Pick up almost any bag of fertiliser and the three letters ‘NPK’ will be listed as major ingredients. You may have to think back to high school chemistry to recall that these letters stand for the elements called nitrogen, phosphorus and potassium.

These are the three major elements needed to fuel basic plant growth. While most fertilisers contain these three nutrients they are present in different amounts depending on the formulation of the fertiliser. These amounts are shown as ratio of nitrogen to phosphorus to potassium (N:P:K).

Plant cells use these nutrients to grow and develop. They help root and shoot development, flower and fruit formation and assist natural pest and disease resistance.

If plants lack adequate nutrients they may cease to grow, may not flower or may fail to form fruit or seeds. They may also be more susceptible to pest and disease problems. Plants that continue to lack nutrients may die or have extremely stunted growth.

The nutrient path

Nutrients are present in the soil as ions and are absorbed by the plants roots and transported to cells through the plant’s vascular system. Nutrients are also absorbed through foliage from liquid feeding.

Ions are readily soluble. While this means they are easily taken up by plants it also means they can also be leached from the soil through the action of water percolating through the soil. It also means that plants have difficulty extracting nutrients when their roots are in dry soils and during drought periods.

Fertiliser formulations

While most soils can grow plants, most garden plants and particularly productive plants such as vegetables and fruit trees, grow better with added fertiliser applied at least annually. Plants growing in containers also need regular fertilising.

Fertilisers may be balanced – that is composed of equal proportions of the major nutrients of nitrogen, phosphorus and potassium – or tailored to suite specific growth needs. Lawn foods, for example, tend to have a higher proportion of nitrogen whereas citrus foods tend to have a higher proportion of phosphorus. Fertilisers formulated for Australian native plants are generally very low or lacking in phosphorus.

Plant problems relating to nutrition

Plants exhibit problems if their nutritional needs are not being met. Too much or too little of a nutrient can show up as discoloured or oddly shaped or sized leaves, lack of flowering, poor fruiting or poorly developed fruit. Nutrient starved plants may die or their growth may be stunted.

Leaves are often the first part of a plant to flag that the plant is lacking nutrients. Deficiencies may show up at a certain stage of development or as a distinct yellow patterning in the leaves.  A plant that is lacking one of its key nutrients may exhibit the following symptoms.

Nitrogen (N) deficiency symptoms appear as yellowing of older leaves. To rectify the deficiency apply any readily absorbed fertiliser such as lawn food. Sources of nitrogen include fertilisers containing ammonium, nitrate or urea. Manure also provides nitrogen. The yellow older leaves may not re-green but new growth will occur.

Phosphorus (P) deficiency symptoms show up on the leaf tips, which may appear burnt, or show up as dark green or purple colouration of foliage. Apply any fertiliser for flowers or fruit. Sources of phosphorus include fertiliser containing phosphate, blood and bone. Fertilisers for Australian native plants contain no phosphate.

Potassium (K) deficiency symptoms include wilting of older leaves, a scorched appearance moving inwards from leaf edge, chlorosis (yellowing) in the leaf veins that begin at the base of the leaf. Add potash, potassium, or a fertiliser for flower or fruiting.