The prudent farmer will take measures to prevent the escape of this moisture into the air. All the film-moisture (on the soil particles) needs to be carefully conserved or saved, for the plants will need very large amounts of moisture before they mature, and they can draw their supply only from this film-moisture. We can again apply the lesson learned in the woods. The soil there is always moist; the leaves form a cover, or blanket, which prevents the evaporation of moisture. Underneath an old plank or board, the soil will be found moist. If we can break the connection between the soil and the air, we can check the escape of moisture. A layer of straw over the soil will serve to prevent the loss of moisture; yet a whole field cannot be thus covered. It has been found that the surface soil, if kept loose, say about three inches of the top soil can be made to act as a blanket or covering for the soil underneath. Although this top layer may become as dry as dust, yet it prevents the escape, by evaporation, of moisture from below. It is a matter of common observation that if tracks are made across a freshly cultivated field, the soil where the tracks are will become darker (Fig. 37). This darker appearance of the soil in the foot-marks is due to the moisture which is there rising to the surface. The implement of tillage makes the soil loose, breaking the capillary connection between the lower layers of soil and the surface; thus the upward passage of the water is checked. Where the foot-print is, the soil has been again pressed down at the surface, the particles have been crowded closer together, and capillarity is restored to the surface so that the moisture is free to escape (Fig. 38). In caring for flower-beds, or even in growing plants in a pot in the school-room, it is important that the surface of the soil be kept loose and mellow. Far better in a flower garden is a garden rake than a watering pot. _Experiment No. 8._--To show the importance of the surface mulch, fill several pots with a sandy loam soil, putting the same weight of soil into each pot. In one pot, pack the soil firmly; in another pot, pack the soil firmly and then make the surface loose. These pots of soil may then be put away to dry; by daily weighing each it can be readily determined what effects the various methods of treatment have upon the moisture-holding power of soils. _Experiment No. 9._--The above experiment may be varied by covering the soil in some of the pots with leaves, or straw, or paper, care being taken that the added weight of the foreign matter is properly accounted for. SOIL TEMPERATURE. [Illustration: _Fig. 39. The moss-grown lawn or grass plot._]