050 HALL: THE MECHANICAL ANALYSIS OF SOILS.

XCVL-The Mechccizicnl A m l y s i s of Soils and the Composition of the F ~ a c t io ns Resulting TILe re f ro m .

By ALFRED DANIEL HALL, M.A.

Irztroduation.

THE mechanical analysis of soils, by which is meant the sorting out of the particles of the soil into groups, each of which contains material lying bet-ween specified limits of size, has long occupied the attention of agricultural chemists, although in this country, as compared with Germany or the United States, it has been but little practised. The importance of this determination lies in the fact that the behaviour of the soil towards water, its power of retaining and handing over the rainfall to the plant, and also its physical texture and amenability to cultivation-factors which are of greater importance in the nutrition of the crop than the amount of plant food present-are determined by the sizes of the particles of which the soil is composed. Without referring in detail to the voluminous literature which has grown up round the subject it mill be sufficient here to quote one or two of the more important papers illustrating the principles on which the process is based. The various methodii in use may be divided into t w o groups; in one, the separation is effected by a stream of running water, the velocity of which is increased to carry over successively coarser particles, in the other by leaving the turbid mixture of soil and water to settle for given periods of time, the longer the time interval the finer being the group of particles remaining suspended. The former method was made quantitative by Schone (Uber Schlamm- analyse, Berlin, 1867), and as modified by A. Meyer (Wollny’s Forsch. der Agricultur. physik., 1882, 5, 228) is in general use in Germany (Wahnscha ff e, Anleitung GU Vissenschaftliclien Bodenuntersucluny, Berlin, 1903). The apparatus has been still further improved and the whole method-reduced t o an accurate form by Hilgard (Amer. J. Sci., 1873, 6, 288 ; U.S.A. Division of Chemistry, 1893, Bulletin 38, p. 60) to whom we are indebted for a considerable elucidation of the whole subject, The second method, depending on sedimentation for a given time, was elaborated by Wolff (Untersuchung. Zandw. Stofle, Berlin, 1875) and critically discussed by Osborne (Conn. Agric. Exp. Station Report, 1886, p. 141 ; 1887, p. 144) who showed that it would yield the same result as the running water method of Hilgard. Wi th sundry modifications in detail, Osborne’s method is now employed by the United States Division of Soils in the extensive soil survey which that Department is executing (U.S. A. Dept. of Agriculture,

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HALL: THE MECEANICAL ANALYSIS OF SOILS. 951

Division of Soils, 1896, Bulletin 4). I n all the foregoing methods the soil is freed from its coarser particles by a system of sieves, and then the loose aggregates of the finer particles are, as far as possible, broken up either by long continued boiling with water or by rubbing the paste of soil and water with a soft pestle formed from an india-rubber bung.

A fundamental modification of the whole process was introduced by Schlcesing (Compt. rend., 1874,78, p. 1276), who began by a preliminary treatment of the soil with dilute acid, then removed the salts and excess of acid by washing, and finally added a little ammonia to the liquid in which the soil was afterwards suspended. Schlcesing himself employed a simple sedimentation process which divided the residue into two fractions only, but it is clear tha t any method of fractionat- ing the soil residue may be used, the essential difference being that in one case the soil is first washed with acid, whereas in the other the process is carried out on the raw soil. Schloesing’s method has not been widely adopted out of France ; Petermann however followed it in his examination of Belgian soils (BeclLemhes de Chemk, Brussels, lS98), and it is also employed oEcially in Italy (Stax. S‘erimentaZe Agrarie Italiane, 1891, 17, 672).

