76 MASSACHUSETTS HORTICULTURAL SOCIETY. 



4. Soils vary greatly in their capabilities of supplying food to 

 crops. Different ingredients are deficient in different soils. The 

 chief lack of one may be potash, of another phosphoric acid, of 

 another several ingredients, and so on. 



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 manur- 

 ing ; others, without irrigation, or amendment by draining, tillage, 

 the 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 

 mechanical condition of the soil and rendering its stores of plant 

 food available is often very important. Hence cheap materials, 

 like bone and plaster, are frequenth' 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 



