Sep 042008

Gardening is a bit like life really… the more you know, the more you know you don’t know. It can be so full of hits and misses, trials end errors, successes and failures… I guess that’s what makes it so much fun. There is not one day that passes when I don’t learn something new about life outdoors, and I hope there never is!

It is this ever-evolving and seemingly endless supply of information that can make gardening a hobby that seems daunting and overwhelming to the newbies and brown thumbs amongst us. Fear of asking ‘stupid questions’ at garden centres or of our gardener friends can prevent us seeking out the information we need. And so many of us experience a string of failures in the patch before we have many successes. Some of the most commonly asked queries that come across my desk involve fertilising – essentially the “who, what, where, when and why” of feeding our flora. So, without further ado, we bring you a comprehensive guide to do with all things (plant) food.

Feed Me, Seymour!

You know, while I was researching this article, I wondered to myself why it is that we think plants need so much food. Are we told this by gardening personalities? Do our plants look at us with sad petals and rumbling stems? Do we assume plants need as much food as we do? I decided, after much deliberation, that the movies were to blame. Think about it, most movies featuring flora involve mutant plants with voracious (and generally blood-thirsty) appetites. Think Little Shop of Horrors, Day of the Triffids, and, my personal favourite, Attack of the Killer Tomatoes (and the numerous sequels on You Tube paying homage to this 1978 classic)! But, like most things we see in the movies, this just ain’t true… plants are not nearly as hungry as we think, and, quite often, we are killing our plants (and the wider environment) with kindness. The obesity epidemic is alive and well, and coming to a backyard near you soon!

So, how much feeding do plants actually need? Well, how long is a piece of string? It depends on oh- so-many factors, that it is impossible to give a straight answer. What kind of plants are you growing, what kind of soil are they growing in, what are you feeding them, what do you want them to do, how often do you irrigate, is there organic matter in your soil, do you have a never ending budget to purchase pre-packaged store bought fertilisers… the list goes on. It’s time to demystify feeding time, and, in order to do so, we need to look at why plants feed.

Why do Plants Need a Feed?

All living organisms need to feed in order to remain healthy and happy (some of us just do it more often than we should!!). Plants are no exception to this rule. So generally speaking, over many thousands of years, plants evolved and adapted to the environments within which they were rooted. And if they were stuck in impoverished soils, then the plants that survived were those that made do with less. Conversely, plants that grew in moist, loamy, nutrient rich soils (think Tasmania) gobbled up as many nutrients as they could, and now, when grown away from their natural environments, can need some serious feed! And, unless the plant is growing in its natural environment, and all plant waste product is being returned to the soil, they are essentially removing nutrients from the soil without replacing them. Ever gone to the fridge when you’re absolutely starving, only you find that there is a heap of empty packets and no tucker? Same deal with plants, except they can’t nip down to their local independent grocer for more food. So that’s where we come in, especially if we are attempting to grow “hungry” plants, like annuals and edibles.

So, where do we start? Well, straight off the bat we need to understand the differences between fertilisers, soil conditioners, mulches and the like. Fertilisers can be defined as a substance containing nutrients essential for plant growth, and are generally available as either organic or inorganic (more on that shortly). Fertilisers are NOT plant food… plants make their own food very happily, but I guess they could be referred to as ‘vitamins’ for plants… they should supplement a plants normal feeding in a positive way, just like a Berocca after our Wheaties does.

So, if that’s a fertiliser, what’s a soil conditioner? Soil conditioners are essentially those organic based items that are added to soil to keep the soil itself in good shape, which in turn benefits our plants. Confused? Don’t be. A soil conditioner is a product that will alter the structure of the soil, improve the water and air content in the soil and increase the organic matter content. All good things if you’re a plant! Think composts, manures and rock dusts… all excellent (and fairly widely utilised) soil conditioners.

Fertilisers – What’s the go?

Trot on in to any garden centre, and you’ll notice that fertilisers come in many forms and in a multitude of brands. There are two main types of fertilisers

  • Inorganic, or artificial fertilisers that have been synthesised through a human manufacturing process
  • Organic or natural fertilisers that have been manufactured from one or a combination of organic ingredients

From a chemical point of view there is little difference between the quality of any nutrient in an inorganic fertiliser (e.g. nitrogen) and that in an organically derived fertiliser. However, there are differences in the quantities of nutrients supplied and how freely available they are to the plants for growth.

Choosing Fertilisers

  • Try and match the need to feed with the plant’s need for growth. Over-fertilisation can cause plants to produce lush, sooky green growth that can lead to a serious infestation of pests and diseases. This in turn means you have to get out there and manage these damaged plant parts. All in all, not fun, and a serious waste of time and money!
  • Think about the growth needs of the plant before you decide how much it needs to be fertilised. If it is a vegetable or flowering annual or a tree coming into fruit, then it is likely to require regular feeding. However, if you have a garden bed full of indigenous or native plants growing in a well structured healthy soil within the correct pH range, the need to add additional fertilisers is probably not necessary. In fact, by feeding, you could very well be killing them with love!
  • Check out the N:P:K rating of the fertiliser you want to purchase. This shows the amount of macro elements contained in the product i.e. Nitrogen (N): Phosphorus (P): Potassium (K). For example, the amount of nitrogen in organic fertilisers generally ranges from 1% to 9% of the total content but in artificial fertilisers it can range from 10% to 50%. Truly organic fertilisers usually have a low nutrient rating, e.g. 3:2:1 whereas artificial fertilisers can be 24:7:18 depending on how they are formulated. There is no perfect formulation of nutrients that will cover all plants as different plants have different requirements. And even this will vary due to seasonal requirements such as flowering or fruiting. The trick is to ensure that adequate, but not excessive, levels of nutrients are available to maintain plant health and necessary vigour throughout the growing season and any periods of dormancy.
  • When fertilising Australian natives that have low phosphorus tolerance, ensure that you select a fertiliser produced specifically for Aussie natives or use an organic fertiliser at half the recommended rate.
  • Supplementary fertilising will be required when plants are grown in containers or pots. Plants in pots are grown in a ‘soil-less’ medium or potting mix. In Australia this is generally composed of pine bark chips of various sizes. However, unlike natural soils, pine bark offers very little in the way of nutrients. Must modern potting mixes come with enough fertiliser to last for the first six months but thereafter, potted plants must be fertilised throughout the year to maintain plant growth.

It is important to only purchase fertiliser as needed as nutrients will be lost over time. Always store fertiliser in a cool, dry area, clearly labeled and dated. Small amounts of excess organic fertilisers can be added to compost heaps but adding artificial fertilisers is not recommended as they may damage composting micro-organisms.

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