The amount of the ash increases with the age of the plant, though the percentage of ash may vary at different times in the different members of the plant. The following table taken from Vines will give an idea of the amount and composition of the ash in the dry solid of a few plants:
CONTENT OF 1000 PARTS OF DRY SOLID MATTER.
| Clover, in blossom | Wheat, grain | Wheat, straw | Potato tubers | Apples | Peas (the seed) | |
|---|---|---|---|---|---|---|
| Ash | 68.3 | 19.7 | 53.7 | 37.7 | 14.4 | 27.3 |
| Potash. | 21.96 | 6.14 | 7.33 | 22.76 | 5.14 | 11.41 |
| Soda. | 1.39 | 0.44 | 0.74 | 0.99 | 3.76 | 0.26 |
| Lime. | 24.06 | 0.66 | 3.09 | 0.97 | 0.59 | 1.36 |
| Magnesium. | 7.44 | 2.36 | 1.33 | 1.77 | 1.26 | 2.17 |
| Ferric Oxide. | 0.72 | 0.26 | 0.33 | 0.45 | 0.2 | 0.16 |
| Phosphoric Acid. | 6.74 | 9.26 | 2.58 | 6.53 | 1.96 | 9.95 |
| Sulphuric Acid. | 2.06 | 0.07 | 1.32 | 2.45 | 0.88 | 0.95 |
| Silica. | 1.62 | 0.42 | 36.25 | 0.8 | 0.62 | 0.24 |
| Chlorine. | 2.66 | 0.04 | 0.9 | 1.17 | .... | 0.42 |
[CHAPTER IX.]
HOW PLANTS OBTAIN THEIR FOOD. I.
[1. Sources of Plant Food.]
170. The necessary constituents of plant food.—As indicated in [Chapter 3], investigation has taught us the principal constituents of plant food. Some suggestion as to the food substances is derived by a chemical analysis of various plants. In [Chapter 8] it was noted that there are two principal kinds of compounds in plant substances, the organic compounds and the inorganic compounds or mineral substances. The principal elements in the organic compounds are hydrogen, carbon, oxygen and nitrogen. The elements in the inorganic compounds which have been found indispensable to plant growth are calcium,[12] potassium, magnesium, phosphorus, sulphur and iron. ([See paragraphs 54-58], and complete observations on water cultures.) Other elements are found in the ash of plants; and while they are not absolutely necessary for growth, some[13] of them are beneficial in one way or another.
171. The carbohydrates are derived, as we have learned, from the CO₂ of the air, and water in the plant tissue drawn from the soil; though in the case of aquatic plants entirely submerged, all the constituents are absorbed from the surrounding water.
172. Food substances in the soil.—Land plants derive their mineral food from the soil, the soil received the mineral substances from dissolving and disintegrating rocks. Nitrogenous food is chiefly derived from the same source, but under a variety of conditions which will be discussed in later paragraphs, but the nitrogen comes primarily from the air. Some of the mineral substances, those which are soluble as well as some of the nitrogenous substances, are found in solution in the soil. These are absorbed by the plant, as needed, along with water, through the root hairs.