5° 



GARDENERS' MAGAZINE 



H 



its Relation to Soil F 



The term humus is applied to a large class of compounds derived from the decay 

 of former animal and plant life. The animal and vegetable materials (organic 

 matter) undergo decomposition in the soil, the final result of which is the dis- 

 appearance of these substances, leaving only a few gases and a small amount of 

 mineral matter. When the organic matter is in its intermediate stages of de- 

 composition, and mixed with the soil, it is known as humus. Opinion as to the 

 fertilising value of humus has swung, pendulum-like, from one extreme to the 

 other. The alchemists taught that the spirits left the decaying animal and vege- 

 table matters and entered plants. By many of the earlier chemists, humus was 

 considered as supplying the larger part of the materials necessary for the develop- 

 ment of the crop, but when the combined labours of De Saussure, Boussingault, 

 I >umas, and Liebig demonstrated that the air supplied plants most of their food, 

 particularly that part which was supposed to come from humus, scientists, as a 

 rule, assigned a low value to humus. From the very earliest times, however, 

 farmers have assigned a very high value to humus as a factor of soil fertility, and 

 this belief was strengthened by the observed facts that soils rich in humus were, 

 as a rule, highly productive, and that such materials as animal excrement or barn- 

 yard manure, which supplied the soil with an abundance of humus, possessed a 

 marked fertilising power. Although many 'of the old theories which were supposed 

 to account for the value of humus are no longer tenable, recent experiments have 

 shown that there are sound scientific reasons for ascribing to humus a high value 

 as a factor of soil fertility, and have demonstrated that " farmers are wholly right 

 in attaching great importance to the preservation of humus in their soils." As the 

 account by Prof. H. Snyder, B. Sc., Minnesota University, given in the Year 

 Book of the U. S. Department of Agriculture will show, humus performs a number 

 of different functions in the soil which are of the highest importance in crop pro- 

 duction. It influences the temperature, tilth, permeability, absorptive power, 

 weight, and colour of soils, and directly or indirectly controls to a high degree 

 their supply of water, nitrogen, phosphoric acid, and potash. 



Loss of Soil Humus and Decline in Fertility. 



A virgin soil or one recently cleared may show a high state of productiveness 

 for a number of years after it is brought under cultivation. Gradually, however, 

 a decline in fertility is observed, which is slight at first, but more marked after a 

 lapse of fifteen or twenty years. Experiments have shown that the decline in 

 fertility is not entirely a result of the removal from the soil of the essential fertilis- 

 ing elements— nitrogen, phosphoric acid, potash, or lime— but is due in many 

 cases to getting the land out of condition through a loss of humus. Experiments 

 conducted by the Minnesota Agricultural Experiment Station on different types 

 of soils worn by continuous grain cropping have shown that when a fertiliser was 

 used containing nitrogen, phosphoric acid, potash, or lime, or when any one of 

 these materials was applied alone, there was " in no case an increase of over three 

 bushels per acre of wheat and two of flax. * * * With soils that have been 

 cropped for twenty years, the largest increase was four bushels per acre." The 

 difference between the grain-producing power of new soils and of worn soils of the 

 same original character was about fifteen bushels per acre. These results, as well 

 as many others which could be quoted, make it clear that the decline in fertility of 

 the soils was not entirely due to a loss of the essential elements of fertility, and 

 that we must seek the cause elsewhere. 



The most important difference, physical or chemical, between the composition 

 of old, worn soils and new soils of the same character is in the amount of humus 

 which is presents. That the loss of humus is an important factor in the decline of 

 fertility is also indicated by the fact that with methods of farming in which grasses 

 form an important part in the rotation, especially those that leave a large residue 

 of roots and culms, the decline in productive power is much slower than when 

 crops like wheat, cotton, or potatos, which leave little residue in the soil, are 

 crown continuously Under grass and similar crops the soil humus increases 

 from year to year, while the continuous culture of grain, cotton, or potatos gradu- 

 ally reduces the original stock of humus. Grass and grain crops in rotation result 

 m alternately increasing and decreasing the humus of the soil and keep the land 

 in a higher state of productiveness, although more nitrogen, phosphoric acid, and 

 potash is removed from the soil than when grain, cotton, or corn is raised con- 

 tinuously. In no case, however, do those systems of farming which return humus- 

 formmg materials to the soil reduce the land to so low a state of productiveness as 

 do those systems in which there is a continual loss of humus from the soil. Agri- 

 culturally considered, the two most important points regarding the composition of 

 numus are (i) the presence of nitrogen as a constant constituent, and (2) the 

 chemical union of the humus with " " ' 



humates. 



potash 



Nitrogen in Humus. 



