Lime, used to reduce soil acidity, is one of the most common of soil amendments. Much has been learned about lime since 1900, and soil scientists continue to learn more. Men like F.T. Shutt, working on the chemistry of soils, learned much about soil acidity and its causes and were able to provide information on how to amend such soils for improved crop production. Many wet, low-lying, and poorly drained soils contain large amounts of organic matter, as do muck and peat soils. These soils contain no calcium carbonate causing them to be sour or acidic. Conversely, many light upland soils may be acidic because the carbonates have been leached and, if cropped, the lime present has been used by the plants. Shutt prepared a number of bulletins on the subject, the first in 1914 as part of C.E. Saunders' summary of results with cereals. One of his last bulletins (86), published in 1928, provides farmers with comparative values of different sources of lime and gives examples of improved yields (fourfold increases with barley at Cap Rouge, Quebec) following liming. Subsequent departmental publications on the same subject were written by L.E. Wright and H.S. Hammond (No. 585 in 1937), C.D.T. Cameron and L.S. Hamilton (No. 1086 in 1960), H.J. Atkinson (No. 869 in 1961), P.B. Hoyt, M. Nyborg, and D.C. Penney (No. 1521 in 1974), and K. Bruce MacDonald (No. 1731 in 1981).
In 1914, one of the first experiments laid out by superintendent Blair at Kentville dealt with the application of lime and fertilizers to a 3-year rotation. Blair and Leefe (15) reported that, after 24 years, the application of limestone was the greatest single factor influencing the yields of all crops. They also noted that potato scab increased as the the soil was neutralized with lime. A similar experiment (106), started at Sainte-Anne-de-la-Pocatière in 1932, produced only slightly increased yields with lime, whereas manure and lime or any combination including phosphatic fertilizers increased the yield of turnips three to five times over that without added amendments.
At Nappan in 1957, on dikeland soils, MacLeod (62) found increases of 10 percent or more in grain yields, and an astonishing increase in clover yields of four to eight times on limed soils in comparison with that of unlimed soils. The amount of lime applied in order to obtain optimum results depended upon both soil and crop. Similar results were obtained by Jasmin and Heeney (40) on organic soils in Quebec. However, they took their 1957 research a step further and found that although liming increased the available soil phosphorus, it materially reduced the amount of nitrogen, phosphorus, and potassium in the plant's tissue, even though, as might be expected, calcium levels were increased. They concluded that unless adequate amounts of lime were used, high applications of fertilizer could in fact be detrimental to plant growth.
One of the questions to answer now was how did the increase in available soil phosphorus occur? Other scientists had shown that organic forms of soil phosphorus underwent mineralization, its extent depended upon various soil conditions. The nature of the release of phosphorus from its organic form following the application of lime, however, was limited. Halstead, Lapensee, and Ivarson (31) of the Soil Research Institute resolved the problem in 1963 when they learned that there was only a slight mineralization of organic phosphorus in unlimed soil over a 9-month period, whereas in limed soil there was a marked increase in microbial activity, which was associated with a reduction in the amount of extractable organic phosphorus and an increase in available soil phosphorus.
Professor F.A. Wyatt, at the University of Alberta, had learned by 1945, that liming wooded soils in the central part of his province doubled the yield of alfalfa hay. Not until Hoyt, Hennig, and Dobb (38) studied 28 wooded and parkland soils of the Peace River region was it realized that liming affected yields of both alfalfa and barley under the climatic conditions existing in these northern areas. The two crops were affected in different ways, however. Unless phosphorus fertilizer as well as lime was added to barley, crop yields were actually depressed. Alfalfa benefited from lime whether phosphorus was added or not. Hoyt, Hennig, and Dobb also found that the amount of soluble aluminum in the soils related closely to the yields of alfalfa and barley. This confirmed an observation made in 1947 by V.A. Chernov, a Russian soil scientist. In the coastal region of British Columbia, John, Case, and Van Laerhoven (46) provided further evidence on the ways in which lime reduces the toxicity of aluminum and manganese, hence increasing yields of alfalfa.
For some time, soil scientists have recognized that most acid soils are saturated with aluminum rather than hydrogen ions. The acidity of the soil is therefore a result of hydrolysis of aluminum. Canadian soil scientists, particularly at the Land Resource Research Institute, have studied this phenomenon in relation to the liming of soils. Turner and Clark (98) gained international reputations in 1966 for introducing the concept of "corrected lime potential" to define the degree of base saturation in soils. This became the basis for procedures now used in soil-testing laboratories to determine the "lime requirement" of soils. More recently Singh (89) investigated the chemistry of aluminum in the presence of sulfur and found that a substantial amount of aluminum is in a form that can move through the soil into water courses, causing aluminum toxicity to aquatic plants and animals.
In Prince Edward Island, where soils are known to be acidic and crops respond to applications of lime, U.C. Gupta suspected from his earlier research and also from some done in other countries that the reaction of plants to molybdenum might be affected by the acidity of the soil, and hence to liming.
His judgment was correct, for when he put his theory to the test in 1969 (30) he showed that both cauliflower and alfalfa crops failed on two of the three soils tested unless both molybdenum and lime were applied. On the third soil, as found by some other workers, lime alone gave a yield response, indicating that the soil contained sufficient molybdenum.
Climatic conditions are major factors in the development of soil acidity, as are a number of agricultural practices such as fertilization, irrigation, and improved drainage. One current threat to further acidification of soils is acid rain. Singh and Coote (90) have recently concluded that up to 1985 the impact of acid rain on agricultural soils, and hence crops, is modest when compared to natural processes of acidification. They warn, however, that a continued watch needs to be kept on the rates and amounts of acid-forming pollutants affecting agricultural soils.