NUTRIENTS AND SOLUTES. 89 



is best seen in sand and gravel, where the addition of a small amount of 

 hmnus greatly increases the vra,ter-holding capacity. This is due to the 

 minuteness of the particles of humus by which the aggregate surface for holding 

 water is maiteriaUy augmented and partly, perhaps, to a direct power of 

 imbibmg water. The total effect is to decrease loss by percolation and evapora- 

 tion, and at the same time to raise the amotmt of non-available water. In 

 more compact soils it increases the absorption of run-off, and possibly breaks 

 up excessive loss by evaporation in consequence of capillarity. In the stiffest 

 soils it also reduces the echard, correspondingly increasing the amoimt of 

 water available to the plant. Hiunus is also associated with other reactions 

 which affect the holard, such as weathering, preventing erosion, and protecting 

 against evaporation (plate 24 b). 



(11) Reaction by decreasing water-content. — Plants decrease the holard 

 directly only by absorption and transpiration. This is a universal reaction 

 of plant communities, and is often critical in the case of the seedlings of woody 

 plants. It is characteristic of the ecotone between grassland and forest, and 

 plays an important part in the persistence of the grassland subclimax, as 

 in the prairies and plains. It doubtless has a similar effect on the seres of a 

 forest region, but its influence is much less marked. The holard is also 

 diminished as a result of other reactions. This is most striking in the case of 

 the shallowing of the water by plant remains and by the deposition of silt in 

 consequence of the obstruction of vegetation (plate 24 a). 



NUTRIENTS AND SOLUTES. 



The reactions of plants which affect the soil solution are least understood, 

 and hence most debated. The actual existence of some of them is still in 

 controversy, and in but one or two cases has an actual relation to succession 

 been demonstrated. The possible reactions upon the content of the holard are 

 as follows: (1) by adding nutrients or actual food, (2) by decreasing nutrients, 

 (3) by producing acids, and (4) by producing toxins. 



(12) Reaction by adding nutrients or foodstuffs. — ^This reaction is the direct 

 consequence of the armual fall of leaves and the death and decomposition of 

 plants or plant parts. In this way a large supply of mineral salts is returned 

 to the soil, and sooner or later these are freed to enter the soil solution. It 

 seems clear that this process favors plants with a high nutrient requirement, 

 but this may be negligible where there is an abundance of nutrients in the 

 soil. The whole question really hinges upon the relation between the amoimt 

 returned each year and the amoimt already available in the soil. At any 

 rate, we have no convincing evidence that humus plays an efficient r61e in 

 succession apart from its fundamental relation to water-content. Experi- 

 ment only can decide this matter, since nutrients and water are absorbed 

 together and both would necessarily tend in the same direction. Cowles 

 (1911 : 176) has suggested that glucose and other soluble food in the humus 

 may be absorbed by green plants, but as yet there is no direct evidence of 

 such utilization. 



(13) Reaction by decreasing nutrients. — ^The inevitable effect of the absorp- 

 tion and use of solutes by growing plants is to decrease the total supply. 

 Actually, however, this reduction is insignificant in natvu-e, and probably also 

 in cultivation. The amount absorbed each year is a very small part of the 