The object of the preliminary treatment; with acid is twofold, in the first place calcium carbonate is removed, secondly the “humates ” which are present in surface soils are decomposed and the liberated humic acid becomes soluble in the ammoniacal liquid in which the sedimentation is afterwards conducted. I n soils containing any large proportion of organic matter, such as the pastures and many of the arable soils of this country, the amount of the humates is sufficient to bind together a considerable quantity of the finest particles into loose aggregates which resist disintegra- tion by the boiling or pestling preliminary to working on the raw soil. I n other soils, calcium carbonate serves the same purpose of a temporary cement for the finest particles, even the weathering and management o€ the soil has the same effect of causing the material of a clayey nature to flocculate into loose aggregates, which behave as larger agglomerations. Again the power possessed by soluble salts of inducing flocculation of the clay particles is well known (compare Joly, MAcanisme intime de la Sedimentation, V H I Congress Ge‘ologique International, 1900) ; hence Schlwesing’s method presents an advantage in that it removes all soluble salts and conducts the sedimentation in a practically uniform medium containing free ammonia in order to induce a maximum of deflocculation. How potent the presence of salts may be in causing flocculation may be seen from the following experi- ments, in which two pure clays were separated into fractions by

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952 HALL: TEE MECHANLCAL ANALYSIS OF SOILS.

sedimentation in pure water and in solutions containing 0.04 per cent. of sodium chloride and calcium sulpbate respectively.

250 C.C. water with

Falling in 1 minute ...... Suspended for 1, but fall.

ing in 25 minutes ...... Suspended for 25 minutes,

but falling in 24 hours.. Suspended for 24 hours . . ,

To tnl.. . . . . . . .

10 grams china clay.

0.

2.22

3.91

2.08 2 *oo

10.21 -

Salt

gram). (0 *1

2-54

6.04

1.60 -

10*08

Gypsum (0'1

grjm).

2.28

7.30

0.37 - - _-

9 95

10 grams modelling clay.

0.

0.53

1'45

2.33 5.53

9'84

Salt Gypsum (0.1 (0'1

gram). gram).

0.45 ~ 0.41

8-76 i S.30

0.9

____ _ _ - _

- i- 1 2 8 ___-

10.11 i 10.09

Speaking generally, it is found that the preliminary treatment with acid gives a greater proportion of the finest order of particles (clay) than is obtained by working on the raw soil. Yet the aoid employed is too weak to dissolve any appreciable amount of mineral material other than calcium carbonate, and the attack must be mainly directed towards those very fine particles which are so small as to approach the condition of dissolved matter. As then the acid treatment cannot create clay particleP, its action would seem to consist in completing the work of resolving the temporary aggregates which is largely but not wholly brought about by the boiling or pestling processes in tho other method ; it, therefore, must give a more accurate measure of the proportion of these finest particles present. in the original soil. For these reasons it seemed desirable to submit the two processes to a critical comparison, for Osborne (Zoc. cit.), who tried both Schlesing's method and his own on the same soils, did not attempt to divide the eoil after washing with acid into the Fame series of fractions as he obtained from the raw soil, but followed Schloesing's division into a clay fraction and a single sand fraction, with the result that the two sets of results are not comparable, I n order to secure a critical ex- amination of the methods it was necessary to work on soils which are identical as regards their original physical structure, but which by processes of cultivation bave assumed very different textures, so tha t in some there may be little and in others much of the finest material bound into the temporary aggregates wbich it is the function of the acid to resolve. For this purpose the Rothamsted soils provide examples of exceptional value; the management of each of the

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HALL: THE MECHANICAL ANALYSIS OF SOILS. 9.53

Rothamsted plots has been unchanged for 50 or even 60 years, some of them have been receiving organic manures annually during the whole period, others have been entirely unmanured or have been re- ceiving only mineral salts, and are, therefore, much depleted of their original store of organic matter. As a, natural consequence, t h e phy- sical texture, as shown by the tilth, by their tendency to dry into a stony or a friable condition, by the ease with which they can be cul- tivated, and by their water-retaining power, varies considerably in passing from plot to plot, yet i t cannot be considered that the funda- mental physical constitution of the soils has been sensibly altered, so small a fraction of the whole weight even of the surface soil is the sum of all the manures added during the experimental period.

