INSTITUTE, PUSA, FOR 1917-18 5l 



separate cultures has to be devised. Ordinary field plots 

 are out of the question, due to the permanent alteration in 

 the physical texture of the soil that would be involved., The 

 ordinary pots used for culture work are also unsuitable for 

 such investigations, for the following reasons : — 



(a) There is inadequate space for root development, 



unless the pots are very large and deep. 



(b) The normal circulation of air and water, such as 



occurs in the soil, is impossible in a culture pot 

 even when unglazed and even when a sunk pot 

 is used. 



(c) Artificial drainage has to be provided in pots, 



which in itself acts as an aerating agent. 



(d) Culture pots frequently introduce the temperature 



factor. 



(e) Such cultures have to be constantly watered, an 



operation consumptive of much time. 

 (/) It is almost impossible to explore in detail the root 

 system of a plant grown in a culture pot. 



These difficulties can be overcome by using very small 

 plots in the place of culture pots. Pits from three to six 

 feet square are dug to the depth of two and a half feet, 

 the upper nine inches of soil being kept separate from the 

 sub-soil. Various aerating agents are mixed with the soil, 

 which is then replaced, care being taken to refill the pits a 

 little at a time and to compress the earth to the required 

 ■degree. If prepared a few weeks in advance and irri- 

 gated immediately, the contents of the plots resemble very 

 nearly the conditions obtaining in field culture. The diffi- 

 culties connected with the circulation of air and water are 

 in this way removed and the temperature factor does not 

 interfere with the results. In cases where the soil is mixed 

 with large quantities of aerating agents, such as potsherds, 

 broken bricks, sand, or powdered charcoal, it is advisable 

 to spread from half to an inch of ordinary soil on the sur- 

 face of the beds to prevent any undue heating or cooling 

 due to the higher conductivity of the aerating materials. 

 Control plots of undisturbed soil and also of soil which has 



