SOILS AND MANURES 357 



(3) Alternate shallow-rooting crops with deep-rooting CHAP. 



VIII 



crops. 



(4) Where there is danger of loss of plant-food from leach- 



ing of the soil owing to the slope of the land, plan 

 to have a growing crop on the land all the summer. 



(5) Bare summer fallow can be used to advantage if the 



weeds are allowed to grow during the rain) 7 season 

 and are ploughed in before they seed. 



(6) Do not rotate cereals with cereals. 



(7) Plan the rotation so as to have about the same amount 



of forage, hay, and roots each year. 



(8) Unless it is thoroughly rotted, so that the weed-seeds 



which it contains are killed, apply the stable manure 



to the root crop (if one is used in the rotation) or 



to a rank-growing crop like maize. 



312. Organic Matter. — By organic matter is meant matter 



composed of substances that are or have been living organisms, 



in contradistinction to the inorganic matter derived directly 



from rocks, metals, etc. Organic manures include farmyard 



manure and humus, which are valuable sources of plant-food, 



especially nitrogen. Humus and organic matter are not 



synonymous, for humus includes only that part of the organic 



matter which has passed the most active stage of decomposition 



and completely lost the physical structure of the materials 



from which it is made ; it has thus become, as a rule, 



thoroughly incorporated with the soil mass {Hopkins, 5). 



Beside returning plant-food to the soil, organic matter im- 

 proves its mechanical condition ; when it is in the proper state 

 it may materially modify the water-content. Soil which has 

 been manured with stable manure is usually moister than un- 

 manured ground ; this may be for some or all of four 

 reasons : — 



(1) It may absorb more rain-water; 



(2) It may draw up more water from below, by capillary 



action ; 



(3) It may lose less water from the surface by evaporation ; 



or 



(4) It may lose less water by drainage. 



Briefly, then, organic matter improves the texture of the soil, 

 adds to its moisture-retaining power, and furnishes nitrogen. 



