top 

 soil. 



Effect of certain crops on the permeability and certain other physical 

 properties of the soil, and on the loss of plant nutrients. 



The loss of potassium and other substances occasioned by manuring 

 with lime. 



Loss of plant nutrients caused by clean cultivation. 



