EXERCISE IX. 

 POTASSIUM, PHOSPHORUS, CALCIUM, AND NITROGEN. 



Supplies for a Laboratory Section of Twelve. One- fourth ounce of potassium metal; wide-mouth bottle; litmus 

 paper: one pound of potassium chloride; twelve test tubes; twelve evaporating dishes; one ounce of yellow stick 

 phosphorus; glass rod about J4"x8"; 500 cc. beaker; one pound of primary calcium phosphate; one ounce of 

 calcium metal; one pound of calcium oxide-lime not slacked; one pound of sodium nitrate; a pair of forceps for 

 handling potassium, phosphorus, and calcium. 



INTRODUCTION. Of about eighty known chemical elements ten only are considered 

 absolutely necessary to plant growth. These ten are hydrogen, oxygen, carbon, nitrogen, 

 iron, sulphur, phosphorus, potassium, calcium, and magnesium. While all ten are essential 

 only a few are apt to be deficient in ordinary soils. The others are so abundant or are needed 

 in such small quantities that the farmer as a rule needs to give no particular care to them. In 

 ordinary field soils nitrogen, potassium, and phosphorus are most apt to be deficient. It is 

 also true that some soils are deficient in lime — a compound containing the element calcium. 

 As the lime grows deficient, the soil becomes acid. Acid soils are unfavorable to the growth 

 of such legumes as alfalfa and red clover. The legume family has the power to take nitrogen 

 directly from the air through the medium of the nodule-forming bacteria found on their roots. 

 Alfalfa and red clover are extremely important legumes and since they cannot be grown in 

 distinctly acid soils it is somewhat difficult to maintain the nitrogen supply of such soils with- 

 out resorting to less satisfactory legumes or artificial fertilizers. 



The ten essential plant food elements, with the possible exception of oxygen, are never 

 used as food material by the plant unless they are combined with one another or non-essential 

 elements. Chemical combinations of elements are called compounds. For example — water is 

 a compound made up of the elements hydrogen and oxygen — potassium nitrate is a compound 

 made up of the elements potassium, nitrogen, and oxygen. 



Part A. Potassium. 



1. (Classroom experiment.) Potassium as an element is very energetic in its action upon 

 other substances and so does not occur free in nature. It is a constituent of many common 

 rocks and minerals, and is therefore rather abundant. Feldspar which occurs both by itself 

 and as a constituent of granite, contains considerable potassium. It is a constituent of nearly 

 all soils. Since potassium as an element is very rapidly oxidized in the air and also will unite 

 with water it must be kept under naptha or some liquid having no oxygen. 



Place a piece of potassium a little smaller than a pea in a wide-mouth bottle about one- 

 fourth full of distilled water or rain water, (a) What action takes place? (b) Test the solu- 

 tion with litmus paper to determine whether it be acid, neutral, or alkaline in action. 



2. Examine a sample of potassium chloride, a compound of potassium commonly used 

 as a fertilizer on soils deficient in potassium, (a) Describe its appearance, (b) Test its 

 solubility, (c) Is it acid, neutral, or alkaline in its action? 



Part B. Calcium. 



3. (Classroom experiment.) Calcium is much like potassium in its action and so for sim- 

 ilar reasons does not occur free in nature. Since it is the principal constituent of limestone 

 and many other rocks the total amount of calcium in the earth's crust is very great. 



Perform an experiment with calcium as you did with potassium. (a) How does its 

 action on water compare with that of potassium? (b) Does the solution have acid, neutral, 

 or alkaline properties? 



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