A gardener’s guide to plant nutrition

Plant nutrition is a cornerstone of healthy gardens. There’s some sciencey stuff involved, but don’t turn the page, I promise I can make it easy to understand! Like humans, plants need nutrients to grow big and strong. Ample nutrition helps build resilience from pests and diseases, too.

Basics

Nitrogen, phosphorus and potassium are the three main macronutrients plants need to function and are often referred to as N (nitrogen), P (phosphorus) and K (potassium) or NPK. Each are involved in lots of different plant functions but to keep it simple think of nitrogen for leaf development, phosphorus for root and shoot growth and potassium for maximising flowers and fruits.

Australia generally has soils low in nutrients but they are not always easy to correctly identify. As a general guide nitrogen deficiency looks like yellowing on old leaves with stunted growth and foliage. Phosphorus deficiency can also show as stunted or spindly growth with dull greyish leaves turning purplish red as they age. Potassium deficiency slows growth, produces leaves with yellow edges (margins) and reduces flowering and fruiting.

The easiest way to ensure strong healthy growth and to avoid deficiency issues is to use things that contain a mix of these nutrients. Homemade or bagged compost and aged animal manures are excellent ways to recharge your soil particularly at the beginning of a new planting cycle. Worm castings are also very good. Another option is compressed organic fertiliser pellets available from nurseries and hardware stores. Most use poultry manure as their base and some have additional ingredients like seaweed, fish meal and blood and bone. Always check that these products carry an organic certification logo to avoid those that have synthetic chemicals added.

If for some reason you still want to boost a particular macronutrient then consider the following options:

• Nitrogen boosters: Blood and bone, sold as powder or pellets, is very high in nitrogen which is readily available to plants. It’s also high in phosphorus.
• Phosphorus boosters: Super phosphate is well known but not permitted in organic gardening. Instead use rock phosphate or guano based products. Blood and bone is another good option. Keep in mind that some Australian and southern African native plants are sensitive to phosphorus, especially those in the Proteaceae family (for example, grevilleas, banksias, hakeas, proteas and leucadendrons).
• Potassium boosters: Potash (potassium sulphate) is commonly scattered over vegie patches given its importance to flowering and fruiting. Ash from your own wood fire is another option.

Liquid seaweed products are very popular but they are not true fertilisers as the level of NPK in these products are generally too low. Instead they are tonics or stimulants that can help reduce stress in plants and promote better growth. Unprocessed seaweed collected from the beach (check your council allows this) can be rinsed and roughly chopped before adding to the compost or used as a mulch.

Beyond

N, P and K are the essential macronutrients for plant function. The other components of fertiliser are micronutrients or trace elements – these are equally vital but needed in smaller amounts by the plant. Micronutrients include boron (B), chlorine (C), copper (Cu), iron (Fe), magnesium (Mg), molybdenum (Mb) and zinc (Zn). Too much or too little of one or more of these can lead to stunted growth, leaf discolouration and lack of flowering and fruit. Complete organic fertilisers provide appropriate amounts into soils for root uptake.

Soil pH affects how nutrients dissolve in soil water and, therefore, impact the plant root uptake. Most plants thrive in a pH range of 6.5–7.5. Below 6.5 is getting quite acidic, whilst above 7.5 it’s quite alkaline. Both situations influence the nutrient availability differently and deficiencies or toxicities of one can impact the uptake of others. That’s why it’s important to test soil pH and add the following amendments as needed:

• Garden lime (calcium carbonate): neutralises acidic soil (raises pH).
• Dolomite lime (calcium carbonate and magnesium carbonate): neutralises acidic soil (raises pH) with the added bonus of supplying magnesium to plants. Isn’t as fast acting as garden lime.
• Sulphur: neutralises alkaline soil (lowers pH).
• Compost and mulch: can help neutralise an acidic or alkaline soil (magic!).

Remember you can also select plants that can tolerate your particular soil pH. Speak to your local nursery for help.

A healthy garden is about more than just applying the right fertiliser. Approach soil health holistically; treat your garden as a living ecosystem beneath your feet. Beneficial fungi, bacteria, and microfauna are vital for processing organic matter into nutrients that become available for plant root uptake. Keep that ecosystem fed by getting organic matter into your garden:

• Avoid raking leaves and let branches lay where they fall, except for paths and driveways.
• Scattering or mixing compost into garden beds several times a year. Anything from a couple of handfuls to a bucket per square metre is beneficial.
• Mulch all bare soil with things such as pea straw, sugarcane or bark mulch.

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