148 THE CHEMICAL ANALYSIS OP SOILS [chap. 



quart bottle with a disc of copper gauze, 2 inches in 

 diameter resting in the neck, but this may be replaced 

 advantageously by a Buchner funnel 6 inches in 

 diameter. In either case the funnel is connected with 

 an exhaust - pump, the disc is covered with a good 

 filter paper wetted, then at least 500 grams of the soil 

 are packed carefully into the funnel and pressed down a 

 little, care being taken to avoid plastering if the soil is 

 clayey. The soil sample as it comes from the field is 

 spread out, roughly crumbled, and mixed ; from this the 

 500 grams or so are taken and weighed before putting 

 on the funnel. Another portion is weighed out and 

 dried in the steam oven, to ascertain the proportion of 

 water in the sample. 50 cc of hot distilled water are 

 now poured on the soil, allowed to stand a few minutes, 

 and the pump started. When the liquid has been drawn 

 through, successive small portions of hot water are put 

 on, and the pump started afresh; it will be found 

 possible to wash out practically the whole of the nitrate 

 with 100 cc. of water. 



If the liquid shows any tendency to come through 

 the filter turbid, this can be obviated by adding a few 

 drops of sulphuric acid to the water. In the filtered 

 liquid the nitrates may be determined by reducing with 

 the zinc copper couple, distilling off the ammonia and 

 determining it either by Nesslerising or by titration, 

 according to its amount. The couple is prepared by 

 dipping half a dozen strips of thin sheet zinc, 6 inches 

 long by I J broad, successively into dilute caustic soda, 

 very dilute sulphuric acid, and then into a 3 per cent, 

 solution of copper sulphate, in which it is allowed to 

 remain until it has acquired a good black deposit of copper. 

 They are washed by immersion in water, and finally in 

 ammonia -free distilled water, and placed in a bottle 

 with 200 cc. of the soil extract and a crystal of oxalic 



