328 CROP GROWING AND CROP FEEDING 



of changing the solubility and availability of the plant food which 

 may be put upon them in fertilizers. It is well proved that 

 phosphoric acid applied in water-soluble form, becomes, in many 

 soils within a few days or weeks, quite, insoluble in water, and 

 for a considerable time gradually diminishes in availability. Cer- 

 tain soils contain enough phosphoric acid to serve many large crops if it 

 were freely accessible to their roots, but that this phosphoric acid is not imme- 

 diately available is demonstrated by the fact that moderate dressings of plain 

 superphosphate strikingly increase the yield. What has just been stated 

 of phosphoric acid is equally true of potash. As to nitrogen, we know much 

 but not nearly enough of its incomings and outgoings. We know that the 

 soils of forests, meadows and moist pastures gain in nitrogen, while dry, 

 naked or tilled ground loses nitrogen from year to year. We know that 

 clovers and legumes generally rapidly enrich or may enrich the soil they grow 

 upon, as respects nitrogen, while the culture of cereals, root and fibre crops, 

 and garden truck, diminishes and exhausts the soil nitrogen. As a rule 

 in case of soils that have a fair proportion of fine clayey matters, all the phos- 

 phoric acid and potash that may be needed to aid any crop, if once applied 

 cannot escape from the soil and will be retained near the surface, will not 

 in any event descend much below or spread from where it has been placed. 

 With nitrogen it is very different and loss of this element may occur in three 

 ways: First, by leaching out in the drainage water as nitrates; second, by 

 escaping into the air as nitrogen gas, and, third, by conversion into compara- 

 tively inert forms, such as exist in leaf mold, swamp muck and peat, or in the 

 cell tissues of fungi and shells of insects. For this reason, soluble and active 

 and therefore costly^ fertilizers are best applied in small doses, at or near the 

 surface of the ground and at short intervals ; while cheap, insoluble and 

 slowly-acting manures may be used in large applications and deeply mixed in 

 order to establish a more permanent state of fertility. The amount of any 

 needed fertilizer element to be supplied annually, must be learned by experi- 

 ence and experiment, since soils vary greatly in their composition and quali- 

 ties ; and the supply must commonly be several or many times larger than the 

 amount annually taken off in the crop. One fertilizer element that is scarcely 

 noticeable in the export of the peach crop, is nevertheless important to its 

 production. The chief ingredient of the ash of the wood, bark and leaves 

 of all trees is generally lime. The wood of healthy peach twigs of one year's 

 growth contained 1.87 per cent, of ash, of which 54.2 per cent, was lime, 9.5 

 per cent, magnesia, 16.3 per cent potash, 4.3 per cent, phosphoric acid and 6.9 

 per cent, sulphuric acid. The mature leaves of oak and chestnut trees con- 

 tain about 30 per cent, of water, 3 to 4 per cent, of ash, and of the latter 30 



