18 BULLETIN 521, U. S. DEPARTMENT OF AGRICULTURE. 
dents have become familiar with the common fertilizing materials 
they should examine the commercial brands to determine, if possible, 
of what they consist, and to determine their value. 
Record and report: Each student should describe each of the fer- 
tilizing materials and state its source and value. After deciding 
upon a certain value or unit for the available plant food contained, 
the market value of each fertilizer sold on the/ local market should 
be determined. 
Note. — In connection with a study of fertilizers in classroom and laboratory 
a number of problems should be assigned in which the students work out the 
value of certain fertilizers when applied to the land. 
Exercise 16. — Effects of Fertilizers upon Plant Growth. 
Purpose: To demonstrate to the students the effect of commercial 
fertilizers on local soils. 
Directions: Secure unproductive soil of the most common type 
in the district. (If more than one type is common, the test may be 
duplicated.) Supply fertilizer to five 8-inch flower pots filled with 
this soil, as follows: (1) Left as a check; (2) add 5 grams of a com- 
plete fertilizer containing from 2 to 3 per cent nitrogen, 8 to 12 per 
cent available phosphoric acid, and 2 to 5 per cent potash; (3) same 
as 2 without the nitrogen ; (4) same as 2 without the phosphoric acid ; 
and (5) same as 2 without the potash. The fertilizer should be 
mixed thoroughly with the soil in the upper half of the pot. Moisten 
the soil with rain water and plant six grains of wheat in each pot. 
Keep moist in a warm, sunny place and note development of the 
plants for at least one month. 
Record and report: Each student should keep a record of the 
growth of the grain in the several pots and make a written report 
of the test in which he makes explanation of the difference in 
growth. 
Note. — If the school owns land or has use of land near the school, a number 
of plats may be used profitably for testing fertilizers sold in the community 
and to demonstrate to the students the effects of fertilizers on the growth of 
various crops. ' 
Exercise 17. — Home Mixing of Fertilizers. 
Purpose: To apply principles relating to the application of fer- 
tilizers and to give practice in their mixing. 
Directions : The value of this exercise will depend to a great ex- 
tent upon the amount of material available and its application to 
local needs. Each student should have an opportunity to partici- 
pate. If the school does not own land upon which commercial fer- 
tilizers are to be applied, it may be possible for the class to do the 
mixing for some patron of the school. The most popular complete 
fertilizer on the local market should be duplicated as far as its 
