274 SOIL AND FERTILIZER ANALYSIS 



of soils, together with the publication of a large number of 

 predictions. Some of these predictions have proved correct; 

 some of them incorrect; and some of them have never been 

 put to the proof. The difficulties in the way of determining 

 accurately fertilizer deficiencies of a soil are so many that 

 an analysis alone cannot give the information desired in a 

 great majority of cases. Fertilizer tests in the field are the 

 best single way to ascertain the plant food needs of a soil 

 (Fig. 69). 



The total amount of each of the several constituents can 

 be determined accurately, and the result is a complete 

 inventory of the plant food supply in any given soil, pro- 

 vided the sampling has been done carefully. It is very 

 important to obtain a sample of soil which represents as 

 nearly as possible the soil of the whole field in question. 

 This information in connection with other data, such as 

 topography, physical condition of the soil, kind of seed, 

 cultivation, temperature, rainfall, and appearance of crop, 

 will give an expert a good idea of the needs of a given soil. 



The difficulty with this kind of analysis in interpreting 

 plant food deficiencies is that by no means all of any par- 

 ticular element is available or will even become available 

 readily. A soil may contain a large amount of phosphoric 

 acid, but may have it in such unavailable form that plants 

 cannot obtain enough for normal growth, and a phosphate 

 fertilizer is actually needed. On the other hand, another 

 soil may contain a very small amount of phosphoric acid 

 and yet have it in readily available form. Of course, if 

 a constituent is present in ridiculously small amounts a 

 deficiency in that constituent may be suspected. 



Various methods have been developed for determining 

 available or readily available plant food. Strong hydro- 

 chloric acid has been used more largely than any other 

 reagent to extract those constituents which it is assumed 

 will be most readily available to plants. Most of the soil 

 analyses published have been made by this method. But 

 the results mean very little for interpreting fertilizer de- 

 ficiencies. There is no accurate standard or minimum 

 amount known, below which a fertilizer need is indicated. 



