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GARDENERS' CHRONICLE 



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I A Lesson on The Production of Available Plant | 

 I Food in the Soil I 



i Being One of a Series of Lessons of a Home Study Course on Gardening, Appearing Regularly in The Gardeners' Chronicle 1 



I Under the Direction of ARTHUR SMITH | 



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Diirinj; recent years scientific investigators along the lines of 

 crop production have made soils a prominent feature of their 

 work, and rightly so, since the soil wealth in connection with 

 plant growth is the greatest and the most important form of 

 the wealth of a nation. 



Our knowledge regarding the chemistry, physics, geology and 

 bacteriology of soils is still far from complete, but a great many 

 fundamental principles, of soil fertility are, however, well es- 

 tablished, and the cultivator today has far wider possibilities 

 of intelligently increasing and conserving the fertility of soils 

 than was the case in the days of Pliny, who wrote some 2,000 

 years ago. "The Earth, gentle and indulgent, ever subservient 

 to the wants of man, spreads his walks with flowers and his 

 table with plenty ; returns with interest every good committed 

 to her care, though consiantly teased more and more to furnish 

 the luxuries and the necessities of man." 



There is no e.xact science of the soil, and it lies on the 

 borderland where the chemist, botanist and bacteriologist meet 

 the gardener and the farmer. We propose this month to dis- 

 cuss briefly one phase only of the relationship between the soil 

 and the plant, and to set forth some of the results of investiga- 

 tions which have been made by scientists in the above respective 

 branches of science. 



By long custom gardeners and' farmers give the name "plant 

 food" to those substances in the soil which help the plant to 

 grow, and while the term could be criticized from the stand- 

 point of plant physiology, physiologists have never supplied a 

 term equally as expressive and as simple. At the same time 

 most of us are aware that what are called plant foods are 

 strictly speaking only the raw material out of which — if such 

 material is in an available condition — the plant forms its food 

 by processes infinitely wonderful and complex. Plant food in 

 the soil we shall understand to be those substances — available 

 or unavailable, as the case may be — which a plant requires, and 

 in speaking of the production of available plant food in the soil 

 we refer to the changing inorganic and organic plant materials 

 naturally in the soil in a condition of availability. 



The processes of soil formation have been previously briefly 

 set forth in these lessons and we cannot at this time afiford space 

 to repeat them. In connection with our present subject, however, 

 it may be mentioned that a soil is, roughly speaking, composed of 

 two parts: 



(1) Substances which were part of the original minerals 

 from which it was derived. 



(2) Substances which have come in as the results of vege- 

 tation. 



Earth composed entirely of mineral matter is not soil, the for- 

 mation of the latter is not complete until vegetation has sprung 

 up and died and its remains have mingled with the mirieral 

 fragments and have be.a.un to decay. Further, soils contain a 

 large population comprising various species of bacteria and 

 other micro-organisms which are necessary to the growth of 

 higher plant life, these organisms not being found in merely 

 pulverized rock. It is generally known that the earliest form 

 of vegetation upon the hard rock is those minute plants called 

 Lichens, but how this soil population springs up in the first place 

 wc do not know, but we do know that the production of avail- 

 able plant food in the soil is, in the case of one at least of the 

 plant's necessities, entirely dependent upon it (excepting so far 

 as we may apply plant food to it in an available state, such as 

 nitrogen in the form of nitrate), and indirectly to a greater_ or 

 less degree in the case of most of the plant's other food require- 

 ments also. 



While soils vary in their richness, potential or otherwise; in 

 their physical characters, whether light or heavy, and in their 

 depth; we can, for all . practical purposes, assume that all 

 soils are capable of supporting vegetation— otherwise they are 

 not soils at all — except in the case of vegetation being_ impos- 

 sible on account of an arid climate — and that they contain more 

 or less of all the ten or twelve ingredients necessary for plant 

 growth. As regards depth, ordinary tillage rarely goes below 

 eight inches, although it might in many instances go deeper with 

 advantage. 



