Dec. 2 7 ,191 s Petrography of Some North Carolina Soils * 579 
The 8-year average with cotton given in Table III again, shows that 
phosphoric acid is the controlling element in these fertilizer tests. When 
potash and nitrogen are used in quantities, as in this experiment, only 
slight increases in yield are produced. The former constituent gave a 
slightly greater average than did the latter. The average “bulk analy¬ 
ses” of many samples of soil from these two fields, as well as from Norfolk 
fine sandy loam, will be found in Table IV. 
TabbE IV. —Average quantity of the total plant-food constituents per acre in various types 
of soil 
SURFACE son, TO DEPTH OF 6 % INCHES (2,000,000 POUNDS) 
Soil type. 
Nitrogen 
(N). 
Phosphorus 
pentoxid 
(P2O5). 
Potassium 
oxid (K2O). 
Calcium 
oxid (CaO). 
Cecil clay loam. 
Cecil sandy loam. 
Norfolk fine sandy loam. 
Pounds. 
I, 141 
769 
853 
Pounds. 
h *55 
503 
953 
Pounds. 
7i 2I 3 
2, 994 
3, 087 
Pounds. 
4, 656 
5> 542 
3, 220 
SUBSOIB TO DEPTH OF 28 INCHES (8,000,000 POUNDS) 
Cecil clay loam. 
2.378 
9.169 
25,090 
i9> 933 
Cecil sandy loam. 
r > 993 
4,007 
i9> °73 
26, 512 
Norfolk fine sandy loam. 
1,36° 
1,573 
453 
8, 880 
A comparison of the yields of cotton on the two fields shows marked 
similarity in fertilizer requirements though the fields are over ioo 
miles apart. These soils belong to the same series, though of decidedly 
different texture, one being a rather heavy clay, the other a medium 
sandy loam. Unquestionably there are numerous other factors than 
the amount of plant food carried by the two soils which enter into 
their productiveness; nevertheless, some relationships exist between 
this question and their requirements for these fertilizer elements. As 
shown in Table IV, the phosphoric-acid content of both soils is low; 
until this element has been added in sufficient quantities there can be 
no increase yields. Although the nitrogen supply in the two soils is 
found in about the same proportion as the phosphoric acid, it is evidently 
changed into a more available form faster than the latter element. 
The potash content of the Cecil clay loam is about double that of the 
Cecil sandy loam, both soils showing that potash is in no way the limit¬ 
ing element. Indeed, it is doubtful whether this element can be applied 
to the former at a profit. A glance at Table I, which gives the mineral 
composition of the Cecil series, shows that in the fine sand and silt 
separates the potash minerals predominate and that biotite mica is 
found among the abundant minerals in all five samples. 
