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SCIENCE. 



[Vol. XVII. No. 437 



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THE STRUCTDEE AND PHYSICAL PEOPERTIES OF 



SOILS.' 



There is no more important economic problem to-day tlian 

 the production of food and textile fibre to support the life of 

 a rapidly increasing- population and to supply their rapidly 

 increasiug wants in this age of advancing civilization. 

 Agriculture is the basis of all manufactures, trades, and com- 

 merce, and the soil is the basis of all agriculture. This was 

 not generally recognized until Liebig's brilliant generalization 

 of the mineral theory of plant growth, and there was in con- 

 sequence no material advance in agricultural methods or 

 practices until his time. Since then this mineral theory has 

 been the subject of a vast amount of the most patient re- 

 search, carried on in the field, laboratory, and plant house. 

 At first it was only considei'ed necessary to determine the 

 chemical composition of a soil and the composition of a given 

 crop to indicate whether the soil had all the elements of 

 plant food in the relative amount contained in the plant, to 

 show whether the soil was well adapted to the crop, or how 

 it could be made so by the addition of chemical substance*. 

 Then it was found that all soils have sufflcieut plant food 

 for ages to come, and that continued cropping during the 

 lifetime oi a man would not reduce this amount materially. 

 Then it was claimed, and is still held by many, that only a 

 very small part of the plant food in the soil is in a condition 

 to be readily available to plants, and if this available food is 

 not used up it quickly reverts to a rocky and insoluble form. 

 Then it was endeavored, by the use of solvents of various 

 strengths, to determine how much of this plant food is at any 

 time available to plants; and failing in this, the work has 

 been pushed blindly forward with plat e.\-periments, trying 



' Abstract of a paper read by Professor Milton Whitney of the Maryland 

 Agricultural Experiment Station before the University Scientific AsEOciatlon, 

 March 35, 1891. 



all kinds of mixtures of all kinds of fertilizers, on all kinds 

 of soils, indiscriminately, as one might go into a drug store 

 in the dark and blindly try all the drugs to cure dyspepsia, 

 for it is dyspepsia that affects the plant more often than any 

 thing else, — an inability to appropriate and assimilate the 

 food within reach. We are spending vast sums of money 

 for commercial fertilizers, which are used indiscriminately 

 on all classes of soil, whether they be light and sandy, or 

 stiff with clay. 



The physical character of the soil has been considered, in 

 all or nearly all the investigations I have ever seen, a vague, 

 complex, but, on the whole, a relatively unimportant factor. 

 The soil is considered a unit. Soils differ physically, just as 

 men differ physically. There is a type of soil suited to grass, 

 another to wheat, others to the different grades of tobacco, 

 and still others to trucks and vegetables. The whole appear- 

 ance and aspect of the soils differ to the eye and touch. 



It is a notorious fact that changing seasons of wet or dry. 

 or hot or cold, have far more effect on the crops than any 

 combination of manures. This in itself is a significant 

 fact. 



In ten years a soil may be so worn out as to become a 

 barren waste. This is not from any loss of plant food, for 

 the aniount so removed from the soil is relatively so small 

 that it cannot be detected with any certainty. But the fact 

 confronts us, that the wheat and corn lands of the great 

 North-west are deteriorating, and the wheat and tobacco lands 

 of our own State are deteriorating, both as to quality and 

 quantity of product. 



I come now to the main point of this paper, that the ex- 

 haustion of soils is physical rather than chemical; that vege- 

 tation, under given climatic conditions, is dependent upon 

 the circulation or movement of water in the soil, and that it 

 is possible to change the physical conditions of the soil so as 

 to control this water circulation, and so control the growth 

 and development of the plant. Nay, further, that the chief 

 benefit derived from the use of commercial fertilizers and 

 manures is their physical effect on the soil, which modifies 

 the relation of the soil to water, rather than, as heretofore 

 supposed, to the actual amount of food they supply the plant. 

 The soil is to be considered as a vast irrigating pump which 

 provides standing room for the plant and supplies it con- 

 stantly with nutritive fluids. If too much water is supplied 

 the plant is inclined to develop leaf in large excess; if too 

 little water is supplied the growth is stunted, but it puts on 

 relatively more fruit. It'is a mean between them that is 

 desired for all plants, but a different mean for each class of 

 plants. 



The soil is composed of minute fra,gments of rocks and 

 minerals, with varying quantities of organic matter. Even 

 the poorest and most barren soils are shown by chemical 

 analysis to have sufficient plant food for countless genera- 

 tions of plant life, while in ten years or less a soil may be 

 " worn out," and made for a time a barren waste. 



However compact and continuous and close textured a soil 

 or sub-soil looks, there is still about fifty per cent by volume 

 of empty space between the solid particles. That is, a cubic 

 foot of soil will hold half a cubic foot of water, if all the 

 space is filled. Clay soils have more empty space than sandy 

 soils. We have found on the average about forty-five per 

 cent by volume of empty space in sandy soils and fifty-five 

 per cent in clay lands. The amount of empty space in the 

 soil may readily be calculated by dividing the weight of soil 

 by the specific gravity, which gives the actual volume of the 

 soil grains, and subtracting this from the total volume oecu- 



