THE SOIL. 17 



while coming between the fine grains of clay it has the 

 opposite tendency by lessening the tenacity with which 

 they are held together so as to decrease the tendency 

 to puddle. Salts which are developed in arid regions 

 are apt to cause this to cement so firmly as to produce 

 hard pan. Moreover hard pan is often produced at 

 the depth at which the plowing runs if the ordinary 

 mold board plow is used on the heavy clay soils always 

 plowing to the same depth. The plowing of such 

 soils rather deeply in the fall, leaving them to be acted 

 on by the frosts of the winter tends to produce the 

 crumb-like condition of good tilth. This effect of 

 freezing and thawing is, of course, more marked in 

 the north than in the south, and the northern soils 

 usually have a better tilth than do those of the south. 

 Careful attention to the tilth of the soil along these 

 lines greatly increases the readiness with which the 

 soil can be penetrated by the roots. This is an im- 

 portant factor in the growth of crops, because unless 

 the roots are able to penetrate the soil thoroughly in 

 all directions they are unable to obtain sufficient mois- 

 ture or the supply of the various chemical substances 

 so necessary for their growth. 



EXERCISE. Make a physical analysis of your soil by putting some 

 in a bottle partially filled with water, shaking thoroughly ; then let 

 the soil settle. By the different sizes calculate the percentage of 

 clay, silt, sand and gravel. Name your soil. Compare the pore space 

 in sand with that in clay. Which will hold the more water and 

 why? Compare a puddled soil with one in good condition, in regard 

 to its water-holding capacity. 



SECTION V. WATER REQUIREMENTS OF CROPS. 



Forms of Water in Soil. We must now examine 

 the soil from the standpoint of the water requirements 

 of the crop. If the neck of a funnel be stoppered, the 

 funnel filled with soil in a granular condition, and then 

 water poured on until the funnel is full, we will have 

 the soil in a saturated condition. The soil allowed to 



