NATURE OF PLANTS 51 



Let us now consider how the root is related to the soil water. 

 You have seen that the soil particles have an attraction for water. 

 This attraction is due to a force termed surface tension. Each 

 soil particle has a definite surface tension or capacity for retaining 

 water that is dependent upon the extent of its surface. The 

 retention of the hygroscopic water uses up a small portion of the 

 particle's capacity and then it adds capillary water until no more 

 can be retained. We say that the particle is in a saturated state- 

 It is evident that the last water added to the particle will not be 

 held very firmly and it is this water that the root hair can readily 

 absorb. But now follows a very remarkable movement of 

 water. No sooner has a particle lost some of its water than it is 

 able to replenish its supply from adjacent particles that are in a 

 saturated state. This transfer of water from particle to particle 

 goes on for surprising distances, depending upon the amount of 

 the surface tension. So we find particles four to six feet away 

 from the root hair contributing a portion of their capillary water' 

 to replace the water thus absorbed. So you can think of the 

 capillary water moving as films from particle to particle towards; 

 the points of absorption. It is for this reason that the gravita- 

 tional water may be of great value. If it is only a few feet below 

 the absorbing roots it will furnish a constant supply of capillary 

 water to the overlying soil particles. So one of the most im- 

 portant features about a soil is the size of its particles. The 

 term texture refers to this condition of a soil and we speak of fine 

 or coarse textured soils. The finer the particles the greater the 

 surface exposed in a given unit of space; and consequently the 

 greater the surface tension and water-holding capacity. A' 

 cubic foot of coarse sand can not retain as much water as a cubic' 

 foot of clay because the clay particles are 200 times smaller than 

 the sand particles. A fine textured soil may lose some of its 

 capacity to retain water owing to the compacting of its particles 

 into larger ones. It is evident that two separate particles possess 

 more surface area than when combined into one particle. This 

 relation of the particles to one another is termed structure and 

 we see that soil structure is of equal importance with soil texture. 

 Now we can understand the chief advantage of tillage. Plowing ' 



