Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions

by Eugene W. Hilgard

The Nitrogen-Content of Humus.—Since soil-humus is doubtless the chief depository of soil-nitrogen, and the main source from which, through the process of nitrification, the nitrogen-supply to plants is usually derived, its content of that element is a matter of great interest. It has been customary to estimate approximately the nitrogen-content of soils by the proportion of humus-substance present; and as the light tints of the soils of the arid region indicate a small humus-content, a scarcity of nitrogen seemed to be also indicated for these lands. As this in a number of cases did not seem to accord with actual experience, an investigation of the subject was made at the California experiment station,[51] with the results shown in the subjoined table. In considering these results it must be kept in mind that while arid conditions can rarely be fulfilled in the humid region, humid conditions are quite frequently locally represented in the arid, in lowlands and on high mountains; while moderately moist benchlands represent the semi-arid regime.

HUMUS PERCENTAGE AND NITROGEN CONTENT IN
SOILS OF THE ARID AND HUMID REGIONS.

Station Number	Soils arranged in order of nitrogen percentages in humus.	Humus in soil, per cent.	Nitrogen in Humus, per cent.	Nitrogen in soil, per cent.
	Soils of the Arid Region (California).
2061	Dark clay loam, Arroyo Grande Valley,	3.06	22.00	.670
	San Luis County
2291	Red soil, Orland, Glenn Co.	.71	21.10	.150
1904	Sediment Soil, Porterville, Tulare Co.	.90	19.50	.180
1901	Sandy soil near Ceres, Stanislaus Co.	.64	18.75	.120
704	Sandy soil of plains, near Fresno, Fresno Co.	.60	18.66	.112
6	Black adobe soil, Stockton, San Joaquin Co.	1.05	18.66	.196
1679	Black adobe soil, Berkeley, Alameda Co.	1.20	18.58	.203
2324	Clay soil of desert, Imperial, San Diego Co.	.38	18.40	.070
1167	Black clay loam soil, near Tulare, Tulare Co.	1.66	18.19	.302
1536	Brown loam soil, Windsor Tract, Riverside,	.20	18.00	.036
	Riverside County
1126	Sandy loam soil, Paso Robles,	.55	17.27	.095
	San Luis Obispo Co.
2301	Red hill soil, Upper Lake, Lake Co.	.81	16.90	.137
1607	Plateau soil of desert, Lancaster,	.25	16.80	.042
	Los Angeles Co.
1159	Sandy plains soil, Tulare, Tulare Co.	.37	16.75	.062
1900	Sandy soil, near Modesto, Stanislaus Co.	.84	16.65	.140
1113	Clay loam soil (slate), Jackson, Amador Co.	.54	16.60	.090
1149	Adobe clay soil, near Paso Robles,	.47	16.18	.074
	San Luis Obispo County
1538	Mesa soil, Chino, San Bernardino Co.	.65	16.08	.105
1147	Sandy loam soil, Paso Robles,	.66	16.06	.106
	San Luis Obispo County
2403	Valley Soil, Wheatland, Yuba Co.	1.50	16.00	.240
1281	Red Mesa soil, Pomona, San Bernardino Co.	.58	15.50	.090
1117	Sandy granitic soil, near Jackson, Amador Co.	.80	15.27	.123
1406	Red loam soil, Arlington Heights, Riverside,	.30	15.00	.045
	Riverside County
1172	Red clay loam soil, east of Tulare, Tulare Co.	.72	14.75	.106
1958	Sandy Mesa soil, Nipomo, San Luis Obispo Co.	.85	14.45	.122
1423	Chocolate-red soil, Carisa plain,	.39	14.36	.056
	San Luis Obispo County
