Plant Food in the Soil 87 



that it will retain the potash from a combination like chlo- 

 ride of potash while the chlorine may be leached away. 

 That is, the absorptive power of the soil is such that it 

 will rob a complete solution of the mineral elements and 

 keep them till some plant roots need them. 



Lime exists in rocks and in the shells of marine shell- 

 fish as the carbonate of calcium. By burning the rocks 

 or shells we get the oxide, and this, rapidly slaked with 

 water, makes a hydrated Hme which is still quite caustic 

 and active, but rapidly returns on exposure to the air to 

 the more insoluble form of the carbonate. Lime is essen- 

 tial to plant life, but while large percentages of Hme are 

 found in the ashes of plants a comparatively small per- 

 centage is really used as plant food. It is taken in because 

 it abounds in the soil water and is useful for the thickening 

 of the cell walls in woody plants, and is also used by plants 

 to render harmless certain acid that is formed as a sort of 

 excretory or waste matter. This was discussed in Chapter 

 II. Though Hme is essential to plant Hfe there are few 

 soils in which it is not abundantly present for this purpose. 

 Nevertheless fresh appHcations of Hme are often very 

 beneficial. Soils through long cultivation are apt to be- 

 come acid, and many of our crops wiH not thrive in an 

 acid soil. Lime, then, is useful to correct the acidity of 

 the soil and render it slightly alkaHne in its nature. Lime 

 also has a good effect on heavy clay soils, preventing the 

 clay from forming hard masses, by flocculating it into 

 smaU masses that aggregate loosely together. The use of 

 lime on a sandy soil has been spoken of in Chapter I. 

 While lime to a smaU extent is plant food, it differs from 

 the other elements which we have mentioned as plant food 



