Some history Around seven thousand years ago, some civilisations grew
alongside rivers where the soil of the alluvial valleys was deep
and fertile. This fertility was replenished by the silt deposited
during seasonal flooding. The Tigris/Euphrates rivers (Mesopotamia)
and the Nile river (Egypt) gave rise to two such civilisations built
on the nutrients that the rivers carried with them from distant
lands. The depositing of this river-sourced ‘fertiliser’
was the key factor that allowed agricultural-based civilisations
to grow a crop surplus, and therefore put more time into developments
such as the invention of writing, creating the wheel, writing literature,
and developing law books. In ancient times, Egypt was able to export
large amounts of food and helped to sustain the Roman Empire. Not
all of these civilisations survived though. For example, the Sumerians
from Mesopotamia (present day Iraq) could not sustain their agricultural
advancements because they caused extensive environmental degradation.
The seasonal flooding was less reliable than that in the Nile valley
and therefore irrigation became the crux of the agricultural system.
Deforestation in the catchment resulted in irrigation channels being
blocked up with silt. In time, human labour was insufficient to
cope with removal of the silt and the cities were abandoned.
Thousands of years later, in 1842, a young English squire called
John Bennet Lawes of Rothamsted (Hertfordshire, England) became
interested in why ground-up bones improved the fertility of some
soils but not others. Lawes dissolved some bones in sulfuric acid
and found the answer was relatively simple – bones provide
phosphorus, which is essential to good root growth. Acid soils (those
with a low pH reading) could dissolve bone material whilst alkaline
soils (with a high pH reading) could not (because calcium phoshate
is insoluble in water). He patented a process for treating phosphate
rock with sulfuric acid to make the phosphate soluble, creating
‘ superphosphate’ and founded the world’s first
artificial fertiliser industry.
The problem of satisfying a plant’s hunger for nitrogen still
remained, however, as shipping sodium nitrate deposits from Chile
and accumulated seabird dung from Peru was proving expensive. The
problem was solved in two ways. Firstly, scientists discovered that
certain plants (known as legumes) had a relationship with naturally
occurring soil microbes that resulted in the microbes converting
nitrogen from the air into nitrogen in the plant in return for a
meal. As the importance of these legumes (such as clover) became
much better understood, farmers began to grow more of them, e.g.
as components of pasture mixes to provide them with a natural source
of nitrogen fertiliser. Secondly, in the early 1900s two German
chemists – Fritz Haber and Carl Bosch – thought that
it should be reasonably easy to combine nitrogen with the hydrogen
in the air to make ammonia (as this is done regularly by lightning
in nature). They found that a powerful charge of electricity could
do the trick and the process involved became known as the Haber-Bosch
Process. This discovery ultimately led to factory manufacturing
of nitrogen fertilisers and gave way to the modern day nitrogen
fertiliser industry.
A soil pH test tells you whether a given soil is acidic or alkaline
and this will determine to a large extent the plants that can be
grown in a given soil. A soil with a pH of 7.0 is termed neutral;
below 7.0 is acidic and above 7.0 is alkaline. It is possible to
adjust the pH of your soil by adding appropriate materials. For
example, if you wish to increase the soil pH level you will need
to add a liming agent such as crushed limestone, dolomite, quicklime
or slake lime. The change in soil pH will be proportional to the
amount of material that you add. Similarly if you find that the
soil pH is too high it can be reduced by adding appropriate amounts
of elemental sulfur, aluminium sulfate or even sulfuric acid.
Plants vary widely in their overall tolerance to different soil
pH levels – with some better able to cope with pH changes
than others. The pH of the soil can also affect the availability
of nutrients, with some nutrients becoming inaccessible to plants
at certain pH levels.
Many flowering shrubs do well, but not the heather
group, azaleas, rhododendrons, lupins or most lilies. There
is a reduced availability of phosphate, potassium, manganese
and iron.
6.0 - 6.5
Optimum for most plants. Maximum availability of mineral nutrients.Phosphates
fixed by soil; potassium, calcium, magnesium, and trace elements
suffer loss by leaching. Bacteria affected more than fungi.
< 5.5
Many plants suffer from acidity; roots short, stubby often
fanged. Phosphate becomes less available.
4.0
Heaths and moorland plants and rhododendrons
do well, but many other flowering plants fail. Soluble aluminium
appears in harmful quantities.
