58 SMITH, COMRIE AND SIMPSON: THE EVALUATION OF LIMING [Vol. 76 The Evaluation of Liming Materials for Agricultural Purposes BY A. M. SMITH, A. COMRIE AND K. SIMPSON (Presented at the meeting of the Society on Wednesday, October 4th, 1950) Experimental evidence is submitted to show that the' value of a burnt lime for agricultural purposes may be seriously under-estimated by the method prescribed under the Fertiliser and Feeding Stuffs Act. Calcium silicates and magnesium oxide are able to neutralise soil acids, and the latter is also important as a source of magnesium in certain soils. Various methods of estimating the neutralising value have been examined and a simple technique, involving a short treatment with dilute acid and applicable to both carbonates and limes, has been found t o give results in agreement with the effects produced on soil acidity in pot and field experiments.THE question of the assessment of the agricultiural value of a burnt lime has been raised many times in the last twenty years. The official methodl makes use of the fact that a sugar solution dissolves calcium hydroxide only, and gives a value for the free calcium oxide in a sample. It is thus quite satisfactory for the analysis of a lime obtained from a properly- burned calcium carbonate limestone, where the calcium oxide is the only material of neutralising value. The method does not take iinto account any carbonate that may be left as a result of under-burning and it ignores almost entirely the presence. of any magnesiumFeb., 19511 MATERIALS FOR AGRICULTURAL PURPOSES 59 oxide, which has a greater neutralising value than calcium oxide and, incidentally, is invaluable as a source of magnesium in magnesium-deficient soils.Many b u n t limes contain other compounds that are of value in reducing soil acidity and increasing the amount of exchangeable calcium in the soil. These compounds are of the nature of calcium silicate and may be produced when limestones containing considerable amounts of siliceous material are burned. They are insoluble in sugar solution, but are so readily hydrolysed that they decompose ammonium salts and neutralise soil acids to an equivalent extent; they also dissolve easily in dilute acid and are, without doubt, of value as “liming material.” The agricultural value of many burnt limes is, therefore, seriously under-estimated by the official method.Attention was first directed to this question in 1931 in a communication to the Agricultural Education Association2 concerning the behaviour of a waste carbonate after burning. It had long been appreciated that basic slag had a “lime value” greater than that corresponding to the small proportion of free calcium oxide it contained. Indeed it has frequently been claimed that a high-grade slag has the same lime value as an equal weight of limestone. This is an exaggeration, but a slag with a high solubility in citric acid does reduce soil acidity to the same extent as about half or two-thirds of its weight of calcium carbonate, because it contains compounds which readily provide an acid soil with exchangeable calci~m.~ A large proportion of the calcium in slag occurs in combination with phosphorus and silica as a result of the reaction at a high temperature between lime and the impurities in pig iron.Similar types of siliceous compounds are probably produced in a lime kiln and so it was not surprising to find that the lime value of certain burnt limes was indeed greater than indicated by their contents of free calcium oxide. METHODS OF ANALYSIS A new chemical method of evaluating an agricultural lime must be based upon convincing data obtained from experimental work with soils. It is difficult to obtain precise results on the effects of liming materials in the field on account of sampling errors, but it is nevertheless important that the accurate observations from laboratory and pot culture experiments should be supported by figures found under natural conditions.In a preliminary approach to the question a number of burnt limes in use as liming materials were examined by five methods and the results are summarised in Table I. To measure the effect on soil acidity, a series of flasks each containing 20g of soil, 50ml of boiled distilled water. and different quantities of the burnt lime were shaken at intervals TABLE I Burnt lime A €3 C D E F G H I THE EVALUATION OF BURNT LIME BY DIFFERENT METHODS Equivalent CaO by CaO by f A I -------- Soil titration NH,Cl distillation T.N.V. Cone. acid Sugar method (a) (b) (4 % % % % 54 66 67 83 86 88 92 90 97 57 65 68 81 82 83 91 90 89 