MULCH
Posted by Phytopath on Jun 19, 2011
The Good, The Bad & The Dangerous.
Gardeners who live in hot climates are used to hearing the advice; mulch, mulch, mulch.
But is it the best thing to do in YOUR garden?
The main types of mulching material are;-
Organic materials which decompose readily. For example, straw, hay, leaves, manures and seaweed.
Organic material that decomposes more slowly, for example, pine bark, wood chips, twigs and thin branches.
Minerals such as pebbles, gravel and crushed bricks, just to name a few.
Synthetic material such as weed mat.
You can also use ground covers as a living mulch.
The idea of using a layer of mulch around your plants or throughout your garden is to reduce the loss of water
from the soil via evaporation.
Mulch can also protect the soil surface from heavy rain and reduce the risk of crusting and erosion.
It can slow the flow of water across the soil surface and increase the amount of water moving down through the soil profile.
Mulch can change the temperature of the soil beneath it and it can sometimes suppress weed growth.
Organic mulches that decompose rapidly also help to improve the soil structure.
All in all, mulching sounds like a good thing to do.
So why is the title of this blog post ‘Mulch. The good, the bad and the dangerous?’
Well, some mulching material can be detrimental to your plants.
If mulch is spread too thickly and forms a watertight layer, it can prevent gases leaving and entering the soil
around plants.
Carbon dioxide levels can build up in the soil and plant roots can ‘suffocate’ from lack of oxygen.
The same situation can occur if you are using plastic mulching material.
Plants may develop Nitrogen deficiency when certain materials, such as sawdust and straw, decompose
rapidly. If you intend using these types of material that are high in carbon, place a layer of nitrogen fertilizer on the soil before applying the mulch.
Mulches can aggravate anaerobic conditions in soil where the drainage is poor. This can lead to denitrification
(loss of Nitrogen) in the soil which will show as a yellowing of older leaves in some plants.
Many types of mulch are used to decrease the soil temperature in hot climates, so the plant roots that are
growing near the surface of the soil don’t ‘cook’. But air temperatures are more extreme just above the mulching material. Young seedling plants growing through a layer of mulch in the summer can be injured by the higher temperature.
The opposite is also true for night temperatures. Organic mulches are not good at absorbing heat during the day and the surface of mulch cools very quickly under night radiation, sometimes being three or four degrees colder above a mulch than above bare soil.
Another factor to watch out for is the possibility of toxicity or phytotoxicity.
Not all plants get along with each other (much like people) and some plants even cause other plants to get really sick if they are in close proximity to each other.
If material that has been designated for mulch has not been composted properly, it can cause toxicity to other
plants once spread around the garden.
Bark and sawdust from certain trees have been reported to be toxic to other plants. Some examples are Eucalyptus, redwood, cedar, larch and spruce.
The toxic substances from these trees can be removed through proper composting. 
So there you have it.
The Good, The Bad and The Dangerous.
Soil pH
Posted by Phytopath on Jan 21, 2011
Soil pH. Why is it important?
Most gardeners are aware of soil pH and the corresponding pH scale of zero to 14.
Gardeners also know that certain plants like growing in soil at a particular pH.
For example, rhododendrons, azaleas, camellias, blueberries and conifers have a preference for acidic soil while peas, beans and hardenbergia prefer to grow in slightly alkaline soil.
So what is acid soil and what is alkaline soil? And what affect does it have on plant growth?
An acidic soil is one that contains a high concentration of Hydrogen ions, H? (an ion is an atom with a charge, either positive or negative) and an alkaline, or base soil, is one which has a low concentration of Hydrogen ions (H?) but a high concentration of hydroxyl ions (OH?).
Without going into too much chemistry, this basically means that the soil pH will affect the health and growth of the plant, by limiting the availability of certain nutrients or by supplying too much of a nutrient, causing toxicity.
Soil contains nutrients that the plant needs to grow. Think of these nutrients as ‘food’.
If the soil pH is outside of the preferred range for a particular plant – it means that the plant cannot take up the nutrient, even though it is in the soil.
An analogy would be you going to the refrigerator for a late night snack and finding a padlock on the door.
You know there is food in there – but you can’t get at it.
Generally, most garden plants can exist in soil with a pH ranging from about 4.5 to 8.5 but some plants are fussy.
Most plants grow best in a soil pH around 5.5 to 6.5.
Within this range, plant nutrients are at optimum availability.
