Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions - Eugene W. Hilgard

(Rept. Cal. Exp. Sta. 1894-5, page 27.)

	Oahu.		Hawaii.
Number of Sample.	No. 21.	No. 22.	No. 24.	No. 26.	No. 27.
Coarse Materials. 0.55ᵐᵐ	2.00	2.50	4.00	3.00	5.00
Fine Earth	98.00	97.50	96.00	97.00	95.00
CHEMICAL ANALYSIS OF FINE EARTH.
Insoluble matter	15.84	14.49	26.99	28.66	21.07
Soluble Silica	14.07	30.37	10.26	7.35	2.68
Potash (K₂O)	.45	.26	.40	.61	.44
Soda (Na₂O)	.14	.08	.26	.17	.25
Lime (CaO)	.26	1.04	.52	.68	.28
Magnesia (MgO)	.65	.80	.96	1.04	.60
Br. ox. of Manganese (Mn₃O₄)	.05	.03	.21	.20	.07
Peroxid of Iron (Fe₂O₃)	39.05	19.68	19.10	18.23	30.10
Alumina (Al₂O₃)	14.61	18.29	21.41	20.18	14.38
Phosphoric acid (P₂O₅)	.19	.32	.64	.70	.97
Sulfuric acid (SO₃)	.03	.09	.32	.21	.29
Carbonic acid (CO₂)
Water and organic matter	14.18	14.59	18.60	21.65	28.60
Total	99.52	100.04	99.67	99.61	99.73

Humus	3.35	3.24	4.84	5.43	9.95
“ Ash	3.12	2.22	2.76	3.56	6.70
“ Nitrogen, p.c. in Humus	3.30	9.800	2.800	3.100	1.71
““ , p.c. in soil	.112	.314	.134	.168	.17
Phosph. acid in humus ash	.110	.166	.580	.500
Soluble in 2% Citric acid	.004	.020	.035	.037	.025
in Nitric acid, 1.20 sp. g	.190	.320	.640	.700	.970
in Chlorhydric acid (1.115 sp.g.)	.430	.350	1.600	1.280
Hygroscopic moisture 15°C.	18.50	21.25	23.07	23.14	23.81

Unavailability of Ferric Phosphate.—It will be noted that in the soils from Oahu with an overwhelming amount of ferric oxid (mostly in the form of hydrate or rust) the citric acid has taken up only an insignificant amount of phosphoric acid; nitric acid took up 40 to 50 times as much, and chlorhydric doubled even this. In the much less ferruginous Hawaiian soils, though containing more alumina, the citric acid extracted nearly ten times as much; proving that it is chiefly ferric oxid, and not the alumina as has been supposed, that causes the insolubility of phosphoric acid in soils and doubtless also in fertilizers. The very unusually high content of phosphoric acid in the Hawaiian soils, exceeding all others on record, so far as known to the writer, emphasize the effects of ferric hydrate upon soluble phosphates; while the fact that these very soils are greatly benefited by the use of phosphate fertilizers, proves that the Dyer (citric acid) method for the determination of available phosphoric acid which in soils Nos. 21 to 26 yielded results largely in excess of the established limit in European soils, cannot be successfully applied to these highly ferruginous soils. It should also be noted that the amounts of phosphoric acid found in the humus extracted by the Grandeau method is in the first two Hawaiian soils over ten times the amount extracted by citric acid, but that while they rise and fall together, no definite quantitative ratio exists between the two.

It is obvious that in such soils, fertilization with water-soluble phosphates would be likely to result in the quick partial withdrawal of the same from useful action, and that any excess not promptly taken up by the crop, is likely to become inert and useless. It will evidently be desirable to use the phosphates in the form of bone meal or basic slag (Thomas Phosphate), which because of their difficult solubility will be acted upon but very slowly, if at all, by the ferric and aluminic hydrates.