57 59 33 65 69 46 68 73 46 82 81 61 80 84 66 82 86 66 88 81 70 87 86 71 87 86 77 METHODS EMPLOYED- 0.2 g of material + 40 ml of N ammonium chloride $- 700 ml of water boiled for 90 to 120 minutes, the distillate being collected in standard acid and the NH, expressed as Cs.0.Total nezctralisiwg vdue-0.5 g of material + 50 ml of 0-5 N hydrochloric acid boiled for 3 minutes. cooled and excess of acid titrated with 0-5 N sodium hydroxide with phenolphthalein as indicator; the result is expressed as equivalent CaO. Suitable quantity of material boiled with concentrated hydrochloric acid for 10 minutes and calcium in filtrate determined by standard method. The official method of determining caustic lime. (a) (b) (c) ( d )60 SMITH, COMRIE AND SIMPSON: THE EVALUATION OF LIMING during several days until an approximate equilibrium was attained, and the final pH values of the suspensions were plotted against the amounts of lime added.A similar curve was obtained with a standard, usually precipitated calcium carbonate, and the relative value of the lime in reducing the acidity was estimated from the two curves. Each figure given in Table I is the average for three acid soils and represents the percentage efficiency or lime value of each material compared with a standard treatment. To take a particular example, 54 parts of lime (CaO) reduced the soil acidity to the same extent as 100 parts of sample A, at a point on the curve corresponding to an application of about 1 ton of lime per acre. This method is a refinement and extension of that used in the routine estimation of the “lime requirement’’ of soils, where equal portions of a particular soil are shaken overnight with equal volumes but different concentrations of lime water and the final pH values are plotted against the amount of lime added.4s6 The amount of lime required to bring the pH value of the soil to any level desired in practice can be interpolated on the curve and the dressing required in the field is calculated with or without a factor according to circum- stances.Although the method is not suitable for the routine examination of liming materials it has been employed frequently in special cases and it has given valuable evidence in testing the reliability of the measurement of total neutralising value (T.N.V.). The method cannot be as precise as a chemical determination because the time required to reach equilibrium depends upon the nature of the soil and the physical and chemical properties of the liming material concerned.But it is possible to obtain smooth titration curves (some examples are shown in Fig. 1) on which interpolations at a particular pH value may be made. [Vol. 76 Fig. 1. Soil neutralisation curves and pot results The results clearly indicated that the ability of the lime to reduce the acidity of different soils was equivalent to its power (a) to decompclse a solution of an ammonium salt and (b) to neutralise dilute hydrochloric acid. These values were in turn only slightly less than (c) the percentage of calcium oxide extracted by boiling concentrated hydrochloric acid, but were always much greater than (d) the percentage of calcium oxide obtained by the official method.For many years, the values obtained by method (d) have, without exception, been low in comparison with those obtained by methods (6) and (c). For example, the last 30 samples of “lime” examined have given the results shovin in Table 11. Many of these samples were under-burned, but in the majority the amount of carbon dioxide was’less than 3 per cent. These results fully confirm the evidence in. Table I, and show that on the average the official method has under-estimated the “liming” value or T.N.V. of the burnt lime by aboutFeb., 19511 MATERIALS FOR AGRICULTURAL PURPOSES 61 18 per cent., the range of differences being from 1 to as much as 30 per cent. However, the T.N.V. figure was always within 4 of the percentage of bases dissolved by boiling concentrated acid and expressed as CaO.TABLE I1 ANALYSES OF 30 SAMPLES OF BURNT LIME Fraction measured (1) Total CaO by concentrated acid . . .. (2) Total MgO by concentrated acid . . .. (3) CaO equivalent to (1 + 2) . . . . .. (4) Free CaO (official method) . . . . . . (5) CaO equivalent (T.N.V. method) . . .. Value (5) less value (4). . .. . . .. Value (3) less value (5). . .. . . .. Range Average 39.7 to 95.3 79.1 0 to 3.1 1.1 41.4 to 95.3 80.7 8.7 to 91.1 60.6 40.0 to 94.0 78.3 4-1 to +30 fl to +a - - Of the various methods mentioned above, that giving the so-called total neutralising value is by far the simplest. It has been employed regularly by us during the past six years as a supplement to the other methods because it was felt that it gave a reliable estimate of the agricultural value of the liming material.It can equally well be used for samples of ground limestone or waste carbonate or shell sand, the result then being calculated in terms of equivalent CaCO, instead of CaO. For example, the last 30 samples of such materials examined have given the results shown in Table 111. TABLE I11 ANALYSES OF 30 SAMPLES OF VARIOUS CARBONATES Fraction measured Range Average (1) Total CaO by concentrated acid . . . . 