Nature of previous Manuring.
A consideration of equal importance is the previous treatment of the soil with manure. For example, where a soil has been liberally treated with farmyard manure, it has been found that mineral manures have a very inferior effect to that obtained by nitrogenous manure. Lawes and Gilbert have found this to be strikingly the case in their experiments on the growth of wheat. In these experiments it was found that the application of mineral manures was accompanied with little or no benefit to the crop, whereas very striking results followed the application of nitrogen. This they attributed to the fact that the supply of mineral fertilisers in the straw of the farmyard manure is largely in excess of the supply of nitrogen. The nature of the action of the manure previously applied is also to be taken into account in determining how long its influence may probably last. Where, for example, the manure has been nitrate of soda or sulphate of ammonia, it may be safely concluded that its direct influence is no longer felt a year after application. The influence of superphosphate of lime, while scarcely so temporary, may be said to last only for a comparatively short time.[242] On the other hand, when the manure applied is of a slow-acting nature, such as bones or basic slag, its influence will probably be felt for a number of years.
Nature of the Crop.
But more important than any of the above-mentioned conditions is the nature of the crop itself. Our knowledge of the requirements of the different farm crops is still very imperfect. A very wide experience, however, of the effect of different manures on different crops, has conclusively proved that their manurial requirements differ very considerably. The subject is complicated by other considerations, such as the nature of the soil, &c.; but notwithstanding this fact, certain points seem to be pretty well established.
In seeking to understand the respective requirements of the different crops for different fertilisers, two important considerations must be borne in mind. These are—(1) the quantities of the three fertilising ingredients—nitrogen, phosphoric acid, and potash—which different crops remove from the soil; and (2) the different power crops possess of assimilating these ingredients.
Amounts of Fertilising Ingredients removed from the Soil by different Crops.
The most convenient way of instituting a comparison between the requirements of the different crops in this respect is by calculating the amount, in pounds, of nitrogen, phosphoric acid, and potash, which average amounts of the different crops remove per acre. The following table shows this for the common crops:—
| Nitrogen. | Phosphoric Acid. | Potash. | ||
| Mangels | Root, 22 tons | 87 | 36.4 | 222.8 |
| Leaf | 51 | 16.5 | 77.9 | |
| Total crop | 138 | 52.9 | 300.7 | |
| Turnips | Root, 17 tons | 63 | 22.4 | 108.6 |
| Leaf | 49 | 10.7 | 108.6 | |
| Total crop | 112 | 33.1 | 148.8 | |
| Beans | Grain, 30 bushels | 77 | 22.8 | 24.3 |
| Straw | 29 | 6.3 | 42.8 | |
| Total crop | 106 | 29.1 | 67.1 | |
| Red clover hay, 2 tons | 102 | 24.9 | 83.4 | |
| Swedes | Root, 14 tons | 70 | 16.9 | 63.3 |
| Leaf | 28 | 4.8 | 16.4 | |
| Total crop | 98 | 21.7 | 79.7 | |
| Oats | Grain, 45 bushels | 38 | 13.0 | 9.1 |
| Straw | 17 | 6.4 | 37.0 | |
| Total crop | 55 | 19.4 | 46.1 | |
| Meadow hay, 1-1/2 ton | 49 | 12.3 | 50.9 | |
| Wheat | Grain, 30 bushels | 33 | 16.0 | 9.8 |
| Straw | 15 | 4.7 | 25.9 | |
| Total crop | 48 | 20.7 | 35.7 | |
| Barley | Grain, 30 bushels | 35 | 16.0 | 9.8 |
| Straw | 13 | 4.7 | 25.9 | |
| Total crop | 48 | 20.7 | 35.7 | |
| Potatoes, 6 tons | 47 | 21.5 | 76.5 | |
| Maize | Grain, 30 bushels | 28 | 10.0 | 6.5 |
| Stalks, &c. | 15 | 8.0 | 29.8 | |
| Total crop | 43 | 18.0 | 363 | |
From the table it will be seen that the crops which remove the largest quantities of all three fertilising ingredients are the root crops—mangels and turnips; that beans remove twice as much nitrogen as the cereals—oats, barley, and wheat—which, in this respect, practically differ very little from one another; while potatoes remove about the same quantity of nitrogen as the cereals. It will further be noticed that the amounts of phosphoric acid removed by the different crops differ very much less than those of nitrogen and potash. Mangels remove slightly more, and turnips slightly less, than double the amount removed by cereals. Meadow-hay, it will be seen, of all crops removes the least phosphoric acid.