for }uh. 1<)20 



251 



The Living Soil 



Al'ERTILE soil, especially well provided with 

 hunnis. is teeming with life ; indeed, it must 

 contain these living organisms in order to be 

 fertile and to support plant life. 



The soil, then, is not a lifeless lump of clay. When 

 well tilled, it is full of pores like a sponge ; and when 

 in the best condition for plant growth, these pores 

 ■contain an abundance of air as well as water. 

 We may thus think of the soil as a honey-combed, 

 spongy mass, made up of a hard framework com- 

 posed of bits of mineral matter, the rock-particles, 

 plastered over with a jelly-like substance (the decay- 

 ing organic matter, or "humus"), containing countless 

 billions of bacteria and other germs. 



The great majority of these living organisms are 

 present in the surface soil, where the humus is, and 

 where the tilling of the soil has provided the porous, 

 well-aerated condition necessary for the growth of 

 the beneficial germs. 



Further, their activity is greatest as a rule in late 

 Spring and in Autumn, and lowest in .Summer and 

 Winter. For the Winter cold checks them, as well as 

 the Summer dryness. The warming of the soil in 

 Spring, together with the Spring rains, bringing an 

 abundance of oxygen washed down out of the air, as 

 well as the needful water, apparently cause the great 

 outburst of germ activity in late Spring. The Autumn 

 maximum may be attributed to the effect of the Fall 

 rains coming after the heat and drought of the 

 Summer. 



There are both good and bad soil organisms. Some 

 bring about the decay of plant and animal remains and 

 the consequent liberation of plant food; others do 

 much harm, especially in causing "sick soils." The 

 most of them are microscopic in size ; others can be 

 seen with the naked eye, such as the beneficent earth- 

 worm which burrows through and feeds on the soil 

 humus, letting in air and drawing down leaves and 

 •other debris from the surface. 



Beneficial soil organisms. — These, fortunately for us, 

 ■■are readily controlled and their growth encouraged 

 by certain proper agricultural practices. These bene- 

 ficial organisms, in the first place, need a well-aerated, 

 light, loamy soil for their best developinent. In a 

 heavy, sticky, clay soil, devoid of humus, or a water- 

 logged, sour, mucky soil, they will not grow. But in 

 the soil made sweet by the addition of lime, v.ell- 

 stirred and oxygenated by cultivation, with a plentiful 

 ■supply of decaying vegetable matter (humus), and, 

 finally, with a sufficient amount of mineral nutrients 

 in soluble form, the beneficial soil organisms flourish, 

 and the haimful ones are killed off or at least kept in 

 the backgrfiund. 



The decay of plant and animal remains is brought 

 about by beneficial bacteria and soil fungi ; by this 

 means, finally crtmibling into dust, they again become 

 ■dissolved in the soil water and thus available for plant 

 food. Plant and animal bodies are composed of at 

 least ten elements tmited into complex organic com- 

 pounds: Carbon, Hydrogen, Oxygen, Nitrogen, Sul- 

 phur, Phosphorus. Potassium. Iron, Calcium and 

 Magnesium. Those in italic are the ones which 

 are most apt to need replenishing in soils, and these 

 the farmer often supplies to impoverished soils in the 

 form of "artificial fertilizers." Of course if he would 

 add to his poor soils manures and plow in legumes, the 

 'final decomposition of these organic materials would 

 supply all the necessary elements. Manures, however, 

 ■ are often poor fertilizers, from having been carelessly 



preserved and allowed to leach away and otherwise 

 lose ammonia and other valuable materials. 



Ammonitication is the production of ammonia by 

 bacteria and molds from manures and other decom- 

 posing organic bodies. This is accomplished b\- break- 

 ing-down processes comparable to digestions, per- 

 formed by the enzymes secreted by certain bacteria 

 and soil fungi. 



Nitrification is the breaking down of this ammonia, 

 accomplished by two very important sets of soil 

 liacteria — the nitrite and the nitrate bacteria. One 

 group converts ammonia into nitrites ; another 

 changes the nitrites into nitrates, the only form of 

 nitrogen which green plants can tise directly. 



Fi.ration of nitrogen front the air by bacteria. The 

 air is made up largely, nearly 80 per cent, of nitrogen. 

 If green plants could use this nitrogen directly we 

 would not need to use sodium nitrate from Chile nor to 

 employ other expensive means of nitrogen fertiliza- 

 tion. 



There are, however, two general groups of soil 

 bacteria that have the power of taking the free nitro- 

 gen out of the air and "fixing" it, by changing it to 

 nitrates and possibly other nitrogen compotnids. 



One group constitutes the "legume bacteria" (Pseu- 

 dcuiouas radicicola) of which there are said to be six 

 varieties (the alfalfa — sweet clover, the clover, the 

 vetch — garden pea, the cow-pea, the soybean, and the 

 garden bean bacteria), which grow on the roots of 

 plants of the legume family, in tinj- swellings called 

 "root-tubercles," or "nodules." These will not grow 

 well except in soils which are sweetened with lime and 

 well aerated. Further, the application of gypsum (cal- 

 cium sulphate) to soils seems to have a remarkable 

 stimulating effect on these legume bacteria, causing 

 an increase, sometimes, equal to 100 per cent, and caus- 

 ing the ntunber of root nodules to be greatly increased. 



The second group of nitrogen-fixers live free in the 

 soil, not associated with the roots of plants. These 

 bacteria are often called Azo-bacteria, and the process 

 of nitrogen-fixation Azofication. There are two kinds 

 of these Azofiers: one which lives under conditions of 

 j>oor aeration (the anxrobic species), and one which 

 requires plenty of air (the aerobic species) and called 

 .Izotobacter. 



The harmful organisms of the so-called "sick" soils 

 in greenhouses are thought by Professor Russell, of 

 the famous English Rothamsted Experiment Station, 

 to be protozoa, such as the am-^ba and other forms of 

 minute animal life, which he says apparently destroj- 

 the useful bacteria. 



Also may be classed as harmful, the so-called 

 "(/( nitrifying" bacteria, which tlourisli in water-logged 

 soils from which the air is largely excluded, and which 

 destroy the nitrates which plants require and thus 

 allow the escape of valuable plant food in the form 

 of ammonia and nitrogen gas. 



Some of the soil-decay organisms are the main 

 cause of the acidity of low-lying fields, of swamp and 

 muck lands. In fact, cultivated soils in general tend 

 to become more and more sour, due to the accumula- 

 tion of the more inert humic and other organic acids 

 and the more rapid removal by drainage of the lime 

 and other bases. Or, the use of acid phosphate and 

 similar fertilizers also is said to cause an increase in 

 soil acidity. I-ime (calcium carbonate — ground lime- 

 stone : or calciimi hydroxide — slaked lime) is used for 

 the Durpose of sweetening such acid soils. — H. W. Olive 

 in Brooklvtt Botanic Garden Bulletin. 



