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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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| 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.