MANUAL OF GENEEAL AGEICULTUEE. 27 



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in many parts of California and the West. The most vital 

 factor in California agriculture today is the maintenance 

 of humus. This may be accomplished by crop rotation 

 including in the rotation a legume (pea, bean, clover, al- 

 falfa, etc.,) which has the ability to obtain its supply of 

 nitrogen from the air through the bacteria which this 

 order of plants harbors in its roots. Humus may be di- 

 rectly added to the soil in the form of manures and in 

 green crops plowed under. It is customary to estimate 

 approximately the nitrogen content of soils by the propor- 

 tion of humus present. 



26. DIRECTIONS FOR OBTAINING SOIL SAMPLES. 



Materials: Spade or post-hole augur, sack or board 

 or oilcloth, quart jar. 



From a representative part of the field from which soil 

 is to be analyzed, remove the leaves and twigs from the 

 surface and dig with the spade or bore with the post-hole 

 augur, down to the depth of four feet. Put all spade or 

 augurs-ful of soil on a clean sack or board. Mix all the soil 

 thus taken out, thoroughly on the sack or board, and keep 

 about a quart of this mixed soil, which will represent an 

 average of four feet in the field. 



To obtain a more representative sample, several sam- 

 ples may be taken in the same way from different places 

 and then a quart from all of them saved. 



PREPARATION OF REAGENTS FOR SOIL ANALYSIS. 



(The following should be prepared by the teacher or some 

 trusted pupil and are enough for an entire class.) 



Ten per cent solution of caustic potash. Dissolve 20 

 grams of solid caustic potash (potassium hydroxid) in 200 

 c.c. of water. 



Dilute hydrochloric acid. Dilute two quarts of the 

 commercial acid by pouring the acid into eight quarts of 

 water. 



One-half per cent solution of phosphoric acid. Dis- 