An acre of land eight inches deep, calculated on an air-dry 

 basis, weighs upon the average one thousand tons. Most people 



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would consider an application of half a ton of complete fertilizer, 

 or its equivalent in stable manure (not forgetting that stable 

 manure is seventy-five per cent water) an ample dressing each 

 year, although in truck farming and in gardens double this 

 quantity is generally used. Based upon the above weight of an 

 acre of soil an application of one ton of fertilizer gives only two 

 pounds to a ton of soil. Probably few of us have realized what 

 an insignificant and infinitesimal proportion this really is, as, at 

 the rate of a ton of moisture-free fertilizer to the acre we do 

 not supply more than one grain to each pound of soil. It would 

 scarcely appear possible that such a minute quantity, only one 

 part in seven thousand, could have any effect upon a crop, and 

 yet we know that it does have effect. Further in this connection, 

 we must remember that with one ton of high grade complete 

 fertilizer we only apply about five hundred pounds of actual 

 plant food in the forms of nitrogen, phosphoric and potash, 

 which may or may not be all available. From this we have 

 forced upon us the important part which the natural plant food 

 in the soil must play in crop production. 



It has long been known that practically all tillable soils are 

 naturally rich in all plant food elements. The average of the 

 results of fort>--nine analyses made a few years ago of the typical 

 soils of the United States showed per acre for the first eight 

 inches of surface, 2,600 pounds of nitrogen, 4,800 pounds of 

 phosphoric acid, and 13,400 pounds of potash. If all of this poten- 

 tial fertility w-ere rendered available, there is present in such an 

 average soil, in the first eight inches, enough nitrogen to last 

 the average crops for 100 years (without taking into considera- 

 tion nitrogen brought into the soil from the atmosphere by 

 clovers and other legumes), enough phosphoric acid for 500 

 years, and enough potash for 1,000 years. In addition to this 

 there is the unestimated plant food in the earth below the depth 

 of eight inches. 



Most people will at once ask, "Why, in the face of all this 

 plant food in the soil is it necessary to fertilize at all?" 



In the first place we must remember that only a very little of 

 the natural plant food in the soil is in a condition available for 

 the plant. Further, in spite of all the scientific research which has 

 been going on for many years and the wonderful advancement 

 in our knowledge of the relationships between the growing plant 

 and its environments, the above question even today cannot be 

 fully answered. We know that manuring generally increases 

 crops, and we know that, especially in fertilizers of an organic 

 nature, some of their benefits are outside and beyond those de- 

 rived from their actual plant food content; but we also know 

 that there is a great deal which in the present state of our knowl- 

 edge is une.xplainable. At the same time, while giving manuring 

 all the credit it is entitled to, we can by devoting more attention 

 to the rendering of the plant food naturally in the soil available, 

 greatly reduce the cost of crop production without decreasing the 

 crops, by reducing the expenditure for e.xpensive fertilizers. 



It must be borne in mind that a chemical analysis of a soil is 

 no criterion upon which to base an opinion as to its fertility at 

 the time the analysis is made, because soil analyses do not dis- 

 tinguish between what is available and what is unavailable. 

 While there is naturally a difference in the quantities of the 

 various plant constituents soils contain we may for all practical 

 purposes assume that those constituents are present in all tillable 

 soils. A dozen different elements are required by plants as food, 

 but the only ones necessary to trouble about are nitrogen, phos- 

 phorus and potash ; lime is only required as actual food in a 

 ven.- small degree, but it has a greater importance in other 

 directions which will be alluded to later on. 



While every ingredient is important in the sense that plants 

 must have all of them or suffer in their growth, nitrogen gen- 

 erally has the first place on account of its expcnsiveness when 

 purchased in fertilizers ; its tendency to be easily lost from the 

 soil, and to the fact that it only exists in the soil as the result 

 of vegetation, that is to say, that merely pulverized mineral 

 matter practically, never contains nitrogen. 



In a pure and uncombined condition nitrogen forms a large 

 proportion of the atmosphere, but in this state nitrogen is un- 

 available as plant food. To become available it has to undergo 

 in the soil the process known as nitrification. The work of 



