Liming is the application of calcium to soil in various forms such as marl, chalk, limestone, mushroom compost or hydrated lime. This increases the alkaline balance of the soil, improves texture, and can increase activity of soil bacteria. However, over-application may result in plant damage.

Adding lime is particularly useful to improve yields in the vegetable garden; however, it is rarely worth changing the pH of the soil for ornamental plants; it is much better to choose plants to suit the existing conditions.

Types of Lime

  • Calcium carbonate (lime): Easy to handle and relatively safe to use.
  • Calcium oxide (quicklime): Efficient at raising pH, but it is very caustic and may scorch plants.
  • Calcium hydroxide (slaked or hydrated lime): Less efficient than quicklime, but also less caustic.

Never apply lime at the same time as manure; the lime will react with the nitrogen-rich manure and release ammonia gas, damaging plants and wasting nitrogen. If necessary, apply lime and manure in alternate years.

Lime Application

Although lime may be applied at any time of the year, it is best to spread it as far in advance of planting as possible. Take a number of pH readings across the plot to work out how much lime to apply. The following table shows the average amount of lime needed to achieve an optimum soil pH:

Type of Soil
Soil Starting pH
4.5 5.0 5.5 6.0
Clay 400 330 260 215
Loam 285 235 190 155
Sand 190 155 130 118

(Above) Amount of lime (in g/sq m) required to raise soil pH to 6.5

Lime may be applied during cultivation of the soil in preparation for planting; simply spread the lime over the soil and then distribute evenly with a rake. If using this method, do not plant or sow for about four weeks. Lime may also be applied as a top-dressing for established plants and watered in.