SOILS AND FERTILIZERS 355 



There are in the United States soils which, so far as known 

 and within historic times, have never been productive, although 

 under climatic conditions favorable to crops. Besides some of the 

 very dry coarse sands and gravel we have 'as examples the- Elm- 

 wood loam of Connecticut, the Conowingo loam of North Carolina, 

 the Monroe silt loam of Louisiana, and the scrub lands of Florida. 

 Other soils are noted for their great and lasting fertility. Examples 

 of this class are the Dunkirk soils of New York, Hagerstown soils 

 of Pennsylvania, Maryland and the Valley of Virginia, and the 

 bluegrass regions of Kentucky, the Marshall soils of Iowa, the Hous- 

 ton soils of Alabama, and the Yazoo soils of the Mississippi Valley. 



Plants in their growth make use of thirteen chemical ele- 

 ments, nine of which they secure directly from the soil. These are 

 called the mineral plant foods; they are phosphorus, potassium, 

 calcium, magnesium, sodium, iron, silicon, chlorin and sulphur. 

 The rock particles are of many kinds, but nearly all kinds contain 

 more or less potassium, calcium, phosphoric acid, etc. Every year 

 the soil water dissolves off a thin surface layer from each particle. 

 Plants appropriate this water and thus secure mineral plant food. 



Many generations of plants have thus been collecting their 

 small toll of food from the soil and storing it up in their tissues. 

 The amount of plant food made ready for plant use during each 

 growing season through the slow solution of the mineral particles 

 of the soil is doubtless supplemented to a considerable degree by 

 the same kinds of material set free from the organic matter also 

 found in the soil that is, the mineral matter originally secured 

 from the dissolved minerals, but built into plants during some 

 former season, may again be used by other plants when the old 

 matter is given an opportunity to decay in the soil. These foods 

 derived directly fronifthe mineral matter of the soil and indirectly 

 from it through the growth, death, decay, and return of former 

 crops are also supplemented, in many cases by the application of 

 mineral matter in the form of commercial fertilizers. 



In addition to the nine elements already mentioned, the grow- 

 ing plant requires four other elements, as follows: Hydrogen, 

 which it secures from water (water is a compound of hydrogen and 

 oxygen) ; oxygen, which it secures partly from water and partly 

 from the air; carbon, which is secured from carbonic-acid gas in 

 the air; and nitrogen. 



Nitrogen is in many respects the most important of all the 

 plant-food elements. It is not found in appreciable quantities in 

 the rock particles of the soil. Ordinary plants depend for their 

 nitrogen entirely on decaying organic matter. As decay proceeds 

 nitrates are formed from the nitrogen contained in organic matter. 

 The nitrates are exceedingly soluble, and unless soon made use of 

 by growing crops are washed out of the soil. Nitrogen is therefore 

 usually the first element to become exhausted in the soil. 



Plants take up these mineral plant foods only in solution in 

 the soil water. In other words the mineral grains in which the 

 bulk of this plant food is locked up must gradually dissolve and 



