Soil pH

What is pH?

pH is the measure of how acid or alkaline something is.  The pH scale goes from 0 to 14 with 7 being neutral,  less than 7 is acid (sour), higher than 7 is alkaline (sweet).  Some examples:  Lemon juice has a pH of 2,  Baking Soda however has a pH of 8.5.

Why pH matters

If the soil pH differs too much from a plants requirements then the plant may not be able to take up nutrients, the nutrients become ‘locked’.  For example a pH below 7.5 means that iron, manganese and phosphorus are less available to a plant.  A pH below 6 can cause Nitrogen, Phosphorus, potassium to be less available.

To complicate matters – some plants prefer a different pH to others.  For example while potatoes do well with a pH of 6.5, Rhododendrons prefer a pH of around 5.

Very Acid Moderately Acid Slightly Acid Very Alkaline
pH 5.0 to 5.8 pH 5.5 to 6.8 pH 6.0 to 6.8 pH 7.0 to 8.0
Blueberry

Celeriac

Crabapple

Eggplant

Endive

Potato

Raspberry

Rhubarb

Shallot

Sorrel

Sweet potato

Watermelon

Bean

Brussel’s sprouts

Carrot

Corn

Garlic

Parsley

Pea

Pepper

Pumpkin

Radish

Squash

Sunflower

Tomato

Turnip

Asparagus

Beet

Bok choy

Broccoli

Gooseberry

Grape

Kale

Kohlrabi

Lettuce

Mustard

Okra

Onion

Pansy

Peach

Pear

Spinach

Silverbeet

Cabbage

Cauliflower

Celery

Chinese cabbage

Cucumber

Olive

Pomegranate

Thyme

 

Why does pH Vary?

Many things can change the pH of a soil, including temperature, rainfall and the type of vegetation previously grown can all effect the soil pH

How to correct the pH in your soil

Applying additives to your soil such as compost, mulch, lime or gypsum will adjust your pH.  Organic matter (such as compost) will neutralise the soil, whereas chemical fertilisers tend to make the soil more acidic.

To raise pH if soil is acidic add:

Lime

Wood ash (calcium carbonate, potassium, phosphorus, trace elements)

TO lower pH if soil is alkaline add:

Pine needles

Leaves

Sulphur

Sawdust

Peatmoss