Beating the frost, for free!

It’s April here and the days are growing shorter and cooler. The summer vegetables are dead stalks, having born their fruit. Soon the frosts will hit, so any leafy green vegetables will blacken and die off when the water in their cells expands as it freezes.

I have some mignonette lettuce and some basil plants I want to keep going a little longer. I need to protect them from frost. I made up a little tunnel from some old number 8 fence wire and some clear polythene (from an old mattress bag). That’s not enough protection from a heavy frost but enough to counter a light frost but I have a secret weapon!

My polythene tunnel made from fending wire and an old mattress bag,

My polythene tunnel made from fencing wire and an old mattress bag and held together with PVC packaging tape.

Have you ever noticed how a pile of grass clippings gets warm inside, as it decomposes?

That’s due to all the bacteria consuming the nutrients in the grass clippings, releasing excess energy. I utilise this warmth with my plastic tunnel, to combat the heavy frosts. In past years it’s even beaten the odd snow fall, as long as it doesn’t last too long on the ground afterwards.

This trick only works if the clippings are deep enough, so I remove the soil from between the rows of plants, to form a trench about as deep as the width of my hand. I’m careful not to expose the roots. Into this trench I put fresh grass clippings to form a mound. It’s important not to get the clippings too deep, touching the stems of the plants, I’m protecting. I need the clippings to be at least a total depth of 160mm or as long as my hand. This will generate enough warmth for a few days, up to a week. Finally, I place my tunnel over the clippings and plants.

This does two things:

  1. The decomposing grass clippings generate some heat. It only needs to be a few degrees, just enough to prevent water freezing overnight. The warmer humid air acts as a cold barrier for the plants.
  2. The decomposing clippings release nutrients back into the soil. When I have harvested the plants, I simply turn the lot over with a pitchfork, adding some animal manure and it will turn the worst soil into good soil for the winter garden.

Using micro climates

It’s important to realise that plants rely heavily on humidity. Their roots draw water and nutrients up from the soil to the leaves. Evaporation of the water at the leaves, creates the suction for more water to be drawn up from the roots. If it’s humid, the water evaporation rate drops and the plant gets time for full synthesis of the nutrients. On hot dry days, the plant can reach a point where it cannot draw enough water through the roots. The evaporating moisture then draws water from the plant’s cells and they shrink, wilting the plant. The difference in humidity can create a much healthier plant, able to withstand hot weather. We keep humidity high by reducing the air flow, preventing wind from drying out the area.
However, some plants are prone to moulds, like zucchinis and cucumbers. To combat this we need a better air flow around the plants. Often trimming off some of the larger leaves can combat this but it pays to not plant these plants in a warm humid corner. Using shelter, you can create a whole range of climate conditions. In Melbourne I had a narrow strip between a high fence and the brick house, on the afternoon sun side. I planted a Tamarillo tree, that everyone said would die in the cold Melbourne frosty winters. It did exactly the opposite, fruiting prolifically on our side and the neighbours side of the fence. They complained about all the fruit until they saw it fetched $13.00 a kilogram!

Tamirillo tree heavy with red tamarillos

Normally frost prone, our tamarillo tree was laden with fruit in Melbourne, due to the micro-climate we created.

Look at your block of land and plan your garden to utilise any micro-climate areas. Look for regions sheltered from winds by a fence. House walls on the sunny side of the building will act as heat banks, releasing warmth into the early evening after the sun has sunk below the horizon. Brick walls will still radiate warmth well into the night but they also absorb humidity. Areas near an outdoor hot water cylinder will be frost free in winter.

Symbiotic relationships and fertility – Fungi.

With the boom in plantation forestry, silvaculturists noticed that some trees grew well in some areas but not in others, even though the soils were identical. Chemical analysis couldn’t explain the difference. Finally it was discovered that a fungi, at the time dubbed mycorrhiza, was causing the difference. It was colonising the root tips and increasing the plant’s ability to absorb nutrients over a wider area. In some cases it formed extensive networks of fibres extending well beyond the normal root system. Since then we have discovered many other fungi that share the same relationship with plants. While we know a lot about plants and insects, we know very little about fungi.

Biologists estimate we have only identified and classified around 12% of all the fungi species. Because soil is actually a living microcosm, peak fertility will not come from chemicals alone. Fertility is the result from a variety of organisms interacting within a favourable environment.

a collection of red spotted toadstools

We still have a lot to learn about fungi. Some can extend the ability of plant roots to get nutrients.

So far we have focused on the chemicals. Let’s move on to the biologicals.

You can’t go down to the garden shop and buy a bag full of beneficial organisms for your soil. Even if you added a kilogram of worms, without the vegetable content in the soil, they will starve and die out. However there is an easier way to add these beneficial organisms.

