330 ECOLOGY 



of humus affect (i) the height to which the water rises 

 and (2) the water-holding capacity of the soil ? Continue 

 the records for several days and plot the results in curves 

 as in Fig. 215. Notice how very slowly water ascends in 

 dry peat in spite of the latter's great capacity for water. 



A similar experiment with dry cotton-grass peat will 

 show that water may rise in the column only 5^ inches in 

 thirty-nine days. The diagram, Fig. 216, shows the 

 amount of water absorbed by the different kinds of soil in 

 this experiment. 



Half-fill another set of tubes with similar soils, but in these 

 cases add 50 c.c. of water from above. Note and mark at 

 short intervals the rates of percolation (Fig. 217) of (1) fine 

 sand, and (2) fine sand mixed with humus. Collect the 

 water that escapes at the lower end of the tubes and notice 

 in each case (1) rate of percolation, (2) time of appearance 

 of the first drop of water from the bottom of the tube, 

 (3) amount of water which escapes in a given time, (4) 

 amount of water held by the soil, observing again the 

 effect of humus on the water-content of a soil. Note that 

 sand is made more coherent and less pervious when mixed 

 with humus. If we consider these experiments and 

 observations in connexion with our previous ones, we 

 are able to understand why dark soil is better for plants 

 than subsoil. 



Make similar observations on cotton-grass peat. Note 

 that it is greasy to the touch. Test samples with litmus 

 paper and determine whether it is acid, alkaline, or neutral. 

 How do peat and humus compare in these respects ? Peat 

 is acid, humus is neutral or alkaline. 



Weigh out 10 grammes of fresh bracken peat, dry it, and 

 determine the water-content ; then burn it and weigh again. 

 How much is lost by burning ? Ten grammes of bracken 

 peat contained 66-2 % water, 17-4 % organic matter, and 

 i6'4 % mineral matter. 



