Soil pH and vegetable growing – Dave Hamilton

pH describes how acid or alkali a soil is. Certain plants will thrive in a more alkali soil and others will do well in acid soil. Most vegetable plants like a soil which is neutral to acidic. The exception to this rule is plants in the cabbage family which prefer things to be slightly alkali.

Alkali

If you have ever been to the Dorset coast you may recall wild cabbage growing on the cliff tops, this is an easy way to remember cabbages like alkali soil.  The white cliffs of Dover, the Oxford Ridgeway or the Chalk downs of Salisbury and Wiltshire are also good examples of alkali soils. Alkali and calcium are strongly linked as it is the presence of calcium in large amounts which makes these areas alkali.

Too much calcium will inhibit the intake of other nutrients in both plants and humans. In humans high calcium diets can be linked to anaemia and we can appear pale and listless, plants suffer in a similar way as they are unable to produce chlorophyll and they develop ‘interveinal chlorosis’ or a pale colour between the veins of the leaves.

Alkali soils can be described as limey or chalky soils.  It is a myth that pine needles will alter the soil pH to make it more acidic. If soil is too alkali it can be improved over time using compost and or manure both of which are slightly acidic. If you require a more alkali soil then it can be improved by liming.

Acid

Moorland and peat bogs are both good examples of acid soils. At its extreme little survives as roots can be literally burnt away. Thankfully, you are very unlikely to have a strongly acid soil away from the moors and the mildly acidic soils you may find in a back garden or allotment will actually help a lot of fruit and vegetable plants.

Acid describes a pH of 1 to 7, the pH of Dartmoor lies somewhere between 3.5 and 4.5. On the moors you will see bilberries, a close cousin of the blueberry growing quite happily but other crops don’t seem to do so well. In the garden some fruit such as raspberries and strawberries will do well in an acid soil but you may struggle with other crops. Potatoes prefer it to be slightly acid as they can be more prone to certain pests and diseases in an alkali soil.

Acid soils can be described as peaty or an ericaceous soil.  Wood ash contains calcium carbonate and can be used in moderation to higher the pH in order to achieve a more Alkali soil. Garden lime, calcified seaweed and dolomite can all also be added but only when needed, over use of lime will damage soils. Lime is also helpful in breaking up a heavy clay soil.

Crops for soil types

The chart below shows examples of crops which will grow well in different soil pH’s.

 

Acid lower than 6.2

Neutral 6.2 – 7

Alkali higher than 7

Blueberry Artichoke Broccoli
Cranberry Beans Cabbage
Endive Carrot (also slightly acid) Turnip
Fennel Celery Radishes
Potatoes Corn Asparagus
Rhubarb Courgettes, Marrow (also slightly acid) Beetroot
Nasturtium Squash Lettuce
Parsley Leek, Onion Spinach
Raspberry Peas Echinacea

 

A healthy soil

An old teacher of mine used to say ‘It doesn’t matter what the problem is, the answer is always Add bulky organic matter’.  Not only does this encourage soil life, key to a healthy soil, it also helps balance a soil’s pH.

If the soil is too high or too low either way or if you want to grow a range of crops then raised beds may be the answer. With raised beds you only improve the soil where it is needed.

As mentioned earlier most vegetable plants will survive in a neutral to slightly acid soil. Many plants will do okay in a soil which veers towards the acid or alkali but it is best to aim for something closers to the centre over time.

Weeds as indicators of soil pH

A soil pH test is the best way to find out the soil’s pH. Don’t be tempted to buy an electronic test as they cannot be calibrated and are therefore very inaccurate. In the absence of a soil pH testing kit you can tell a certain amount from what weeds are growing on your vegetable plot before you start. The chart below gives you a general idea of some common weeds you may find.

Acid lower than 6.2

Neutral, healthy soil 6.2-7

Alkali higher than 7

Buttercups (Ranunculus spp.), ,

Burdock  (Arctium minus),

Bellflower (Campanula sp.), and

Docks (Rumex spp)

Chickweed (Stellaria media),

Wild Carrot (Daucus carota),

Mosses (Musci class)

Dandelion (Taraxacum officinale) (Also slightly acid)

white Mustard (Brassica hirta),

Common Mullein (Verbascum thapsis)

Chicory (Cichorium intybus),

Pennycress (Thlaspi arvense),

Nettles (Urtica dioica)

Fat Hen/Goosefoot ( Atriplex hastate and Chenopodiums)

Scarlet pimpernel Anagallis arvensis

Plantain (Plantago major) – Can also indicate compacted soil

Groundsel ( Senecio vulgaris),

Salad Burnett (Poterium sanguisorba),

Cinquefoil (Potentilla monspeliensis),  Silvery Cinquefoil (Potentilla argentea),

Queen  Anne’s lace (Daucus carota),

Nodding Thistle (Carduus nutans),

Wild Strawberries (Fragaria  species

Spear plantain (Plantago lanceolata)

True Chamomile (Anthemis nobilis)

 

Article written by Dave Hamilton. Dave has now left Selfsufficientish but you can catch up with him on davehamilton.me.uk or on twitter @davewildish

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1 Comment on Soil pH and vegetable growing – Dave Hamilton

  1. We have an excellent soil improver made from post-consumer urban food waste that is quite high alkaline so good for balancing clay-based soils. This article is very interesting but some images would be helpful, especially to quickly identify weeds and vegetables that like each soil type.

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