What is soil nutrition for gardens?

Nutrients are essential for a plant to carry out a normal life cycle. There are at least 16 essential elements. The most important of these – carbon, hydrogen and oxygen – are provided by air and water. The rest come from minerals in the soil and decaying matter. Nutrients are needed for different plant functions – any deficiency or excess results in a vast range of specific symptoms.

Most soils provide the majority of nutrients but, because plants are constantly drawing them from the soil, they must be replenished. The main nutrients that need continual replenishing are nitrogen (N), phosphorus (P) and potassium (K) – the 'N:P:K' listed on fertiliser labels.

A good compost will provide most of these requirements and the majority of soils deliver most other nutrients at adequate levels. However, Australia's ancient soils are notoriously low in phosphorus. If soil is nutrient deficient it is best to address this when establishing a garden bed by adding compost and fertiliser and then maintaining this level.

Fertilising is not a matter of just adding more nutrients. Plant growth is limited by the supply of the scarcest nutrient. Many gardeners make the mistake of just adding more and more fertiliser; you can have too much of a good thing. For example, nitrogen rich urea makes a lawn green but when applied in excess, say by a legcocking canine, it can burn and kill. By using compost as the main fertiliser it is very hard to over-fertilise our gardens.

Nutrients also react one with each other. For instance, copper aids in processing nitrogen. Also, nutrients need to be present in forms that plants can access – this is determined by chemical form, soil moisture and pH.

Only fertilise when plants are actively growing

Applying fertiliser is about timing and knowing the needs of your plants. Plants only require fertilising when they are actively growing. For most this means fertilising in autumn and spring.

In spring, use a fertiliser high in nitrogen to encourage leaf growth. In autumn, use phosphorous and potassium to strengthen the plant and encourage root development. Nutrients are delivered through moisture in the soil. In summer there is no point in fertilising dry ground – drought affected plants are more likely to be harmed than helped by fertiliser. Always fertilise moist soil and water after application.

We also need to synchronise our fertilising with the life cycle of our plants. Fertilise with a high nitrogen fertiliser to encourage initial growth and leafy foliage. To encourage fruit set and ripening, back off the nitrogen and use a fertiliser higher in potassium and phosphorus (look for the relative levels of N:P:K on the fertiliser packet). Like people, plants have different eating requirements. Those adapted to sandy, low nutrient soils are light feeders often requiring no extra fertiliser. Many food plants are gross feeders, especially leafy vegetables and fruits like citrus. These plants will need regular composting and fertilising. They may also need direct boost. In organics this is best delivered as a foliar feed delivered straight to the leaves to avoid leaching through the soil.

At Heronswood we are aiming to close the nutrient cycle by focusing on compost rather than purchased inputs. We have sandy soils but with years of improvement, through compost and mulching, we have good levels of organic matter. We do not fertilise our kikuyu lawn, clippings are left, so no nutrients are being removed. All garden beds are fed through mulching with compost in spring.

The more intensive production and flower beds receive additional applications of Diggers organic Veggie & Herb fertiliser in spring and Diggers organic Fruit & Citrus as flowers and fruit are forming and again in winter. Rock dust is applied sparingly to ensure supply of micronutrients. Leaf vegetables and flowers in early growth are given organic foliar fertiliser and liquid seaweed every 2-3 weeks.

However, we do not have a fixed schedule. We look to the plants and the weather to guide us. By not overfertilising we avoid pests attracted by sappy leaf growth and limit our impact on the surrounding waterways.

We feed the soil, not the plant

Following an organic approach to soil fertility, we let the soil itself sort out the complex requirements of the plants. We feed the soil, not the plant. Rather than treating specific symptoms with a specific product we take an holistic approach to soil health.

Organic fertilising is like the Slow Food of fertilising versus the Fast Foods of the synthetic fertiliser industry. Synthetic fertilisers are quick fixes; they rely on their volatility to be readily accessible to plants. They move rapidly through the environment, washing into waterways, causing algal blooms and degradation of aquatic environments.

Organic fertilisers are slowly released into the soil. They are less concentrated and tend to come in a form bulked out by organic matter that also helps to improve the structure of the soil. Organic fertilisers are generally recycled products like manures, straws and composts, repurposing what would otherwise be waste.

Synthetic fertilisers require large amounts of energy and fossil fuel to produce, negating many of the carbon benefits of the plants they fertilise. Keeping garden soil fertile is about providing it with a well-rounded meal, not junk food and vitamin pills. Remember, the nutrients we feed our garden soil are ultimately the nutrients we are feeding ourselves!