Tony Dickerson answers your questions
Q Why shouldn’t I add lime and manure to the soil at the same time? Stephen Scarfe, Alphington, Exeter
A If you add lime and manure to your soil at the same time the nitrates in the manure will interact with the hydrogen produced by liming. This converts the nitrates, which are the valuable nutrients in manures, into ammonia gas, which simply escapes into the atmosphere. If you’re adding manure just to improve the soil structure, this won’t matter because you can add fertilisers later.
A soil pH test measures the acidity or alkalinity of the soil. A pH7 is neutral, above pH7 is alkaline and below pH7 the soil is acid. Most garden plants grow in a range of soil pH but some, such as camellias and rhododendrons, need an acid soil while brassicas do best in alkaline soils.
It’s worth checking soil pH before planting a new garden, making vegetable beds, planting fruit or where growth is disappointing. You can test the pH of your soil at any time, but not within three months of adding lime, fertiliser or organic matter because the test is likely to give misleading results.
Liming is only necessary if your soil is particularly acid (6.5pH) or below, or if you’re growing brassicas, which prefer an alkaline soil above pH7. The vast majority of fruit and veg do best on a slightly acidic soil. To assess if you need to lime, do a pH test first. If liming is necessary, do it in autumn because lime’s slow acting, and then manure in spring. This also reduces the leaching of nutrients from the manure by winter rains.
Apply garden lime by digging half of it into the soil to a depth of 20cm (8in) and sprinkling the rest on the surface after digging. Calcified seaweed and ground chalk are other forms (calcium carbonate).
The amount of lime to add depends on the pH and soil type. Clay resists changes in pH (called buffering capacity) so that more lime is needed to change the pH in soils with high clay content than in sandy soils. As a guide, to raise the pH from 5.5 to 6.5 you’d need 1kg of lime per sq m (2lb per sq yd) for a clay soil, 0.8kg per sq m (1½lb per sq yd) for a loam soil and 0.7kg per sq m (1¼lb per sq yd) for a sandy soil.