Calculating Lime Advice
Lime advice will depend on such factors such as soil type and crop type (refer to Chp. 8, page 32 to 37 of the Teagasc Green Book for target pH levels). The soil pH test measures the soil water pH which is shown on the soil test report as pH. This corresponds to the pH value that we refer to on a regular basis for particular crops. For example the target soil pH for grassland is pH 6.3, for cereals is pH 6.5.
Lime advice shown on the soil test report relates only to the crop that is entered initially when the soil sample details are logged on the nautilus programme. Lime applications are applied within a year or 2 of taking the soil sample. If the crop changes (e.g. from Grass to Barley) from that initially entered it will therefore be necessary to calculate new lime advice.
Determining Lime Requirements (LR)
Lime advice is determined by the SMP pH test result reported on the soil test report as SMP (for lab use only). The SMP pH determines the soils buffering capacity (a measure of the soils ability to resist pH changes) which depends on such soil factors as soil texture and soil organic matter content.
To calculate the crops lime requirement two values are required - the target SMP pH required for each crop group as shown in the table below and the measured SMP shown on the soil report. The closer the measured SMP pH is to the target SMP pH the less lime is required. The formula for lime requirement is shown in the yellow box below.
For example where a soil report for a grass field has a measured SMP pH 6.3 on a mineral soil, the lime advice is calculated as 5t/ha. See calculation below
6.7 – 6.3 x 12.5 = 5.0t/ha
Mark Plunkett & Linda Moloney Finn - April 2023