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

ANALYSIS AFTER DIFFERENT TIMES
OF DIGESTION.

Ingredients.	No. of Days’ Digestion.
Ingredients.	1	3	4	5	10
Insoluble residue	76.97	72.66	71.86	70.53	71.79
Soluble Silica	8.60	11.18	11.64	12.30	10.96
Potash	.35	.44	.57	.63	.62
Soda	.06	.06	.03	.09	.28
Lime	.26	.29	.28	.27	.27
Magnesia	.42	.44	.47	.45	.44
Br. Ox. Manganese	.04	.06	.06	.06	.06
Ferric Oxid	4.77	5.01	5.43	5.11	4.85
Alumina	5.15	7.38	7.07	7.88	7.16
Phosphoric acid	.21	.21	.21	.21	.21
Sulfuric acid	.02	.02	.02	.02	.02
Volatile matter	3.14	3.14	3.14	3.14	3.14
Total	99.63	100.68	100.55	100.69	99.80

Amount of soluble matter	19.67	24.88	25.57	27.02	24.87
Amount of soluble bases	11.05	13.68	13.91	14.49	13.68

While these results pointed clearly to the five-day period as being sufficiently effective so far as the plant-food ingredients are concerned, it was not easy to understand why a ten-day digestion should be less incisive than a five-day one. Instead of repeating the ten-day experiment, it was thought preferable to re-treat the residue from the five-day digestion for five days more. The result was that only more silica and alumina went into solution—in other words, additional clay was alone being decomposed. This being of no interest in the matter of plant nutrition, the five-day period was definitely adopted by the writer for his work; and it, together with the acid of 1.115 density, is the basis of all the results given in this volume, except where otherwise stated. There appeared to him to be no good reason for the acceptance of the arbitrary method of soil-extraction suggested by Kedzie and since adopted by the Association of Official Agricultural Chemists; the more as to do so would throw out of comparison all the previous work done by Owen, Peter, and himself and his pupils, which had already been definitely correlated with the natural conditions and with cultural experience.[117]

Virgin Soils with High Plant-food Percentages are Always Productive.—In strong contrast to the contradictory evidence deduced from the analysis, by any method, of cultivated soils when compared with cultural experience, it seems to be generally true that virgin soils showing high percentages of plant-food as ascertained by extraction with strong acids (such as hydrochloric, nitric, etc.), invariably prove highly productive: provided only that extreme physical characters do not interfere with normal plant growth, as is sometimes the case with heavy clays, or very coarse sandy lands.—To this rule no exception has thus far been found. The composition of some representative soils falling within this category is given in the annexed table, which at the same time conveys some idea of the proportion of acid-soluble ingredients usually found in the best class of natural soils.

TABLE EXEMPLIFYING HIGH PLANT-FOOD
PERCENTAGE IN SOILS.

(A) = Buckshot soil. Yazoo Bottom.
(B) = Black Prairie. Rankin County.
(C) = Loamy Sediment. Houma, Terrebonne parish.
(D) = Rio Grande Bottom. Sandy Sediment.
(E) = San Diego Co. Colorado Bottom. Silt Sediment.[118]
(F) = Riverside Co. Palm Valley. Micaceous Sandy Soil.
(G) = Tulare Co. Experiment Station. Plains Loam.
(H) = Solano Co. Putah Valley. Dark Loam.
(I) = San Luis Obispo Co. Arroyo Grande Dark Loam.

