Acidification and Lime Decisions on a Long-Term Pasture

The soil is strongly acidic at both depths, and the subsurface pH of 4.0 with 18% aluminium saturation is the critical constraint, because aluminium toxicity restricts root growth and clover nodulation. Lime moves down the profile very slowly, typically only a few centimetres per decade, so a single large surface application will not correct the 10-20 cm layer quickly. The finer, higher-neutralising-value lime is likely to react faster and be more effective per tonne because finer particles have greater surface area, but at 1.5 t/ha it delivers less total neutralising capacity than 2.5 t/ha of the coarser product unless its neutralising value is substantially higher. The grower should compare the two products on neutralising value and fineness, not just price per tonne. Given the $150,000 cash limit, liming all 300 ha at 2.5 t/ha would cost $71,250 for the lime alone at $95/t, which is actually within budget, but the finer lime at 1.5 t/ha would cost $54,000. The real issue is that spreading a single pass across the whole property spreads the benefit thinly. A better strategy is to prioritise the most acidic and most productive paddocks, apply a higher rate there, and plan a second application in subsequent years. Borrowing at 7% may be justified if the expected pasture response exceeds the interest cost, but the response to lime takes two to three years, so cash flow must be planned accordingly.

The plan is premature. Lime raises pH slowly, and the pasture response typically appears in the second and third seasons, not immediately. The subsurface acidity at 10-20 cm will take much longer to correct, so root growth and clover performance will improve gradually. Increasing stocking rates immediately would remove the pasture before it has recovered and could worsen the decline, especially if the season is dry. The grower should also consider that the 20% production decline may have causes other than acidity, such as phosphorus deficiency, waterlogging, or pasture pests, and these should be checked before attributing all gains to lime. A better plan is to apply lime to priority paddocks, maintain or slightly reduce stocking rates for one to two seasons, monitor pH and pasture composition, and increase numbers only when clover establishment and pasture growth are demonstrably improved. The 18% aluminium saturation also means that some sensitive species may not establish until pH rises, so a phased approach with monitoring is more defensible than an immediate stocking increase.

Liming 300 ha at 2.5 t/ha requires 750 t of lime. At $95/t delivered and spread, the total cost is 750 × $95 = $71,250. If the grower borrows the full amount at 7% interest, the annual interest cost is $71,250 × 0.07 = $4,987.50, or about $5,000 per year. A 20% pasture recovery on a flock of 4,000 ewes at a gross margin of $60 per ewe would be worth 4,000 × $60 × 0.20 = $48,000 per year, if the recovery translates fully into carrying capacity and the gross margin holds. On these figures, the interest cost is easily covered, and the investment looks attractive. However, the comparison is optimistic: the response is not immediate, it may be less than 20%, and the gross margin per ewe may not rise proportionally if the extra pasture is used to increase stocking rate rather than to improve lambing rates or reduce supplementary feeding. The grower should also account for the cost of the second lime application needed to correct subsurface acidity. A staged approach, liming the most acidic paddocks first and measuring the response, would reduce the risk of borrowing against an uncertain return.