20.2 to 53.1 41.8 (2) Total MgO by concentrated acid . . .. 0 to 9.0 1-1 (3) CaCO, equivalent to ( 1 + 2) . . .. . . 36.0 to 94.8 77.5 (5) CaCO, equivalent (T.N.V. method) . . .. 43-4 to 96.5 77.7 (4) CaCO, equivalent to CO, . . .. .. 36.0 to 96.1 77.6 Value (3) less value (5).. .. . . .. -2.8 to +4*0 - The agreement would seem to be adequate for this type of agricultural material, since the limit of variation permitted by the Fertiliser and Feeding Stuffs Act is, for limestone, 5 per cent. of the CaCO, stated, and for quicklime, 10 per cent. of the CaO. The total neutralising value method is, therefore, applicable to both carbonates and limes and so gives a fair valuation for agricultural purposes of a badly burned or partially burned limestone ; moreover, it takes account of the neutralising value of the magnesium oxide and other basic substances. P O T EXPERIMENTS Having established that the total neutralising value was in fact a good measurement of the ability of a liming material to reduce soil acidity in the laboratory, it was used as the basis of comparison for different liming materials in pots and in the field.In one set of pot experiments, two highly unsaturated soils-a mineral soil, W, derived from glacial sand, with a pH value of 4-7 and a loss on ignition of 9 per cent., and a thin peaty soil, P, overlying acid andesite, with a pH value of 3.3 and a loss on ignition of over 80 per cent.-were each treated with calcium hydroxide, at two rates, and with fine and coarse fractions of three different limestones. The “fine” fraction consisted entirely of particles passing a 100-mesh sieve; the particles of the “coarse” fraction lay between 100-mesh and 1 mm in diameter. All the materials were used in equivalent amounts based upon the total neutralising values that are given in Table IV in terms of CaCO,; the calcium hydroxide was also used at half that rate.The liming materials were thoroughly mixed with the air-dry soils before potting, and the treatments were replicated. The pots were cropped in successive years with peas, barley and peas; the pH value of the soil in each pot was determined from time to time. For comparison, a range of treat- ments with each material was tested in soil suspensions under laboratory conditions to obtain titration curves. A selection of the results is presented in Table IV and a few typical curves are shown in Fig. 1.62 SMITH, COMRIE AND SIMPSON: THE EVALUATION OF LIMING [Vol. 76 TABLE IV EFFECT OF DIFFERENT LIMING MATERIALS ON SOILS IN POTS p1-I of soil W pH of soil P -7 r--------h-_--\ T .N.V. Weeks in pots Weeks in pots Equiv. as Lab. ,-A-, Lab. /-A-, Material CaO MgO CaCO, CaCO, expt. 35 73 116 expt. 35 73 116 A1 Ca(OH),-half-rate . . 75.6 - 136.0 131.8 6.1 5.5 5.9 5.5 4.8 5-0 4.9 4.8 A2 &(OH),-full-rate . . 75.6 - 135.0 131.i3 6.9 6.2 6.3 6.0 6.0 5.7 5.6 5.6 B1 Limestone-fine . . 40.2 6.5 87.8 90.5 6.7 6.5 6.4 6.3 6.1 5.7 5.6 5.7 B2 Limestone-coarse . . 44.6 5.2 95.8 92.0 6.2 5-9 6.3 6.4 5.3 5.7 5.7 5.8 D1 Limestone-fine . . 49-8 1.8 93.0 90.’7 6.7 6.3 6.5 6-3 6.3 5.6 5.5 5.7 D2 Limestone-coarse . . 52.6 1.8 98.0 95.!i 6-2 5-9 6.5 6.5 5.5 5.7 5.2 5.8 F1 Limestone-fine . . 31.1 20.3 106.4 105.3 6.4 6.4 6.6 6.4 6.2 5.8 5.8 6.1 F2 Limestone-coarse . . 31.7 20.1 106.8 107.0 5.7 5.7 6.1 6-3 5.3 5.5 5.4 5.9 The results show that- (a) The T.N.V.figures were substantially the same as the equivalent CaCO, calculated from the total calcium and magnesium. (b) With hydroxide and fine limestone, the soil pH values reached higher levels in the laboratory suspensions than in the pots; compare curves and points A, D1 and Fl in Fig. 1. This was possibly the result of leaching of the fine material from the pots that were kept outside throughout the experiment. With the “coarse” fractions, however, the soils attained higher pH values in the pots than in the laboratory; see points D2 and F2 in Fig. 1. This was probably because the duration of the laboratory experiment was too short for complete reaction between soil and coarse particles. In the pots, the fine and coarse fractions ultimately gave the same result but more quickly in the peaty soil than in the mineral soil.