CHAPTER XXI.--INDIRECT MANURES--GYPSUM, SALT, ETC. Gypsum 462 Mode in which gypsum acts 462 Salt 465 Antiquity of the use of salt 465 Nature of its action 465 Salt not a necessary plant-food 466 Can soda replace potash? 466 Salt of universal occurrence 467 Special sources of salt 468 The action of salt 468 Mechanical action on soils 470 Solvent action 470 Best used in small quantities along with manures 472 Affects quality of crop 472 Rate of application 473 CHAPTER XXII.--THE APPLICATION OF MANURES. Influence of manures in increasing soil-fertility 474 Influence of farmyard manure on the soil 475 Farmyard manure _v._ artificials 476 Farmyard manure not favourable to certain crops 477 Conditions determining the application of artificial manures 477 Nature of the manure 478 Nitrogenous manures 478 Phosphatic manures 480 Potash manures 480 Nature of soil 481 Nature of previous manuring 482 Nature of the crop 483 Amounts of fertilising ingredients removed from the soil by different crops 484 Capacity of crops for assimilating manures 486 Difference in root-systems of different crops 488 Period of growth 489 Variation in composition of crops 490 Absorption of plant-food 490 Fertilising ingredients lodge in the seed 491 Forms in which nitrogen exists in plants 491 Bearing of above on agricultural practice 492 Influence of excessive manuring of crops 492 CHAPTER XXIII.--MANURING OF THE COMMON FARM CROPS.