328 



ECOLOGICAL FACTORS: 



rived from pre-existing organic matter, but most or- 

 ganic matter is far from usable. When an organism 

 dies, it decays and becomes humus, but even this 

 humus must be further simpUfied chemically before 

 minerals can be extracted by most plants. 



Soil humus has certain peculiarities that are mostly 

 of benefiit to life. It is a source of food when trans- 

 formed to minerals, but its potential nutrients are not 

 limited to minerals. Various saprophytic plants and 

 certain soil animals obtain nourishment directly from 

 humus. Humus also has physical properties that 

 cause it to hold both water and minerals in the soil 

 and to yield these nutrients freely to plants. In addi- 

 tion, humus or organic matter in the uppermost 

 layers of the soil insulates underground vegetative 

 structures against temperature extremes, retards soil 

 compaction by raindrops, and reduces soil erosion by 

 runoff. 



SOIL MOISTURE 



The spaces between soil particles, 40-60 per cent 

 of most soil volumes, contain soil moisture and gases, 

 but the actual amount of water in a given soil is re- 

 lated to many factors. Basically, soil moisture reflects 

 the amount of precipitation in a locality; however, 

 equal amounts of precipitation over a widespread 

 area do not necessarily result in homogeneous soil 

 moisture. Possible variables modifying water content 

 are evaporation rate before precipitation strikes the 

 ground, vegetation "trapping" moisture prior to its 

 infiltrating the soil, slope or frozen ground increasing 

 runoff, greater soil porosity or dryness increasing in- 

 take, topographic depressions collecting water, and 

 greater soil water evaporation because of coarse soil 

 texture (all factors in the hydrologic cycle). In addi- 

 tion, water is lost by being chemically "tied" to soil 

 compounds (adsorption) and by penetrating below 

 the level where it is available to plants (percolation). 

 These various relationships are summarized by an 

 equation of water availability. The water available to 

 a plant equals the total precipitation plus condensa- 

 tion of water minus that portion lost by air evapora- 

 tion, interception, runoff, absorption, collection, soil 

 evaporation, adsorption, and percolation. (Figure 

 17.4). 



Soil moisture is the main source of plant water, the 

 most critical need of these organisms. Plants require 

 water to take in, transport, and expel materials and to 

 carry on photosynthesis and transpiration. Also, 



plants must have moisture to maintain cell turgidity, 

 because less than normal water causes wilting. If 

 sufficient water is lost, permanent wilting and often 

 death result. 



SOIL REACTION 



Soil reactions may be acid, neutral, or alkaline. 

 Reactions are said to be acid when there are more 

 hydrogen ions (acid producing) than hydroxyl ions 

 (alkali producing) in a soil solution. The reaction is 

 measured in terms of the negative logarithm of the 

 concentration of hydrogen ions and is called the pH. 

 All that must be known about this is that there are 

 three possible reactions, acid, neutral, and alkaline, 

 and that a pH of 7.0 is neutral, values below 7.0 are 

 progressively more acid, and values above 7.0 are pro- 

 gressively more alkaline. The pH values for most soils 

 range from 4.0 to 9.0 and the extremes of pH are 

 and 14. 



Soil acids and alkalis form in many ways. Acids 

 usually are derived from acidic igneous rocks, soil 

 organisms, and many organic remains. Alkalis usu- 

 ally come from decomposition of limestone or basic 

 igneous rocks and evaporation of water from drainage 

 basins such as semidesert or desert lakes. 



Soil reaction is related to the availability of soil 

 minerals to plants. Most minerals are available at a 

 pH of 6.5, a slightly acid condition. At a pH much 

 below or above this figure some minerals may become 

 poisonous; at a pH of 4.0, hydrogen ions and at a 

 pH of 9.0, hydroxyl ions become injurious to plant 

 tissues. 



To summarize, slightly alkaline or acid conditions 

 seem to have little effect on many animals and (prob- 

 ably) some plants. However, plants in general appear 

 to be more restricted by the reaction of the soil in 

 which they live. In both plants and animals there are 

 species that can live in only a narrow range of alkali 

 or acid near neutrality; but some species actually 

 might prefer one of these media, and a few, such as 

 tapeworms, can withstand wide ranges. 



SALINITY 



Certain plants, called salt plants, live naturally 

 only in places too salty for most life. In some respects 

 calling the plants of saline environments salt plants 

 creates a misconception, because these organisms also 

 are adapted to the poor aeration related to either 

 excess moisture or salinity. Such saline habitats pre- 