1291	Sandy hill land, near Jackson, Amador Co.	.76	14.34	.109
585	Wire-grass loam soil, Visalia, Tulare Co.	1.00	14.10	.146
863	Red ridge loam soil, Grass Valley, Nevada Co.	2.89	13.91	.402
1907	Dark loam soil, near Chino, San Bernardino Co.	.92	13.26	.121
1115	Sandy granitic soil, near Jackson, Amador Co.	.85	13.20	.112
332	Plateau desert soil, Mojave, Los Angeles Co.	.28	12.50	.035
2126	Gravelly soil, East Highlands,	.62	11.75	.070
	San Bernardino Co.
1910	Ojai Valley soil, Nordhoff, Ventura Co.	1.64	11.21	.183
2187	Sandy loam soil, Soledad, Monterey Co.	.97	11.10	.110
1759	Sandy soil, Perris Valley, Riverside Co.	.53	11.04	.059
774	Bench slope soil, Ontario, San Bernardino Co.	1.29	10.85	.140
1984	Red soil, East Highlands  “ ““	.58	10.50	.060
2325	Silt soil of desert, Imperial, San Diego Co.	.65	10.70	.070
1906	Light sandy soil, Pomona, San Bernardino Co.	.95	9.80	.093
2430	Hillside adobe, Berkeley, Alameda Co.	1.85	8.70	.160
	Average of arid uplands	.91	15.23	.135
	Sub-irrigated Arid Soils (California).
586	Sandy plains soil, Tulare, Tulare Co.	1.14	10.79	.123
1466	Loam soil, Miramonte, Kern Co.	.60	10.66	.064
1284	Moist land loam soil, Chino, San Bernardino Co.	1.99	10.20	.203
1148	Swale soil, near Paso Robles,	1.16	9.65	.112
	San Luis Obispo Co.
1714	Bench soil, Santa Clara River, Piru,	.78	9.56	.074
	Ventura Co.
77	Alluvial soil, Tulare Lake bed, Tulare Co.	.47	9.37	.045
1880	Creek bench soil, Niles, Alameda Co.	1.19	8.90	.109
1903	Sediment soil, Porterville, Tulare Co.	1.12	8.50	.140
168	Alluvial soil, Santa Clara river, Santa Paula	.84	7.99	.067
	Ventura Co.
1760	Green-sage land, Perris Valley, Riverside Co.	.91	7.70	.070
506	Alluvial soil, Colorado River, Yuma,	.75	7.47	.050
	San Diego Co.
1636	Red soil, Manton, Tehama Co	2.00	6.86	.137
1758	Alkali soil, Perris Valley, Riverside Co.	.60	6.83	.071
1963	Sandy loam soil, Willows, Glenn Co.	.36	6.05	.022
2080	Sandy soil, Santa Maria Valley,	1.64	5.36	.090
	Santa Barbara Co.
	Average of sub-irrigated arid soils	1.06	8.38	.099
	Humid Soils From Arid and Humid Regions (California).
207	Eel River Alluvial soil, Ferndale, Humboldt Co.	1.25	6.96	.085
2319	Alluvial soil, Hupa Valley, Humboldt Co.	7.83	6.70	.514
213	Marsh soil, Novato, Meadows, Marin Co.	1.54	6.36	.089
1704	Valley soil, Hollister, San Benito Co.	.94	5.21	.049
2295	Tule soil, Upper Lake, Lake Co.	1.70	4.50	.077
110	Alluvial soil, Putah Creek, Dixon, Solano Co.	1.71	4.25	.072
37	Redwood Valley soil, Pescadero, San Mateo Co.	2.28	3.07	.070
	Average for California	2.45	5.29	.135
	Other States.
26	Bog soil, Michigan[52]	33.02	6.08	2.012
	Back-land clay loam, Houma, Louisiana	5.07	4.20	.218
	Duff soil, Oregon	13.84	3.49	.483
	Sandy prairie soil, Harris Co., Texas	2.13	3.66	.184
	Average for other States	7.01	3.78	.295
23	Red soil, Oahu Island, Hawaii (maximum)	1.57	5.07	.078
27	Guava soil, Hawaii Island (minimum)	9.95	1.71	.170
	Average of 5 soils, Oahu Island	3.01	6.07	.237
	Average of 2 soils, Maui Island	9.07	2.13	.286
	Average of 4 soils, Hawaii Island	6.17	2.54	.146
	Average for Hawaiian Islands	5.26	3.69	.169
	Total for Humid soils, average	4.58	4.23	.166

