For adequate plant nutrition, gardens should be fed with fertiliser twice a year at the very minimum. Once at the start of spring (September in the southern hemisphere, March in the northern hemisphere) for warm season crops, and then again at the start of autumn (March in the southern hemisphere, September in the northern hemisphere) for cool season crops.
After the first month of autumn has passed, and temperatures have started dropping in cool and temperate climates, the conventional gardening wisdom is to stop fertilising the garden, and not fertilise again until the start of spring. We’re told that there’s no point fertilising during the cold seasons because the plants can’t use the fertiliser, and that’s sound advice.
Gardener’s with inquisitive minds may ask, “why can’t plants use the fertiliser in winter?”
Cold Weather Nutrient Availability and the Nitrogen Cycle
The reason why we don’t fertilise in cold weather in cool and temperate climates is because low soil temperatures reduce soil microbe activity. Just like most living things, the microorganisms in the soil are more active when its warm, and less active when it’s cold.
Microbe activity plays a very important role in the nitrogen cycle, the process in nature where nitrogen is converted to into various forms and exchanged between the air, soil and water.
There are two parts of the nitrogen cycle that play an important role when it comes to feeding plants, and nutrient availability, these are the processes of mineralisation and nitrification.
Mineralisation is the process whereby microbes decompose organic nitrogen (N) from manure, plant materials and soil organic matter into inorganic forms, first to ammonia (NH3), and then to ammonium (NH4+)
Since mineralisation is a biological process, the rate of mineralisation varies with soil temperature, moisture and soil aeration (which determines the amount of oxygen available to the soil microbes).
Mineralisation occurs most readily in warm, well-aerated, moist soils with a temperature of 20-35°C (68-95°F),
Nitrification is the process whereby microorganisms convert ammonium (NH3) to nitrite (NO2–) and then nitrate (NO3–) in order to obtain energy.
The process of nitrification occurs most readily in warm, well-aerated, moist soils with a temperature of 19.5 – 30°C (67-86°F), but virtually halts below 5°C (41°F) and above 50°C (122°F).
Feed the Soil, Not the Plants
In the ecological design system of permaculture, there’s a saying “feed the soil, not the plants”.
When we add fertiliser to the soil, we’re not feeding the plants, were feeding the soil-food web, the complex soil ecosystem which breaks down the organic matter and natural fertilisers into a form that the plants can use.
Organic matter is converted into nitrate, the form of nitrogen which plants can use the most, through a series of chemical steps as shown below:
R-NH2 -> (NH3) -> (NH4+) -> (NO2–) -> (NO3–)
organic N ammonia ammonium nitrite nitrate
In the chemical equation above, we represent organic nitrogen chemically as R-NH2 because the nitrogen in organic matter is the form of amino acids, which are typically made up of an amino group (NH2). The amino groups are attached to an R-group here, which is an abbreviation in chemistry for any group in which a carbon or hydrogen atom is attached to the rest of the molecule.
With respect to organic matter, the symbol R-NH2 is telling us that nitrogen is in the form of an amino group (NH2) attached to carbon-containing molecules, it’s that simple.
It may be worth mentioning that the chemistry definition of organic compounds are molecules that contain carbon in them. All living things on the planet are carbon-based, they’re primarily made up of molecules containing carbon. Just like nitrogen, carbon is also converted and recycled though many forms in a cycle of nature known as the carbon cycle.
To summarise, as part of the nitrogen cycle, the processes of mineralisation and nitrification are carried out by soil microorganisms that are part of the soil-food web ecosystem. Through these two processes, microorganisms convert organic matter, materials that were once living into nitrate (NO3–) which is the form of nitrogen that is most available to plants. Nitrogen in nitrate form is water soluble is highly susceptible to leaching out of soils. Soil nitrogen levels drop due to uptake from plants, leaching and volatisation into the atmosphere, and therefore need to be topped up periodically. The best time to add more nitrogen rich materials such natural fertilisers is when the soil temperature are high enough for the mocrobial activity to take place.
- Cornell University Cooperative Extension, Agronomy Fact Sheet Series, Fact Sheet 2, Nitrogen Basics – The Nitrogen Cycle , http://nmsp.cals.cornell.edu/publications/factsheets/factsheet2.pdf
- University of Nebraska, Institute of Agriculture and Natural Resources, Documents – Section D: What happens when nitrogen is applied to the soil? https://water.unl.edu/documents/Section%20D.pdf
- University of Arkansas Cooperative Extension Servi, The Nitrogen and Phosphorous Cycle in Soils, https://www.uaex.edu/publications/PDF/FSA-2148.pdf