l8 l-EKTILITV AND FKRTILIZKR IIIXTS 



It has been found that sodium and potassium may replace 

 each other, to a Hmited extent, in correcting the acidity that may 

 take place in plants, although they cannot replace each other 

 in supplying plant food. There are some elements which have 

 common functions, but each element has its work to perform for 

 the complete development of plants. 



2. Physical Condition of the Soil. — There are some soils which 

 contain sufficient amounts of available plant food for the needs 

 of crops but this food cannot be utilized because of other fac- 

 tors Avhich affect the physical condition of soils. Some of these 

 factors will be briefly discussed. 



Temperature. — The temperature of the soil depends upon the 

 heat of the air and the nature of the soil. It is a very important 

 consideration in plant growth.* In summer the sunshine causes 

 the upper soil to be warmer than the lower or deeper soil. In 

 winter the deeper soil is warmer than the surface soil. In other 

 words the temperature of the air affects soil warmth. 



The germination of seed, the transference of soil water, which 

 contains the available plant food, the movement of the soil air, 

 the development of organisms are all greater when the soil is 

 warm. The coarser soils seem to warm up more readily than 

 the heavy clays. The location of the land influences soil warmth. 

 A soil with a southern exposure is naturally warmer than one 

 with a northern location.* 



Mechanical Composition.— Should we examine a few different 

 soils we would find that there is a great difference in the size 

 of the particles or grains that make them up. For example, 

 when lumps of dift'erent soils are broken up and passed through 

 sieves of various sizes, or shaken in bottles with water, parti- 

 cles varying in size from gravel to fine dust are apparent. The 

 grains or particles of soil are usually classified into four groups : 

 gravel, sand, silt and clay. Sandy soils predominate in the larg- 

 est particles, gravel and sand ; alluvial or silt soils contain more 

 particles the size of silt, and clay soils have more of the finest 

 particles, clay. It should be understood that all soils contain 

 large and small particles. A loam soil contains all the particles 

 in about equal proportions.* 



