202 SOILS. 



water will bring about this undesirable condition in the dif 

 ferent kinds of soil. 



To determine this point we may either employ the deter- 

 mination of pore space by a comparison of the density of the 

 soil constituents (see chap. 7, p. 107) with the volume weight 

 of the soil; or we may measure directly the amount of water 

 required to fill the pore-space. For the latter purpose it is 

 only necessary to measure the amount of water (conveniently 

 flowing from a graduated pipette) which, rising slowly from 

 below in a U-shaped tube so as to expel all the air before it, 

 is required to fill a definite weight or volume of the soil en- 

 tirely full, so as to rise to its surface. We thus ascertain the 

 amount of empty space existing within the soil, 1 which in the 

 absence of water will ordinarily be filled by air. 



In most cultivated soils, as already stated, the air-space con- 

 stitutes about 25% to 50% of their volume; and this space 

 when filled with water represents what is commonly termed 

 their maximum i^atcr capacity or saturation point. It is of in- 

 terest to know this, because it has been ascertained from ex- 

 perience that in order that plants may reach their best develop- 

 ment, the capillary water present should not amount to more 

 than 60 r /c, or less than 40% of its maximum water-holding 

 capacity ; thus leaving about half the pore-space filled with air. 

 This optimum, however, varies somewhat for different plants, 

 some, like celery, being more tolerant of excess, and others 

 being more tolerant of a deficiency of moisture, as is the, c. g., 

 egg-plant, originally a desert growth. 



Capillary Ascent of Water in Soil Columns. \Yhcn a col- 

 umn of dry soil (e. g., contained in a glass tube closed with 

 muslin at the lower end) is brought in contact with water, the 

 latter is soon seen to ascend in the soil, wetting it and thus 

 changing its color so as to permit of ready observation of its 

 progress. At first the rise is comparatively rapid, in some 

 cases as much as an inch in one minute; but it soon slows 

 down and after a time ranging from a few days to many 



1 Simple as this operation appears to be, it is found to be by no means easy to 

 expel with certainty every small air bubble without resorting to means which 

 would destroy the natural condition of the soil ; such as boiling, or the use of the 

 air-pump. These determinations cannot therefore lay claim to gieat accuracy. 



