26 PLANT SUCCESSION AND CROP PRODUCTION 



gion represents a chemical residue, and like the residue on a filter 

 paper, it is frequently washed clean. 



Plants need such small amounts of soluble salts for their de- 

 velopment that the sui-prise is not that they are able to secure these 

 raw materials in most soils, but that there may be found soils in a 

 humid region which do not support abundant plant growth. The 

 pine barrens are apparently an example of this. If a plant cover suf- 

 ficient to accumulate salts, especially nitrates, in excess of what is 

 carried away in solution and by oxidation, could be started on these 

 barrens, there is little doubt but that a rapid cycle of succession 

 would be initiated. 



The mineral portion of a soil is also of importance ecologi- 

 cally because of the fineness of the particles and the nature of their 

 arrangement. These properties are known as the texture and struc- 

 ture of the soil, respectively. The former is a relatively perma- 

 nent property of the soil, that is, the size of the particles is not 

 materially reduced in a short time. Certainly, cultivation and till- 

 age produce no alteration in the size of the soil particles. The tech- 

 nical classification of the soils has been made largely on the basis 

 of the texture. The structure of soils, the arrangement of the par- 

 ticles, is undergoing constant change. Frost, rain, plant roots, and 

 burrowing animals are a few of the agencies which effect a change 

 in the structure of soils. 



The humus portion of the soil is rich in colloids, which are 

 highly retentive of water. For this reason humus soils, or soils in 

 which the humus portion predominates over the mineral portion, 

 are proverbially productive soils. The old evolutionary idea, "all 

 life from pre-existing life," may be translated to the environment 

 when we speak of humus soils, for a soil full of humus contains the 

 accumulated remains of previous generations of plants, and the 

 future generations have this as their heritage- The humus furnishes 

 the energy to bacteria, which in turn leave a newly molded form 

 of energy for green plants. 



The nitrifying bacteria do not add to the total soil nitrogen, 

 however. They are unable to use free nitrogen and live solely upon 

 dead organic material, converting the nitrogen into a form useful 

 for the green plants. Some soil bacteria can draw on the free nitro- 

 gen supply, but the nitrogen thus accumulated must be converted 

 into organic nitrogen compounds before it is available to green 

 plants. This is discussed in the following section on the biotic 

 factors. 



