33 
Table XI. — Effect of soil on immediate availability of phosphoric acid in guanos. 
Efficiency of phosphoric 
acid in — 
Difference 
between 
efficiency 
in clay and 
in sand. 
Difference 
between 
efficiency 
Source of phosphoric acid (P 2 5 ). 
Red clay 
soil. 
Sandy 
soil. 
in clay 
and sand 
expressed 
as per- 
centage of 
efficiency 
in sand . 
Guano Xo. 797A 
50- 
48 

26 
47 
31 
26 
29 
50+ 
89 
20 
65 • 
75 
153 
123 
5 
a 16 
3 
14 
36 
a 34 
20 
4 
o94 
3 
33 
a68 
a 31 
ai 
+45 
+32 
- 3 
^12 
+ 11 
- 3 
+ 6 
+21 
+46 
- 5 
+ 17 
+32 
+ 7 
+22 
+ 19 
Per rent. 
+ 900 
+ 200 
- 100 
Guano Xo. 92^ 
+ 86 
+ 31 
- 9 
Guano Xo 966 \ 
+ 30 
Guano Xo 966B 
+ 263 
+ 1,150 
Guano Xo. 977 
- 5 
+ 567 
Guano Xo. 9S2 
- 97 
Slag 
+ 10 
+ 71 
Floats 
+ 450 
a Average from several determinations. 
Guanos Xos. 917, 957, 966 A, and 977 were no more or only slightly 
more, effective in clay than in sand, while all other guanos were far 
more effective in clay. In some cases guanos of such low availability 
in the sand as to be practically worthless were highly efficient fertili- 
zers in the clay. Relative to acid phosphate, basic slag had about 
the same efficiency in clay as in sand, while bone meal and floats were 
much more efficient in the clay. 
It should be considered in judging these results that the efficiencies 
were measured against acid phosphate, that is, they were relative, 
not absolute. The increased efficiency of some of the phosphates in 
the clay soil may therefore be due to a depression in the effective- 
ness of the acid phosphate, or an increase in the effectiveness of 
the other phosphate, or a combination of the two changes. Which 
of these changes occurred does not affect the choice of what phosphate 
to use on a certain soil although it is important for a knowledge of 
the reactions of the soil. 
Efficiency of the phosphoric acid as affected by remaining in the 
soil. — Certain phosphates are supposed to become more available 
through various reactions in the soil. Thus finely-ground rock phos- 
phate is supposed to be more effective after it has remained in the soil 
for a period than it is when applied immediately to the crop. 1 As 
nearly all guanos contain most, or a large part, of their phosphoric 
acid in a form which is not immediately available, it was important 
i Xumerous investigators, including P. Wagner, have not been able to establish this, while others have. 
Possibly the nature of the soil is the determining factor, although some affirmative conclusions have 
been based en inadequate data. 
55016°— 18 5 