Nitrogen.—In determining the nitrogen-content of the soil, a great variety of methods has been followed. Some include all that can be obtained by the combustion of the organic matters of soil and from the nitrates present in the same; while others, the writer among the number, believe that the mainly important source of nitrogen to the plant being the nitrification of the humus-nitrogen, the determination of the humus by the method of Grandeau, and of the nitrogen contained in it, should be the standard; the unhumified vegetable matter being of no definitely ascertainable value, and the nitrates varying from day to day and being liable to be lost by leaching at any time; therefore forming no permanent feature of the soil. Considering the variety of methods, the unanimity with which about one-tenth of one per cent (.10) has been assumed as the ordinarily adequate percentage is remarkable. In view of the extremely variable amount of nitrogen in the humus (ranging from 1.7 to nearly 22%), the amount of the latter cannot, of course, afford even an approximation to the nitrogen-content; except that as in the humid region, the nitrogen-percentage is not known to exceed about 5 or 5.5%, an approximate estimate can be made on that basis. In the arid region, according to location, the nitrogen-percentage may be from three to six times greater for a similar amount of humus. ([See chap. 8. p. 135]). In the writer’s experience, a nitrogen-percentage of .1% in the arid region is a very satisfactory figure, indicating that the need of nitrogen-fertilization is not likely to arise for a number of years.

Nitrification of the Organic Matter of the Soil.—In order to test the question whether or not the nitrogen of the unhumified debris existing in surface soils is directly nitrifiable, the writer selected a soil which in its natural condition sustains intense nitrification, so that at some points it contains as much as 1200 pounds of sodic nitrate per acre. The composition of this soil, representing the land of the “ten-acre tract” of the southern California substation, is as follows:

SOIL FROM “TEN-ACRE TRACT,”
SOUTHERN CALIFORNIA SUB-STATION, NO. 1284.

Coarse Materials. > 0.55ᵐᵐ	1.00
Fine Earth	99.00
	100.00

CHEMICAL ANALYSIS OF FINE EARTH.
Insoluble matter	62.62	70.92
Soluble silica	8.30	70.92
Potash (K₂O)	.95
Soda (Na₂O)	.50
Lime (CaO)	5.07
Magnesia (MgO)	.84
Br. ox. of Manganese (Mn₂O₄)	.06
Peroxid of Iron (Fe₂O₃)	6.43
Alumina (Al₂O₃)	3.88
Phosphoric acid (P₂O₅)	.21
Sulfuric acid (SO₃)	.06
Carbonic acid (CO₂)	3.66
Water and organic matter	6.02
Total	99.70

Water-soluble matter, per cent.	.137
Sodic nitrate, per cent.	.020

Humus	1.99
“ Ash	1.13
“ Nitrogen, per cent. in Humus	10.30
““ , per cent. in soil	.203
Total Nitrogen in soil	.330
““ in unhumified matter	.127
Available Potash{ citric }	.03
Available Phosphoric acid {method}	.03
Hygroscopic Moisture absorbed at 15° C.	5.81

It will be noticed that this is a rather strongly calcareous soil, (nearly 9% of calcic carbonate), slightly impregnated with alkali, of which about one-ninth is saltpeter. One portion of this soil was thoroughly leached with distilled water until not a trace of nitrates could be detected in the leachings. Another portion was treated for the removal of humus according to the Grandeau method ([see chapter 8, page 132]); the extracted soil showed under the microscope an abundance of vegetable debris, some slightly browned as from incipient humification.

The calcic and magnesic carbonates withdrawn in the humus-extraction were then restored to the soil in the form of finely divided precipitates and thoroughly mixed in, first in the dry and then in the wet condition; the extracted soil being repeatedly wetted with turbid water from the leached soil, in order to replace and reinfect it with the nitrifying bacteria. Both soils were then spread out in flat glass dishes and placed in a wooden box containing also a similar flat dish with distilled water, upon which played the draught from the inlet pipe opening into the outer air, with outlet-holes in the cover at the opposite end; thus keeping the air within fairly moist. In addition, the soils themselves were moistened with distilled water every three days and restored to a loose condition by stirring. The whole was placed so as to maintain, during the greater part of the 24 hours, a temperature of from 30 to 35 degrees C. At intervals the samples of both soils were leached and color-titrated for their nitrate content by the picric-acid test. The results, calculated as sodic nitrate, during two years were as follows: