Acid layers within the topsoil can cause crop failure in acid-sensitive plants like faba beans and chickpeas. Conventional agriculture acidifies soil through ammonium fertilisers, nitrate leaching, and product removal e.g. harvesting grain.
Topsoil acidity is usually measured as a 0-10 cm core. This provides an average pH for the topsoil that can hide acid layers. For example, the top few centimetres might have a pH of 6 while there is an acid layer of pH 4 at around 5 cm depth.
If you’re planning on growing an acid-sensitive crop, test soil pH in 0-5 cm and 5-10 cm increments. Make sure there is no mixing between layers when sampling. Use the results to refine your lime application strategy, or decide to plant an alternate crop.
How to improve lime applications
If possible, place lime into the acid layer. Incorporating lime is more effective than topdressing. In the short term, liming without incorporating is unlikely to treat layers below 3 cm depth. If you can’t incorporate, add enough surface lime so alkali can move into the soil. Keeping the surface pH above 5.5 (in a CaCl2 extract) maximises the opportunity for alkali movement downwards. Adding organic matter with lime may enhance the rate of alkali movement.