Soil nutrients occur naturally in the soil and their presence and availability
are without doubt major factors affecting both crop yield and quality.
Plants growing on nutrient-deficient soils tend to be weak with low immunity,
attracting pests and disease. Therefore, it is always best to keep soil
fertility at a sufficient level rather than wait for signs of deficiency
to occur, i.e. prevention is the best cure.
Most New Zealand soils are naturally deficient in phosphorus (P) and
sulfur (S) because soil is developed from both melted bedrock and weathered
bedrock, which were eroded under high rainfall conditions.
The pH of a soil will determine to a large extent the plants that can
be grown on it. It can also affect the availability of nutrients, with
some nutrients becoming inaccessible to plants at certain pH levels. However,
by adding appropriate amounts of materials the pH of your soil can be
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.
Many plants suffer from acidity; roots short, stubby often fanged.
Phosphate becomes less available.
Heaths and moorland plants and rhododendrons do well, but many other
flowering plants fail. Soluble aluminium appears in harmful quantities.
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.
(Pic) Stunted growth; small yellow, pale green or possibly
bluish leaves. Thin weak stems. Nitrogen deficient plant
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
(Pic) Stunted roots and growth. Small purple leaves
and stems. P deficient plant. (www.worldinter.net)
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
(Pic) Fruits are poorly coloured, lacking in flavour. Leaves
will appear scorched at edges, mottled, spotted or curled. K deficient
N (nitrogen), P (phosphorus) and K (potassium) are the nutrients that
plants need in the largest quantities so it is a good idea to feed small
amounts of fertiliser often, rather than large amounts infrequently because
too much fertiliser can be lost to the air or groundwater.
N – Hen manure, animal urine, green lucerne hey,
fresh grass clippings and legume green manures (e.g. cereal straw residue).
P– all plant wastes, blood and bone, eggshells
and animal manures.
K – most New Zealand soils have sufficient potassium
reserves so returning plant residues to the soil prevents the need to
apply other sources of K.
Gardeners be aware!
Not all plants have the same nutrient requirements so you must
match fertiliser and/or compost application to a given soil with the needs
of the plants that are to be grown in that soil. Oversupplying nutrients
can harm the environment and sometimes be lethal.