§ 108. EXPERIMENTS WITH PLANTS AND SOILS. 201 



observed in one part of a field than in another, although 

 the nature of the soil and the mode of treatment are ap- 

 parently the same throughout ; in such a case, much may 

 possibly be learned, by a careful comparison of the amounts 

 of the crops taken from the two parts of the field, and a 

 search for the cause of the difference by a careful exami- 

 nation of the soil. Of course the nature of the subsoil 

 should be ascertained, down to the depth of a metre or 

 so, in order to be sure that the cause of the phenomenon 

 does not lie there, perhaps in some accumulation of water, 

 or a great difference in the mechanical or physical charac- 

 ters of this subsoil. 



Actual experiments with manures and growing plants, 

 to be combined with soil analysis, are best made in boxes 

 of soil that has been carefully sifted and mixed ; a perfect 

 sample of such a soil can be obtained without difficulty. 



The wooden boxes to contain the soil for these experi- 

 ments may be made about one metre deep, and half a 

 metre square, and with several holes through the bottom. 

 They should be set in the ground in a grass plot, so that 

 they will project but 3-5 cm. above the surface. The 

 soil with which they are to be filled should be pulverized 

 when in a very moderately moist condition, by rubbing 

 it between the hands, or with a wooden pestle in a porce- 

 lain mortar, and passed through a sieve with meshes 6-8 

 mm. wide ; an ample quantity of it should be provided, 

 so that there may be enough for the analysis and for all 

 possible contingencies, besides what is required to fill up 

 the box. 



To fill the box, put a layer of gravel 5-8 cm. thick 

 over the bottom, and then add the soil, pressing it down 

 gently, as it is put in in small portions at a time ; then 

 pour a quantity of rain-water over the soil, equal to 

 about half that which it can retain in its pores ; stir up 

 the surface and fill with more soil, up to the edge of the 

 box, and, if possible, before sowing the seed, or letting 

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