146 Principles of Plant Culture. 
phosphoric acid. In leaching, the potash is mostly washed 
out, but the phosphoric acid is largely retained. 
266. Farm and Stable Manures should be the first 
dependence of the cultivator, but these may often be profit- 
ably supplemented by commercial fertilizers. Aside from 
farm and stable manures, leguminous crops are undoubtedly 
the cheapest source of nitrogen for the farm, and, with un- 
leached wood ashes, furnish all the needed fertilizing ingre- 
dients for grain crops grown in rotation. For garden crops, 
however, if sufficient stable manure can not be obtained, 
more nitrogen can often be profitably used than leguminous 
crops alone can furnish and for such, commercial fertilizers 
may often be advantageously added. 
267. Crops Suggest Their Own Needs to some ex- 
tent, so long as they are not suffering from drought. Asa 
rule, a lack of nitrogen is indicated by pale-green foliage, 
or small growth of leaf or stalk. Excess of nitrogen is indi- 
cated by excessive growth of leaf or stalk, with imperfect 
bud-, flower- and fruit development. Lack of phosphoric 
acid is indicated by scanty crops of light or shrunken seed, 
on plants of normal size. Lack of potash is indicated by 
small crops of inferior fruit, accompanied by satisfactory 
growth. 
268. Crop Rotation Economizes Plant Food, because 
some crops use more of a given food constituent than others. 
Alternating crops having different food requirements tends 
to prevent the exhaustion of special food substances. 
269. A Growing Crop Tends to Conserve Fertility 
because it reduces drainage by taking up water from the 
soil, and at the same time, appropriates the available plant 
food, thus preventing the loss of the latter by washing out. 
