• Pliospliatic Manures for Root-Crops. 
43 
either wanting^ altogether or contains an insufficient supply of 
one or more of these essential ash-constituents. 
Thus we may learn from the composition of the ash of turnips, 
and the amount of mineral matters in an average crop, that at 
the very least 177|- lbs. of potash, 104;^^ lbs. of lime, and 40 lbs. of 
phosphoric acid must be present in an available condition in the 
soil or the manure, or both together, if we wish to grow 20 tons 
of bulbs and 6 tons of tops. If, for instance, the soil be de- 
ficient in either lime or potash, and none is applied, the turnips 
are likely to become diseased, or the crop deficient in quantity, 
though the other mineral and organic substances useful to turnips 
are present in abundance. 
It has indeed been observed that the exclusive use of super- 
phosphate, however beneficial it may be in the majority of in- 
stances, has in some soils led to a complete or partial failure, or 
the presence of disease in the turnip-crop. Again, instances are 
on record in which neither superphosphate nor bone-dust have 
had anv effect on turnips ; and good root-crops have been ob- 
tained without any phosphatic manures. 
It thus appears that the effects of superphosphate of lime 
upon the turnip-crop vary according to the soil it is grown 
upon ; and it is a subject of considerable practical importance to 
investigate the reasons of these variations, and to point out under 
what circumstances phosphatic manures may be advantageously 
applied, either alone or in conjunction with farmyard-manure, 
guano, or other more complex fertilizers. 
1. Superphosphate, it appears to me, is efficacious in many 
instances as a manure for root-crops, because soils in general 
seldom contain more than one to two tenths of a per cent, of 
phosphoric acid, which acid is required in a considerable 
quantity by turnips, and because the short period of active 
growth of the crop necessitates a far more abundant supply of 
phosphoric acid than is contained in the portion of the soil 
reached by the fibres of the turnip. The direct and exclusive 
supply of phosphoric acid, however, is attended with beneficial 
results only when the soil, in addition to a fair amount of organic 
matter, contains an abundance of available potash, lime, and the 
other mineral matters entering into the composition of the ash 
of turnips. In that case the only deficiency in the soil is avail- 
able phosphoric acid ; and the want is best supplied by soluble 
phosphate. 
2. Soluble phosphate of lime, the most active constituent of 
commercial superphosphate, made entirely from mineral phos- 
phates, may either be washed out by heavy or long-continued 
rains, or undergo changes in some soils which render it in- 
effective. 