	Mississippi				Louisiana.		Texas.		California.
	(A)		(B)		(C)		(D)		(E)		(F)	(G)	(H)		(I)
	(Heavy Clay).				(Loam).
Number of Sample	390		188		240		37		506		1092	1159	110		2061
Chemical Analysis of Fine Earth.
Insoluble matter	51.06	71.77	69.95	74.40	35.48	56.24	36.04	53.30	58.57	63.90	71.45	72.98	67.33	71.00	53.43	72.43
Soluble silica	20.70	71.77	4.46	74.40	20.76	56.24	17.26	53.30	5.33	63.90	5.50	5.50	3.67	71.00	19.00	72.43
Potash (K₂O)	1.10		.90		1.03		1.31		1.18		1.42	1.20	.93		.67
Soda (Na₂O)	.33		.24		.13		.22		.16		.18	.52	.12		.18
Lime (CaO)	1.35		1.04		.72		14.43		8.67		2.20	1.86	.77		2.11
Magnesia (MgO)	1.67		.91		.88		1.53		2.97		2.09	1.81	2.29		2.26
Br. ox. of Manganese (Mn₃O₄)	.12		.12		.014		.07		.03		.05	.08	.11		.06
Peroxid of Iron (Fe₂O₃)	5.82		4.77		7.10		4.09		4.14		6.68	6.86	8.01		5.23
Alumina (Al₂O₃)	10.54		7.25		15.45		9.11		8.40		5.78	5.66	9.16		7.40
Phosphoric acid (P₂O₅)	.30		.47		.15		.20		.13		.35	.10	.11		.71
Sulfuric acid (SO₃)	.02		.16		.25		.04		.15		.01	.03	.12		.22
Carbonic acid (CO₂)							9.91		7.82		.18				1.82
Water and organic matter	7.37		10.74		18.52		?		3.34		4.29	2.54	7.12		6.63
Total	100.38		101.01		100.48		100.22		100.89		100.18	100.24	99.74		99.72

Humus															3.06
Nitrogen in humus															22.00
Nitrogen in soil															.67
Hygroscopic moisture. absorbed at 15°C															10.70
Available phos. acid															.14
Available potash															.14

Discussion of Table.—It will be noted in this table that while the total of the matters soluble in acids (inclusive of silica) ranges from a little below 50 to over 77 per cent, the total of directly important mineral plant-food ingredients (potash, lime, magnesia and phosphoric acid), constitute in moderately calcareous soils only from about 2.5 to somewhat over four per cent of the whole. Yet if all these were in available form, the supply would be abundant for many hundreds and even thousands of crop years. For, one-tenth of one per cent in the case of the clayey soils of the preceding table would amount to about 3500 pounds per acre-foot, and to 4000 in the case of the sandy ones. Hence the amount of phosphoric acid in e. g., the Mississippi delta soil from Houma would suffice for the production of about 440 crops of wheat grain (at 20 bushels per acre) if only one foot depth were drawn upon; but as the roots of grain easily penetrate to twice and half and three times that depth even in the humid region, the number might be tripled. As a matter of fact, however, that soil has produced full crops for from forty to fifty years only; yet this is considered an exceptionally long duration of profitable production without fertilization.

The first and last soils in the above list represent probably the highest types of productiveness known. The Yazoo bottom soil has produced up to one thousand pounds of cotton lint per acre when fresh, and is still producing from four to five hundred pounds after thirty years’ culture. The Arroyo Grande soil of California with its extraordinary percentages of phosphoric acid and nitrogen, as well as exceptionally high proportion of available phosphoric acid and potash, has made such a record of productiveness, and high quality of the seeds produced, that it has for a number of years been excluded from competition for prizes offered by seed-producers elsewhere, in order to give other sections a chance. Both these soils are rather heavy clays, but readily tillable in consequence of their abundant lime-content. The remarkably high content of acid-soluble silica, indicating the presence of much easily available zeolitic matter, is doubtless connected with the exceptional productiveness.

Experience, then, proves that lands showing such high plant-food percentages will yield profitable harvests for a long time without fertilization, or with only such partial returns as are afforded by the offal of crops. Also that when fertilization comes to be required, instead of supplying all the ingredients usually constituting fertilizers, only one or two of these will as a rule be actually needed, and even these in smaller amounts than in “poor” lands; thus materially reducing the expense of fertilization. The high production and durability of such lands therefore amply justify their higher pecuniary valuation; for which there would be no rational permanent ground if they required fertilization to the same extent as poor lands. In other words, if the entire amount of soil-ingredients removed by crops had had to be currently replaced equally in all cases (as is implied in the hypothesis, advanced by some, that the chemical composition of soils is of no practical consequence), the high prices which from time immemorial have been paid for black prairie and rich alluvial lands as against meagre uplands and barrens, would have been so much money wasted.