Humus, as ordinarily obtained from the soil, contains from 3 to 12 per cent 

 of nitrogen. According to Professor Hilgard, the soils from arid regions are poor 

 a humus, containing from 1 to 2 per cent., but this humus is correspondingly rich 

 in nitrogen, in many cases containing 14 per cent. In many of the prairie regions 

 tne soil contains about 5 per cent, of humus, and this humus contains about 10 per 

 cent, of nitrogen. Since, therefore, nitrogen is one of the prominent constituents 

 or numus, it is easily understood how a loss of humus has also resulted in a loss of 

 nitrogen. This decline in the nitrogen content of the soil is one of the most 

 serious results of the loss of humus from the soil. A virgin soil containing 4 per 

 cent, of true humus and 0.35 per cent, of nitrogen will after twenty years of grain 

 cropping show about 2-5 per cent, of humus and 0-2 per cent, of nitrogen. In 



en^vT'^ y T S, / herefore ' there has a loss of **5 P er ^nt. of humus, 



equivalent to about 3,500 pounds per acre, and 0*15 to 02 per cent, of nitrogen, which 



l.!r U 'f ! nt t0 to 5>oco P° unds of nitrogen per acre. Since fifty pounds per 

 crons W° 8 f a! * gC c * uantit y for an Y ordinary grain crop to remove, the twenty 

 2 ?m nlf i m Z St ! emoved ni ^e hundred pounds of nitrogen. At least 

 SfSS^^S^^ ^ en ,ost b y the decomposition of the humus, the 



in the drainage waters. For every 



by the 6^^^^*™ « P° unds of *"* *e soil 



are 



2 ^A m S£ TriLri tHe ChangCS that ^dergoes 



nitrates, proaucing iree nitrogen gas, which escaDes in* *u 



is one of the most important natural provisions for renderinV^ ****** 

 of the soil available to plants, and a certain amount of it I i the ^faS 

 growth, but it can be readily seen that under iniudir.W ne <*ssiry to ZJ 

 tion of the soil it may work a positive injury by SSS^SK °' «C 

 nitrogen, or, m case of rich soils, it may supply the ^ ry Wastc of 

 much nitrate and thus produce a rank growth of straw TJPiSS cro P with too 



straw and leaves 

 and 



litroge 



kneficul to 

 frequently 



crop, 



The available nitrogen is thus mcim^Tw^t the mS??* 

 decreased. Exoenments at th* at; 1^ ; . ne tot al mtrop*» l. 



foil 



Station indicated that one year of fallowing caused 



Eiperim- 



— • T 1-1 T J r ""^vvmg caused a gam of o-nn?-, !Z 



available nitrogen and a loss of 0*0114 per cent, of total it • ^ 001 



taining originally 0-1536 per cent, of total nitrogen and 0 S 2 °°* 



it— ~ j -*T1^ * u u (JD02 per cent, of anil. 



able nitrogen. 



— 1 



I 1 J. — _ I 



COfr 



For every pound of ifitfO^w^^ ^^^-5 S* 

 treatment there was a loss of over five pounds of id^^^ ^ 

 summer fallowing is, therefore, only temporarily beneficial at thl - ^ 

 total humus and nitrogen of the soil. When a soil is non S v, Cxpen * of 

 the loss of nitrogen is much smaller, but even then ^idJSa^S^ 

 summer fallowing is a wise practice. In no case should summer fell • T? 

 practised on a new soil. Autumn cultivation keeps the humus anrf „ ! ^ , 

 the soil in better condition than late spring. NLficatioi * « T* rf 

 until quite late in the fall, and in the lonth tS ^^ £? £ 1^ £ 

 year. The change is most rapid near the surface, where there is plenty rf 0 S2 

 from the air. In early fall ploughing the available nitrogen formed from the hE 

 is near the surface, where it does the sprouting seeds and the young crops the SI 

 good. With late spring ploughing this available nitrogen is ploughed under T 

 inert organic nitrogen is brought to the surface. In old soils the process of nirA 

 cation does not go on rapidly enough to furnish available nitrogen to the croo. fc 

 a new soil the process of nitrification is liable to go on too rapidly. Deep Glu - 

 ing and thorough cultivation aid in nitrification. Hence the longer the sofffc 

 cultivated, the deeper and more thorough must be its preparation. Ploughing nn* 

 be done at the right time, preferably in the fall, so as not to interfere with ne* 

 year s water supply. The application of lime and wood ashes aids in the reductioi 

 of nitrogen of humus to available forms and prevents the formation of sour modi 

 Good drainage is also necessary to nitrification in the soil. In water-logged '* 

 the humus does not decompose normally, but peat is produced on account of the 

 absence of oxygen. We thus see that nitrification, although sometimes a serioui 

 source of loss, may be largely controlled by the management of the soil. 