It is assumed tha t the preferable method of mechanical analysis is that showing the essential identity of soils from neighbouring plots which may have acquired very dissimilar textures through differences of cultivation, since information is thus obtained as to the permanent rather than the temporary characteristics of the soil. It has been argued that the problem is not to ascertain the fiindamental constitution but the current condition of the soil ; that if, as is assumed, the working nature of the soil depends on the size of the particles, it matters little whether they are solid as with a sandy soil, or aggregates of smaller particles behaving like larger units as with a clay soil in a good state of tilth. But in the one case, unskilful management will ruin the tilth, which in the other is practically independent of the treatment, and although the two soils may for the time be similar the analysis should aim at gauging their capacity and behaviour under all condi- tions by determining their real nature. If once the mechanical analysis provides an accurate description of the mineral framework of the soil, a consideration of other factors, such as the proportion of humus and calcium carbonate, may enable the analyst t o form an idea of their actual texture. But no method of mechanical analysis can actually measure the texture, a problem which if i t is soluble at all in the laboratory must be attacked by other processes.

The Xoils Exumined.

The soils selected for analysis bave been derived from the Rotham- sted experimental fields; these fields are scattered about the estate and differ somewhat in texture, so that the results for a given field are only comparable among themselves. The soil is a strong flinty loam derived from the “clay with flints ” which covers much of the chalk plateau in Hertfordshire ; the subsoil is distinctly irregular, a neigh- bowing exposure showing besides the true ‘‘ clay with flints ” pockets of almost unaltered ‘‘ plastic clay ” and of “ tertiary ” sand. The fields

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954 HALL: THE MECHANICAL ANALYSIS OF SOILS.

with the exception of Barn Field, are nearly level, and there is evidence that the experimental portions were sensibly uniform at the start.

Of the soil samples examined Nos. 1, 2, 3, 4, were drawn in 1893, and Nos. 25, 26, 27, in 1904. No. 1. Plot 2, received 14 tons of farmyard manure every year

(50 years). No. 2. Plot 3, unmanured every year (50 years). No. 3. Plot 6, received 34 cwt. of superphosphate, 200 lbs. of

potassium sulphate, 100 Ibs. each of sodium and magnesium sulphates, 100 lbs. each of ammonium sulphate and chloride every year since 1852 (42 years).

Plot 9, received a similar manuring to Plot 6, except tha t the ammonium salts were replaced by an average of 500 lbs. of sodium nitrate added annually over the 42 years.

The Broadbalk .Field has grown wheat every year since 1844.

No. 4.

No. 25. Plot 3, unmanured (61 years). Ns. 26. Plot 6, manured as described above (53 years). No. 27. Plot 9, manured as described above (53 years). The Hoos .Field has grown barley every year since 1852. The

No. 5. Plot lo, unmanured every year (52 years). No. 6. Plot 2A, received 34 cwt. of superphosphate, 100 Ibs. each

of ammonium sulphate and chloride every year, No. 7. Plot 2AA, received 33 cwt. of superphosphate and 275 lbs.

of sodium nitrate. Boos Potato Land is a part of the same field which grew potatoes for

26 years, 1876-1901, after which it has grown barley without further manuring.

No. 8. KO. 9.

No. 10.

No. 11. The Palb is grass-land mown for hay each year and has received the

The samples were drawn in

samples for analysis were drawn in 1903.

Samples were drawn in 1903. Plot 1, unmanured continuously. Plot 3, received 14 tons per acre of farmyard manure every

Plot 5, received 200 lbs. each of ammonium sulphate and

Plot 6, received 550 Ibs. of sodium nitrate.

year (26 years).

chloride.

same manures since 1856 (43 years). 1904. No. 12. No. 13.

Plot 3, unmanured continuously. Plot 9, received 34 cmt. of superphosphate, 500 lbs, of

potassium sulphate, 100 lba. each of sodium and magnesium sulphates, 200 lbs. each of ammonium sulphate and chloride. No. 14. Plot 14, received the same t.reatment as Plot 9 except that

the ammonium salts were replaced by 550 lbs. of sodium riitrate. The Barn Field has been under cultivation for roots since 1843, and

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HALL : THE MECHANICAL ANALYSIS OF SOILS. 056

for the last 28 years has grown mangels, the manurial treatment has been practically continuous for each plot since 1856 (48 years). Soil samples were drawn in 1903.