I had a small hobby farm just out of Melbourne. There was a small enclosure close to the house that was sheltered but too small for grazing and the soil was not very good either. At the time I was working for a subcontractor for Melbourne’s parks and gardens. Each day one of the grounds contractors would dump about 75 Kg of lawn clippings. As an experiment, I asked him if I could have them all for about six weeks. I simply dumped them in rows and covered them with polythene to retain the moisture. The following spring, I planted six black currant bushes in one row and two red current bushes in another.

The bushes grew well, possibly extremely well but I had never seen a current bush to compare them to. What I did see was a huge yield of currents only in the top half of the bushes. We were swamped with currants. Being ignorant of currants, I assumed this was normal for currents. We also noticed our free range chooks were consuming less feed but the eggs were rich orange yolks and plentiful. I never connected the two until several weeks later when I was collecting the last of the season’s currents and flushed out several fat hens from the currant bushes. The reason we had no currants below 1 metre high, was the chooks were eating them! I wonder what our yield would have been if it wasn’t for the chooks?

Although I added no chemical fertilisers to that enclosure, the grass clippings had decomposed into compost and enriched the tired soil with both chemicals and organisms from the composted grass clippings. Many trace elements can be present in soil but are in an insoluble form. The fungi and other organisms that colonise the soil can break down these insoluble compounds into soluble ones, releasing the trace elements and nutrients for plants.

The role of fungi is a relatively new area of study for science. We think it aids plants by extending their root systems ability to extract nutrients. In some cases the fungi extracts nutrition from the plant but in many cases we don’t know what the fungi gains from the plant however the fact that the plant and the fungi exist and flourish better together would indicate there is some mutual benefit for each other – a symbiotic relationship. There are even predatory fungi that trap nematode worms.

We see fungi only when they fruit, forming fruiting bodies like mushrooms and toadstools. What we never see is the vast network of filaments that exist below ground, often extending hundreds of metres in all directions, that is the real fungi, supporting that fruiting body. There is a lot more to be discovered yet.

We might know how all the organisms work together to increase the soil’s fertility but there is no doubt that introducing your own composted material will boost your soil. The compost you buy in bags from the garden store is sterilised, usually with super heated steam, so although it is high in humus, it has virtually no micro-organisms. Making your own compost is best.

To make your own compost is very simple. Vegetable scraps can be buried directly in the soil that is later to be used as a garden. Alternatively create a compost heap by forming a low wall on three sides of a rectangle and throw all your grass clippings and vegetable scraps inside. In dry climates, it pays to water the heap every so often, the organisms perform best in a damp environment.


Symbiosis and fertility

So far we have focused on soil structure the chemicals. The trend today, is to take a piece of land, strip the top soil and build on the sub soil. Then to replace the top soil for landscaping, they bring in what they call “sandy loam”. This is usually mostly sand with minimal loam. It looks good and is easy to dig for a garden (which is why it’s a favorite with landscapers) but poor when it comes to moisture retention and fertility.

The plants you planted that were surviving in wintwer, wilt and die in summer. The soil just can’t retain water. The problem is easily fixed with the addition of humus. Leaf litter, lawn clippings or even fine bark mulch will assist with moisture retention and as they decompose in the soil, will realease valuable nutrients, however they will break down very slowly because your soil is lacking in the naturally occurring organisms that decompose this material, recyling the nutrients.

Soil is not just sand fibre and a few chemicals; it is a living environment with all sorts of organisms, from visible sized insects to microscopic ones, all interacting together. To do this effectively they need a warm moist environment. The process almost stops when the soil temperature drops to zero and when it tops 35ºC but peaks around 15ºC.

These organisims perform one or more of three functions:

  1. Decocmposition – the rotifers – they consume material and release nutrients back into the soil, in a form that can be used by the plants. Unfortunately they are often not too particular which material they consume and can consume living and dead material, causing harm if not in balance with the other organisms. These are the bacteria, fungi and many different types of nematodes (worms) and insects.
  2. Predators – these organisms prey on others, keeping the system in some sort of balance. Some predators can also feed on plant material when food is in short supply.
  3. Aerators – the beneficial organisms usually perform best when there is oxygen present and to aerate the soil requires organisms to create tunnels for air flow and drainage.  Excess water can also prevent soil oxygen.

When these organisms are in balance your garden will flourish but you can’t buy them from the garden shop. They have to be grown on site and the fastest way is in compost.

Trace Elements

These are elements, that effect soil fertility but are present in tiny amounts, too small to measure without some very sophisticated equipment. Different trace elements favour different plants. We are still discovering what trace elements do in plants. In many cases they are present in such small amounts, it is hard to determine if they are a trace or a contaminant!