Another source of loss of humus in the prairie and forest regions is the freque* 

 burning over of the land. Soils covered with pine, in which sand largely pre- 

 dominates, frequently lose half or three-quarters their total nitrogen when visited bf 

 forest fires. The sand, being of an open and porous nature, aids in the more 

 complete combustion of the humus. In the timbered regions of the North-wes 

 the great forest fires of 1894 resulted in the average destruction of 1,500 poundiof 

 humus nitrogen per acre, to say nothing of the nitrogen lost in the burning of the 

 timber. Analyses of soils, before and after the fire, made by the Minoearti 

 Agricultural Experiment Station showed a loss in some cases of 2,500 pounds per 

 acre of nitrogen, equivalent to a loss of 75 per cent, of the total amount in the 

 soil. The prairie fires have not been so destructive upon the humus ;is the foctH 

 fires, because the burning has been confined more to the surface. An aver 

 prairie fire, however, will remove more nitrogen from the soil than five Ofduwr 

 crops of wheat. 



Mineral Matter in Humus. 



Besides being a great reservoir of nitrogen, humus is an indirect means « 

 supplying the plants with other feriilising constituents. Humus as it occurs ifl tt» 

 soil is combined with potash, lime, phosphoric acid, and other comp mods whkk 

 are essential as plant food. The decaying animal and vegetable matters km 

 various organic acids, which combine with the potash, lime, iron, and alumina, » 

 well as with other elements, and form a series of compounds known as hunutcv* 

 which but little is definitely known. By some, the potash, line, a ™J*j£ 

 mineral constituents of the humus are regarded as simply associated with W 

 humus and not organically combined with it, but there are a number of ficts wttj 

 indicate that the union is chemical and not simply mechanical. The m««» 

 matter combined with the humus is characteristically rich in phosphoric aadW 

 potash, two compounds which are of great value agriculturally. The m\mm 

 matter combined with the humus from different soil types, however, is not ti^ 

 of the same nature, and the amount of plant food thus combined with nuniM ■» 

 not been extensively investigated. In the case of rich prairie Mitt over i.£ 

 m ' ' 000 pounds of potash per acre to the depUi ol m 



E-ltL- .-.-fi, LknmiK. In the case of soils p* 



foot have been found to be in combination with the humus 



pounds pej 



acre. The average of anatyseTof the mineral matter of the humus from saapk* I 

 product' 



potash 



ictive prairie soils yielding 25 per cent, of humates showed 7.50 pa 

 h and 12.37 per cent of phosphoric acid. In these soils, which * 



•^A mJtwAi l- .... r _ r * r r 1 t :„ ~<*\A npr acre out 01 * 



BBC* 



werewd 



tottiorf 



supplied with humus, 1,500 pounds ot pnospnonc wu y — "7^.^ 

 8,750 was combined with humus, and 1,000 pounds of potash out 01 ^ 

 12,250 pounds. According to Hilgard, the amount of P^ s P h0 ^ a ^l t m * 

 found associated with humus varies from 0.1 to 0.5 of the total ,an i£T 

 soil, indicating in many cases the amount of this element available 10 y 



Value of Humates as Plant Food. ^i*** 



The value of these various forms of humates as plant food has been ^ ^ 

 of extensive investigations and many of these ^P^f 1 ^ 5 ^ v^jabkh* 



ui pianc 100a. At the Minnesota Agricultural «Pc»V iCM \r"S [ wereh^ > " 

 have been successfully grown when the only forms of mineral M y 

 potash, lime, magnesia, iron, and humic phosphate and suip ^ 

 material obtained from rich prairie soil was mixed with pure fg*'^ 

 practically no plant food, and gypsum was added to prevent tnc^ ^ 

 humus. The mixture was watered with teachings from 



o*is 



which might be mentioned are 2 t »M many ° thers sixt y tim <* »ore phosphoric Tdd. ' The only source from * 



Humus furnishes a SS '^SET 12£T&SS? *** *** subsLces was the humates adJed^ to the *tt 



Such changes as nitrifica- 



ally carry on the ^ rf *Ski£* 

 : paied finally produced fcrtUe seeOj ^ 



potash than was_.n *^X#*°£ 



organisms." Thel t common SSS2 tfSH ? , ^ , aCt ^ ty * W= ^STSS wSe om 



chiefly to the combined BoA tfftS Y f°\ % ^ lts J n ' tro g en » » growth, plainly sho^ng That 3 



The nitrifying oreinism fol u ^organisms of nitnhcation and denitrification. Z,ith t\?S„„l , " nU 



genous conshtuents h the . humus ' breaking down its organic 



drainage, and ti^d^fS^T^ - Wtntts Which may be washed out J 



ne acnitnrying o r g ;imsm completes the work by feeding c 



in the 

 by feeding on the 



to use them. There are a number of fact ^ l rcmmu .* tr - . 

 plants are capable of feeding on humates. The roots o. p.-^» ^.^bb 

 of grains, will always be found clustering around any dec*y»» 



potash, Ph^pnonc^. h ^ 

 'the microorganism ^be p . ^ * 

 GM nnctice wh ^icobrly** 



