PRACTICAL ADVICE
Making sense of fertilisers
Sap is flowing, buds have burst and seedlings leap into growth. All this activity needs food to keep it going. Spring is feeding time for almost everything: vegetable gardens, fruit trees, lawns, hedges, trees and shrubs, indoor plants, outdoor containers, roses and perennials.
What do plants need?
The NPK on a fertiliser pack represents the three ‘major nutrients’ that plants need the most of. Nitrogen (N) is important for leaf growth and for converting sunlight into plant energy. Phosphorous (P) is critical for cell formation particularly in root growth and seedling growth, as well as flowering and fruiting. Potassium (K) plays an important role in a plants strength, water uptake and disease resistance, and also the quality of flowers, fruits and seeds.
Trace elements (aka ‘minor nutrients’), which include iron, calcium, magnesium, sulphur, boron, and many others, are just as essential to plant growth but used in smaller quantities. Plants grow best when they receive all the nutrients they need in the right balance.
Where do plant nutrients come from?
In nature, plants get all their nutritional needs from their environment - from rock minerals held within soil particles built up over millions of years plus a continuous supply of decomposing animal and plant matter. In a garden, we remove nutrients every time we harvest so we need to give back what we take away to keep our plants in peak productive health.
What kind of plant food is best?
Regardless of what we feed our soil, how well it holds and releases nutrients to plant roots depends on the type of soil we are lucky enough to have and, most importantly, how we manage it. A healthy soil contains billions of beneficial fungi, bacteria and other soil microbes busily binding soil particles together and transporting nutrients to plant roots. Compost, mulch and bulky manures sustain a soil’s ability to hold water and nutrients, while encouraging earthworms and beneficial microorganisms.
If added in enough bulk, compost and well-rotted
animal manures can meet all plant nutritional needs in a healthy garden, especially when supplemented by homemade fertilisers like comfrey tea, worm tea and fish fertiliser. This is easiest when there is a good local supply of free or low-cost material. Using a range of different manures can provide a balanced diet, even though the exact nutritional make up of any one organic manure is hard to pin down.
By comparison, manufactured fertilisers offer precision and convenience, giving fast reliable results when applied as directed. Many modern fertiliser products are now based on naturally occurring organic ingredients.
Different plants have different nutrient requirements, but luckily for the bewildered home gardener, this is reflected in the wide range of specifically targeted plant foods we have available to us.
Fast acting or slow release?
A fast-acting plant food applied as a liquid or powder (mixed into the soil with water) will provide a rapid boost to growth. Liquid fertilisers are especially useful for plants growing in containers, when applied little and often. Never use powdered fertiliser on potted plants.
Controlled-release (aka slowrelease) fertilisers release nutrients gradually, more in sync with plant growth. Safe to use on container plants, they also allow the convenience of feeding less often while helping to avoid waste and environmental harm though over use. Some fertilisers combine both fast and slow release action.
What's the buzz about seaweed?
Increasingly popular with home gardeners and commercial growers, seaweed extracts are not plant foods on their own, but ‘growth enhancers’ used alongside fertilisers to boost natural plant function.
Compost, mulch and bulky manures sustain a soil's ability to hold water and nutrients, while encouraging earthworms and beneficial micro-organisms.
Seaweed's magical effect has to do with its gift of some important plant hormones: auxins for long-wise growth, cytokinins for healthy cell division and nutrient transport and gibberellins for seed germination, flowering and fruiting.
Applying seaweed to the soil or foliage increases yields and improves the quality of flowers and fruit (especially if it is a seaweed product with good levels of potassium). Seaweed improves resistance to pests and disease and helps make plants more resistant to frost and drought.
A fantastic aid for vegetable gardeners, seaweed reduces transplanting shock by stimulating rapid root recovery. Soak seedlings in seaweed solution at planting time.
Why lime?
Adding lime (calcium) to the soil has many benefits if used wisely. There are three main forms.
Garden lime (calcium carbonate) is made from natural limestone. It is used mainly to raise soil pH, also known as ‘sweetening the soil’. In general an acid to neutral soil (pH 6.0 to 7.5) suits most plants. If the pH gets too low (acidic) or too high (alkaline), plants can’t access the nutrients they need, even if those nutrients are present in the soil. Lime can be very effective in the vege garden, but use no more than the recommended rate and keep lime away from potatoes and ‘acid-loving’ plants like camellias and blueberries. If in doubt a soil pH test is worthwhile.
Dolomite lime (calcium magnesium carbonate) raises soil pH more gradually than lime. The magnesium in it is useful for certain plants, including leafy vegetables and fruit trees. Dolomite also stimulates rapid decomposition of organic matter, useful in the compost heap.
Gypsum (calcium sulphate) is used to improve soil structure without raising pH. A natural mineral with a pH of 7.0 (neutral), it is safe to use around acid loving plants. Use gypsum to loosen clay and compacted soil and to boost microbial activity in the compost heap.