THE WORK OF ROOTS 157 



us suppose that the precipitate 1 when it has all settled in the neck of the 

 funnel forms a column one millimeter high. We know that this indi- 

 cates a content of one-tenth of one per cent of phosphorus in the soil 

 examined. If we now examine soil from the garden in the same way and 

 find only one-half a millimeter of precipitate (using the same funnel), it 

 indicates only one-twentieth of one percent, which is poor. Such a soil 

 will be benefited by the application of phosphoric acid in some of the 

 forms previously described. 



LIME. We usually know whether a soil contains much lime by the 

 hardness of the water and the amount of scale it deposits on tea kettles 

 and boilers. If a drop of strong hydrochloric acid produces effervescence 

 (i. e., an appearance of bubbles of gas) when placed on a sample of soil, 

 it indicates the presence of an excess of lime. Take 25 grams of the 

 sand which has been treated with dilute hydrochloric acid as described 

 above, and add to it a quarter of a gram of whiting or marble dust, 

 which will give us a content of one per cent of carbonate of lime in the 

 soil: this should be thoroughly and equally mixed throughout the entire 

 mass of sand. Place 5 grams of this sand in a test-tube; add 4 or 5 cc. 

 of hydrochloric acid (chemically pure) ; heat until it just begins to boil ; 

 add strong ammonia water (chemically pure) until the liquid smells of 

 ammonia; filter while hot and add 5 cc. of a saturated solution of oxa- 

 late of ammonia (made by filling a bottle about one -fourth full of 

 oxalate of ammonia, then filling with water and allowing it to stand until 

 saturated). Transfer to the funnel just described and allow the precipi- 

 tate 2 to settle. If the column of precipitate is two millimeters long we 

 know that a sample of soil treated in the same way which gives a column 

 of precipitate one millimeter long indicates only one-half as much lime, 

 or % per cent, etc. In a sandy soil 1 to 2 per cent of lime is about right ; 

 in a clay soil three -tenths to five-tenths of one per cent is good ; 10 to 15 

 per cent is an excess in any soil. 



ALKALI. To 20 grams of the sand which has been treated with dilute 

 hydrochloric acid add 4 cc. of a solution made by dissolving in 100 cc. 

 of water 1 gram sodium carbonate 3 (washing sodal, 1 gram sodium chlo- 



1 The precipitate is molybdo-phosphate of ammonium. 



2 The precipitate is oxalate of lime. 



^Allowance has beeii made for water of crystallization. 