(c) All the materials, added in equivalent amounts according to their total neutralising values, produced essentially the same reduction in acidity as measured by the pH values of the soils. FIELD EXPERIMENTS It is easy to prepare an intimate mixture of soil and liming material for a pot experiment but it is very difficult to get an even distribution of a liming material to a particular depth in the field. Furthermore, sampling errors in the field are relatively large, so that a smooth curve depicting the change in pH value is not to be expected from a series of samples. The trouble may be particularly acute for an application of ground burnt lime; as a result of slaking and carbonation, small pockets of the inaterial can form quite hard lumps, which do not disintegrate easily and remain very inefficient from the point of view of neutralising soil acidity.A limestone ground to pass a 100-mesh sieve may, therefore, provide a larger surface in contact with soil particles than a ground lime, and so reduce soil acidity more rapidly. Large particles of limestone have slower rates of reaction according to their size and hardness. Eventually, however, all forms will exert an effect depending upon their relative contents of available calcium and magnesium. Waste carbonates from paper works may contain some sodium hydroxide, but the amount is generally less than I per cent. In recent field experiments, designed to compare different forms of liming materials, the dressings have been based on the total neutralising values of the materials used.The primary object of these experiments was to measure the response in crop yield, but it was sometimes possible to sample the soil of the individual plots at intervals during two or three seasons, and to compare the pH values with the results expected from the titration curve obtained in the estimation of the “lime requirement” of the original soil. A selection of average results from four experiments on differei-it soil types is given in Table V. The letters L, LS and w represent respectively ground burnt lime, ground limestone and waste carbonate, and the dressings are equivalent amounts of CaO calculated from the total neutralising values (T.N.V.). For example, 2 5 ~ means that ground burnt lime was added in amount sufficient to supply 25 cwt of CaO per acre according to its T.N.V.The reduction in acidity was responsible for large increases in the yields of barley and roots in experiment 1, but the difference between equivalent amounts of lime and limestoneFeb., 19511 MATERIALS FOR AGRICULTURAL PURPOSES 63 was not significant. Treatment did not produce a significant increase in roots in experiment 2 and only the smaller dressings produced a larger yield of beet tops and sugar in experiment 3. In experiment 4, equivalent amounts of lime and of two fractions of limestone produced the same yields. The highest pH values reached in experiments 1, 2 and 3 were quite in accord- ance with the dressings applied and with the calculated values.In experiment 4, equivalent amounts of lime and finely ground limestone had effected the same increases in pH value, but it was doubtful if equilibrium had been reached with the other materials in the period of less than 8 months. TABLE V EFFECT OF DIFFERENT LIMING MATERIALS ON SOIL AND CHOP 1. 2. 3. 4. NIDDRIE MAINS-sandy loam, drainage good, original pH value 4.7. Cwt of CaO per acre, 25.3.1947 . . . . nil 25L 50L 1947 CYOP- Barley grain (cwt/acre) . . . . 0.61 2.82 3.87 Barley straw (cwt/acre) . , . . 0.89 2.87 3.56 Turnips y t s (tons/acre) . . . . 13.3 24.0 25-6 1948 GYOP- { ops (tons/acre) . . . . 2.27 3.82 4.54 pH at 11.2.1948 . . . . . . . . 5.0 5.1 5.2 19.11.1948 . . . . . . . . 4.8 5.3 5.8 21.3.1949 . . . . . . . . 4.7 5.3 5.8 14.9.1949 .. . . .. . . 4.7 5.2 5.9 Calculated pH* . . . . . . - 5.4 6.0 Rainfall was low in 1947 and high in 1948. CAMMERLAWS-Heavy loam, drainage poor, original pH value 5.8. Cwt of CaO per acre, 18.3.1948 . . . . nil 2 0 ~ s 5 2 ~ s Swedes { pH at 16.11.1948 . . . . .. . . 5.7 6.0 6-3 22.3.1949 . . . . . . . . 5.6 6.0 6.8 Calculated pH* . . . . . . . . - 6.1 6.5 AYTON LAW-Medium loam, drainage fair, original pH value 6.1. Cwt of CaO per acre, 2.2.1948 . . . . nil 1 4 ~ s 3 0 ~ s roots (tons/acre) . . . . 22.1 23.5 21.0 tops (tons/acre) . . . . 5.37 5.11 4.95 roots (tons/acre . . .. . . 9.9 11.3 10.1 tops (tons/acre) . . . . . . 