A pH test measures soil acidity or alkalinity. A pH 7.0 is
considered neutral. An acid soil has a pH value below 7.0. Above pH 7.0 the
soil is alkaline. Most of us want our soil in between 6.1-7.0 on the acidity and a 2 or 3
on the other tests. Hopefully that makes sense.
- Very acid soil
- Most plant nutrients,
particularly calcium, potassium, magnesium and copper, become more soluble
under very acid conditions and are easily washed away
- Most phosphates are
locked up and unavailable to plants below pH 5.1, although some acid
tolerant plants can utilise aluminium phosphate
- Acid sandy soils are
often deficient in trace elements
- Bacteria cannot rot
organic matter below pH 4.7 resulting in fewer nutrients being available
to plants
- Action: Add lime to
raise the pH to above 5.0. The addition of lime can help break up acidclay soils
pH 5.1 - 6.0
- Acid soil
- Ideal for ericaceous
(lime-hating) plants such as rhododendrons, camellias and
heathers
- Action: Add lime if
other plants are grown
pH 6.1 - 7.0
- Moderately acid soil
- A pH 6.5 is the best
general purpose pH for gardens, allowing a wide range of plants to grow,
except lime-hating plants
- The availability of
major nutrients is at its highest and bacterial and earthworm activity is
optimum at this pH
- Action: It is not usually
necessary to add anything to improve soil pH at this level
pH 7.1 - 8.0
- Alkaline soil
- Phosphorus availability
decreases
- Iron and manganese
become less available leading to lime-induced chlorosis
- But an advantage of this
pH level is that clubroot
disease of cabbage family crops (brassicas) is reduced
- Action: Sulphur, iron
sulphate and other acidifying agents can sometimes be added to reduce
pH. Clay
soils often require very large amounts of acidifying material
and soils
with free chalk or lime are not usually treatable
