CHAPTER IV 
MANURES, LIME AND FERTILIZERS 
Need of plant food.—Greenhouse vegetable forcing is 
the most intensive type of agriculture. The plants are sct 
very close together, so that a maximum draft is made on 
the supply of available plant food. One crop follows 
another in close succession, and in a well-managed house 
there is practically no loss of time or space from Sep- 
tember 1 to August 15. Continuous heavy cropping 
under glass requires much more plant food than any line 
of outdoor cropping that can be followed in temperate 
regions. 
Again, the greenhouse vegetable grower raises products 
of high moncy value, and the cost of the plant food re- 
quired for maximum crops is so insignificant, compared 
with the net returns, that he cannot afford to take chances 
by not supplying sufficient nourishment. It is not un- 
common to see greenhouses which are properly heated 
and ventilated filled with crops that are small and inferior - 
because the plants have not been properly fed and per- 
haps watered. There must be perfect cultural conditions 
in every respect in order to realize the utmost returns. 
No greenhouse soil has yet been found which does not 
need frequent and liberal applications of plant food. 
Value of manures.—Numerous investigations have 
shown that the crop-producing power of a soil is more 
dependent upon its physical than upon its chemical 
composition. In other words, if a soil possesses the best 
physical properties, plant foods are not likely to be want- 
ing to any considerable extent. The probabilities are 
that this conclusion of the soil specialists does not apply 
so much to the artificial conditions of the greenhouse as it 
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