Fruitful life of fertilisers
One of the greatest of challenges for gardeners is knowing when that last application of fertiliser has started to run out and more needs to be applied. There’s no easy answer.
Gardening would be so much easier if we humans could see what’s happening in our soils and potting mixes on a microscopic scale, maybe doing a “Honey, I Shrunk The Kids” on ourselves, and embarking on a journey of discovery through the air spaces and water-laden pores of our (hopefully) well-structured soils.
Apart from fighting off all the greeblies in the soil that’d want to eat us (we are organic matter), any remaining skerricks of nutrients would be able to be seen and we’d have an idea about what was needed for the plants above.
Soon after we’d be pursued by a fangy fungus, a nasty nematode or an edacious earthworm and no doubt turned into nutrients as well.
Unfortunately, full-sized gardeners have little to rely on for an indication of a lack of nutrients apart from physical deficiency symptoms, which can be quite misleading even if they manifest themselves just as the textbooks suggest.
The only true method of determining nutrient levels in the soil is to take a sample of your soil and send it to a laboratory for analysis, which I’d recommend every few years or so.
From an everyday gardener’s perspective, it’s best to understand the type of fertiliser you’re applying to help determine its effective life once it is incorporated into the quite variable material that soil is.
Liquid fertilisers are first cab off the rank, with most types regarded as those with the shortest life in a soil.
These fertilisers are usually either in a concentrated liquid form, or a soluble powdered form, both to which water is added to dilute for application.
Salt-based liquids or powders are usually in a form that can be readily absorbed by plants immediately they enter the soil, but this usually means that they don’t last long as well. They’re either taken up by plant roots, quickly leached through irrigation or rainfall, or absorbed by some organisms.
Application intervals are therefore quite small, with instructions normally stating a re-application frequency of every two weeks or so.
Liquid organic fertilisers may last a little longer in the soil as they contain materials that may need to be broken down by soil organisms before nutrients are released.
This means that re-application intervals will vary from two weeks up to a month.
Granular fertilisers will generally last longer than the liquid forms.
Granular products can be purchased as fully manufactured, salt-based forms, a mix of salt-based and organic granules, or fully organic granules/pellets.
Some salt-based granular forms are readily soluble in water, so will be the shortest-lived in your soil.
Urea is an example of a soluble granule which supplies only nitrogen, which must be watered in for immediate availability.
It may only supply nitrogen for a few weeks or so. Other salt-based fertilisers are manufactured in a form that can be applied dry to a moist soil but will then gradually release nutrients based on moisture levels and temperature.
These may only need re-application every two to three months or so.
Some lawn fertilisers are formulated to release a range of nutrients over three months, without the need to water them in or apply to a moist lawn.
Some granular fertilisers will contain a large percentage of organic nutrients, which can extend the time between reapplications, as organic compounds take time to be broken down by soil microbes.
This may mean that you may only need to apply these every four to six months.
This applies for fully organic, dry fertilisers as well. Arguably, the longest-lasting form of fertiliser is the controlled-release product.
These are usually complete fertilisers that have been encapsulated in a prill which releases nutrients based on soil temperature – the object being more nutrients are released in the warmer months when plants are growing, and less in the cooler months when growth slows.
Formulations for garden beds and landscaped areas vary with their release times from four months up to nine months.
These are mostly salt-based nutrients, however there are newer formulations that also incorporate microbes that are supposed to help improve soil structure and increase nutrient availability.
These may also contain seaweed extracts as well as other organic plant “tonics” and may last up to five months in the soil.
As with all fertiliser products, always follow the manufacturers’ instructions, and never exceed the recommended application rates as this may lead to physical damage such as burnt roots and foliage, or a toxic level of some nutrients.
A final note – remember that nature is variable and chaotic, so the time periods mentioned in this article will vary depending on climate and weather, soil type, amount of cultivation and other factors.
GARDEN CUTTINGS
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