PRACTICE XXII 



EFFECT OF ORGANIC MATTER ON RETENTION OF 

 CAPILLARY WATER 



Two students may work together, using the same tubes as in 

 Practices XX and XXI. 



Mix the soil and organic matter thoroughly for each tube in 

 a large pan. 



Fill tubes as follows : 



No. 1, sand or other soil. 



No. 2, 95 per cent sand and 5 per cent organic matter. 



No. 3, 90 per cent sand and 10 per cent organic matter. 



No. 4, 80 per cent sand and 20 per cent organic matter. 



No. 5, 60 per cent sand and 40 per cent organic matter. 



The soil and organic matter should be mixed by weight on 

 basis of water-free material. 



The apparent specific gravity of these mixtures may be deter- 

 mined as in Practice XI or calculated from average figures fur- 

 nished by instructor. 



Treat as in Practices XX and XXI. 



Determine the weight of capillary water retained. Express in 

 per cent on water-free basis and in acre inches. 



Find the weight of water held by 1 g. of soil in tube No. 1. 

 Find the weight of water held in the mixtures that was due 

 to soil on this basis and credit the remaining water to the 

 organic matter. 



Divide the weight of water (in grams) retained due to the 

 organic matter, by the weight (in grams) of organic matter. 

 This gives the grams or unit of water held by 1 g. or unit of 

 organic matter. 



On the basis of the mixture of soil and 5 per cent of organic 

 matter, how many tons of organic matter will be required to 

 increase the water-holding capacity of this soil by 1 inch ? 



To secure more data on the effect of organic matter one group 

 of students may use sand and organic matter loose, another 

 group the same mixture compact. All of the stock soils should 

 be treated in the same manner. 



Each student should secure from members of the class this 

 data for each of the stock soils, loose and compact, and tabulate 

 in his manual and on data sheet for comparison. 



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