of the plant. As starch itself is insoluble, it must be converted into sugar before it can be transported. Iron is necessary to the production of chlorophyll. A plant grown in a soil devoid of iron contains no chlorophyll and, therefore, does not possess the power of fixing carbonic acid gas and manufacturing starch. Lime probably performs a number of functions, one of which is to neutralize the poisonous oxalic acid formed in the plant and render it harmless by producing the insoluble calcium oxalate. Of the part played by the other elements practically nothing is known.

Other Ways in Which Plant Food is Lost.—In the first chapter it was suggested that the decrease in fertility of a soil might be due to the fact that the crop removes from it something that is essential to plant growth, and the following paragraphs have been devoted to determining what these essential elements are. Before proceeding to apply the knowledge thus gained, brief mention will be made of two or three ways in which plant food may be lost, other than by removal of the crop:

First, by leaching of the soil, or removal of plant food in the drainage water. For practical purposes nitrogen may be said to be the only element lost in this way. As the nitrogen removed by leaching is all in the form of nitrates, any loss from this cause is extremely unfortunate. The soil has the power of fixing most of the mineral elements, so that only traces of them are lost in the drainage water. The fact that certain mineral fertilizers are fixed by the soil can be shown by a simple experiment. A tall cylinder is filled with soil. and to it is added a quantity of water in which are dis

solved compounds containing nitrate nitrogen, phosphoric acid and potash. If the water that leaches. through this soil is analyzed, it is found that the potash

It is a dangerous practice to allow soils to remain bare and exposed to washing rains. Thousands of acres of good lands have been destroyed in this country in the manner shown in this illustration.

and phosphoric acid have been removed by the soil, but that the nitrogen all remains in the leachings.

Second, by surface washing. In hilly countries this may be a very important factor. As the soil is removed bodily from the surface of the field, it follows that the loss in this case falls on all the food elements. It affects nitrogen and phosphoric acid more than the

other ingredients. Most of the nitrogen is in the organic matter which is near the surface, and, being lighter than the rest of the soil, is more easily washed away. In most soils the first foot contains a larger proportion of phosphoric acid than the subsoil.

Third, by denitrification. This has been referred to in a previous chapter and may be of great moment in a soil that is not properly managed. The conditions. that are desirable in the soil are such as best prevent denitrification, so that the farmer who understands his business need not fear this source of loss.

It is evident that in all these cases the heaviest loss falls on the nitrogen, the most expensive element to supply, and emphasizes a former statement, that the maintenance of fertility is largely a question of an adequate supply of nitrogen.

A Small Part of the Plant Food is Derived from the Soil. Attention is again called to the fact that the atmosphere is the original source of 981⁄2 per cent of the materials found in the green plant; the carbohydrates, fats and fiber being composed of elements supplied in the form of water and carbonic acid gas. These substances are furnished free of cost in humid climates, the supply being practically beyond control, and their use by the plant results in no impoverishment of the land. The subject of practical importance to the farmer is the supply of the other 11⁄2 per cent of the plant, consisting of nitrogen and the ash elements which are derived directly from the solid particles of the soil. It has been shown that seven of these elements are essential to plant growth. Experience has proved that only three of these elements (i. e. nitrogen,

phosphoric acid and potash) are likely to become exhausted, or, in other words, that nothing is gained by adding to the soil any of the other elements of plant food. This is due to the fact that the plant uses nitrogen, phosphoric acid and potash in rather larger quan

What immense quantities of plant food this hill must contain, but who can tell how much of it is available to plants?

tities than the other elements, and that they exist in smaller quantities in the ground, and not because they are any more essential to vegetation. Occasionally soils are found that are actually deficient in lime, but in most cases lime is present in sufficient abundance for the growth of the plant. In this study of the effect of the removal of the crop upon the amount of plant food in the soil, then, it will simplify matters to confine

attention to the three substances nitrogen, phosphoric acid and potash, assuming that all other elements are present in the earth in abundance.

Amount of Fertility Removed by Crops.—The different crops vary greatly in the amount of the three valuable fertilizing ingredients which they contain. The following table gives the amount of nitrogen, phosphoric acid and potash in 1,000 pounds of some of the important crops, the different materials being selected to show something of the range of composition.

Notice the great difference in the amount of fertilizing materials removed in 1,000 pounds of the various crops as shown in the table, especially under nitrogen and potash. For the purpose of this discussion the