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5. Soils fail to furnish enough food for crops not so much 

 because they have not abundant stores, as because the materials 

 are not in available forms. A soil may have thousands of pounds 

 of phosphoric acid within reach of the plants, but locked up in 

 fragments of rock so that the roots cannot absorb it, and then the 

 crop will fail for lack of phosphoric acid. 



6. The infertility of many soils is due more to their mechanical 

 condition, their texture, and relations to heat and moisture, than 

 to lack of plant food. Such soils want amendment first and 

 manures afterwards. Some soils will give good returns for 

 manuring ; others, without irrigation, or amendment by drainiug, 

 tillage, use of lime, marl, or muck, or otherwise, will not. 



7. The chief use of fertilizers is to supply plant food which 

 crops need and soils fail to furnish. 



8. But the indirect action of fertilizers in improving the me- 

 chanical condition of the soil and rendering its stores of plant food 

 available is often very important. Hence, cheap materials, like 

 bone and plaster, are frequently more profitable than manure or 

 artificial fertilizers. 



9. Plants vary greatly with respect to their demands for food, 

 their capabilities of gathering the ingredients from soil and air, 

 and the effects of different fertilizers upon their growth. Hence, 

 the proper fertilizer in a given case depends upon the crop as well 

 as upon the soil. 



10. The only ingredients of plant food which we need to con- 

 sider in commercial fertilizers are potash, lime, magnesia, phos- 

 phoric acid, sulphuric acid, and nitrogen. Of this list, magnesia 

 is generally abundant even in " worn-out" soils. Sulphuric acid 

 and lime are more often deficient, and hence one reason of the 

 good effect so often observed from the application of lime and 

 plaster. The remaining substances — the phosphoric acid, nitro- 

 gen, and potash — are the most important ingredients of our com- 

 mercial fertilizers, because of both their scarcity in the soil and 

 their high cost. 



11. The chief use of commercial fertilizers, such as guano, 

 phosphates, bone, potash, salts, and special fertilizers prepared 

 by formulae for different crops, is to supply nitrogen, phosphoric 

 acid, and potash. 



12. These materials are expensive, but the right ones in the 

 right places are nevertheless very profitable. But the same fer- 

 tilizers in other cases may bring little or no return. 



13. It is not good economy to pay high prices for materials 

 which our soils themselves ma}' furnish, but it is good economy to 

 supply the lacking ones in the cheapest way. Farmers cannot 