The acid loving plants previously mentioned, don’t actually love acid – they like the fact that iron (as opposed to ion) is in abundant supply in soils with a low pH (acidic).
These acid-lovers are not very good at extracting iron from the soil, so they like to grow in an environment where their roots are exposed to a plentiful supply.
So what would happen if you planted a lime-loving plant that prefers alkaline soil, into an acid soil?
If it is a legume, it would most likely die.
In acidic soils below pH 5, aluminium and manganese become soluble.
Legumes are very intolerant of soluble aluminium and would show signs of toxicity if grown in this soil.
If you want to amend the soil and make an acidic soil less acidic, lime (calcium carbonate) or dolomite (calcium carbonate and magnesium carbonate) can be added.
Note ; Gypsum will not change the soil pH
Seek advice on the rate of application according to your soil texture and current pH.
If your soil is alkaline and you would like to make it less alkaline, it may be possible to change the pH by adding agricultural sulphur or sulphate of iron. But if there is lime present in your soil, this will not work.
Nutrient deficiencies found in alkaline soil include phosphorus and most of the micronutrients (trace elements)
My current soil pH is around 8. When I first moved here 21 years ago the soil pH was 7. Can anyone guess why it has become more alkaline?
Rainfall also affects soil pH. It is usual for a soil to be more acidic in high rainfall areas, and more alkaline in arid conditions.
If you live in an area of high rainfall with acidic soil, you may need to apply lime to the soil on a regular basis, depending on the plants or crops grown.
So in summary, pH is very important and should be measured or checked regularly.
The home gardener can check the pH of a soil by using one or more of several methods.
Spike – these are useless, don’t waste your money
Colour metric chart – these are the most popular in Australia and are readily available from large plant nurseries and hardware stores.
A small sample of soil is taken and then a dye indicator (liquid) is added to the sample. Next, you sprinkle the sample with Barium sulphate (a powder) and wait for the colour to change.
The down side of using this method is that some people are colour blind and the rest of us have to try and match the soil sample to the colour card provided.
The colours on the card can differ between manufacturers depending on which printing company they use to print the colour cards.
If your interpretation of the colour is one pH unit out, this could be substantial considering that the pH scale is logarithmic. Meaning the difference between pH 7 and pH 8 is not one – but ten times more alkaline.
pH Electronic paper – these are nifty little gadgets that fit in your pocket. They provide an electronic reading of the soil pH sample and are used a lot by the viticulture and floriculture industries.
There are other methods of taking pH readings, some would be too expensive for the home gardener and others you can make yourself, but the results of self made pH testing devices would be similar to the spike.
If you are serious about gardening, don’t go there, but if you want to have fun with the kids just ask me for the recipe.
My final question to readers is this – “Why is pH always written with a lower case p and an upper case H?”
If you can answer this question correctly in the comments section below, I will send you a prize.
Soil Amelioration
Posted by Phytopath on Nov 23, 2010
Ameliorate: – to improve or make better.
I believe that virgin soil needs no amelioration. It is fine just as it is.
The problem we face is, the soil we farm or garden in, is not as we want it, for the plants that we would like to grow in it and has probably been altered through previous farming or gardening practices.
So we add fertilizers, lime and gypsum etc. in the hope that we can improve the existing soil conditions.
My question is “What’s to improve on Mother Nature?”
I think a better term or word (other than ameliorate) would be ‘change’ or ‘manipulate’ – to our advantage.
So, here are some suggestions to change your current soil conditions and why you would consider such changes.
Most of the soil amendments or additives used in the garden alter the soil structure (gypsum) or the soil pH (lime).
Good soil structure is important because it allows the free movement of water through the soil profile as well as allowing harmful gases to escape from the soil around the root zone of plants.
A soil with a good structure also allows easy movement of oxygen into the pore spaces between the soil particles.
Why is this important?
If your soil has a poor structure due to compaction (stop parking the car on the front lawn), the air and water cannot move freely through the soil profile and there is a good chance that newly planted trees and shrubs would find it difficult for their roots to penetrate the soil.
For existing trees and shrubs, it would mean that any water added to the soil, through irrigation or rain, would travel extremely slowly through the profile and possibly linger like a bad smell. The plant roots would start to die from lack of oxygen (nothing to do with the bad smell) because the water is being held in all of the pore spaces. Poor drainage, we call it.
Many clay soils are considered to have poor structure and the usual recommendation is to add gypsum.
Be aware that gypsum does not react with all clay soils.
There are two predominate types of clay soil. Slaking clay and dispersing clay.
