360 HOW THE NEEDS OF SOILS MAY BE DETERMINED 



of acid phosphate or bone meal. Later on rock phosphate may 

 be used. 



Phosphorus determinations of ordmary sands, mucks and peats 

 are valuable in ascertainmg the quality of these soils. 



If any one doubts that a soil can ever become depleted by 

 exhaustive cropping, let him compare the phosphorus analyses of 

 long-cropped and depleted lands with those of virgin soils of the 

 same kinds, or of similar soils that have had their phosphorus 

 supplies maintained or increased through good farming methods. 

 No arguments are so convincing. 



Potassium Analyses. — On many soils, especially the heavier 

 types, potassium determinations are important largely because 

 of their inventory value. On other soils analyses show surprisingly 

 low supphes of this element, particularly in case of peats. Usually, 

 when a soil is found to contain a low amount of potassium, potash 

 fertihzers are required. Soils abundantly supplied should be so 

 managed as to enable them to render available sufficient amounts 

 to meet the needs of crops (see "Potash^' in index). 



POT TESTS 



Pot Tests Not Fully Reliable. — It has been the hope of chemists 

 to determine by pot tests what chemical analyses do not or can 

 not show as regards the fertilizer needs of a soil or its cropping 

 possibilities. This method consists in filling several jars or cylinders 

 with the soil to be studied, treating the pots in different ways, 

 and growing a crop to indicate the proper treatment (Fig* 198) . 



This method seems most reasonable and sure. Moreover, it 

 can be carried on under absolute control as regards moisture and 

 temperature conditions. The results of some pot tests are of special 

 value in determining the fertilizer needs of any particular soil 

 and serve to reinforce field observations and chemical analyses 

 (Figs. 135 and 136). In general, however, such results can not be 

 fully relied upon as guides in the practical operations of the farm, 

 because it is impossible in such tests to reproduce the natural 

 conditions found m the field.^ 



FIELD OR PLOT TESTS 



Field Tests Are Best. — ^Fertilizer tests made on the farm, or 

 on the land in continued plot experiments, are the most reliable. 



3 Tests made in large cylinders usually extend over several years. It 

 is not practicable to make pot tests on the farm. 



