16A1 Calculated lime rate – from exchangeable Al³⁺

Several investigators have proposed that lime rates for acidic, highly weathered soils should be based on the amount of exchangeable Al³⁺ present (e.g., Kamprath 1970), rather than on raising soil to a predetermined value such as pH 6.5. This method is based on findings (Mendez and Kamprath 1978) that liming rates equivalent to exchangeable Al³⁺ × 1.5 can neutralise most of any toxic Al present, thus creating suitable soil conditions for the growth of most plants.

The method has limitations, especially when the presence of sulfides and the likelihood of manganese toxicity contribute to acid soil infertility. Moreover, calculation-factors other than 1.5 may be necessary in some situations (e.g., Cochrane et al. 1980). This method’s main advantage over Method 16B1 is that an estimate of exchangeable Al³⁺ by Method 15G1 is the only analytical requirement.

Theory

Assuming field moist soil bulk density is 1.0 g/cm³, then 1 ha to a depth of 10 cm = 10⁶ kg

Calculation and Reporting

Lime requirement (kg CaCO₃/ha 10 cm) = 750 × cmol_cAl/kg of the soil segment

where:

•   cmol_cAl/kg is for a specified 10 cm profile segment, and

•   rates of CaCO₃ (agricultural grade; see Note 1) are additive for each 10 cm profile segment under consideration.

Make appropriate adjustments if soil bulk density varies from 1.0 g/cm³ and/or if the expected efficiency of agricultural limestone varies from 100%.

Report lime requirement (kg CaCO₃/ha 10 cm).

Note

1.  The effectiveness/efficiency of liming materials depends on particle size and chemical composition, in addition to the amount applied (e.g. van Lierop 1985; Oldham 2007).

Fine material reacts much more quickly and is therefore more effective than more coarsely ground limestones, coral sand and the like. Lime that passes a 60 mesh sieve is commonly rated as having an efficiency rating (ER) for fineness = 100%; that passing through a 20 but not a 60 mesh sieve = 60%; that passing an 8 but not a 20 mesh sieve = 20%; while that not passing through an 8 mesh sieve = 0. These ERs take account of the time required to dissolve the limestone. Assuming a limestone contains 78.8%, 10%, 2.4% and 7.8% of each sieve size fraction, then the fineness ER would equate to 85.28%.

The other key product variable is the product’s neutralising value (NV) or neutralising capacity, which is determined chemically and expressed as a percentage of pure CaCO₃ (chemically pure CaCO₃ has a NV of 100%; dolomitic limestone has a theoretical NV of 108%). Impurities such as clays in limestone reduce NVs.

To determine the effectiveness/efficiency or AV of limestones, the particle size ER has to be adjusted by the NV, plus an additional adjustment for moisture content (MC) of the limestone. For example, a limestone having an ER of 85%, a NV of 95%, and a MC of 13%, has an AV = {85% ER × 0.95 NV × [1.00 – 0.13 MC]} = 70.25%.