Applying lime is one of the most beneficial but also the most under-valued things you can do for grassland.
Soils naturally become acidic over time, the process greatly hastened by application of ammonium nitrate fertilisers. As soil becomes more acidic the availability of elements, both good and bad, becomes greater. The optimum pH is a balance between sufficiently acidic to allow uptake of important elements, but not so acidic as to have toxic levels of others, such as aluminium. Plants have varying tolerance to both deficiencies of important trace elements and toxicity and unfortunately it tends to be the most productive grasses (which in plant breeding trials are grown in optimum pH conditions) that are the most sensitive, whilst weeds and less productive grasses cope with acid conditions, gradually taking over a sward.
It is not inevitable that grass swards need regular reseeding – if cared for they can be highly productive for many years. The purpose of a reseed should be to take advantage of advances in plant breeding and genetics to improve a sward rather than because the grass has died out. On many farms the practice is to only apply lime before reseeding, and it is little surprise that the useful grass species gradually start to struggle to compete with weed plants - not because they are inherently short-lived, but because the conditions in which they are expected to grow steadily deteriorate.
The target pH range for productive grass swards (i.e. any grass which receives bagged fertiliser, slurry or manure) is 6.0 – 6.2. It’s important that soils are tested on a three to five year basis to correctly balance nutrients and pH.
The soil type dictates how much and how often lime is required to maintain optimum pH – generally light soils need limed little and often, whilst heavier soils need larger applications but less frequently.
Of the liming material available the most cost-effective is the most basic – ground limestone. Allow at least 12 months for it to work fully, so apply the year before you intend to reseed. Prilled/granular lime is limestone which is ground into a very fine dust and then prilled to facilitate spreading – you will need the same overall quantity as ordinary ground lime but you apply a little each year. Since the finer the material the quicker it acts, prilled lime, despite being relatively expensive, is excellent in an emergency or for use on short term lets.
Chloe McCulloch, SAC Consulting
This article from Farming for a Better Climate was funded by the Scottish Government as part of its Climate Change Advisory Activiey and first published in the Farming Scotland Magazine during June 2016.