Fertilisers and the Environment


Fertilisers and the Environment


Don Burke says the best thing you can do for the environment, as a gardener, is not abuse fertilisers. Don’t overdo it, don’t use the wrong ones and if you’re close to bushland, think about the water running off your property.


Modern fertilisers have revolutionised both gardening and agriculture. Without them, millions of people would starve to death all over the world. But they come at considerable environmental cost.

Fertilisers have been over-used on farms and in gardens. This has led to pollution of soils and groundwater and also to major collapse of local eco-systems. Most weed problems in Australia are due to fertiliser abuse and also to over-use of water. Weeds are merely colonisers of degraded land. It is estimated that more than 80% of lawn fertiliser that you apply is either washed away or seeps into groundwater. Less than 20% is used by the lawns.

Classic problems caused by over-fertilising of farms and gardens include:

1. Death of most bushland plants that remain in gardens.

2. Major bushland dieback and luxurious weed infestations downhill from housing estates and roads.

3. Floating weed problems such as salvinia, duckweed and azolla in rivers, dams and fish ponds.

4. Toxic blue-green algae infestations of rivers, dams and lakes.

5. Weed problems like fireweed, capeweed, stinging nettles and Patterson’s curse in pasturelands.


What can you do?

I am sure that no responsible person wants to do anything that would cause land degradation. Gardeners and farmers have the opportunity to do their bit in minimising land degradation. Despite what city people believe, most farmers are well advanced in dealing with environmental issues. So this article is addressed to the naughty people: gardeners. Well, perhaps they’re unintentionally naughty. We’ll look at fertilisers, their use and abuse in each section of the garden.


Lawn foods

This is the big end of town. These do heaps of damage. Old-fashioned lawn foods that look like bags of dirty sugar are a bit nasty. These are salts that give instant greening-up of lawns but most of what you apply washes away and causes environmental harm. Equally, most people put far too much on their lawn anyway.

Lawns that are not fertilised each year still grow and perform perfectly well. So, you can all quite safely either reduce your fertilising or cut it out completely. By far the best lawn foods to use are the slow-release ones, such as Scotts Lawn Builder. These work for three or four months, releasing a little bit of fertiliser daily over a long period. Put these on (maybe at half the recommended rate) in September each year – and leave it at that.

If you are on sloping land that sheds water towards a river, creek, dam or even bushland you should seriously consider never fertilising your lawn.


Garden fertilisers

Most garden plants will grow very well without ever being fertilised unless you have very sandy soils. Plants that are not fertiliser-hungry include:

• Rhododendrons

• Azaleas

• Camellias

• Lavenders

• Rosemary

• Almost all native plants – in fact almost all garden plants except those in the next section.

For these plants, you could use a quarter-strength dose of most fertilisers every second or third September. Or a very light dose of animal manure such as pelleted poultry manure (eg, Dynamic Lifter) every second year.


Plants that need fertiliser

Some plants are what we call ‘gross feeders’, that is, they guzzle fertilisers. Without fertilisers they don’t do so well.

Vegetables: vegies need fertilising to produce lots that you can eat. A pre-planting fertilising with animal manures and/or compost is a good idea. That is usually enough for most root crops (carrots, beetroot, parsnips, etc) and also for fruiting vegies such as tomatoes, eggplants, beans, peas, etc. You can also jolly them along with liquid fertilisers such as Miracle Gro, Nitrosol, Aquasol, Thrive, etc – but stop once the plants are half-grown – otherwise you won’t get good roots or fruit.

Leaf vegies: these include silverbeet, spinach, lettuces, cabbages, Asian greens, rocket, etc – and they all need much more fertiliser. Use the pre-planting fertilisers such as manures and/or compost, but they need regular fertilising at least every two or three weeks with liquid foods such as Nitrosol, liquid manures (homemade), Aquasol, Miracle Gro, etc. You must continue to fertilise them until harvest, otherwise they will go to seed.

Fruit trees: all fruit trees need annual fertilising with any complete fertiliser (eg, citrus food or rose food) in spring. Citrus in particular need fertilising in August and February with citrus food or pelleted poultry manure to ensure healthy growth and lots of fruit.

Some flowering shrubs: the fertiliser guzzlers amongst popular flowering shrubs include roses, hibiscus and gardenias. Gardenias love duck or chook poo, but the rest can do very well on rose or citrus food.


Pot plants

All potting mixes sold these days are soil-free. This enables the mix to drain better and to be mostly free of plant diseases. But the biggest downside is that they contain no long-term reservoir of fertiliser (as soils do). Most Standard potting mixes have a small amount of added fertiliser and the Premium potting mixes contain more fertiliser that will last significantly longer – maybe six months.

However, all potting mixes require slow-release (controlled-release) fertilisers for sustained plant growth. Even premium mixes need fertilising after the first year. Fertilise all pot plants with a six- or nine-month controlled-release fertiliser in September each year. These products are very efficient and contribute very little to environmental degradation.


Septic tanks

Septic tanks cause horrific injuries to local ecosystems. The runoff of water plus fertilisers from absorption trenches is lethal to local plants, and this in turn devastates local wildlife. This damage causes a plume of death downhill from houses in native bushland which remains toxic to native plants for over 20 years. In rocky areas with shallow soils the destruction can extend for over a kilometre.

Over time septic runoff will totally degrade and destroy bushland areas. The best method for minimising septic runoff is to use a twin-tank, biological non-septic system that is joined to an irrigation system. The system should spray onto level grassy areas which may be able to absorb the water and nutrients.

The key bit here is that overly-wet and over-fertilised land is toxic to the original native species yet it is the preferred habitat for weeds such as privet, lantana, asparagus weed and Pittosporum undulatum. These, and many other weed species, are colonisers of degraded land. You can replant the original native species as much as you like, but they won’t prosper in wet, over-fertilised soils. Here, of course, I am referring to dry bushland (dry schlerophyll) areas that are widespread in Australia.


Vulnerable areas

As stated above, shallow soils on rocky landscapes are very vulnerable. Probably the most vulnerable of the lot are sandy soils, especially those on the coast of Western Australia. Sandy soils do not store a reservoir of fertiliser as normal soils do. Hence they need fertilising, but the fertiliser goes straight through them and into the groundwater below. WA has new regulations to try to deal with this problem, but in general any slow- or controlled-release fertiliser is best. Animal manures, blood and bone or Osmocote or Nutricote are all useful.

Even just water runoff from roads and houses poses a serious risk to remnant bushland. Commonly, in areas downhill from roads and houses, the weeds soon choke out the bushland. It’s tragic that local councils fail to steer water runoff into areas that tolerate it.


Grey water

As you may have guessed by now, grey water is rather nasty in its effect on plants. It can poison soils with too much sodium and phosphorus, but the sheer constancy of the wetness from outflows alone can kill plants.


The bottom line

If we are all mindful of the environment when we use fertilisers, we can drastically reduce environmental damage. This is probably the biggest contribution that the average person can make to the environment. So it is worth a thought.