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| Chalky soil |
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Chalky soils contain calcium carbonate or lime and often behave in a similar manner to sandy soils
In fact chalk is a form of limestone, 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).
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, namely;
By testing it yourself with an inexpensive soil test kit and /or you can send samples away for analysis.
You may know your soil contains a full range of plant foods, because you put them there, but the level of alkalinity will determine whether these can be used by the plants.
For example; several 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;
Symptom; leaf yellowing.
By pH 7.5, potash and manganese are severely restricted, affecting photosynthesis and chlorophyll synthesis;
Symptom; leaf scorch and veining.
At pH 8.O and above, all nutrient supplies are compromised.
Practical solutions for chalky soils;
Use lime loving plants (calcicoles) as opposed to calcifuge (lime-hating) plants
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.
n.b. Alkaline conditions support large populations of worms and bacteria that make short work of organic matter, so such soils tend to become low in humus, with the resultant loss of moisture retention if this process is not maintained.
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 it 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 with sulphur powder use;
75g per sq (2oz per sq yd) for light soil,
100g (3oz) on loamy soil.
150g (4oz) for 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.
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