Plants need food, just as you and I do, and those grown in poor
soils that are low in nutrients will go hungry and are likely to
become unhealthy and get sick. Pests and diseases have an uncanny
knack of being drawn to weak and sick plants with ‘low immunity’.
When it comes to the fertility and health of your garden soil, it
is always best to opt for prevention, aiming to keep the soil fertility
at an adequate level rather than to wait for signs of deficiencies
to occur – ‘prevention being better than cure’.
Maintaining a physically and chemically healthy, biologically active
soil will aid in preventing problems of ill health. It is this concept
that led to the saying ‘feed the soil, not the plant’.
Thomas Jefferson was on the mark when he wrote the following in
a letter to his daughter regarding a pest problem, in 1793: “When
the earth is rich, it bids defiance to droughts, yields in abundance,
and of the best quality. I suspect that the insects which have harassed
you have been encouraged by the feebleness of your plants, and that
has been produced by the lean state of your soil ”.
The nutrients required to provide a balanced meal to the crop must
be present in soluble, inorganic forms in order for plants to be
able to use them. Whether you provide your nutrients in ‘organic’
or ‘chemically synthesised’ forms, it is important not
to neglect the general health of the soil. Soil organic matter contains
plenty of nutrients but they are not available to plants until microbes
degrade the organic matter and excrete nutrients in inorganic forms
that plants can take up. Thus the topic of a soil’s fertility
is intimately tied to biological health of the soil. The many armies
of microscopic organisms and other soil organisms such as earthworms
continuously work away, decomposing organic matter and converting
nutrients from forms that are not available to plants into forms
that are.
Soils vary in their natural ability to supply plants with essential
nutrients. Some soils are deficient in one or more nutrients because
they are formed from rocks that naturally contain little of those
elements. Other soils are deficient because they are very old and
have lost their nutrients as a result of either weathering or overuse.
Soils with high amounts of clay and humus will have a good ability
to supply nutrients because their overall negative charge holds
onto the positively charged nutrients such as potassium and calcium.
Too much clay, however, can mean that such nutrients are held on
to so tightly by the soil that the plant may have trouble getting
access to the nutrients. Similarly, a sandy soil has a low ability
to hold on to nutrients, and may benefit from organic matter additions.
It is a therefore a good idea to know your soil’s capacity
to retain nutrients as doing so can save you both time and money
in the long run. Fertiliser needs for gardens also depend on the
type of plants that you intend to grow, as some plants have very
specific requirements or tolerances.
Where soil nutrient deficiencies do occur, we can get a clue as
to what might actually be lacking by looking at the symptoms that
appear on the plants (see table below) and can add appropriate materials
to correct the identified problem.
The rate and vigour of growth and colour
of leaves. Protein building, enzymes and photosynthesis
Occurs during early spring owing to the leaching
effects of high rainfall and again in late summer when prolonged
drought conditions result in soil nitrogen being trapped and
concentrated in the dry surface soil above the absorptive root
zone.
Stunted growth; small yellow, pale green or possibly
bluish leaves. Thin weak stems.
Phosphorus (P)
Root growth, ripening of seeds and fruit.
Phosphorus deficiencies are most pronounced in the winter
and early spring when soil forms are immobile and relatively
unavailable for growth.
Stunted roots and growth. Small purple leaves and stems.
Potassium (Potash or K)
.
Assists photo-synthesis and the production of carbohydrates.
Protects plants against diseases and environmental stress.
May occur at any time of the year. It is easily leached
from the soil and consequently deficiencies are common during
periods of high rainfall.
Note: Close links with nitrogen.
When nitrogen is increased, so must potash or deficiency will
appear
Fruits are poorly coloured, lacking in flavour. Leaves will
appear scorched at edges, mottled, spotted or curled.
Calcium (Ca)
Essential for sturdy young plant growth. One of the most important
soil foods.
Occur when plants are water-stressed during dry periods, the
former when high levels of soluble salts are present in the
soil and the latter when soils are over-limed.
General lack of vigour, growing points die, growth stops.
Without calcium, some species are unable to assimilate nitrogen.
Magnesium (Mg)
Part of the chlorophyll molecule (which makes leaves green).
It is necessary for the formation of amino acids and vitamins.
Essential to germination of seed and synthesis of sugar.
These deficiencies are disorders characteristic of acid soils,
especially of pumice land or soils of recent volcanic origin
as, for example, on the central plateau of the North Island.
