This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Perhaps the most misleading thing about the application of certain manures in experimental gardens is that what is found to yield good results in one particular soil may prove to be quite useless on another soil. If the increased yield in a crop could be really attributed to the application of a certain manure, the experiments would be of immense value. A reference to the article on the "Manuring of Potatoes", in Vol. IV, will, however, convince any cultivator that no real reliance can be placed on a particular manure applied to any soil. It will be noticed from the figures that the very manures which are claimed to yield large crops in one county are the very same that give results even poorer than the soil to which no manure at all has been applied. If it is fair to claim an increase in yield for certain manures in one place, it is equally fair to attribute a decrease in yield to the same manure applied in another locality. And yet all manurial experiments are carried out on this illogical basis. The only reliable thing about the application of special manures is that results are true only for the particular place in which they are obtained.
Another misleading method of applying manures is to assume, first of all, that a certain crop requires certain manures, and must have them at all costs. Take, for instance, such a crop as the Turnip, which is said to take up 112 lb. of nitrogen, 33 lb. phosphoric acid, and 148 lb. of potash from an acre of soil. The uninitiated are apt to come to the conclusion that large supplies of nitrates, phosphates, and potash must be applied to the soil, no matter what its chemical or physical condition may be, whenever Turnips are to be grown.
Some laboratory experimenters, indeed, make a difference between "light" and "heavy" soils, and recommend a variation in quantity of the manures which analysis has indicated as being essential for a certain crop. It is, of course, possible that such recommendations will prove effective in certain cases, but that will be more by accident than design, and will depend upon circumstances.
Before applying special manures to the soil the cultivator has to consider, first of all, the physical nature of his particular soil; the amount of manure, organic or otherwise, he has already given it; the crops grown and harvested from it; and the system of cultivation practised, whether deep or shallow. He must also remember that every cultivated soil, even one that has not been manured for fifty years, like that at Rothamsted, contains almost inexhaustible supplies of certain foods. His main object ought to be to bring as much of this food supply as possible into use by good and deep cultivation, and then to supply the deficiency (if any) by organic or inorganic manures, or by both in certain proportions. This is the wise and economic policy to pursue, and it will pay much better to turn up the soil to a good depth than merely to scrape up a few inches of the crust, which has been perhaps cropped over and over again for years until it has become either sick or exhausted.
In business the grower must pay from £4 to £20 and more per ton for special manures to supply food which probably exists in abundance in his own soil if he would only liberate it. A ton of special artificial manure, costing say £20, will dress from 3 to 7 ac. The same amount of money spent in digging the soil 1 ft. deep would bring from 7 to 10 ac. into a fertile condition, or, if double dug, from 3 to 5 ac, with more lasting effects. The insoluble stores of nitrogen, phosphates, potash, and lime that are locked up in it in the dark are likely to be liberated and made available when brought up to the light and exposed to the action of the weather. Indeed, in actual practice it is so, and the man who turns his soil up most frequently and most deeply is the one who reaps the largest and best crops at a minimum of expense.
The point, therefore, for the commercial grower to consider is, which is better - to spend more money on labour and get his plant foods out of the soil, or to spend less money in labour and more in artificials and leave the natural food supplies in the earth untapped for many years?