[Illustration: Fig. 20. Rye growing in surface soil (Pot 3), subsoil (Pot 4), and sand (Pot 5)] {43} Figs. 20 and 21 are photographs of sets of plants grown in this way; the weights in grains were:-- Green weight After drying Rye Mustard Rye Mustard Plants grown in top soil (Pot 3) 14.5 17.7 5.6 2.6 " " " subsoil (Pot 4) 2.9 5.1 1.6 1.1 " " " sand (Pot 5) 2.0 4.6 0.8 1.0 [Illustration: Fig. 21. Mustard growing in surface soil (Pot 3), subsoil (Pot 4), sand (Pot 5)] The plants in the soil remained green and made steady growth. Those in the sand never showed any signs of getting on, their leaves turned yellow and {44} 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:-- {45} Green weight After drying Mustard growing in soil previously cropped with rye, Pot 1 17.8 62.3 Mustard growing in soil previously uncropped, Pot 2 3.3 8.6 [Illustration: Fig. 22. Mustard growing in surface soil previously cropped with rye (Pot 1) and in surface soil previously uncropped (Pot 2)] {46}