MATERIALS IN THE SOIL 65 



Capacity of Soil for holding Water. — Soil containing organic 

 materials holds water much more readily than inorganic soil, as 

 a simple experiment shows. If we fill vessels of equal size (such 

 as the one shown in the figure) with gravel, sand, barren soil, rich 

 loam, leaf mold, and pulverized leaves — all dry — then pour equal 

 amounts of water on them and measure all that runs through, the 

 water that has been retained will represent the water 

 supply that plants could draw on from such soil. 



Soil Water a Solution of Mineral Salts. — Water, 

 as it passes through the soil, gradually dissolves 

 very minute portions of the chemical compounds 

 of which the soil is composed, so that soil water 

 is really a dilute solution of mineral salts. 



Capillarity. — Water moves against the force of 

 gravity by means of a physical phenomenon, that 

 of capillar' ity. We know that water will rise against Apparatus for 

 the force of gravity between two closely placed glass +^^?^? ^°^l ^^^^ 

 plates or in a tube of very small bore. This is due 

 to the fact that the particles or molecules of water are attracted to 

 or adhere to the glass. Soil water adheres in the same way to the 

 soil particles, and thus rises in the ground. 



Nitrogen in a Usable Form Necessary for Growth of Plants. — 

 A chemical element needed by the plant to make protoplasm is 

 nitrogen, but this element cannot be taken in an uncombined state 

 from either soil water or air. It is usually obtained from the organic 

 matter in the soil, where it exists with other substances in the form 

 of ni'trates. Ammonia and other organic compounds which con- 

 tain nitrogen are then changed by microscopic plants called 

 bacteria, first into nitrites and then into nitrates, 



A Plant needs Mineral Matter to make Living Matter. — Liv- 

 ing matter (protoplasm) , besides containing the chemical elements 

 carbon, hydrogen, oxygen, and nitrogen, contains very minute 

 proportions of other elements which make up the basis of certain 

 minerals. These are calcium, sulphur, iron, potassium, magne- 

 sium, phosphorus, sodium, and chlorine. 



That plants will not grow well without certain of these mineral 

 substances^ can be proved by the growth of seedlings in a so-called 



1 See Hunter's Laboratory Problems in Civic Biology for list of ingredients. 