Gypsum will only react with dispersing clay. It flocculates (look it up in the dictionary) the soil particles, kind of clumps them together, to create larger pores for better drainage.
The best way to treat slaking clay is to add organic matter.
How do you know if you have slaking or dispersing clay?
You could ask an Agronomist or you could attend one of my talks. Or hang in there and I will eventually cover it in a future blog post.
Sandy soils can also have poor structure.
Unlike clay soil, water can move very quickly through sandy soil.
Many sandy soils are referred to as ‘gutless’, meaning they have little ability to sustain good plant growth.
The reason for this is the free movement of water through the profile. The water leaches or takes with it, any nutrients that were previously available for plant growth.
The lack of ability to hold on to moisture for any length of time, also means that plants can suffer from water stress more often than if the same plants were grown in a good loamy soil.
The best way to address these problems is to add organic matter.
The organic matter will help to retain moisture in the soil and it will also help to retain and release nutrients to the plant.
So why do gardeners add lime?
You sometimes hear the terms ‘sweet soil’ and ‘sour soil’. This is a reference to the acidity (sour) or alkalinity (sweet) of a soil.
Many farmers add massive amounts of fertilizers to their crops. Over time this can cause the soil to become acidic. To address this problem, they add lime to the soil.
The reason for doing this, whether on a farm or in a suburban backyard, is the relationship between soil pH and nutrient (fertilizer) uptake by plant roots.
Plants have a preference for the amount and type of nutrients available to them. Just like you and I have a preference in what we like to eat.
If the soil pH is outside of the limit or tolerance of a particular plant, then the plant is unable to extract the particular nutrients that it requires for good growth. The nutrients are often there but the plant cannot access them for various reasons.
Think of yourself looking to go out for a meal but the restaurant is closed. The food is there but you cannot access it unless someone opens the door to the restaurant.
Changing soil pH is sort of like opening the door to the restaurant. All of a sudden the plant has access to the required nutrients.
If your soil is acidic and you are not growing acid loving plants, then the addition of lime will help to reduce the acidity. The amount of lime used will depend on your soil type. Seek professional advice.
If your soil is alkaline and you would like to grow azaleas – forget about it.
Alkaline soil can be encouraged to be less alkaline with the addition of sulphur. This is not however a cheap fix like lime. Again, seek professional advice.
This post is getting quite lengthy and more like a lesson than a blog post. I must stop somewhere, so here it is.
If you have a real interest in soil science, there are many books available on the subject, from beginners to advanced.
The more you understand your soil, the easier gardening becomes.
Soil is Not Dirt
Posted by Phytopath on May 10, 2010
Red-brown Earth
One thing that I find mildly annoying is calling soil, dirt.
Dirt is what politicians put on each other, soil is a valuable resource.
We use soil to grow food, grow timber for building materials, make beautiful pottery and to build mud brick homes.
Soil health is very important to our own health. Food that is nutrient dense is grown in a soil that is very much alive and brimming with soil microbes.
When we talk about plant nutrition we often think of feeding the plant with major and minor nutrient elements such as the big three; nitrogen, phosphorus and potassium.
What we really should be thinking about is feeding the soil. If we do this, the plants will thrive.
A healthy organic soil has the ability to ‘hold’ on to nutrient elements until the plant requires them. Humus, the end product of decomposed organic matter, has a negative ionic charge that attracts and holds any nutrient element that has a positive ionic charge (called cations).
Think of a magnet that has a north pole one end and a south pole the other end. Now instead of thinking north and south poles, change that to positive and negative. When you place two negative magnets together they will repel and the same happens for two positive magnets. But place a negative and a positive together and they will be attracted to each other and form a bond.
The interesting thing is, organic matter and clay particles have a negative charge. That is super special if you want your soil to be nutrient rich. The ability of the soil to hold on to these nutrients is called the cation exchange capacity.
The plant roots can then treat the soil almost like a supermarket, and go shopping for the nutrients they require for growth. The plant roots do not just take the nutrients they want, they pay for them in the form of bartering. The plant will take up a nutrient in exchange for a nutrient that it already has in its system. Cool hey?
What is also interesting is that plants really don’t care whether the nutrients are in organic form or chemical form. Nitrogen will always be nitrogen and calcium will always be calcium, but if you supply the nutrients (fertilizer) in an organic form, then you are feeding the soil and the myriad of soil micro-organisms. This of course is great for the life of the soil.
To be continued……..
