Raising soil pH
Soil pH can be raised by the addition of lime
. Lime is most commonly
applied as ground chalk, ground limestone or slaked lime. When lime
is added to an acid soil it neutralizes the soluble acids and the calcium
cations replace the exchangeable hydrogen on the soil colloid surface
(see cation exchange). Eventually hydrogen ions are completely
replaced by bases and base saturation
is achieved, producing a soil
of pH 7 or more. However, care should be taken not to overlime a soil
because of its effect on the availability of plant nutrients.
The lime requirement
of soil can be estimated from knowledge of
the required increase in pH and the soil texture (see buffering capacity). A pH of 6.5 is recommended for temperate plants on mineral
soils; pH 5.8 on peats. The amount of a liming material needed to meet
the lime requirement will depend on the neutralizing value of the lime
chosen and its fineness.
Liming materials can be compared by considering their ability to
neutralize soil acidity, fineness, and cost to deliver and spread. The
neutralizing value (NV) of a lime indicates its power to overcome
A neutralizing value of 50 signifies that 100 kg of that material has the
same effect on soil acidity as 50 kg of calcium oxide. The fineness
lime is important because it indicates the rate at which it affects the soil
acidity (see surface area). It is expressed, where relevant, in terms
of the percentage of the sample that will pass through a 100 mesh sieve.
Liming materials commonly used in horticulture are listed below with
some of their properties. The relationship between the different forms of
calcium is shown in Figure 20.5.
|Figure 20.5 Forms of calcium
is the most common liming material. Natural soft chalk
) that is high in calcium carbonate is quarried and
ground (NV = 48). It is a cheap liming material, easy to store and safe to handle. A sample in which 40 per cent will pass through a 100 mesh
sieve can be used at the standard rate to meet the lime requirement.
Coarser samples although cheaper to produce, easier to spread and
longer lasting in the soil, require heavier dressings. Shell sands
calcium carbonate, have neutralizing values from 25 to 45, i.e. whilst the
purest samples can be used at nearly the same rate as chalk, up to twice
as much of a poorer sample is required to have the same effect.
(also known as quicklime, burnt lime, cob lime or
caustic lime) is produced when chalk or limestone are very strongly
heated in a lime kiln. Calcium oxide has a higher calcium content than
calcium carbonate and, consequently, a higher neutralizing value. Pure
calcium oxide is used as the standard to express neutralizing value (100)
and the impure forms have lower values (usually 85–90). If used instead
of ground limestone, only half the quantity needs to be applied.
|Figure 20.6 Lime kilns. Used to ‘burn’ (heat) chalk or
In contact with moisture, lumps of calcium oxide slake, i.e. react
spontaneously with water to produce a fine white powder, calcium
hydroxide, with release of considerable heat. This was an effective way
of obtaining a fine lime from the quarried material before there was heavy
rolling machinery to grind the coarse lumps. The lime kilns that were
used are still a common sight, especially in small ports round the coast
(see Figure 20.6) Although rarely used now, calcium oxide has to be used
with care because it is a fire risk, ‘burns’ flesh and scorches plant tissue.
, hydrated or slaked lime, is derived from calcium
oxide by the addition of water. The fine white powder formed is popular in
horticulture. It has a higher neutralizing value than calcium carbonate and
its fineness ensures a rapid effect on the growing medium. Once exposed
to the atmosphere it reacts with carbon dioxide to form calcium carbonate.
It should be noted that all forms of processed lime quickly revert to
calcium carbonate when added to the soil. Calcium carbonate, which is insoluble in pure water, gradually dissolves in the weak carbonic acid of
the soil solution around the roots (see Figure 20.5).
, also known as Dolomitic limestone, is especially
useful in the preparation of composts because it both neutralizes acidity
and introduces magnesium as a nutrient. Magnesium limestone has a
slightly higher neutralizing value (50–55) than calcium limestone, but
tends to act more slowly.
Liming materials also provide the essential nutrients calcium and,
when present, magnesium to the soil. Bicarbonate is formed from the
carbonate in carbonic acid, e.g. rainwater or soil water, around respiring
roots to provide a soluble form that can be taken up by plants (see
Unless very coarse grades are used, lime raises the soil pH over a oneto
two-year period, although the full effect may take as long as four
years; thereafter pH falls again. Consequently lime application should
be planned in the planting programme. It is normally worked into the
top 15 cm of soil. If deeper incorporation is required, the quantity used
should be increased proportionally. The lime should be evenly spread
and regular moderate dressings are preferable to large infrequent
applications. Very large applications needed in land restoration work
should be divided for application over several years.
Care should be taken that the surface layers of the soil do not become
too acid even when the lower topsoil has sufficient lime. Top layers
are the first to become depleted with consequent effect on plant
establishment. This tendency has to be carefully looked for in turf
management as this can lead to the formation of ‘thatch’ (see Figure
18.6). Applications of organic manures or ammonium fertilizers should
be delayed until lime has been incorporated. If mixed they react to
release ammonia which can be wasteful and sometimes harmful.
Decreasing soil pH
Soil pH can be lowered by the addition of acids or sulphur to reduce
the base saturation of the mineral soil. Some acid industrial by-products
can be used, but the most usual method is to apply agricultural sulphur
which is converted to sulphuric acid by soil micro-organisms. The
sulphur requirement depends on the pH change required and the soil’s
buffering capacity. The application of large quantities of organic matter
gradually makes soils more acid. Acid fertilizers such as ammonium
sulphate reduce soil pH over a period of years in outdoor soils and can
be used in liquid feeding to offset the tendency of hard water to raise
pH levels in composts. In some circumstances it has been appropriate to
grow plants in a raised bed of acid peat
or to work large quantities of
peat into the topsoil; an approach that is not sympathetic to avoiding the
destruction of peat wetlands.