588 



Journal of Agriculture, Victoria. [15 Oct., 1919. 



It is estimated that wliere a proper system of drainage exists the soil 

 retains from 10 to 12 per cent, of its weight in water, which is distri- 

 buted through the soil as films surrounding the particles of soil. These 

 may be considered as minimum amounts in contrast to the maximum 

 amount represented by the complete saturation of soil after heavy rain, 

 or other cause. A glance at plate No. 19 will show how moisture is 

 held in the soil. The films of water surrounding the soil particles 

 naturally vary in thickness, and by their contraction or expansion the 

 air movements in the soil are regulated. 



The moisture-holding capacity of the soil is directly increased by 

 proper soil drainage, as will be seen by a reference to plate l^o. 20. 

 For instance, if the depth of soil above the natural water-table is, say, 

 12 or 18 inches, and by laying of drains it is increased to 3 oi 4 feet, the 

 capacity of the soil for holding moisture is increased accordingly. In 

 fact, such increase is really making an underground moisture reservoir 

 for the use of trees when they are in an active growing condition — the 

 time when they are in most need of it. . 



SURFACE SOIL 



Plate 20. — Show^ing depth of -water-tatoie affected by distance 



between drains. 



A, B, and C. — Drains. 1 and L'. — Water table. 



Influence of Drainage upon Soil Temperature. 



It is estimated that well-drained land is from 10 deg. to 12 deg. 

 warmer in winter time than land in need of drainage. The colder 

 condition obtaining in the undrained soil is due to the sunlight falling 

 there being employed in evaporating the water, instead of warming the 

 soil, as in the case of well-drained land, where the surplus water passes 

 off through the soil to the drains below. 



Soil warmth in winter and early spring is beneficial, as it provides 

 suitable conditions for the activities of nitrate forming organisms, which 

 do not become active till a soil temperature of over 50 deg. is reached. 

 As these bacteria build up the nitrogen content in the soil, the earlier 

 the essential warmth is brought about the sooner will the beneficial 

 activities of these organisms commence producing an enriched soil 

 for the betterment of the trees. 



Planning the Drainage System. 



Many different factors will need to be considered in the planning of 

 the drainage system of the orchard according to the nature of the site. 