A high trace of iron, for example will promote citrus growth, especially oranges. Iron is essential to all plants because it is used to produce chlorophyl – the green compound that plants use to convert sunlight into enough energy to combine carbon dioxide with water to get glucose.

Calcium is also an important trace mineral for plants, just like it is for animals. It is used in the formation of cell walls and a range of other functions.

Molybdenum is vital in the cellular reactions of all plants but especially in nitrogen fixing plants like peas, lupins and clover. A field of pasture of mixed grasses will become dominant with clover if molybdenum is added in trace quantities.

Magnesium is an important trace element in plants. If defficient, the leaves will appear yellow or blotched with yellow patches. The plants will be vulnerable to pest borne diseases. It is required to break down glucose and release energy at the cellular level in all life forms but it serves a double role in plants because it is also required to make chlorophyl.

Cobalt is not regarded as an important trace element in plants but if defficient in grazing animals, they become emaciated because it is a key ingredient in the enzyme rumen bacteria create to digest the cellulose in plants.

There are many other elements that appear as trace elements and we don’t know what they do. Some are thought to assist bacteria in breaking down compounds for use by plants. Others could be used as catalysts by the plants themselves. Because they are present in such tiny amounts and are not actually consumed by the plants it’s difficult to decide if they are contaminants, inert compounds or essential to some function in the plant.

Soil fertility – The NPK group – Nitrogen


To produce leafy growth plants need more nitrogenous compounds. Through photosynthesis they break these down to extract the nitrogen required to create leaf cells. This means they need a compound that is rich in nitrogen. Usually we use urea or ammonium sulphate based fertilisers to increase the nitrogen available for plants. Chicken manure is high in nitrogen but is too strong to add directly to the plants. Instead it’s a great additive to compost. The bacteria will break it down to make it even more readily accessible to plants.

It’s important to realise that plants require nutrients in minute quantities. A little goes a long way. We currently have a farming attitude of saturating the soil with an excess of fertiliser for a bumper yield. Over time the level of nutrients rises to the point where the water run off is toxic to aquatic life. The best method is to rely on natural organic fertilisers. Usually they are slower to release their nutrients, often lasting several growing seasons and the run off water is less toxic to aquatic life. An added benefit is that they usually break down to release other nutrients in lesser quantities, unlike artificial chemical fertilisers that leave a residue that builds up with each application.

a field of mainly blue flowering lupins

A field of lupins in Canterbury New Zealand, grown as green fertiliser.

Another way of increasing nitrogen in the soil is to grow plants that “fix” nitrogen in the soil. These are plants that have nodules on their roots with special bacteria that trap nitrogen in a form readily usable by plants. The most common plants used are all the members of the pea family and lupins. Sometimes driving around in the country, you will see fields of bright blue or yellow lupins. A farmer is replenishing the nitrogen content of the soil naturally. It’s a lot cheaper than applying artificial fertilisers and it can be done over the less productive time of the year.

As a home gardener, there are a different varieties of lupins that make a bright show of flowers as well as step up the nitrogen in your soil. Simply plant them and when they have flowered for a while (but before they have all died off and developed seed) dig the entire plant into the soil. In a month they will have decomposed into a nitrogen rich humus.

Lupin flowers in a variety of colours

There’s a wide range of lupin varieties for the home gardener. You get the show and the nitrogen fixing – a win-win scenario!

Fertility and the NPK group – Potassium


Potassium is generally linked to seed, flowers and fruit production. It is highly soluble and easily leached out of the soil. For this reason is uis more common in sandy soils. Though it is present in manure in small quantities, it is much higher in ash. Primitive cultures discovered that burning off the vegetation somehow created better soil for their crops. What they were doing was raising the levels of potassium in the soil through the resulting ash.

Two tomatoes on a vine with yellowing over their top halves

Yellow shoulder in tomatoes is often linked to a potassium deficiency.

Typical symptoms of Potassium deficiency in plants include brown scorching and curling of leaf tips. The symptoms generally first appear on older leaves. The mature leaves will begin to show yellowing at the edges, which will eventually turn brown at the extremities. In tomatoes they develop a yellow shoulder on the fruit and the leaves turn brown and wither. There is some debate whether this is directly due to a deficiency in potassium or the fact that it causes the vines leaves to wither and exposes the fruit to too much sun and heat – either way, there is a relationship.

Leaf with yellowed edges that have turned brown.

Potassium deficiency is more noticeable in the older leaves which first turn yellow at the edges then brown and curl.

