249 



shells or their debris. The red sand, on the other hand, is a soil of entirely 

 different origin. It contains practically no lime. 



The soil acidity (pH) does not show any great variation, Math the exception 

 of soil No. 10. All the soils, with the exception of the latter, have an alkaline 

 reaction approximating to that of sea water (pH 8'6). In soil No. 10 the effect 

 in the decrease in the amount of salts and lime is seen. The pH value is just 

 on the alkaline side of neutrality. 



Water retaining capacity of the Soil. — It is obviously desirable to have 

 some means of comparing the water retaining capacity of the soils. This can, of 

 course, be expressed by the v/ilting coefficient;(^^) but the method is unsuited to 

 work of a reconnaissance type. 



Hilgard*^^^^ gives a method of obtaining the maximum saturation of a soil 

 sample. We tried this method and found the results approximately the same 

 as those obtained by the method we finally adopted. The results obtained by 

 the following method have been found consistent and are comparable. The 

 bottom was knocked out of a small test tube 15 cm. in diameter and the hole 

 plugged with a small amount of cotton wool. Soil was then poured in to a 

 height of about 8 cms., and the sides of the tube lightly knocked to tamp down 

 the soil. The tubes were then allowed to stand in a Petri dish of distilled water 

 until the water just reached the top of the soil column by capillarity, temperature 

 and humidity remaining constant. Weighings of the tubes and contents, before 

 and after absorption, allow the percentage of water in the soil to be calculated. 



Soil sample No. 4a — Water, 502 per cent. 

 „ „ „ 9 —Water, 46 3 per cent. 



,, ,, ,, 10 — Water, 363 per cent. 



We took these three samples as representative of the three main zones discussed 

 below ; determinations of the water retaining capacity of the other soils were 

 not made. 



VEGETATION. 



The vegetation may most conveniently be considered by a brief description 

 of the different communities crossed in a transect from the coastline, passing 

 inland, to the low sandy upland. The communities show a remarkably regular 

 zonation. 



Avicennia officinalis consociation. 



Mangroves fringe the shore and come inland to the limit of daily tidal scour. 

 The community is a closed one, and Avicennia officinalis is the only species. The 

 ecotone line between the mangrove consociation and the salt swamp plants is 

 very regular, the determining factor being the limit of regular tidal flooding. 

 Mangroves follow the estuaries of drainage channels into the salt swamp area 

 only so far as the tide regularly ascends. 



Arthrocnemum arbnsciila consocies. 



The dominant species in the salt swamp country is Arthrocnemum 

 arbuscula. The bushes have a rounded habit and are 1-1 '5 metres high. They 

 branch freely, and would form an impenetrable thicket, were it not for the fact 

 that the secondary lignified tissue is curiously brittle, owing to the structure of 

 the ground sclerenchyma. The foliage shoots are green and succulent. This 

 was so at the time of our visit ; in spite of the high percentage of soluble 

 salts in the soil (samples Nos. 1 and 2), the plants showed no sign of a lack of 



(10) Blackmail, V. H., "The Wilting Coefficient of the Soil," Journ. of Ecol., ii., p. 43, 1914 



(11) Hilgard, E. W., "Soils," p. 209, 1919. 



