Afbil 14, 1921 



The Florists^ Review 



27 



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WHY SOILS ARE SICK 



SOIL AND PLANT LIFE. 



What We Are Learning About Plants. 



In my paper read at the recent meet- 

 ing of the Illinois State Florists, pub- 

 lished in The Review March 10, I point- 

 ed out the more important principles 

 underlying the use of fertilizers. Com- 

 mercial fertilizers, when applied to 

 soils, often show a direct effect on the 

 growth of plants and it is unquestion- 

 ably true that there is a direct relation 

 between plant growth and soil fertility 

 as measured by plant food materials. 

 However, plant growth and, hence, pro- 

 duction, are not dependent entirely 

 • upon the chemical composition of soils. 

 Other factors are involved and these 

 often come noticeably into play. 



In my recent paper I emphasized the 

 fact that it is not only necessary to 

 have present, in the soil, certain chem- 

 ical substances in minimum quantities, 

 but that these substances must be in 

 available form. I pointed out also the 

 importance of natural manures. By nat- 

 ural manures I mean not only the usual 

 stable manures, but include in that term 

 all forms of plant material such as 

 cover crops plowed under and also all 

 residues of dead vegetation. 



Manures furnish not only an ideal 

 physical substratum for plant growth, 

 but they contain also food materials, 

 especially nitrogen, in quantity. This 

 nitrogen, however, is largely in an un- 

 available form; that is, in a form that 

 cannot be used directly by plants. The 

 nitrogen in manures must be changed 

 from its organic, unavailable form into 

 an available form. 



Bacteria Busy Workers. 



This change is brought about by 

 means of small living organisms known 

 as bacteria. These forms live in the 

 soil. There are frequently from 30,000,- 

 000 to 100,000,000 of them in one gram 

 of soil. This little army of workers is 

 constantly at work transforming the or- 

 ganic materials into other forms and, 

 curiously enough, there is involved a 

 high degree of specialization of labor. 

 One group of organisms carries out the 

 first step in the process, a second group 

 another step, and still a third carries 

 the process to completion. With 

 the work of this little army of bacteria 

 the growing plant is intimately bound 

 up. In fact, the whole existence of the 

 ])lant is dependent upon it, for it is this 

 little army of workers which transforms 

 manures into nitrates, which are pssen- 

 lial for the growth of the plant. There 

 is a distinct relationship, then, between 

 bacterial activity and the productive- 

 ness of soil. 



Let me impress upon you that the soil 

 bacteria are living forms. As livinj); 

 forms they demand, as do our green 

 plants, certain favorable conditions for 

 growth and for their activities. These 

 little organisms will not thrive in a 

 wet soil. Neither will they thrive in a 



The paper read by Dr. P. A. I>>lienhniier, of 

 tlie division of floriculture, University of Illinois, 

 lit the meetinK of the Chicago Florists' Club 

 April 7. on "What We Are Xx?arning About 

 Plants." 



Dr. P. A. Lehenbauer. 



soil which is acid in reaction, and they, 

 too, have their preferences for a cer- 

 tain degree of warmth. Under unfavor- 

 able conditions not only will they not 

 thrive, but, curiously enough, their ac- 

 tivities are displaced by the work of 

 bacteria which are a detriment. These 

 detrimental forms do not form nitrates; 

 instead, they actually destroy the ni- 

 trates which were manufactured by the 

 useful forms. 



It is quite evident, then, that in grow- 

 ing plants we must keep soil conditions 

 favorable for the nitrogen bacteria. 

 Whatever affects injuriously the nitro- 

 gen bacteria affects, in a similar way, 

 the growth of the plant. The two are 

 intimately related in their activities. 

 When conditions are such that the ni- 

 trate bacteria thrive and multiply, we 

 have reasonable assurance that the or- 

 ganic materials in the soil are being 

 transformed and that the plant receives 

 an adequate supply of nitrogen. 



So-Called Sick Soil. 



Let me call to your attention still an- 

 other factor oi)erating in soils. I men- 

 tioned the group of organisms, the sev- 

 eral forms of which are useful in that 

 they decompose the organic nitrugenous 

 manures in the soil and form available 

 nitrogen compounds, which tJie jilaiit 

 requires. I have stated also that there 

 is a second group which, when condi- 

 tions are unfavorable for the activities 

 of the useful forms, thrive and become 

 harmful because Ihey undo the work 

 of the useful forms. There is still a 

 tliird group which appears to be of 

 great importance. Frequently we meet 

 with a situation in which apjiareiitly 

 the soil contains abundant food mate- 

 rials and these food materials are in 

 available form, and, furthermore, other 

 conditions, such as temperature and 

 moisture, are undoubtedly favorable to 



growth. But, after a period of time, 

 our plants fail to respond under these 

 favorable conditions and productiveness 

 decreases. Such a phenomenon, com- 

 monly known as soil sickness, is prob- 

 ably . of most frequent occurrence in 

 greenhouses. How can we explain this 

 anomalous situation? It is believed, 

 and we have much evidence for our be- 

 lief, that the soil harbors a third group 

 of organisms, which, although they do 

 not affect directly plant growth, are 

 harmful indirectly in that they appar- 

 ently destroy the useful forms, while 

 they themselves multiply rapidly. In 

 other words, so-called sick soils are sick, 

 not because there is a lack of plant food 

 materials, or because there is acidity or 

 possibly even because the temperature 

 or moisture conditions are unfavorable, 

 but because there is an unbalanced 

 condition in the bacterial life of the 

 soil. The useful workers, those which 

 produce available food, are in minority, 

 while the nonproducers, the parasites in 

 the soil world, are in majority. 



This is not fiction. Although many 

 of the details still need solution, the 

 essential facts are based upon scien- 

 tific experiment. To give you all the 

 evidence is not possible, for it is a long 

 story; permit me to say only that if 

 such sick soils are partially sterilized 

 with heat, or if treated with disinfect- 

 ants, as carbon bisulphide or toluene, 

 the productive properties of the soil re- 

 appear. Apparently, this is because the 

 harmful forms are more susceptible to 

 these treatments than are the useful 

 forms and, consequently, on the appli- 

 cation of disinfectants, the parasitic 

 forms are destroyed while the useful 

 workers again multiply and carry on 

 their productive work. 



"Water Cure" for Plants. 



Under natural conditions, that is, un- 

 der conditions outdoors, so-called soil 

 sickness is less conspicuous and prob- 

 ably of less frequent occurrence. This 

 is probably due to the fact that out- 

 doors the soil is subjected to more vari- 

 able conditions, such as changes of tem- 

 perature and changes of moisture con- 

 tent. Outdoor soils are subjected to 

 heavy freezing during the winter 

 months, also to heavy, drenching rains 

 j)eriodieally throughout most of the 

 year, wliicli is not the case in the green- 

 house. Because of the latter fact, the 

 so-ealled "water cure" for sick soils 

 has recently been advocated, based on 

 the assumption that soil sickness is 

 brought about by the accumulation of 

 salts near the soil surface. The latter 

 assumption is not justifiable in all cases, 

 liowever, for soil sickness seems to oc- 

 cur in greenhouses during the winter 

 luoiiths, when soil evaporation is not 

 great and certainly but little concen- 

 tration of the salts near tlie surface oc- 

 curs. However, whatever the underly- 

 ing causes may be, tlie water cure may 

 lie effective in some cases. 



Soil sickness, altliough more conspic- 

 uous and also apparently of more fre- 

 (pient occurrence in greenhouses, is 

 found also outdoors. Cases of soil sick- 



