Soils and fertilizers . and potash is not rhuch morethan two percent of the total weight of the soil, and it wouldbe easy to find analyses that would show much less. Someof the very important substances are present only in tenthsor even hundredths of a percent. The great bulk of thesoil contributes nothing to plant growth other than to furnishmechanical support and to store air and water for the useof roots. 109. Relation of composition to productiveness. — Theproductiveness of a soil is not necessarily directly propor-tiooal to the quantity of plant-food materials that it con- 94 SOILS AND FE
Soils and fertilizers . and potash is not rhuch morethan two percent of the total weight of the soil, and it wouldbe easy to find analyses that would show much less. Someof the very important substances are present only in tenthsor even hundredths of a percent. The great bulk of thesoil contributes nothing to plant growth other than to furnishmechanical support and to store air and water for the useof roots. 109. Relation of composition to productiveness. — Theproductiveness of a soil is not necessarily directly propor-tiooal to the quantity of plant-food materials that it con- 94 SOILS AND FERTILIZERS tains. This is because there are so many conditions, towhich soils are subject, that interfere with the ability ofplants to obtain the nutrients or that, in other ways, inter-fere with plant growth. It is, however, possible for thequantity of some substance required by plants to be so smallthat it is not sufficient to furnish enough nutriment for prof-itable crop production. Probably all of the soils, whose. Fig. 20. — Relative quantities of potash, lime, phosphoric acid and nitro-gen in a sack containing 200 pounds of dry soil, when the percentages presentare respectively , , and analysis is stated in Table 17, would be benefited by theapplication of some fertiKzers, with the possible exceptionof the rich prairie soils. This is not because there is notactually enough plant-food material in the soil, but becauseit is not in a form that is available to plants. 110. Available and unavailable plant-food materials. —The available plant-food materials in soils consist of thoseportions of the total supply that plants are able to securein their growth. We have seen that it is necessary for all PLANT-FOOD MATERIALS IN SOILS 95 substances to be in solution in order that they shall beabsorbed by plants. Soil is not readily soluble. The natu-ral insolubility of soil is modified by various conditionsof the soil itself and by the plants that grow on it. T
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