Photosynthesis is affected and shows as yellowing of leaves,
or purplish brown patches between the veins. Leaves may fall
prematurely.
Nitrogen
The rate and vigour of growth and colour of leaves. Protein
building, enzymes and photosynthesis.
Nitrogen deficiencies are most prevalent Plants under nitrogen
stress show rapid and spectacular recoveries when nitrogen supply
is restored, although the degree of yield restoration depends
on the state of maturity of the crop
Stunted growth; small yellow, pale green or possibly bluish
leaves. Thin weak stems.
Phosphorus
Root growth, ripening of seeds and fruit.
Phosphorus deficiencies are most pronounced in the winter
and early spring when soil forms are immobile and relatively
unavailable for growth. Plants under phosphorus stress grow
slowly, have an abnormally long growing period and only partially
recover when the supply is restored through soil warming or
other means.
Stunted roots and growth. Small purple leaves and stems. Yield
of fruit and seed poor.
Potassium (Potash)
Close links with nitrogen. When nitrogen is increased, so
must potash or deficiency will appear. A fruit-forming fertiliser.
Assists photosynthesis and the production of carbohydrates.
Protects plants against diseases and environmental stress.
Potassium deficiencies may occur at any time of the year.
It is easily leached from the soil and consequently deficiencies
are common during periods of high rainfall. It is common when
plants are growing vigorously, particularly when they are oversupplied
with nitrogen. Restored supply leads to good recovery.
Fruits are poorly coloured, lacking in flavour. Leaves will
appear scorched at edges, mottled, spotted or curled
Calcium
Essential for sturdy young plant growth. One of the most important
soil foods.
These deficiencies are liable to occur when plants are water-stressed
during dry periods, the former when high levels of soluble salts
are present in the soil and the latter when soils are overlimed
General lack of vigour, growing points die, growth stops.
Without calcium, some species are unable to assimilate nitrogen.
Magnesium
Part of the chlorophyll molecule (which makes leaves green).
It is necessary for the formation of amino acids and vitamins.
Essential to germination of seed and synthesis of sugar.
Magnesium and molybdenum: These deficiencies
are disorders characteristic of acid soils, especially of pumice
land or soils of recent volcanic origin, for example, on the
central plateau of the North Island.
Photosynthesis is affected and shows as yellowing of leaves,
or purplish brown patches between the veins. Leaves may fall
prematurely.
Sulphur
Necessary for chlorophyll synthesis.
Similar to nitrogen deficiency, leaves become light green.
Each of these elements is required by plants only in very
small amounts, but they can be vital for certain plant functions,
e.g. iron is mainly needed in the formation of chlorophyll,
copper for nitrogen metabolism, and zinc for seed and starch
formation.
Manganese and iron: These deficiencies also
occur in calcareous or overlimed soils at any time of the year.Copper:
Copper deficiencies are largely confined to peat soils.
If we bear in mind that ‘what goes in is what comes out’,
then it makes sense that if a plant is grown in a soil that is low
in phosphorus, the plant will also be low in phosphorus. Similarly
if dead plant material that contains a large amount of phosphorus
is returned to the soil, then the amount of P found in the soil
will increase accordingly. Nitrogen (N), phosphorus (P) and potassium
(K) are the nutrients that plants need in the largest quantities
and there is a wide range of fertiliser products containing these
nutrients available for the gardener. There are also a host of ‘micronutrients’
that, although plants don’t need them in such large quantities
(indeed some can be toxic if oversupplied), are still essential
for balanced plant diets. The gardener needs to be aware that not
all plants have the same nutrient requirements and must match fertiliser
and/or compost applications to a given soil with the needs of the
type of plants that are to be grown in that soil.
Nitrogen
A lack of N can mean smaller and/or unpalatable plants or crops.
