378 MISCELLANEOUS FARM SUBJECTS 



sterility. Lime is one of the most variable of soil constituents, exist- 

 ing in all proportions, from mere traces to as high as 30 per cent. 



Magnesia. Resembles lime in many respects, and generally 

 accompanies it, as in dolomite or magnesian limestone, present in 

 all cultivated soils. There is abundant evidence that it is needed for 

 the complete development of our more common cereals. Its exact 

 function as a plant food is unknown. 



Potassium. Found in all soils and results from a decomposition 

 of feldspathic rocks. It is an essential constituent of all plant food. 

 As it occurs in soil it is very soluble, consequently is liable to loss 

 through leaching. Ashes of plants contain potash in the proportion 

 of from 10 per cent to 15 per cent. 



Sodium. Is the basis of common salt and as such is every- 

 where abundant. It resembles potassium, but can in no sense take 

 the place of potassium in plant nutrition, i. e., in land plants. 



Phosphoric Acid. Occurs in all fertile soils, but usually in 

 much smaller amounts than the other important ingredients. Never 

 found free, but always combined either with iron, alumina, or lime. 

 It is one of the soil ingredients most likely to be exhausted. 



Sulphuric Acid. Is one of the constituents of most soils, pres- 

 ent in minute quantities, usually combined with calcium in the form 

 of gypsum. 



Chlorine. Is usually associated with sodium in the form of 

 common salt, which occurs in all soils and in natural waters. While 

 chlorine is not a large constituent of soil, nor does it perform a very 

 important part in the soil's work, yet it is found uniformly in plant 

 ash, and is regarded as being in some way essential. 



PLANT FOOD REQUIRED BY CROPS. 



It should be remembered that of the ten different chemical 

 elements required for the growth of agricultural plants, three come 

 directly from air and water in practically unlimited amounts (ex- 

 cept in time of drouth), and that these three, carbon, hydrogen, 

 and oxygen, constitute, as a rule, about 95 per cent of the mature 

 crop. Nevertheless each of the seven elements obtained from the 

 soil, though aggregating only 5 per cent, is just as necessary to 

 the life and full development of the plant, as are these three. 



The two elements, sulphur and iron, are required by crops in 

 euch small amounts and are present in nearly all soils in such large 

 amounts that the supply rarely if ever becomes depleted, thus 

 narrowing the problem essentially to five elements, constituting 

 not more than 4 per cent of the average crop. 



The productive capacity of practically all soils in good phys- 

 ical condition is measured by the available supply of limestone 

 and the three elements, phosphorus, potassium, and nitrogen, be- 

 cause they are required by all crops in very considerable quantities, 

 while in most soils the supply of one or more of them is limited. 

 If the supply of one of these elements is too limited, it must as a 

 consequence, limit the yield of the crop, even though all other 

 factors essential to crop production are well provided. It is because 