10.4 12.3 20.8 sugar (cwt/acre) . . . . . . 33.8 38.8 34.5 . . .. . . 6-2 6.5 6.9 22.3.1949 . . . . . . . . 6.1 6.5 6.8 Calculated pH* . . . . . ... - 6.5 7.0 pH at 30.11.1948 . . 1 OOL 3.59 3.21 30.4 5.22 5.3 5.8 6.4 6.6 6.6 5%- 20.7 5.48 6.6 6.7 6.5 30w 10.5 11.2 36.5 6.9 7.1 7.0 CARSLOGIE-Light gravelly loam, drainage good, original pH value 5.4. Cwt of CaO per acre, 29.3.1945 . . . . nil 3 1 . 0 ~ 3 1 . 2 ~ ~ 3 1 . 5 ~ ~ Beet { (4 (b) roots (tons/acre) . . . . . . 3.71 10.7 12.1 11.0 pH a t 18.11.1945 . . . . .. . . 5.2 6.4 6.4 6.2 tops (tons/acre) . . . . . . 4.89 17-5 20.3 19-4 Calculated pH* . . . . .. . . - 6.1 6.1 6.1 5 0 ~ s S.E. 4.16 f0.43 4.13 f0-99 27.6 f1.17 4-36 k0.17 5.2 5.6 5.9 5.9 6.0 S.E. f 1.03 - S.E. f 0.60 f 1.59 - 2 0 . 7 ~ ~ 2 0 . 4 ~ (4 9.2 15.1 17.1 5.9 6.5 5.8 5.8 (4 8-8 (a) 93 per cent. of 100 mesh. Standard error for roots -~=0.86, for tops fl.50. L = ground burnt linie, LS = ground limestone, w = waste carbonate.* Calculated pH value from the titration curve with the original soil. (b) 37 per cent. of 100 mesh, 88 per cent. of less than 2 mm. (c) Shell (d) Beet factory carbonate, 42 per cent. of water. sand, 1 per cent. 100 mesh, 62 per cent. less than 2 mm. The reliability of the final pH value calculated from the laboratory estimation of lime requirement depends, of course, upon the factor used to convert the laboratory figure to a field figure; this involves assumptions regarding the apparent density and depth of the soil concerned and the uniform incorporation of the liming material. For mineral soils with an apparent density of about 1 in the dry state, a factor of 1.7 has regularly been used to obtain an estimate of the lime required to bring the top 8 or 9 inches of soil to a particular pH value, 18 to 30 months after the application of this dressing.Special consideration must be given to soils containing more than about 10 or 12 per cent. of organic matter.[Vol. 76 The agreement between the values found and those predicted is of secondary importance, however. More important for the present discussion is the fact that different liming materials applied in equivalent amounts calculated front the simple total neutralising value have produced the same effect on the pH value of these soils. Numerous data from a much larger range of soils and liming materials would be required to decide whether this is of general application. In principle, it would appear to ble quite sound for limes from badly burned or from impure limestones, for ground limestone, for waste carbonates from paper works or sugar beet factories, and for certain types of blast furnace slags. The method might have to be modified for those less abundant waste materials that contain sulphides or other decomposable salts and are sometimes used locally for liming soils.The virtue of the method lies in its simplicity and its ability to give a measure of all those compounds of calcium and magnesium which are of value in soil amelioration. It is of interest to observe, in this respect, that in the United States the Association of Official Agricultural Chemists defines a liming material as “any material whose calcium and magnesium content is capable of neutralising soil acidity.”f’ 64 SMITH, COMRIE AND SIMPSON: THE EVALUATION OF LIMING REFERENCES 1.2. 3. 4. 6. 6. COLLEGE OF AGRICULTURE “Fertilisers and Feeding Stuffs Regulations,” 1932, p. 23 Lauder, A., and Smith, A. M., Agric. Prog., 1933, 10, 172. -,- , Ibid., 1934, 11, 93. Smith, A. M., and Coull, R., Soil Res., 1932, 3, 10. Smith, A. M., Tffans. 2nd Comrn. I.S.S.S., 1933, A, 102. “Official and Tentative Methods of Analysis,” A.O.A.C., 6th Ed., Washington, 1948, p. 896. EDINBURGH AND EAST OF SCOTLAND DISCUSSION THE PRESIDENT congratulated the authors on their paper, and mentioned that he had found high values by the Total Neutralising Value method for so-called “lime waste,’ which was essentially calcium silicate. He asked if the authors had tested such silicate materials in the course of their field work.He also asked whether, if the Total Neutralising Valuation be adopted, there should not be a statement about the magnesium content if any is present. He enquired whether the Total Neutralising Value method was efficient for very hard crystalline forms of carbonate. MR. J. G. SHERRATT asked whether a fictitiously high value would not be given by the Total Neutralising Value in the analysis of waste industrial lime containing calcium sulphide. In many places throughout the country there were huge industrial dumps consisting of lime in various forms. Calcium carbonate predominated, but