_Action of Superphosphate sometimes unfavourable._
But even in this case there may be other conditions which render insoluble phosphate a preferable manure. Such a case is where the soil is of a very light nature and is deficient in lime. In this case the acid superphosphate, not having the necessary base to combine with, may prove even hurtful to the young plants. According to the late Dr Voelcker, a concentrated superphosphate may produce a smaller crop than a fertiliser containing only a quarter as much soluble phosphoric acid, when applied to root-crops on sandy soils, greatly deficient in lime. Cases such as the above, however, are extremely rare; and we may say that, in the case of root-crops generally, superphosphate must be regarded as of special value.
In any case, superphosphate ought to be applied to a soil some time before it is likely to be assimilated by the plant, in order to allow neutralisation of its acid character to be fully effected before the plant's roots come in contact with it. Thus Professor S. W. Johnson, one of the greatest living American authorities, states it as his opinion that recent investigations tend to show that soluble and reverted (or precipitated) phosphates are, upon the whole, about equally valuable as plant-food, and of nearly equal commercial value. But as Sir John Lawes, in quoting Professor Johnson to the above effect, remarks, this opinion is based on an experience of American agriculture, in which country soluble phosphate is chiefly applied to cereal crops, while in this country it is chiefly applied to turnips. In the case of cereal crops, the importance of a speedy early growth is not so great, as we have already pointed out, as it is in turnips, where the danger to the young plants from the ravages of the turnip-fly is such that the growth of even a day or two may make a very considerable difference.
A consideration of the action of superphosphate, then, throws a good deal of light on the conditions which determine the value of insoluble phosphates when applied to the soil, and shows that the state of division, intimacy of mixture with soil-particles, and the nature of the soil, are the determining factors. Insoluble phosphates, as we shall have occasion to see when discussing basic slag, have their best action on soils poor in lime and rich in organic matter. Tables have been drawn up with a view to furnishing a guide for the value of phosphoric acid in different manures. In the Appendix[232] we give those of Wolff for 1893, and an American table, drawn up for 1892. The comparative values of mineral phosphates, as well as Peruvian guano and bone-dust, will be further referred to in the following chapter.