SOIL FLORAS AND FAUNAS 149 



in the soil water is not high. There are very few plants that can use 

 a 5 per cent salt solution and many plants are killed by concentra- 

 tions much less than this. There are some plants, however, that can 

 endure considerable amounts of salt in the soil. Thus we find that 

 saline regions have very characteristic floras, and salt waters also 

 have floras that are characteristically different from those of fresh 

 waters. INIany marine algse cannot live at all in fresh w^ater. Some 

 fungi, likewise are able to grow in very high concentrations. A 

 mold, for example, has been observed growing in a can of karo syrup. 



INIany elements in addition to those that have been shown to be 

 essential are absorbed to a certain extent by plants and probably 

 each one has some physiological effect upon the plant. Some ele- 

 ments which are almost universally poisonous to plants, such as 

 copper and zinc, are found in the ash of certain plants, and, in fact, 

 growth seems to be stimulated in some plants by the presence of 

 small quantities of these poisonous elements in the soil. 



9G. Soil Humus.— Humus comprises the total organic matter in 

 soils with the exception of the living plants and animals. It is de- 

 rived almost entirely from the decay of plants or plant parts though a 

 small percentage of it is usually derived from the decay of dead 

 animals and from the excreta of living animals. It occurs in the soil 

 in all stages of decomposition and for this reason the proportions of 

 the various chemical elements in it is constantly changing. Usually, 

 however, a little more than half of humus is carbon and about 3 

 to 6 per cent is nitrogen. Humus is the most important source of 

 nitrogen for the majority of plants. When humus is in a late stage 

 of decay it is made up of such small particles that it exhibits the 

 properties of colloids and for this reason the amount of humus pres- 

 ent in a soil is an important factor in determining the water holding 

 capacity. 



All soils that support plants must contain some humus but the 

 amount may vary from less than 1 per cent of the total dry weight 

 of a dry sandy soil to more than 85 per cent of peat or muck. Soils 

 in humid climates are usually well supplied with humus but it is 

 apt to be concentrated in the upper layers of the soil. In an arid 

 region, on the other hand, there is likely to be a much smaller 

 quantity of humus but it is usually more uniformly distributed 

 through the various soil layers. 



97. Soil Floras and Faunas.— The plants and animals that spend 

 all or a part of their lives entirely within the soil make up a group 

 of soil factors of w'hich w'e are in need of much more study. They 



