MEASURING PLANT FOOD REQUIREMENTS 153 



phosphorus content, and one-fourth of 1 per cent of the 

 potash content of the soil generally can be made available 

 in one year. If we have an analysis of the soil, we can 

 readily calculate the number of pounds of each element of 

 plant food that would become available, and if the com- 

 position of the crop to be grown is known, the limiting 

 factors of crop raising can be determined with some degree 

 of accuracy, provided that the premises are correct and that 

 unusual seasons do not exercise undue influence. 



110. Methods of Soil Analysis, (a) Collecting and Preparing Samr 

 pies for Analysis. A soil sample is collected by taking fifteen or twenty 

 borings at different and apparently representative places on the soil. 

 The borings should be dried, pulverized if necessary, and thoroughly 

 mixed and rolled on a large piece of wrapping paper or enamel cloth; 

 then by means of a spatula or wooden paddle, quarter the mass into 

 four approximately equal parts, discard two-quarters that are diagonal 

 to each other and continue the mixing, quartering and discarding until 

 the residue amounts to about a pint. This residue should be an accurate 

 sample of the field. A 2-inch auger with a long stem makes a good 

 implement for collecting soil samples. Taken to a depth of 6| inches, 

 an average soil weighs 2,000,000 pounds per acre, and talcing the 

 sample to this depth facilitates calculations. 



After air-drying, the sample is pulverized to pass through a sieve 

 with round holes 1 millimeter or J^ of an inch in diameter. The gravel 

 particles which are too large to pass through are weighed to determine 

 the per cent present and then discarded. The sample is thoroughly 

 mixed and placed in an air-tight container for analysis. The obtain- 

 ing of a sample which fairly represents the soil is of the utmost im- 

 portance, and time and care in this process are necessary. 



(6) Acidity or Alkalinity. Ten grams of soil are shaken with 100 

 cubic centimeters of distilled water in a suitable flask and allowed to 

 stand over night. The liquid is then decanted through a filter paper 

 and 50 cubic centimeters are placed in a beaker, 2 or 3 drops of phenol- 

 phthalein added, the beaker covered with a watch-glass and boiled to 

 a volume of 5 cubic centimeters unless a pink color appears before that 

 degree of concentration. If no color appears the soil is neutral or acid, 

 while if a pink color appears it is evidence that the soil is alkaline. 



A very simple test for the reaction of a soil may be made by plac- 

 ing a strip of blue litmus paper and a strip of red litmus paper in the 

 bottom of a tumbler, adding the soil to be tested to a depth of about 

 1 inch in the tumbler and then moistening the soil with either dis- 



