188 SOILS AND MANUBES 



Lower half (Pi KI), phosphate and nitrogen (no potash). 



,, (Ki P 4 ), phosphate, kainite, and nitrogen (complete 



manure). 



,, (P 4 K 4 ), kainite and nitrogen (no phosphate). 



,, (K 4 N 3 ), nitrogen alone. 



It will be seen that there are three plots which re- 

 ceive only one kind of manure phosphates alone, potash 

 alone, and nitrogen alone and three from which these 

 substances are respectively withheld, the other two being 

 added. The former should be compared with the un- 

 manured plot, the latter with the complete manured 

 plot. The special manurial requirements of the soil 

 may thus be judged, both from the effects of adding a 

 particular kind of manure and from the effects of with- 

 holding it. 



The results can best be ascertained by gathering the 

 produce of each plot and weighing it, but if they are 

 not to be recorded, simple inspection will generally be 

 sufficient. If the effects of the manures are not apparent 

 to the eye it may be either because the soil does not 

 require manure at all or because it requires some other 

 treatment as well. 



When the experiment is conducted on purely qualita- 

 tive lines, i.e., when the results are to be ascertained 

 by simple inspection, it is not necessary that the measure- 

 ments should be very exact. They might be dispensed 

 with altogether but for the necessity of adjusting the 

 quantities of the manures, at least approximately, to the 

 area of land. In the arrangement described above, the 

 plots are 10 yards by 20 yards, which is very nearly ^jth 

 part of an acre, and the quantities of manure given are 

 at the rate of 6 cwt. of phosphate, 3 cwt. of kainite and 

 1J cwt. of nitrate of soda per acre. 



If the results are to be recorded, the experiment must 

 be quantitative and should be carried out with the exact- 



