92 GULF TERTIARY OF TEXAS. 



and nitrogen (ammonia) which it has taken into solution in its passage 

 through the atmosphere, and also part of the rain itself. From the earth 

 they take up, in solution, the mineral constituents necessary for their growth, 

 as well as more nitrogen. The organic materials, i. e., the carbon, hydrogen, 

 oxygen, and nitrogen contained in the carbonic acid, water, and ammonia, 

 go to form the main part of the fibre and juices of the plant, while the inor- 

 ganic materials, i. e., the mineral constituents, are associated with them in 

 comparatively very small but yet absolutely indispensable proportions. The 

 mineral constituents vary considerably in different plants, but whatever the 

 mineral requirements of a plant are, they always remain constant. If there 

 is not enough of a certain mineral in the soil, the plant is not content with 

 taking up a smaller percentage of it and growing to its normal size, but it 

 takes up all it can get, uses it at the same rate as the composition of the plant 

 requires, and, when it can get no more, it ceases to grow. The result is a 

 small, deformed, and weak plant; and, in the case of corn, wheat, etc., a 

 small crop of poor, half-sized grain. The atmospheric agencies are continu- 

 ally decomposing the mineral matter locked up in the insoluble part of the 

 soil, and making it available for plant food, thereby constantly replenishing 

 the store of this material. But the process of agriculture generally abstracts 

 mineral food faster than it is supplied. Hence the necessity of returning it 

 to the soil in the shape of marls, manures, or of artificial fertilizers. These 

 minerals are usually potash or soda, phosphate of lime, ammonia, and some- 

 times lime and magnesia. Though the marls of this region contain only 

 small proportions of these ingredients, yet it is sufficient to be of great agri- 

 cultural value. It will be seen by the table of analyses that almost all of 

 them contain potash, soda, phosphoric acid, lime, and magnesia, all of which 

 are ingredients useful to many exhausted soils.* The first three ingredients 

 are of special value, as the supply of them is very apt to run short in East 

 Texas soils, but the supply of lime and magnesia is generally larger, and 

 therefore less apt to need replenishment. The first three of the analyses 

 given below are of altered greensand, such as underlies the iron ore bed of 

 Cherokee, Anderson, and other counties, in the form of the yellow substance 

 known as "yellow sandstone." They are valuable for local use, and their 

 very general distribution makes them particularly accessible. No. 4 is a green 

 clay stratum in the greensand bed of Cherokee County. Besides its alkalies, its 

 forty-nine one-hundredths per cent of phosphoric acid makes it more valuable 



*Much of the soda and potash in these marls is in the form of insoluble silicates, which 

 are not immediately available for plant food. But they gradually decompose under the in- 

 fluence of the atmosphere and the vegetable acids in the soil, and are finally converted into 

 carbonates and taken up into solution. Part of the silica shown in the analyses is also in a 

 soluble form, and is of great service to the growth of plants. 



