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SCIENCE 



[N. S. Vol. LIII. No. 1369 



with closely similar climatic conditions. 

 Earely a native plant shows similar aggres- 

 siveness like ragweed in the north and Eupa- 

 torium capillifolium in the south. Various 

 hypotheses have been advanced to explain the 

 aggressiveness of weeds, but they are simply 

 hypotheses. Certainly attempts to find a cor- 

 relation between weediness and abundance of 

 seeds produced has proved a dismal failure. 

 Nor has any better success been achieved 

 toward understanding the contrasting phe- 

 nomenon of dwindling or " petering-out." 



The distribution of the species of a genus 

 — let us say the oaks of the United States — is 

 an interesting phenomenon. But why is one 

 species circumscribed thus and another de- 

 limited so? To say the species have different 

 adaptations is merely stating the fact in other 

 words. If we are honest, we must admit, I 

 think, our complete ignorance. Now these 

 are samples of a great group of phenomena 

 that confront students of cultivated plants. 

 They must very properly, I think, also be 

 considered problems of ecological botany. 

 But the ecology of the botanists has not thus 

 far developed enough to be an asset to the 

 plant culturist. The details of jKind margins, 

 mountain tops, and seashore strand throw no 

 light on why maize or potatoes or wheat thrive 

 better in some situations than in others. 



Now we come to soils. Surely chemical 

 and botanical science have here rendered 

 signal service to plant culture. Here again it 

 is well to consider primitive plant culture. 

 Undoubtedly our prehistoric ancestors must 

 have observed the greater luxuriance of plants 

 on certain soils, about dung droppings and on 

 landslides. Certain it is that long ago many 

 uncultured tribes had learned to use dung, 

 ashes, fish, leaf mold, seaweeds, and other 

 substances to increase yields. One of the 

 early results achieved by chemists and botan- 

 ists was to determine the chemical elements 

 necessary to plant life. Eventually from this 

 developed the idea that all of these necessary 

 elements were amply abundant everyvrhere 

 except nitrogen, phosphorus and potash. And 

 very naturally substances containing these 

 elements were sought out to use as fertilizers. 



The history of the development leading up to 

 the conclusion stated and its general adoption, 

 was by no means simple. On the agricultural 

 side the great exponent was Liebig. But one 

 can not to-day read Liebig's nimierous works 

 without realizing how much he floundered in 

 the maze of conflicting facts and theories and 

 the many errors into which he was led. In 

 his combatting of the old humus theory of 

 plant nutrition he denied any value whatever 

 to humus except to supply carbon dioxide, but 

 in spite of his teachings the German farmers 

 refused to abandon the use of dung and com- 

 Fwst. Liebig also clung to the idea that 

 nitrogen was of no avail as a fertilizer, as the 

 atmosphere furnished abundant supply. One 

 of the controversies over nitrogen was finally 

 solved by Hebiegel and WiKarth, who estab- 

 lished the fact that legumes by the aid of 

 root nodules were able to utilize atmospheric 

 nitrogen. This is one of the striking land- 

 marks of agricultural science, but it is well 

 to remember that the practical effect of 

 legumes in rotation was well known to the 

 Romans and other ancient pteoples. In Hartes 

 "Husbandry," published in 1764, is written: 



All plaats that bear leguminous flowers (as lu- 

 cerne, saerifoin, trefoils, vetches, etc.) enrioli the 

 ground and of this the husbandman has daily ex- 

 perience in the culture of clover. 



In its final evolution the Liebig theory of 

 soil fertility came to mean that the produc- 

 tivity of a soil was primarily determined by 

 the quantity and availability of the nitrogen, 

 phosphorus and potash which it contained. 

 Indeed many modern writers identified these 

 three substances as fertility. 



Curiously enough, almost any experienced 

 farmer will express an opinion after examina- 

 tion as to the quality of a soil. His standards 

 of measurements are about as follows: Soils 

 decrease in productivity based on correlation 

 with texture in about this sequence — clay 

 loams, loams, silt loams, clays, fine sands, 

 coarse sands, gravel; and in color in about 

 this order — black, brown, red, yellow, gray, 

 white. His judgment is therefore based in 

 part on texture and in part on color. Crude 