sulphate, sulphide and silica also were frequently present. Farmers were encouraged to use these wastes and the material was eligible for a Government grant on the basis of its lime content.In practice, a small amount of calcium sulphide did ncrt seem to be harmful; indeed, it was claimed to be beneficial, although its value in neutralising the soil acidity was not apparent. MR. J. KING asked whether limes of high silica content were as effective as the normal carbonate limes in flocculating clay soils. MR. W. F. ETHERIDGE asked whether any experimLents had been performed on tissue tests for calcium assimilation on the various liming materials mentioned. DR. J. H. HAMENCE said that he had listened tc, the paper with considerable interest, particularly in view of the fact that the Advisory Committee of the Ministry of Agriculture and Fisheries had been revived. He felt that many lime merchants had been unfairly penalised in the past, in view of the fact that lime in the form of chalk and also combined with silica had frequently not been taken into account when selling parcels of lime.The problem was a very important one, since he had found that a very small amount of silica was capable of fixing a relatively large amount of lime, and of rendering it inert in respect of its solubility in the official sugar solution. It may be of interest to note in passing that it was his experience that roughly one molecule of silica was capable of fixing four molecules of calcium oxide. For many years he had held the view that lime loosely combined with silica in this manner was available for soil neutralisation, and i t was therefore very pleasing to find that the authors had produced substantial experimental evidence to support this view.MR. C. J. REGAN asked what was the relative value of calcium carbonate and calcium oxide from the aspect of the practical farmer. Was there any advantage in burning limestone to produce quicklime for agricultural use ? DR. SMITH replied to the President’s questions thak silicate materials had not been specially examined, the field experiments having been carried out only with liming materials in regular use. The Total Neutralising Value should be supplemented by a statement of the content of magnesium oxide or carbonate if that were more than, say, 10 per cent., because instances of magnesium deficiency in plants were becoming more frequent and the best method of correcting such acid soils was to use liming materials containingFeb., 19511 MATERIALS FOR AGRICULTURAL PURPOSES 65 magnesium.Hard crystalline forms of carbonate had not been tested, but the rate of reaction between soil and carbonate depended upon the size and hardness of the particles of the latter. In reply to Mr. Sherratt, he said that the presence of sulphide presented a difficulty that might be overcome by a double titration, but the amounts of waste materials containing sulphides were relatively small and of local interest only, and farmers who used them were advised to leave the material exposed in the open until toxic substances had decomposed. The danger incurred was probably small if the waste by-product was incorporated in the soil some weeks before sowing the crop. He informed Mr. King that no experiments had been made on the flocculating power of high-silica limes. In reply to Mr. Etheridge, he said that the assimilation of calcium from different liming materials was not known. Of the several factors involved, the state of saturation of the soil with bases would probably be the most important and all the experimental evidence pointed to the fact that there was little ultimate difference between equivalent amounts of various liming materials as far as the reduction in soil acidity and the increase in exchangeable calcium were concerned. In reply to Mr. Regan, he said that it did not matter whether quicklime or ground limestone was used provided the farmer appreciated what was meant by equivalent amounts. Contrary to the usual assumption, ground limestone might react more quickly than ground lime, which was liable to be carbonated as hard lumps on the surface of the soil and remain comparatively ineffective. There was little to be said in favour of the old practice of allowing heaps of burnt lime to slake on the field; subsequent spreading was laborious and irregular and might be very bad if rain made the heaps unworkable. Ground lime could be spread evenly, but i t was a most unpleasant job. Ground limestone could be stored indefinitely and was easy to spread but, of course, was required in larger quantities than ground lime.