30 THE PLANT FOOD SUPPLY 



Conditions for Using Green Manure. — Green manuring can 

 be employed more advantageously under the less intensive methods 

 of vegetable growing, such as truck farming, than in connection 

 with market gardening proper. It is the cheapest method of 

 manuring on low-priced land. At a distance from large cities, 

 where stable manure is difficult to obtain, it furnishes the most 

 available means for maintaining the soil in proper condition for 

 vegetable production. However, near large cities, on high-priced 

 land that is kept in vegetable crops from early spring till late fall, 

 there is no time to grow green manuring crops. It is cheaper to 

 employ the time of teams to haul stable manure from the city than 

 to employ the time of the land in growing green manures (Fig. 13). 



Green manures alone cannot be depended upon to produce as 

 good results as animal manures, for they do not add to the actual 

 supply of phosphorus and potassium in the soil, and unless they 

 are legumes they do not add to the nitrogen supply. So far as 

 the plant food supply is concerned, they merely help make avail- 

 able to the plants the plant food already present in the soil. 



COMMERCIAL FERTILIZERS 



Commercial fertilizers are concentrated forms of plant food 

 that can be purchased in the market. They may be either com- 

 plete or incomplete fertilizers. A complete fertilizer is one which 

 contains all three of the essential elements of plant food: Nitro- 

 gen, phosphorus and potassium. An incomplete fertiUzer is one 

 which contains only one or two of these three elements. 



Commercial fertilizers differ widely in the amount of plant 

 food they contain. They are therefore usually sold on the basis 

 of a guaranteed analysis specifying the percentage they contain 

 of each element of plant food. Different fertilizers are manu- 

 factured to meet the demands of different crops and different 

 soils. The separate ingredients from which complete commercial 

 fertilizers are made are usually cheaper than the complete mix- 

 tures in relation to their plant food content. It is therefore more 

 economical for a gardener to purchase the separate ingredients 

 and mix his own fertilizer than to buy the ordinary commercial 

 brands of ready-mixed fertilizers. He can also make a fertilizer 

 to meet his own specific needs. 



Sources of Nitrogen. — Some of the leading commercial forms 

 in which nitrogen can be bought are nitrate of soda, sulfate 

 of ammonia, dried blood, castor pomace, cotton seed meal. 



