But to return to our experiment with Pots 1 and 2. They had been kept moist before the mustard was sown. Did this moisture have any effect on the soil? Take two of the pots that have been kept dry and uncropped, and two that have been kept moist and uncropped, also one of dry uncropped subsoil and one of moist uncropped subsoil. Sow rye or mustard in each pot and keep them all equally supplied with water. It is soon evident that the top soil is richer in plant food than the subsoil, and the soil stored moist is rather richer than that stored dry, although the difference here is less marked. In an experiment in which the soils were put up early in July and sown at the end of September the weights of crops in grams obtained were:-- {48} Green weight After drying Plants grown in top soil stored in 16.9 2.6 moist condition (Pots 10 & 11) 18.9 2.8 Plants grown in top soil stored in 12.1 1.8 dry condition (Pots 8 & 9) 14.4 1.9 Plants grown in subsoil stored in moist condition (Pot 13) 5.5 0.9 Plants grown in subsoil stored in dry condition (Pot 12) 5.6 0.8 The crops on Pots 10 and 11 ought of course to weigh the same, and so should the crops on Pots 8 and 9. The differences arise from the error of the experiment. In all experimental work, however carefully carried out or however skilful the operator, there is some error. There is clearly an increase in crop as a result of storing the surface soil in a moist condition, showing that additional plant food has been _made_, since these pots were put up. On the other hand it does not appear that much plant food has been made in the subsoil during this time. Further evidence on this point is given by an experiment similar to that in Fig. 22, but where mustard is grown in _subsoil_ kept moist, but uncropped for some time, and in _subsoil_ previously cropped with rye. The results in grams were:-- Green weight After drying Mustard growing in subsoil previously cropped with rye 12.6 2.27 Mustard growing in subsoil previously uncropped 12.9 2.26 {49} These should be compared with the figures on p. 45. Although the subsoil lay fallow for a long time it produced no plant food but is just as poor as the subsoil that has been previously cropped. These observations give us a clue that must be followed up in answering our next question.