SOILS. CROPS AND FERTILIZERS. 



19 



Plant Food — Constituents of Plants 



Combustible 

 Carbon 

 Hydrogen 

 Oxj-gen 

 Nitrogen 

 Sulphur 



Part. 



Essential 



Not so 

 EiSsential 



Possibly • 

 Essential 

 to Some 

 Plallts 



II. 

 Asb. 



Phosphorus 



Potassium 



Calcium 



Magnesium 



Iron 



Sodium 



Silicon 



Chlorine 



Manganese 



Iodine 



Fluorine 



Lithium 



Bromine 



Caesium 



Rubidium 



Copper 



Lead 



Arsenic 



Zinc 



Tellurium 



Barium 



Active 

 Constitu- 

 ents 



Partially 

 Dormant 

 Constitu- 

 ents 



Wholly 

 Dormant 

 Constitu- 

 ents 



Balanced Supply. 



A plant must have each and every 

 one of the different plant foods. Each 

 is essential to aid in some particular 

 function, and no one can be substi- 

 tuted for another. Nature has sup- 

 plied an abundance oi most of the 

 essential foods, but sometimes some 

 of them are lacking or are in an un- 

 available condition. Of all the ele- 

 ments of plant foods which are essen- 

 tial, only a few ai-e often In this 

 condition. These are nitrogen ,H?hos- 

 phoric acid, potash and lime. The 

 term, plant food, as ordinarily used 

 by farmers, includes nitrogen, phos- 

 phoric acid and potash only, not that 

 these are any more essential to plant 

 growth than are the other substances, 

 but because of the deficiency of them 



in many soils, and their correspond- 

 ing commercial importance. 



Nitrogen is an essential fertilizing 

 element. It is the most expensive, 

 and consequently, the most important 

 commercially of the plant foods. It 

 exists in that part of the soil com- 

 posed of organic material. The 

 nitrogen originally all came from the 

 atmosphere. Four-fifths of the air 

 is nitrogen. In the pure state it is a 

 gas, but in the soil it is a constituent 

 of the organic materials. Phosphoric 

 acid does not exist in the soil in a 

 free state. It unites with lime and 

 forms phosphates oi lime. 



Lime and phosphoric acid unite in 

 three different proportions: 1. One 

 part phosphoric acid and one lime. 

 2. One part phosphoric acid and two 



U a Known Facjs ^*2- '^2 



Illustrating the law of minimum. 



lime. 3. One part phosphoric acid 

 and three lime. These are called, re- 

 spectively: I, mono-calcic phosphate; 

 2, di-calcic phosphate, and 3, tri-cal- 

 cic phosphate. It is in the form of 

 these lime combinations that plants 

 obtain their phosphoric acid. Thes« 

 three substances vary in the way In 

 which they dissolve in water, and 

 hence are not equally available as 

 plant food. 



Mono-calcic phosphate dissolves In 

 water as does sugar or salt, and con- 

 sequently, its phosphoric acid Is di- 

 rectly available. 