No. 15. Plot lo, received 14 tons per acre of farmyard manure every year,

No. 16. Plot 40, received 3$ cwt. of superphosphate, 500 lbs. of potassium sulphate, 200 lbs. of sodium chloride, 200 lbs. of magnesium sulphate.

No. 17. Plot lc, received 14 tons of farmyard manure, and 2000 lbs. of rape cake every year.

No. 18. Plot 4c, received the same treatment as 40, with the addition of 2000 lbs. of rape cake annually.

No. 19. Plot 4a, received a similar treatment to 4c, except that the rape cake was replaced by 200 lbs. each of ammonium sulphate and chloride.

No. 20. Plot 4 ~ , received a similar treatment to 4 A and 4c, except that the nitrogenous manure was 550 lbs. of sodium nitrate instead of the ammonium salts or rape cake.

No. 21. Plot 5 ~ , received 38 cwt. of superphosphate, and 200 lbs. each of ammonium sulphate and chloride.

No. 22. Plot 5 ~ , received a similar treatment to 5 ~ , except that the ammonium salts were replaced by 550 lbs. of sodium nitrate.

The Barn3eZd Grass-Zund was originally part of the preceding field, but was laid down to grass in 1874; no record has been kept of its occasional manurings.

No. 23. Sample drawn from the portion next to Plot lo. No. 24. Sample drawn from the portion next to Plot lc. All the samples are taken to a depth of nine inches unless otherwise

stated. I n all cases the soil sample is dried at a temperature not exceeding

40°, roughly powdered, and passed through a sieve with round holes 3 mm. in diameter, and all the analyses were made on the air-dried fine earth passing this sieve.

Method of Anulysis.

Ten grams of the fine earth are treated with 100 C.C. of N/5 hydrochloric acid, the acid being renewed if it is not sufficient to dissolve all the calcium carbonate present. The soil is rubbed up for some time without the application of pressure with a soft pestle made of an india-rubber bung at the end of a glass rod ; this pestling may be conveniently effected by the aid of a small water turbine. The undissolved material is then thoroughly washed until free from acid and may be collected on a filter, dried and weighed to ascertain the loss

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956 HALL: THE MECHANICAL ANALYSIS OF SOILS.

on solution, or washed immediately into the sedimenting vessel. In either case after being freed from acid i t is thoroughly pestled again with water to which a few C.C. of strong ammonia have been added. A lipped beaker (about 10 cm. high, and 7 cm. in diameter) is employed with marks a t 8.5 and 7.5 cm. from the bottom; into this the mixture of soil and water is washed through t w o sieves, the first having round holes 1 mm. in diameter, the second being a woven wire sieve No. 100 brass wire cloth, 100 meshes to the inch. The material arrested by the sieves is dried, weighed, ignited and reweighed. The beaker is filled to the mark with water and left undisturbed for 24 hours, The turbid liquid is then decanted from the sediment, which is rubbed up afresh with a few C.C. of ammonia, water is added to the mark, and the 24 hours' sedimentation is repeated. The whole process is repeated day by day until the supernatant liquid is clear after 24 hours; tha t is, until all the particles which will not fall 8.5 cm. in 24 hours are removed. Generally about eight decantations are sufficient, but with some soils as many as fifteen may be required. The turbid liquid poured off is evaporated, and the residue dried in the steam oven, weighed, ignited over an argand burner and weighed again, or the turbid liquid may be coagulated by acidifying and the residue collected by decantation. The sediment remaining in the beaker is worked up afresh in the same way, the water being only added to a depth of 7.5 cm., and left for 2 hours 5 minutes ; the process is repeated as before until all the material re- maining suspended for that period is removed. The decanted liquid is again evaporated, the residue is dried in the steam oven, weighed, ignited and reweighed. A third decantation follows with a water column 7.5 cm. in height, and the time of settlement 12.5 minutes; lastly comes a decantation for 75 seconds with a height of 7.5 cm. as before. Thus t h e whole of the fine earth is divided into seven fractions, the two coarsest by means of sieves, the five finest by settlement from water.

The times of settlement and the height of the water column are purely arbitrary and may be varied at will in order to secure fractions of any desired diameter, the size obtained being determined by actual measurement under the microscope.

