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Volume 48, 1915
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Art. LV.—Studies on the Lime Requirements of certain Soils.

[[Read before the Philosophical Institute of Canterbury, 1st December, 1915.]

I.

Under this heading Hutchinson and MacLennan, in the Cambridge Journal of Agricultural Science for March, 1915, describe a very rapid and simple method for determining the lime requirement of soils. Since so much is spoken and written, while so little is really known about the lime requirements of our New Zealand soils, it seemed desirable to test the validity of this method, which claims to give valuable and accurate information for very little trouble. The present article contains an account of such an inquiry, together with the results obtained from trials with certain Canterbury soils.

The method is based on the absorptive capacity of the soil for calcium carbonate presented to it in solution as bicarbonate. A known quantity of bicarbonate in solution is left in contact with a known weight of soil for a few hours, at the end of which time the amount of lime in the solution is again determined. The quantity of lime lost by the solution is the quantity required to satisfy the given weight of soil, and from these figures the percentage requirement of the soil, and hence the required weight of lime per acre, is calculated. To obtain this last result, the apparent density of the soil must also be determined.

The calcium-bicarbonate solution is easily prepared in an ordinary “sparklet” or refillable “soda-water” apparatus, for which bulbs of compressed carbon dioxide are used. Into this an excess of finely divided calcium carbonate (10 gr. to 12 gr. pure pptd CaCO3) is put before the gas is admitted. The solution of CaH2(CO3)2 thus formed is siphoned off, and about one-third of its volume of water is added, after which it is filtered free from undissolved carbonate, standardized by titration with decinormal sulphuric acid, using methyl orange as indicator, and is ready for use. The strength of a solution prepared in this way is about one-fiftieth normal.

Ten grams of the soil under investigation is placed in a bottle or flask of about 500 c c. capacity, and 200 c.c. to 400 c.c of the solution is added. The air in the bottle is displaced by a current of CO2 to prevent the precipitation of CaCO3, and the bottle is placed in a shaking-machine for three

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hours, or shaken by hand at intervals of about twenty minutes for four hours, at the end of which time a portion of the solution is filtered, and its strength again determined by titration with decinormal acid.

II

In the first trials of the method made in this laboratory the results shown in Table A were obtained

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Table A.
Requirement of CaO indicated
Soil Percentage Pounds per Acre
21* 0 18 2,450
4 0 16 2,178
16 0 18 2,450
29 0 14 1,906
Waipara Downs 0.13 1,770
Motueka 0 42 3,718

It will be noticed that the figures expressing the weight of lime required per acre are higher than the average farmer would care to believe. The Motueka soil is a very unpromising sample from the apple lands, and, as it is known from an analysis to be devoid of calcium carbonate, the extremely high figure appears not unreasonable. The question, however, presents itself, Suppose we give this or any other soil a preliminary dose of carbonate of lime, will the demand as indicated by the method under trial be correspondingly lowered? With a view to testing this, several experiments were made. To 10 grams of the Motueka soil was added 0.06 gi CaCO3 (that is, 0.336 per cent. CaO), 100 c c of water was poured on, and the bottle left for about an hour, with occasional shaking. The bicarbonate solution was then added, and the rest of the operation completed in the regular way. The expected lime demand was then 0.42 less 0.34—that is, 0.08 per cent, the actual requirement indicated was 0.18 per cent. However, it was considered that the short preliminary exposure of the soil to the lime might account for this difference, and so a more comprehensive trial was arranged. Two soils were taken, and determinations were made in duplicate of their lime requirements, both in the natural state and after twenty-four hours' treatment, with varying quantities of CaCO3 in 100 c c of water. The results thus obtained are shown in the following table.

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Table B
Original Lime Requirement Equivalent of CaO added Lime Requirement finally expected Lime Requirement actually indicated
Soil. Percentage Pounds per Acre Percentage Pounds per Acre Percentage Pounds per Acre Percentage Pounds per Acie
21A 0 136 1,854 0.062 845 0.074 1,009 0.076 1,037
21A 0 136 1,854 0.118 1,609 0 018 245 0 019 260
21B 0.181 2,464 0.185 2,520 −0.004 −56 −0.003 −42
21B 0.181 2,464 0.213 2,900 −0 032 −436 −0 037 −498

[Footnote] * Numbers in all cases refer to numbers of fields on the Lincoln College farm.

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These results are very satisfactory; but it is obvious that a more practical test of the method would be to determine by it the lime requirements of two similar and adjacent soils, one of which had received a known dressing of lime at a sufficient length of time previously to allow of its being incorporated with the soil. Unfortunately, such conditions do not obtain on the College farm at the present time; for, though a good deal of liming has been done on grass land during the past winter (1915), it is certainly not yet incorporated with the soil, owing to the abnormally low rainfall. However, the following trials were made with such material as was available :—

  • (1)

    A sample was collected from field 21, consisting of thirty-three cores taken 4 in. deep, the assumption being that the 3 in. of rain received since the field was limed in June had distributed the lime through that depth of soil. Determinations were made on this sample, and, for comparison, on one collected from the same field before the application of the lime.

  • (2)

    It is known that the east end of the College playing-field was heavily dressed with air-slaked lime about ten years ago, with a view to the eradication of rushes. Samples were therefore taken 6 in. deep from the east end, from the west end, and from among a patch of rushes in the northwest corner.

