36 VEGETABLE GARDENING 



the store of life-giving humus. Legume crops are es- 

 pecially valuble for this purpose. In some districts a 

 green crop is planted as soon as potatoes are taken off 

 and is plowed under as soon as large enough. 



Commercial Fertilizers. By commercial fertilizers is 

 meant those manures that are commonly sold by the trade. 

 When of a high price they are generally of a guaranteed 

 composition, and they should be bought at a valuation 

 based on the amount of nitrogen, potash, and phosphoric 

 acid which they contain in an available condition for 

 plants. The more available the form in which these materials 

 exist in the fertilizer, the more valuable it is generally con- 

 sidered. Most states require a guaranteed analysis to 

 accompany the packages in which the fertilizers are sold, 

 and exercise some supervision over the business. Among 

 the most common commercial fertilizers are the following : 



Tankage. Tankage is a refuse product from slaugh- 

 tering establishments, which after being deprived of its 

 fat is brought to dryness and ground. It is very rich in 

 nitrogen and phosphoric acid, but contains very little 

 potash. Most of the nitrogen and phosphoric acid that 

 it contains is immediately available to the roots of plants. 

 It is probably the cheapest source of nitrogen and phos- 

 phoric acid to be found in the Western states. It varies 

 considerably in composition, and this may sometimes be 

 accounted for by the fact that in some establishments 

 the blood is separated from the other offal, thus reducing 

 the percentage of nitrogen in the tankage. 



Tankage is rather a slow-acting fertilizer. It may safely 

 be used in quantities of less than 1000 pounds per acre if 

 applied broadcast and worked into the soil. Four hundred 

 pounds per acre is generally considered a good application. 

 It may safely be used in these quantities around grow- 