The whole process is a tedious one, requiring something like three weeks for its completion and the evaporation of 20 litres or more of water. It can be shortened by the introduction of centrifugal methods whereby the force of gravity causing the fall of the particles is replaced by a much stronger centrifugal force (U.S.A. Department of Agriculture, Yearhook, 1900, p. 399 ; Kilroe, Proc. Roy. BuhZim Xoc. 1904, I. [v], 223). But with systematic working a number of determinations can be carried on simultaneously, and except in the first and final stages, these demand but little time from day to day.

Otherwise it admits of no acceleration.

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HALL: THE MECHANICAL ANALYSIS OF SOILS. 057

The following table gives the approximate size of the particles in each of the fractions obtained by the foregoing method. Examination under the microscope showed tha t whilst no fraction was absolutely free from smaller particles the bulk fell within the specified limits.

TABLE I. Sizes of particles in the groups separated by mechanical anal'ysis.

Diameter in mm.

Max. -3. 1st Sieve ........................... 360 1.0 2nd ,, ........................... 1-0 0 *2 1st Sediment ..................... 0.2 0.04

..................... 0.04 0.0 1 2nd 2,

..................... 0.01 0-004 3rd ? f

..................... 0.004 0.003 4th 9 7

'( Klay " ........................... 0.002 -

The groups of particles obtained in a mechanical analysis do not possess any definite chemical individuality ; in the coarser grained fractions worn grains of quartz predominate, mixed with an increasing proportion of hydrated aluminium silicate and ferric oxide as the size of the particles becomes less, but even the finest fractions contain some free quartz. The following series of determinations show that whilst the coarsest fraction separated by sedimentation contains in the ignited state more than 90 per cent. of quartz and only '7.4 per cent. of aluminium silicate, the finest contains about 10 per cent. of quartz and 67 per cent. of aluminium silicate.

T A ~ L E 11,

Partial analjses of soil fractions separated by mecbanical analysis

Samples ignited to remove organic matter before analysis. involving a preliminary washing with acid.

Nature of sample. SiO,, Fe,OB. A120,. 1st Sediment * ... 94-6 1 . 1 3.4 2nd ) 9 92.0 1.2 6 .2 3rd 9 9 88.3 1'8 8.5 4th I? 61.7 7 *o 23.4

* Mixed samples.

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05s HALL: THE MECHANICAL ANALYSIS OF SOILS.

TABLE I1 (continzled). Nature of sample. SiOl. ‘IKIay” No. 22 45.6

1 7 ;, 21 45*4 9 9 ,, 20 46.3 ?7 ), 19 47.3 9 9 ,, 4 45.2 7 9 ,) 3 44.7 9 ) ,, 8 48.6 9 , ,, 10 44-5

Fe,O,. 12.8 12.2 12.4 12-7 11% 12.4 10.5 12.5

Al,O,. 30.1 30.3 33.8 31-6 29.9 30.3 31.2 29 9

Mean of “Klays” 45.9 12.2 30.9

The presence of organic matter does not permit of an exact deter- minaiiion of the proportion of water of hydration, but by recalculating the analyses (which were all made on samples after ignition) on the assumption that the alumina is combined as A1,0,,2SiO,,2H2O, the approximate percentage composition of the finest fraction would be : kaolinite, 72-75 ; ferric oxide, 11-12 ; quartz, 9-10 ; alkalis and alkaline earths, kc., 4-6; nor is there much variation from this mean, at least in dealing with soils of similar origin, as with these from the Rothamsted fields.

It is probable that the physical proFertics of the soil are dependent on the size rather than on the chemical nature of the constituent particIes, being determined in the main by the extent of the surface they expose. Deposits may be found consisting mainly of very finely divided quart.z, which have all the properties usually associated with clay soils, the opposite case of coarse-grained kaolinitic material-chemically clay -but behaving more like sand, also occurs, Hence, a difficulty arises in giving the name of i‘clay’’ to the finest fraction obtained by mechanical analysis, and i t has been suggested that this fraction should be distinguished by the designation (‘ klay,” this being defined as the material existing in the soil which possesses a diameter of less than 0.002 mm.

