Individual study: The effects of different grades of limestone and depth of incorporation on spoil pH and growth of perennial rye-grass Lolium perenne at a colliery spoil reclamation site at Welch Whittle, Lancashire, England
Costigan P.A., Bradshaw A.D. & Gemmell R.P. (1984) The reclamation of acidic colliery spoil. IV. The effects of limestone particle size and depth of incorporation. Journal of Applied Ecology, 21, 377-385
Acidic colliery spoils may require application of large amounts of limestone for long-term control of acidity to enable vegetation to establish and grow successfully. However, the addition of a very fine agricultural powdered limestone that is normally used may have disadvantages because of rapid dissolution. Therefore one solution may be to apply the limestone in a 'slow-release' form as coarser particles. The effects of different grades of limestone (up to 2.4 mm diameter) and depths of incorporation (15 and 25 cm) on spoil pH and growth of perennial rye-grass Lolium perenne, were measured over 2 years in two field experiments on colliery spoil.
Study sites: Two field experiments were established on adjacent tips at Welch Whittle (National Grid reference SD 545135), Lancashire, northwest England. This tips were reclaimed in 1968 but the pH subsequently fell causing almost complete vegetation die-back. In this study, pH was determined and the amount of limestone required to bring the pH to 6.4 was calculated. Both tips were extremely acidic: Tip A: pH 2.7, limestone requirement 50 t/ha; Tip B: pH 2.9, limestone requirement 44 t/ha.
Limestone: Three grades of limestone were investigated: fine (an agricultural powdered limestone: 60% < 0.3 mm), medium (particles 0.6-1.2 mm diameter) and coarse (particles 1.2-2.4 mm diameter).
Experimental design: The tip A experiment used all three grades of limestone applied in a randomized factorial design at 5, 10, 25 and 100 t/ha at two depths of incorporation (15 and 25 cm) with four replicates. On tip B only two grades of limestone (fine and medium) were used. Within each plot (0.5 x 0.5 m) the limestone was incorporated with a garden fork and NPK fertilizer (125 kg/ha N as ammonium nitrate, 150 kg/ha P as superphosphate and 50 kg/ha K as potassium sulphate) was mixed into the top 5 cm. L.perenne was sown at 100 kg/ha, tip A was seeded in early May 1976 and tip B, 2 weeks later. The summer was exceptionally dry but the grass on tip A was well established before the drought, whereas on B there was poor establishment. All plots were therefore resown at the same rate, in September 1976.
Shoot growth was cut to ground level, dried and weighed in September 1976, April and August 1977 and April 1978. After each sampling the plots received 60 kg/ha N as ammonium nitrate. Surface pH of each plot was determined in June 1976, October 1976 (tip A only) and October 1977.
On Tip A at the end of the first season there were pronounced effects of limestone grade and application rate on grass growth. With the fine grade there was no further growth response to application above the 10 t/ha level at either depth. With the medium and coarse grades there appeared to be a response up to 100 t/ha, but even at this rate growth did not attain that of plots receiving fine limestone. In subsequent harvests growth in coarser grade treated plots gradually increased relative to that of fine limestone plots. By April 1978 the effect of grade was not statistically significant. Limestone incorporation depth had no effect on growth except in the harvest of April 1977 when it was 23% less with 25 cm limestone incorporation than with 15 cm. There appeared to be some advantage to deep incorporation during the summer growth period provided adequate limestone was applied. Liming to 25 cm rather than 15 cm reduced the concentration of limestone within the spoil and had a consequent reduced effect on pH. The finer grades and higher application rates had more immediate neutralizing effects but the coarser grades eventually raised the pH to a level suitable for growth. For all grades at shallow incorporation, only 100 t/ha was sufficient to produce a lasting improvement. The lower rates exhibited an initial rapid increase in pH but this was followed by a gradual return to acid conditions. The fall in pH was too large to be attributed to leaching, demonstrating that acid was being generated within the spoil. Whether this was from pyrite oxidation or slow release from pools of exchangeable acidity is unknown. The coarser the grade of limestone used, the lower the pH sustained at this 'adequate' liming rate.
The results on tip B were very similar, but grass establishment was poorer (possibly due to drier conditions). In later harvests, however, growth was consistently better than on tip A. This was perhaps to be expected, as the surviving vegetation from the unsuccessful 1968 reclamation attempt was better on tip B than on tip A, although the initial surface pH and lime requirement of the two sites were only slightly different. Samples of spoil taken in July 1976 from tips A and B had pyrite contents of 0.95 and 0.50 (as %FeS,) and acid-neutralizing capacities of 0.11 and 0.19 (as %CaCO,) respectively.
Conclusions: The coarser limestone particles appeared to have a long-term neutralizing effect provided that sufficient quantity (100 t/ha) was applied. The coarser grades reacted more slowly than the fine grade and there was no evidence that coarser grades became unreactive. Deeper incorporation improved plant growth over the summer. The incorporation of the same rate of limestone to a greater depth did not raise pH to the same level as concentrations of limestone in the spoil were less.
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