44 The plant food in the soil 



fell off; in spite of the care they received, and the 

 water, warmth and air given them, they looked starved, 

 and that, in fact, is what they really were. Nor did those 

 in the subsoil fare much better. The experiment shows 

 that the top soil gives the plant something that it wants 

 for growth and that it cannot get either from sand or 

 from the subsoil; this something we will call "plant 

 food." 



Further proof is easily obtained. At a clay or gravel 

 pit little or no vegetation is to be seen on the sloping 

 sides or on the level at the bottom, although the surface 

 soil is carrying plants that shed innumerable seeds. 

 A heap of subsoil thrown up from a newly made well, 

 or the excavations of a house, lies bare for a long time. 

 The practical man has long since discovered these facts. 

 A gardener is most particular to keep the top soil on 

 the top, and not to bury it, when he is trenching. In 

 levelling a piece of ground for a cricket pitch or tennis 

 court, it is not enough to lift the turf and make a level 

 surface ; the work has to be done so that at every point 

 there is sufficient depth of top soil in which the grass 

 roots may grow. 



How much plant food is there in the top soil? 

 To answer this question we must compare soil that has 

 been cropped with soil that has been kept fallow, i.e. 

 moist but uncropped. Tip out some of the soil that has 

 been cropped with rye, and examine it. Remove the 

 rye roots, then replace the soil in the pot and sow with 

 mustard; sow also a fallow pot with mustard. Keep 

 both pots properly watered. The soil that has carried 

 a crop is soon seen to be much the poorer of the two. 

 Fig. 22 shows the plants, while their weights in grams 

 were : — 



