12 FUNDAMENTALS OF AGRICULTURE. 



as much phosphoric acid in it as in acid phosphate 

 (raw rock phosphate treated with sulphuric acid), and 

 when used with considerable amounts of organic mat- 

 ter, such as stable or green manure, becomes available 

 rapidly enough to supply crops with this element when 

 the first application is in quantities of a half to three- 

 quarters of a ton per acre. This acid phosphate, how- 

 ever, is more immediately available, and can be used 

 to advantage when organic matter is not available. 



Potash usually exists in the soil in large enough 

 amounts so that when the soil is kept properly sup- 

 plied with organic matter by the decomposition of 

 which carbon dioxide is set free so as to react on the 

 mineral part of the soil containing the potash, this 

 element becomes available. But certain soils, espe- 

 cially sandy soils, are often deficient in this element 

 in which case it must be supplied as a fertilizer. From 

 50 to 100 pounds of potash is a good application. 



EXERCISE. Does a soil in good physical condition or one in poor 

 physical condition have the greater percentage of pore space? Why? 

 What causes humus to be black in color? What effect will this have 

 upon soil temperature? May green manuring ever be detrimental? 

 Make a list of the leguminous plants in your vicinity. From the 

 results in Delaware Bulletin No. 60, calculate the amount of nitrogen 

 left in the soil when 3 tons of cowpea hay were removed. Calculate 

 the number of acres in your county. What per cent, is subject to 

 erosion? Name the states drained by the Mississippi River and 

 tributaries. How many square miles in the above area? The Missis- 

 sippi River deposits 3,702,758,400 cu. ft. of solid material in the Gulf 

 of Mexico annually. If the weight of a cubic foot of soil averages 80 

 pounds, how many tons does the Mississippi River deposit annually? 

 The area of the surface of an acre is 43,560 sq. ft. How many acres 

 of soil one foot deep will be formed at the mouths of the Missis- 

 sippi River every year? 



SECTION IV. SOIL PHYSICS. 



In addition to the influence which the soil has on 

 the growth of plants on account of the chemical ele- 

 ments which it furnishes, it greatly affects the growth 

 of plants through physical factors. Among these are 

 included the water supply of the plant, conditions with 



