532 VARIOUS FERTILITY FACTORS 



where tidal power could fill extensive reservoirs and where a 

 mountain stream could serve to dissolve and remove the salt 

 deposit after the " mother liquor " is drawn off for further con- 

 centration. 



It is sometimes claimed that potassium salts, especially kainit, 

 have some power to prevent damage from fungous diseases and 

 injurious insects, but it may be questioned whether the effect 

 is direct or indirect ; data already given show very conclusively 

 that not only potassium salts, but also the salts of magnesium 

 and sodium (usually to a smaller extent), produce marked benefit 

 in many instances. In most cases, however, any influence which 

 aids directly or indirectly in the proper nourishment of the plant 

 will thus enable the plant itself better to resist attacks of insects 

 or disease. The author has frequently observed that insect in- 

 juries are much more apparent on corn grown on poor land than 

 on adjoining plots treated with phosphorus in connection with 

 farm manure or crop residues. 



Where the soil is markedly deficient in potassium, as compared 

 with normal soils, as is the case with certain peaty swamp soils, 

 and where the application of potassium salts on such soils produces 

 marked benefit, while sodium salts produce practically no benefit 

 (see Table 91), there can be no question regarding the need and 

 value of potassium for its own sake; and even where the soil con- 

 tains normal amounts of potassium, if enormous crops are to be 

 grown that draw very heavily on potassium, as the 40 to 50 tons per 

 acre of mangels on Barn field at Rothamsted (see Table Jib), 

 the time will come when potassium must be returned. (This, how- 

 ever, is also the case with magnesium and calcium.) 



On the other hand, where the plowed soil contains sufficient 

 total potassium to meet the draft upon it for, say, two thousand 

 years, and where sodium or magnesium salts produce about the 

 same effect as potassium salts, and where potassium produces 

 little or no effect if applied in connection with liberal amounts 

 of decaying organic matter, the conclusion may be safely drawn 

 that the addition of commercial potassium is not essential in 

 adopting systems of permanent agriculture, for even the slight 

 erosion that occurs on nearly level lands will possibly provide an 

 absolutely permanent supply. 



