CROP STIMULANTS AND PROTECTIVE AGENTS 535 



that even heavy applications of plaster fail to liberate sufficient 

 for profitable crops, and thus the plastered land is made poorer 

 than the untreated land. 



A common method of advertising has been to write the word 

 PLASTER with a heavy application of the material in large letters 

 on a cultivated field in vi.ew of the public road. In the larger, 

 greener growth of grain crops or grass, the word PLASTER can be read 

 by the passers-by; and thus the landowner is induced to plaster 

 his whole field. If, however, he would only apply plaster year after 

 year where the word was first written, the time would come when 

 the word could not be read; and, if he still continued the applica- 

 tions, ultimately he would again be able to read PLASTER, if we 

 may judge from the testimony of common experience. 



Common salt. Common salt (sodium chlorid) is sometimes 

 used as a crop stimulant, but its beneficial effect is likely to be even 

 less durable than that of land-plaster. However, where common salt 

 or other soluble salts, such as sodium sulfate or magnesium sulfate, 

 or mixtures like kainit, are applied in connection with sufficient 

 supplies of phosphorus, nitrogen, and lime, the effect of the stimu- 

 lant must be confined chiefly to holding the phosphorus or other 

 necessary elements in available form and to liberating potassium 

 from the soil; and where the natural supply of potassium is ex- 

 tremely large, the beneficial effect of the applied salt may continue 

 for many years, as is well shown in the results from Rothamsted. 



Kainit. Kainit, of course, also supplies some potassium, and is 

 thus in some part a fertilizer, though in large part a stimulant. 

 To some extent this is also true of the common acid phosphate, 

 which contains phosphorus mainly in the form of a soluble acid 

 salt, and twice as many molecules of manufactured land-plaster : 



Ca 3 (PO 4 ) 2 + 2 H 2 SO 4 = CaH 4 (PO 4 ) 2 + 2 CaSO 4 . 



It is difficult to conceive of a more effective combination than 

 about 200 pounds per acre of a mixture of acid phosphate and kainit 

 applied twice for each five-year rotation of corn, oats, wheat, clover, 

 and timothy. With all crops removed, such a system would doubt- 

 less as thoroughly deplete the soil as any that could be devised, 

 clover itself, used in this way, being a very powerful soil stimulant. 

 If anything could be added to hasten the action, an application 