  • (3)

    A determination was made on a soil from Weka Pass, formed from limestone rock, and obviously containing a high percentage of calcium carbonate. The results of this set of trials are shown in Table C.

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Table C.
Requirement of CaO indicated.
Soil Percentage Pounds per Acre.
21 (before liming) 0 212 2,887
21 (after liming) 0 176 2,398
Play-field, east end 0.19 2,586
Play-field, west end 0.18 2,450
Play-field, north-west corner 0.24 3,267
Weka Pass −0.05 −680

These results presented several interesting features. First, the difference in lime requirement of the two samples from field 21 is about 4½ cwt., while the amount of lime actually applied was 5 cwt. or 6 cwt., so that the agreement is fairly close. There is practically no difference in the indicated requirements of the two ends of the football-field, one must assume, therefore, that ten years is sufficient time to neutralize the effect of a dressing of lime in the top 6 in. As was expected, the corner of the ground covered with rushes showed a markedly high requirement. The limestone soil was able to give lime to the solution.

III.

Early in the course of the work it was noticed that the determinations made at different times on the same soil gave different results. Thus the following figures for the percentage lime requirement of field 21 have been obtained on different occasions: 0–14, 0.15, 0.17, and 0.18. Expressed in

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pounds per acre, the range is from 1,900 lb. to 2,400 lb.—that is, the highest figure is more than 25 per cent, greater than the lowest. It was recognized, however, that the determinations which failed to agree had been made under different conditions as to volume and strength of bicarbonate solution, and so it seemed necessary to make sets of determinations simultaneously on one soil under uniform conditions. Results of such are shown in Table D.

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Table D
Requirement of CaO indicated.
Soil Set No Percentage Pounds per Acre
21 (before liming) A 1 0.143 1,948
" A 2 0.151 2,056
" A 3 0.143 1,948
" B 1 0.179 2,440
" B 2 0.179 2,440
21 (4 in. deep after liming) C 1 0.156 2,126
" C 2 0 156 2,126

In sets B and C 400 c c of solution was used, in set A only 300 c c, the strength of which was also rather less than in the case of the other sets.

As a further test, three samples of soil from field 16 were treated simultaneously—A with 300 c c of solution of strength 0–02 N, B with 400 c c of the same strength, C with 400 c c. of strength 0–014 N. The lime requirements indicated in these three cases were—A, 0–11 per cent, B, 0–12 per cent, C, 0–05 per cent. It seems, therefore, that the volume of solution may be varied within fairly wide limits without affecting the result, provided that the solution does not at any time fall below a certain concentration. This is an obviously suggestive point, which merits investigation.

IV.

In one experiment a positive, though small, lime requirement was indicated for a limestone-derived soil, and this suggested the possibility of a weakening of the solution (which corresponds to a lime requirement) from physical as well as from chemical causes. To test this idea a series of trials was made. Four soil-samples that had already been in contact with solution for twenty-four hours in connection with previous trials, and which were now presumably satisfied as regards their lime requirements, were filtered from their old solutions, and treated again with fresh solution. A sample of sand was prepared by treating alternately with concentrated HCI and strong ammonia solution, washing thoroughly, and separating a uniform sample by sedimentation. About 9 grams of this sand was treated in the same way as the soils. Another bottle contained a soil derived from limestone from Waikari, and, lastly, a bottle of the bicarbonate solution without any soil at all was put through the same processes as the others samples of this series. The results are given in Table E, the first four soils being which, having been previously treated, were presumably already saturated.

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Table E
Requirement of CaO indicated.
Soil Strength of Soil Percentage. Pounds per Acre.
Nelson* 0.024 N +Means a positive requirement. 0.042 +572
Warkari * 0.024 N +0.025 +342
21 (after liming)* 0.024 N +0.051 +695
Sand 0–019 N Nil. Weka Pass†; 0.019 N − Means that the solution has taken up lime from the soil. −0.05 − 680
Check-bottle 0.024 N Nil
Waikari 0.025 N + 0.05 + 680

These results indicate that all soils remove a certain quantity of lime from the bicarbonate solution independently of their actual lime requirements, provided that the solution is above a certain concentration initially. The Weka Pass soil can scarcely be in a different chemical condition as regards lime to that from Waikari, since both contain a large excess of calcium carbonate; and yet whereas the Waikari soil removed lime from solution, that from Weka Pass gave it up to its solution, the strength of which was increased from 0–019 N to 0–02 N.

No attempt is made in this paper to explain this phenomenon, nor will any attempt be made to give a definition in chemical language of the term “lime requirement.” The aim of the work herein described is to find whether the method gives results for a given soil which agree with what is known from other sources of the lime requirements of that soil. Finding this to be sufficiently near the case for practical purposes, the writer is for the present prepared to accept the statement of Hutchinson that the amount so indicated is actually the optimum for plant-growth. It has been shown, however, that the result for any given soil varies with the strength of the solution, and that in practice it is necessary to make the determination under standard conditions. Either a solution of uniform strength must be employed for all determinations, or a correcting factor must be applied. As sufficient data has not yet been obtained to enable one to select a reliable correcting factor, the use of a solution of standard concentration is recommended, for by this means strictly comparable results are obtainable.

It may be added that this work has brought out many points of more theoretical interest, which will be discussed in another communication.

[Footnote] * Second treatment of sample.

[Footnote] † Result given in Table C, and repeated here for comparison.