624 Circular No. 7. ^ 



iients. Of potash, for example, soils contain from a few thousand 

 pounds to a hundred thousand or more pounds; of phosphorus, from a 

 couple of thousand pounds to forty or fifty thousand pounds in an acre 

 of soil one foot deep. But the plant food is not readily soluble or avail- 

 able and chemical methods have not been able to distinguish with 

 sufficient accuracy between the available and the unavailable portion. 

 (2.) Chemical determinations do not measure the favorableness of the 

 five or six other conditions of the soil which are as essential to plant 

 growth as is the food supply. These include moisture, oxygen, etc. 

 Fertilizers affect these, for their function is not single. 



The two important constituents whose direct availability from the soil 

 particles is most aiTected by lime are potash and phosphorus. A large 

 array of figures and observations shows that lime liberates potash in an 

 available form in the soil. This is to be expected from chemical laws. 

 It is a matter' of substitution. The lime takes the place of the potassium 

 in its insoluble combinations. Probably the most representative figures 

 on this point are those of Frear of Pennsylvania, who found on nine soils 

 of widely different character an increase in the availability of potash 

 ranging from 6 per cent on muck to 55 per cent on red shale clay. The 

 average for the nine soils was 2;^ per cent, equivalent to 60 pounds 

 potassium per acre. 



Phosphorus is the mineral food element present in smallest amounts 

 of any constituent in normal soil. It may exist in three primary com- 

 binations. These are lime phosphates, iron phosphates and aluminum 

 phosphates. The latter two forms are exceedingly insoluble and are 

 not materially affected by carbonated water, so that phosphate in these 

 combinations is largely unavailable to plants. Lime phosphate, on 

 the other hand, has its availability much increased by carbonated 

 water over pure water. Consequently, by the use of lime which will 

 change some of the iron and aluminum phosphates to lime phosphates, 

 the availability of phosporus is increased. Frear found on the same 

 nine soils in Pennsylvania on which the availability of potash was 

 determined, that 200 bushels of lime per acre increased the availabiUty 

 of phosphorus from 16 per cent on muck to 140 per cent on a lime- 

 stone clay ; in only one case — that of a black shale — was its availabil- 

 ity decreased. The average increase on eight soils was 43 per cent, 

 equivalent to 140 pounds per acre. From this and other investigations, 

 it is clear that the use of lime is especially desirable on soils rich in 

 iron and aluminum but low in lime. The net result of the use of lime 