It is a good idea to feed small amounts of high N fertiliser often,
rather than large amounts infrequently, and only when the plant
is growing strongly as N is very mobile in the soil and if too much
N is just sitting around in the soil it can be lost to the air or
groundwater. Some natural sources of N include hen manure, animal
urine, green lucerne hay, fresh grass clippings and legume green
manures (i.e. beans, peas, lupins, clover). Other mulches and composts,
however, (e.g. cereal straw residue) contain only low amounts of
N and their addition to soil may temporarily ‘lock up’
soil N while the material gets broken down. This is because microbes
need to access nitrogen to grow while they break down the such material
and they will need to `borrow’ it from the surrounding soil
if it is not present within the added organic material. The N is
not `lost’ forever from the garden, but while the microbes
are using it, it becomes unavailable for plant uptake. Once there
is no more organic material left for the microbes to eat, the microbes
will die and the N contained in their bodies will be released back
into the soil. If small amounts of high-N fertiliser are added between
layers of mulch, you can prevent plants in the immediate vicinity
of a mulch application from becoming N-deficient. Including legumes
as part of your crop rotation in the garden is a good natural way
to add some N back into your soil.
Phosphorus
Phosphorus is removed from soil when plants and crops are harvested
so it must be replaced. There are a number of P sources that can
be utilised as all plant waste contains some phosphorus, as do bones
(blood and bone is a good source of P), eggshells and animal manures.
However, most organic materials contain only low amounts of P. So
if a P ‘fix’ is needed, you can buy soluble P fertilisers
that will become readily available for plant growth. Phosphorus
tends to behave in a somewhat sneaky way in the soil as it likes
to form insoluble compounds with calcium and other minerals, but
plants need to take up phosphate when it is in the soluble form.
Potassium
Most of the soils in New Zealand have sufficient K reserves, so
that providing plant residues are returned to the soil, K fertiliser
does not need to be applied so often. Potassium fertilisers are
frequently more commonly referred to as ‘potash’.
In many cases too much of one nutrient can ‘lock up’
or interfere with the absorption of another so it is important to
get the overall balance of nutrients in the soil just right. It
is also important to apply nutrients when they are needed the most
or precious nutrients that readily dissolve in the soil water can
get leached away (e.g. washed out of the soil with excess water).
This can of course contribute to environmental contamination of
groundwater and/or local waterways, as well as waste your time and
money.
If you think you are having problems with nutrient deficiencies
in the garden, it is not a bad idea to have a soil test done. It
may help determine which nutrients to add and also indicate if a
soil amendment such as lime or organic matter is needed. If a nutrient
deficiency has been diagnosed, it can be important to find out why
the nutrient is deficient before trying to overcome the problem.
For example, it may not be wise to apply zinc fertiliser to a soil
with a pH of greater than 7, as at that pH level the plants will
have difficulty accessing the added zinc – in this case you
would be best to first address and fix the high pH problem before
adding the zinc.
Regular applications of commercial fertilisers prevent deficiencies
of nitrogen, phosphorus and potassium from occurring, although supplementary
dressings of nitrogen and phosphorus at the middle stages of growth
are beneficial and produce luxuriant growth even in intensive plantings.
In addition, where there is a history of phosphorus deficiency,
care should be taken to raise the pH above 5.5 and summer crop plantings
are best delayed until the soil has sufficiently warmed.
Conversely where cases of boron, manganese or iron deficiency have
occurred, past overliming with the development of alkalinity is
indicated, and acidification would be advisable, using appropriate
fertilisers, e.g. ammonium sulfate.
Calcium deficiencies rarely reflect inadequate
soil calcium except in very acidic soils where the application of
lime will be sufficient. More commonly these deficiencies reflect
dry soil stress and an even watering regime should prove an adequate
remedy.
Magnesium deficiencies, if suspected, may indicate
an acid soil that may be rectified by liming; alternatively soil
levels may have been depleted if the garden is aged or they may
be naturally low, for example, in the pumice lands of the North
Island. Dolomitic lime or magnesium sulfate should be applied if
either is the case.
Molybdenum deficiencies are readily corrected
by applying commercial preparations of superphosphate to which sodium
molybdate has been added as a trace ingredient.
Copper deficiency when encountered may be rectified by applying
copper sulfate at 2 g/m2.
Nutrient deficiencies arise when naturally occurring nutrients
are not adequate to sustain healthy plants or when the nutrients
are not present as soluble, inorganic forms in order for plants
to use, e.g.
soils formed from rocks that are naturally low in nutrients,
very old and have lost their nutrients as a result of weathering
or overuse,
soil pH not suitable for the availability of nutrients,
too much clay (i.e. holds nutrients too tightly),
too much sand (i.e. low ability to hold on to nutrients, results
in nutrient leakage).
However, adding humus to clay or sandy soil can improve a soil’s
capacity to supply and retain nutrients.
