115 



This absence of oxyo^en in the soil will cause also a cessation of the 

 various processes by which dormant plant food is converted into an. 

 active aud available form ; for many changes due to the action of bac- 

 teria — such as nitrification — are dependent upon the supply of oxygen. 

 'Not only does this filliug up of the air spaces in the soil prevent the 

 entrance of oxygen but also prevents the escips of oarbDuic acid gas 

 and other deleterious substances. Such an und ^sirable state of affairs 

 can be remedied by draining, because as the water drains dowaward, 

 room is made for fresh air to e iter and occupy the spaces between the 

 soil particles. It is for such reasons as these that agriculturists are 

 warned against over irrigation and the cons ^.quent badly ventilated 

 condition of the soil. 



Having considered the first question, viz. : How does the soil get 

 its water ? we can pass on to the consideration of the manner in which 

 the water is held in the soil, a knowledge of which should place the 

 agriculturist at an advantage in his enieavour to coatrol his water 

 supply. As the suitability of a soil for agricultural purposes depends 

 largely upon its power of holding sufficient water to supply the needs 

 of the plant, it becomes one of the chief aim^ of the cultivator to im- 

 prove this power and to prevent unnecessary loss of water. An answer 

 to the second question will be more readily obtained, if it be remem- 

 bered that owing to the irregularity in the shape of the sol particles, 

 there are small interspaces which are filled either with air or with 

 water. When a quantity of soil is completely saturated with water 

 all these interspaces are occupied by water, on the other hand if the 

 soil is perfectly dry these interspaces are fall of air. It is the am )unt 

 of water that a soil is capable of holding when saturated that determines 

 its /pater holding capacity. This capacity can be readily estimate 1 by 

 a simple experiment. 



Take an ordinary glass jar with a fairly narrow neck Knock the 

 bottom out. Fasten up the jar with the neck dow awards, putting a 

 glass underneath to catch the drainage water. Block up the neck of 

 the jar with broken earthenware, covered by a layer of pebbles This 

 will ensure perfect drainage. Fill up the jar carefully and evenly with 

 a known weight — say lib — of dry soil, leaving a space of about 2 inches 

 at the top Pour on a known volume of water^say one pint at a time 

 — atregular intervals, measuring the amount that drains through eaeh 

 time. This amount will gradually increase until the whole volume of 

 water poured on at any one time will pass through. Tne difference 

 between the total volume of water poured on and the total volume of 

 the drainage gives the amount of water that one p )und of dry soil is 

 capable of holding. In this way the water holding capacity of soils of 

 different kinds can be compared , and the value of the methods of in- 

 creasing this power experimentally tested. 



Although the amount of water that a soil can hold when saturated 

 will depend upon the volume of the interspaces, this is not thi factor 

 that determines the amount of water ^^^r/n^/^6^n^/^ retained by a soil. 

 This amount is regulated by fineness of the particles. 



The reason of this will not be apparent. It will be necessary to 

 examine more closely intothe way in which the soil holds its water. The 

 water forms films round each individual soil p ir tide an 1 if the s oilis f a' rly 

 dry the films will necessarily be very thin, getting thicker and thicker 



