VI. 1 SOILS REQUIRING POTASH MANURES 177 



account for an inferior plant and a reduced yield on the 

 plots receiving potash salts ; the author has observed 

 a case on heavy land where the application of a rather 

 excessive amount of kainit so altered the texture of the 

 soil, that the draught of ploughs upon it was perceptibly 

 increased, and the crop suffered to a marked degree. 



The examples that have been given to illustrate 

 the specific action of potash must, however, be used 

 with some caution as a guide to the manuring of 

 crops under ordinary conditions of farming. They are 

 extreme cases, drawn mostly from the later years of 

 the Rothamstcd experiments, when the exhaustion of 

 the available potash in the soil had become very pro- 

 nounced through the continuous cropping with the help 

 of a manure containing all the other elements of 

 fertility except potash. Except on special soils and 

 with the specially potash-loving crops, it is not usual 

 to find in this country that the use of a dressing of 

 potash salts has any visible effect on the yield, so large 

 is the stock of potash in the soil, and so well is it 

 conserved by the ordinary systems of cropping. 



On the lighter soils, the sands and the gravels, 

 potash is most likely to be deficient, and the ill-effects 

 arising from its absence are intensified by the dryness 

 of these soils. Even on such soils, potash manures will 

 rarely be found remunerative for cereal crops ; for 

 mangolds and potatoes, and to a less extent for turnips, 

 they are necessary ; while grass land can hardly be 

 maintained in a satisfactory character without potash 

 at regular intervals. On the stronger soils, potash is 

 a remunerative manure for mangolds, and occasionally 

 for land laid up for hay ; but in general, the use of 

 nitrate of soda as a source of nitrogen will liberate 

 enough of the locked-up potash in the soil for the needs 

 of the crop. 



M 