The theory of a particle falling through a viscous liquid has been considered by Stokes (Trans., Camb, Phil. Xoc., 1851, 9, S), who dealt with the motion of small water drops like fog through the air, and by Barus (Amer. J. Xci., 1889, 37, 122). From Stokes’s paper is derived the following formula :

2gfi“v - P) , v = 9 q

for the velocity of the falling particle, where u is the density of the

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HALL: THE MECHANICAL ANALYSIS OF SOILS. 959

Calculated at

LO". 1 16".

-II_

particle, a its radius taken as a sphere, p the density, and T the coefficient of viscosity of the medium. Utilising this formula, the following diameters are obtained for the particles which just fall in the times specified for each group.

Minimum by observatio~i.

Sediment.

1st ............ 2nd ............ 3rd ............ 4th ............

Limiting velocity of fall.

7 5 cm. in '15 seconds 7'5 ,, 750 ,, 7'5 ), 7500 ,, 8'5 ) ) 86400 ),

Diameter in mm.

1 ,-- i

0.040 0'037 0'01 0.0126 0'0116 0.01 0'0040 I 0*0037 1 0.004 0-00126 j 0.00126 0'002

The agreement is as close as can be expected considering that the particles are not spherical, and that no precautions are taken to avoid convection currents due to change of temperature during the longer intervals of sedimentation. The use of a dilute solution of ammonia of uncertain strength instead of pure water might be thought tovitiate the value attached to 77, the coefficient of viscosity, but a series of com- parative measurements shows that various ammonia solutions, up to a much greater strength than is ever attained in carrying out the fore- going method, possess sensibly the same viscosity as pure water through a greater temperature range than that of an ordinary laboratory.

Rxamination of Xoils Xich and Poor in, Humzcs.

Analyses have been made by the foregoing method of eighteen of the Rothamsted soils, six being rich in humus through the accumula- tion of organic manures or after lying for some time in pasture, twelve being deficient because of continuous cropping either without manure or with mineral salts only. I n order to obtain a critical opinion as to the effect of the preliminary washing in acid, the same eighteen soils have also been analysed without such treatment, that is, the raw soil has been rubbed up in distilled water onlyand separated by sieves and by sedimentation, using precisely the same times of settlement and distances of falling.

The results are set out in the following tables, of which Table I11 gives the results when the material is weighed after drying at looo, and Table IV after ignition to remove organic matter.

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9.69

s.z 9 1

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9.9 P. & _-

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L.S 9.5.Z Z.0Z 6.6 9.E

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C*L 6.51

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P.61 9.02

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quam!pas

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....... UlUI 1-E

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.............................. .................. .................. ..................

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'AXXX'I "IOA E22E P w

.".." .-*"

.... .... .... .... ..-. .... .... .... .... .... ... .... .... .... .... .... .... ...

wwa ct

$G ; g.2 i E : 6.g

... ... ... ... ... ... ... ... ... ... ... ... Dung.

Unmanurecl.

Minerals and sodium nitrate.

Unman wed.

Minerals and ammoniuni salts.

Minerals and sodium nitrate,

c @

UV OT

Wntuanurcd.

Dung.

- Minerals

only.

Dung.

Grass-land. I

Minerals and ' rape-cake. ~

Dungand 1 rape-cake. I Fo

Grass-land. 1 Minerals and,

ammonium sslts.

Superphos- pliate and aminonium salts.

Minerals and sodium nitrate.

Superphos- phate and sodium nitrate.

196 'STIOS do SISATBNV WOLNBH33MI 3HL : TIBH

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962 HALL : THE MECHANICAL ANALYSIS OF SOILS.

A high degree of precision must not be expected from the method, partly because errors due to manipulation readily occur, and even more because of the impossibility of drawing true samples of soil for the analysis, first in the field and then in the laboratory. In the field any slope will mean some variation, whilst every cultivation and every heavy rainfall will to some extent rearrange the physical constitution of the soil. It will be seen, however, that these inevitable variations are never great enough t o mask the essential identity of samples drawn from the same area.

