Soil - Chalky
Chalky soils are alkaline, light brown in colour, and often contains large quantities of stones of varying sizes, and has a pH of 7.5 or more,
They also contain calcium carbonate or lime and often behave in a similar manner to sandy soils.
Chalk is a form of limestone and is a sedimentary rock deposited at the bottom of seas in prehistoric times, and is largely made up of prehistoric marine animals shells.
Over many years the rock has weathered, and the calcium carbonate has leached out as free calcium into the adjacent soil by the action of rainwater, subsequently producing an alkaline soil that records a pH level higher than 7.0 (neutral).
Chalky soils, dry out quickly in the summer, and have a tendency to block trace elements such as iron and manganese so that they are unavailable to plants, this in turn causes poor growth and yellowing of leaves.
This type of soil is usually of an extremely poor quality, and generally needs substantial amounts of fertilizers and other soil improvers adding on a regular basis.
It may be quite obvious that your garden has a chalky soil, but just how alkaline is it?
It is worth carrying out soil tests in various areas of the garden to establish this.
This can be done in a couple of ways, for example;
1) Test it yourself with an inexpensive soil test kit.
2) Send samples away for analysis at a Soil laboratory.
You may know your soil contains a full range of plant foods, because you put them there, but the level of alkalinity determines whether these can be used by the plants or not.
Important nutrients, particularly iron and manganese, are chemically changed in alkaline soils into forms that plants cannot use.
At pH 7.2 the availability of essential nutrients such as manganese, phosphorus, boron, iron, zinc and copper is limited.
- Leaf yellowing usually indicates potash and manganese are severely restricted, and is affecting photosynthesis and chlorophyll synthesis, these symptoms usually appear at around pH 7.5
- Leaf scorch and veining is indicates that all nutrient supplies are compromised, and usually appears at pH 8.O and above.
Professional soil testing will usually highlights these problems, and advice is usually forthcoming to rectify the situation.
Practical solutions for chalky soils:
Use lime loving plants (calcicoles) as opposed to lime-hating plants (calcifuge)
Calcicoles are species of plants that have adapted to growing in alkaline soils.
Trying to reduce the pH substantially can be difficult, especially where a shallow rock subsoil or natural hard water source is adding calcium to the soil.
Regular applications of organic supplements such as composted pine needles, leaf mould, garden compost and well-rotted manure will improve the soil texture and make it more acid.
Iron sulphate was once recommended, but in particularly chalky soil the extra iron becomes locked up and unavailable.
Sulphur is the only chemical worth trying, as this causes soil bacteria to convert it into sulphuric acid, which in turn reacts with the lime to form less-soluble calcium sulphate, or gypsum.
- Work the sulphur powder into the top 150-200mm (6”-8”) of top soil at least three months before planting.
Application rates will depend upon the type of soil:
To lower the pH by 1.0 use;
- 75g per sq (2oz per sq yd) on light soil.
- 100g (3oz) on loamy soil.
- 150g (4oz) on heavy clay.
Check annually to monitor the change in pH, and be prepared to repeat the treatment.
- Feeding plants with ammonium-based fertilisers will reduce soil pH over a number of years.
- High-potash feeds help amend natural potassium deficiencies in chalky soils, while sensitive plants respond to better to foliar sprays.
- Sow leguminous green manures such as field beans, clover or winter tares in vacant ground.
These fix nitrogen from the atmosphere and release it again as they decompose, and add acidifying humus to the soil, raising fertility and improving texture.
- Improve the drainage of heavier alkaline soils so that some calcium is leached out of the soil, which gradually brings down the pH.