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Soil pH
Effect on plants & nutrient availability
Why & when plants need some major nutrients
Methods of improving soil N, P & K levels


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.

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Soil pH

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 adjusted.

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Effect on plants and nutrient availability


Soil pH Impacts
> 7.5 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.

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Why and when plants need some major nutrients?

  Positive effects Prevalent time Deficiency effects

Nitrogen (N)


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

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.

(Pic) Stunted roots and growth. Small purple leaves and stems.
P deficient plant.

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 leafs (

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Methods of improving soil N, P and K levels

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.

Natural sources:

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.

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