It thus appears that on the average the humus of the arid soils contains about three and a half times as much nitrogen as that of the humid; that in the extreme cases, the difference goes as high as over six to one (see Nos. 37 and 704); and that in the latter cases, the nitrogen-percentage in the arid humus considerably exceeds that of the albuminoid group, the flesh-forming substances.

It thus becomes intelligible that in the arid region a humus-percentage which under humid conditions would justly be considered entirely inadequate for the success of normal crops, may nevertheless suffice even for the more exacting ones. This is more clearly seen on inspection of the figures in the third column, which represent the product resulting from the multiplication of the humus-percentage of the soil into the nitrogen-percentage of its humus; as appears in comparing the respective averages, or Nos. 1167 and 110 and others. An additional consideration is the probable greater ease with which the nitrifying bacteria can act upon a material so rich in nitrogen.

We must not, then, be misled by the smallness of many humus-percentages in the arid region, into an assumption of a deficiency in the supply of soil-nitrogen.

Decrease of Nitrogen-Content in Humus with Depth.—Since the oxidation of the carbon and hydrogen in the humus-substance, and the consequent increase of its relative nitrogen-content, are manifestly dependent upon the presence of air and heat, it is reasonably to be expected that the nitrogen-percentage of the humus should decrease with the depth of the soil. That this is really the case is plainly shown in the subjoined table, which gives the humus-percentages and the nitrogen-content of the humus from the surface foot down to twelve feet, in a soil on the bench of the Russian River, Cal., which is sub-irrigated, and liable to more or less rainfall during the summer. It will be seen that not only does the absolute humus-percentage decrease quite regularly down to seven feet, at which point there evidently was at one time a strong root development, causing a notable increase of the humus-content; from which again there is a regular decrease down to the twelfth foot. It will be noted that the nitrogen-percentage in the humus, while not decreasing with the same regularity as the humus-content itself, yet exhibits a general recession from 5.30 to 1.15 in the ninth foot, to which direct oxidation doubtless never penetrates.

HUMUS AND NITROGEN-CONTENT OF RUSSIAN RIVER SOIL.
Depth in feet.	Per cent Humus in soil.	Per cent Nitrogen in Humus.	Per cent Humus-Nitrogen. in soil.
1	1.21	5.30	.064
2	1.16	4.32	.054
3	1.14	3.87	.044
4	1.17	3.76	.044
5	.74	2.16	.016
6	.60	2.66	.016
7	.47	2.54	.012
8	.78	1.54	.012
9	.54	2.24	.012
10	.52	1.15	.006
11	.53	1.51	.008
12	.44	1.81	.008

Influence of the Original Materials on the composition of Humus.—The great variability of the composition of humus formed from different substances is well shown in the subjoined table, representing the results of experiments made by Snyder,[53] who caused various substances to humify by mixing the pulverized material intimately with a soil poor in humus, and allowing the process to continue for a year. At the end of that time the humus formed was extracted by the method of Grandeau, outlined above, and analyzed, with the following results.

	Sugar.	Oat Straw.	Green Clover.	Wheat Flour.	Sawdust.	Meat Scraps.	Cow Manure.[54]
Carbon	57.84	54.30	54.22	51.02	49.28	48.77	41.93
Hydrogen	3.04	2.48	3.40	3.82	3.33	4.30	6.26
Nitrogen	0.08	2.50	8.24	5.02	0.32	10.96	6.16
Oxygen	39.04	40.72	34.14	40.14	47.07	35.97	45.63
	100.00	100.00	100.00	100.00	100.00	100.00	100.00