LAND AND OTHER AGENTS OF PRODUCTION 165 



The moisture content of a soil is a function of its structure. A 

 sandy soil soon becomes wet, but dries again rapidly. Its large pores 

 allow rapid percolation of the free water; its relatively small total 

 surface (a consequence of the large size of its particles) holds a pro- 

 portionately small amount of water; it possesses but little colloidal 

 material to absorb and retain water. Addition of easily decomposable 

 organic matter increases the amount of colloid and thus increases the 

 water-holding capacity; addition of clay increases the colloids and 

 the total surface, and also partially blocks up the pores, the last two 

 effects being due to the smallness of the clay particles. Under equal 

 conditions of water supply, clay soils and soils rich in organic matter 

 are, therefore, much moister than sandy soils. 



The whole of the soil water is not generally available for any one 

 plant. Water must be supplied to the plant at least as quickly as it 

 is 'ost by transpiration; otherwise, wilting sets in. Now the rate of 

 supply of soil water is simply the speed at which water can move in 

 the soil, and this depends on the amounts of clay and colloidal matter 

 present; it may easily fall below what is wanted for maintaining 

 equilibrium in the plants growing on soils rich in clay or organic 

 matter. Wilting is so difficult to characterize, and is affected by so 

 many external circumstances, that in any case it affords only a rough 

 method of studying the "availability" of the soil water for the plant. 



Organic matter. The distinguishing characteristic of soil is that 

 it contains part of the complex material synthesized by plants. This 

 material affords energy to numerous micro-organisms, and is gradually 

 converted by them into simple substances appropriate for plant 

 nutrition. In addition, it has important physical effects on the soil. 

 Two great groups are to be carefully distinguished: one furnished by 

 recent generations of plants, the other deposited with soil during its 

 formation and therefore as old as the soil itself. The organic matter 

 furnished by recent vegetation may roughly be classified as: (i) mate- 

 rial that has not yet had time to decompose and still retains its 

 definite cell structure; (2) partially decomposed and still decomposing 

 material; (3) simple soluble decomposition products; (4) plant or 

 animal constituents not decomposable in the soil. 



The partially decomposed material forms a particularly vague 

 and indefinite group, but one extremely important to plant growth. 

 This group possesses at least six properties not shown by the un- 

 decomposed plant residues: 



i. It gives a dark brown or black color to the soil. 