The consideration of the results is facilitated if they are recalculated after eliminating the items most subject to variations unconnected with the problem under consideration, that is, the moisture lost at looo, the loss on solution and ignition, and the coarse material arrested by the sieves, which is too coarse-grained to be accurately distributed in a sample weighing 10 grams only.

The latter fractions constitute only about 10 per cent. of the whole soil and may be dismissed without more consideration in the present case, but they can be determined with accuracy by estimating separately in a sample weighing at least 50 grams,

The figures, however, are not markedly affected by recalculation on such a basis, and it will be sufficient t o confine attention to Table IV, in which the varying proportion of organic matter, sometimes high in the finer fractions, has been removed by ignition.

From a consideration of this table the following points will be apparent :

(1) Preliminary treatment with acid results in a greater propor- tion of ‘‘ klay ” than does the method of working directly on the raw soil, the deficiency in the ‘‘ klay ” fraction in the latter case being generally made up by an increase in the next coarser fraction, and a still smaller increase in the fraction above, the two coarsest fractions being practically identical by both methods.

(2) The differences between the two methods, which are greatest when there is much organic matter in the soil, practically disappear when the soil has been long unmanured, or has been deflocculated by the continued action of saline manures, I n the six dunged or grass- land soils, with an average loss on ignition of 7.S per cent., the pro- portion of ‘‘ klay ” is 1503 per cent, when working on the raw soil and increases to 19.3 per cent. after the acid treatment; on the twelve other soils, with an average loss on ignition of 6.1 per cent,, the in- crease is from 16.6 per cent. in the raw soil t o 17.2 per cent, as deter- mined after treatment with acid.

(3) Soils which from their proximity we have every reason to consider as identical give similar results by the acid treatment, what- ever their manuring has been, but show wide variations when the raw

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HALL: THE MECHANICAL ANALYSIS OF SOILS. 963

soiI is examined. As a striking example the soils 17, 18, and 24, are all within a few yards of one another in a uniform part of the field, but 18 has been receiving but little organic and much saline manure, 17 has had heavy dressings of organic manures, and 24 has been laid down to grass for many years, By the acid treatment the proportions of “ klay ” are 21.6, 23.7, and 22.0 per cent. respectively, whilst the raw soil gives 20.2, 23.9, and 11.1 respectively. The acid treatment brings together into one gronp all the soils of similar origin whatever has been their subsequent management ; the results obtained by work- ing on the raw soil depend also on the treatment the land has been receiving. To this conclusion one exception must be made ; soils 20, 22, and 27, show a lower proportion of L f k l a y ” than the other soils from the same field ; this difference, which is a real one, being visible whether the analysis is made on the raw soil or after treatment with acid, will be considered later.

These results lead to the conclusion that the preliminary treatment with acid gives a truer picture of the ultimate physicaI constitution of the soil than that obtained by working on the raw soil, The acid employed is too weak to cause any reduction in the size of the coarser order of particles such as would bring them within the ‘‘klay” limits ; were it so, the increased propoztion of “ klay ” due to the acid treatment would not be confined to the soils rich in organic matter, but would be equally manifest with deflocculated soils like 20 and 22, or 19 and 21. These soils give practically identical results by both methods, which is the best evidence that no new error is introduced by the acid treatment. But when working on raw soils well provided with organic matter much of the “ klay ” is cemented into larger groups by the humus, and these aggregates so far resist the repeated operations of pestling as to be weighed in the coarser fractions, It is not probable that any amount of rubbing up or boiling will completely resolve them, hence the actual amount of ‘‘ klay ’’ determined will, to some extent, depend on the thoroughness with which these processes are pursued. As however the necessary resolution is fully carried out by the acid treatment, as it also removes all salts capable of causing flocculation, it may be concluded that washing in dilute acid is an essential preliminary to the mechanical analysis of soils,

Further justification is seen in the fact that, when working on the Rothamsted soils, the acid method shows the essential identity of all the soils from the same experimental field, whereas separationp made on the raw soils give very dissimilar results, depending on the treat- ment to which the various plots had been subjected,

THE ROTHAMSTED EXPERIMENTAL STATION, HARPENDEN,

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