68 VEGETABLE FORCING 
commercial fertilizer. It contains about 15 per cent of 
nitrogen. The salt dissolves quickly in the moisture of 
the soil, when it immediately becomes available to plants. 
This is usually the cheapest form of nitrogen. 
Sulphate of ammonia, which is formed from waste 
materials produced in the manufacture of illuminating 
gas, is used sometimes in the fertilizing of greenhouse 
crops. It is more concentrated than nitrate of soda, since 
it contains about 20 per cent of nitrogen. Lime should 
be used in conjunction with large applications of 
sulphate of ammonia in order to prevent unfavorable 
chemical conditions in the soil. 
Of the organic fertilizers, dried blood is probably the 
most popular. It consists of blood from the animals 
slaughtered in the great packing houses, and is prepared 
for market by evaporating, drying and grinding. The 
best grades of dried blood contain from 12 to 15 per cent 
of nitrogen. While dried blood is not nearly so available 
as nitrate of soda, it decomposes very rapidly in the 
warm, moist soils of the greenhouse, and when properly 
applied produces most excellent results. 
Different grades of tankage are also available for 
greenhouse crops. They vary greatly in the amount of 
nitrogen which they contain, and also in the fineness of 
the particles. Tankage consists of all sorts of miscel- 
laneous refuse of packing houses. 
Other forms of nitrogenous fertilizers are used occa- 
sionally in the greenhouse, but they are not important, 
except the various forms of animal bone which contain 
some nitrogen. These are especially popular among 
florists. The bone preparations seldom contain more 
than 4 or 5 per cent of nitrogen. The nitrogen in bone 
meals becomes available very slowly, and this is the 
most serious objection to their use for greenhouse crops. 
On the other hand, large quantities of bone meal may be 
used with perfect safety, and this knowledge adds greatly 