 Most tomato fertilisers are higher in potassium. If you have been growing tomatoes and using a commercial tomato fertiliser, other vegetables planted in the same location may show magnesium deficiency symptoms. This is not due to a deficiency of magnesium, it can be due to plants taking up the more reactive potassium instead of magnesium. Use chemical fertilisers sparingly, especially when they are plant specific.

Ash from treated wood, coal, plastics and general household rubbish can also contain high levels of toxins, like heavy metals and should not be used on food gardens. Ash is also quite caustic and will harm most plants if applied directly. It’s best to steep ash in water and use the water to water the plants. It can be added directly to the soil, if it is left to age a few weeks before planting. This allows the soil bacteria time to break it down and reduce the caustic levels in the soil.

All plants require a mix of all three nutrients but depending on their growth, will require slightly more of one or the other nutrient.

Fertility and the NPK group – Phosphorus

Phosphorus is usually associated with root growth. Plants grown in soils deficient in Phosphorus are usually stunted. Adding phosphate fertilisers to new plants is a common practise that produces rapid lush growth, however it does not increase the final yield in many cases. For this reason grazier farmers add superphosphate (or super as it is called in the agricultural community) to their fields when regrowth begins in spring. Unfortunately much of it is wasted and ends up in the water run off with disastrous results in the aquatic systems. Usually applied as phosphate in horticultural applications, it produces dramatic improvements in growth because the added root growth gives plants more access to nutrients. Unfortunately it is often overused in agriculture, to the point where it leaches into rivers, creating algal blooms and choking aquatic weed growth. Each year the nitrogen and phosphate rich run off from farms in the Mississippi valley runs off into the Gulf of Mexico where the nutrient rich water reacts with sunlight, creating a massive algal bloom that covers 16,000 square kilometres. When the algal bloom dies off, it leaves an area the size of New Jersey, so deprived of oxygen it is lethal to all fish.

Satellite view of Gulf of Mexico

The read region around the mouth of the Mississippi River shows water with oxygen levels below 2 parts per million due to phytoplankton blooms. Courtesy of NASA

Phosphorus deficiency in young plants can appear similar to a potassium deficiency because the roots are stunted and cannot take up enough nutients, making the leaves appear yellow at the edges. As the plants grow sugars can accumulate and cause anthocyanin pigments to develop, producing a reddish-purple color in the leaves. Before you decide there is a shortage of phosphorus, check your plants do not typically have a red/purple tendency. Generally this is rare in most soils except those in tropical regions where monsoonal rains and flooding can leach phosphates out of the soils.

Tomato vine leaf with purple undersides

A tomato vine leaf showing signs of phosphorus deficiency – a purple shade under the leaves.

Animal manure is generally high in phosphorus. It is best aged or steeped in water and the resulting water used to water the plants.

Soil fertility and plant nutrients

We have looked at soil structure which is usually the key to a good garden in most areas but we should also discuss fertility here as well. Fertility is the result of three factors:

  1. Structure – it doesn’t matter how rich your soil is in nutrients if the plant can’t get to them or the soil is so wet it is oxygen deficient and a breeding ground for anaerobic bacteria. Or conversly, too dry.
  2. Nutrients – we’ll look at these in more depth in this session. These are consumed by plants as part of their normal growth cycle.
  3. Trace elements – these are elements, present in minute quantities that usually favour one type of plant more than others. They are present in such tiny quantities that they can only be measured with complex instruments, beyond the scope of the average gardener.

The NPK nutrient group.

Plants need sunlight and water to survive; something we all take for granted but they also need other nutrient compunds for growth too. As a simple overview, we call this the NPK combination. Basically they need all three elements (N = Nitrogen, P = Phosphorus and K = Potassium).

In the next instalment we’ll look at these in more detail, what they do and where to get them.

Soil structure – the wet test

In my case we had a red clay. To see how much of each component was in the soil,

  • I placed a sample of soil in a glass bottle, so it filled it to about half way.
  • I added water and shook it vigorously.
  • I left it to settle overnight and it separated into three layers, below a layer of excess water – humus at the top in a thin dark brown layer. A large band of dissolved clay in the middle and a lower layer of sugar like sand.
  • The next morning I re-shook the sample again and checked it that evening, The humus layer was thicker and the layers were more clearly defined. The clay had not completely dissolved the day before.
Jar of soil and water in layers

Once settled the dissolved soil will settle into layers

This was a perfect visual representation of my soil. Since my block was scraped free of top soils when they built the house, it was highly likely this sample represented the soil structure over my entire block. I would advise anyone to take 4 to 6 samples, as far away as possible from each other, to get an more accurate idea of their block because it can varey widely, especially with established homes.

Ideally you want to see equal layers of each structural element – humus, grit and clay. For root crops a little more grit and for leaf crops a little more humus.