Growing plants can be quite technical if we wish to make it so. There is chemistry, physics and biology to consider if you want to create the
absolute best growing environment for your plants. Luckily for us, it isn't super necessary to consider all of these technical details in order to grow the majority of plants successfully.
Having said this, sometimes a small amount of knowledge can go a very long way. Perhaps nowhere is an understanding of some basic principals likely to pay off as much as in container growing culture.
But first, a story. This has been circulating the internet for years, and I promise there is a point, although I may take a little while to get there!
A
professor stood before his philosophy class and had some items in front of him.
When the class began, he wordlessly picked up a very large and empty mayonnaise
jar and proceeded to fill it with golf balls. He then asked the students if the
jar was full.
They agreed that it was.
The
professor then picked up a box of pebbles and poured them into the jar. He
shook the jar lightly. The pebbles rolled
into the open areas between the golf balls. He then asked the students again if
the jar was full.
They agreed it was.The professor next picked up a box of sand
and poured it into the jar. Of course, the sand filled up everything else. He
asked once more if the jar was full.
The students responded with a unanimous
‘yes.’
The
professor then produced two Beers from under the table and poured the entire
contents into the jar effectively filling the empty space between the sand.The
students laughed..
‘Now,’
said the professor as the laughter subsided, ‘I want you to recognize that this
jar represents your life. The golf balls are the important things—your family,
your children, your health, your friends and your favourite passions—and if
everything else was lost and only they remained, your life would still be full.
The pebbles are the other things that matter like your job, your house and your
car.. The sand is everything else—the small stuff.
‘If
you put the sand into the jar first,’ he continued, ‘there is no room for the
pebbles or the golf balls.
The same goes for life.
If
you spend all your time and energy on the small stuff you will never have room
for the things that are important to you.
Pay
attention to the things that are critical to your happiness.
Spend
time with your children. Spend time with your parents. Visit with grandparents.
Take your spouse out to dinner.There will always be time to
clean the house and mow the lawn.
Take
care of the golf balls first—the things that really matter. Set your
priorities. The rest is just sand.
One
of the students raised her hand and inquired what the Beer represented. The
professor smiled and said, ‘I’m glad you asked.’ The Beer just shows you that
no matter how full your life may seem, there’s always room for a couple of
Beers with a friend."
As great as this story is for representing life, for me at least, it also represents some interesting considerations when choosing your soils for containerised plants. We will come back to this later.
The Role of Soil Media in Container Gardening
Back when I was young and first started growing plants in my backyard, I noticed that all the plants I planted in the ground seemed to do ok, whereas everything I planted into containers tended to be riding a downwards slope to death, rarely living for more than a few months. The problem was, I was using soil straight from the ground in my pots. But why is this so bad?
The answer comes by asking the question, what exactly is the role that soil plays in plant growth?
To the uninitiated the answer may surprise. The media in which a plant grows need only serve three functions;
1. As an anchor to support the plant
2. As a media to access water and nutrients
3. As a media to access air
To this end, the actual form that soil media takes is not so important as fulfilling these three needs. Nowhere is this point more evident than in hydroponics where plants can be grown entirely submerged in water, as in a deep-water culture system, or in aeroponics where a plant is grown with its roots suspended in air, being misted or sprayed, at intervals, with a nutrient solution. In both of these cases the plant itself is supported by the hydroponic systems or inert media such as clay pellets. Air is bubbled through a nutrient solution in a deep-water culture system, and in aeroponics air is available at most times.
It is important to caveat these next points with the following; when growing in the ground, different rules apply to growing in containers. Although the three basic needs still apply, in the ground there are entire ecosystems of bacteria, fungi and other organisms that work in symbiotic and complimentary fashion with the soil. For instance, earthworms and other organisms make tunnels, introducing air into the system, whilst other organisms work to make particular nutrients available, or destroy toxins.
Another key point for consideration is drainage, however this will be covered in depth later.
Of the three needs that plants have of soil media the first is very easy to accomplish. Plants can be anchored in almost anything. You can dig up soil from the garden, or plant them in gravel or potting mix or even shards of glass. The second need is slightly more difficult.
On Nutrient Uptake - Warning Chemistry Ahead!
Plants require a host of nutrients in the form of minerals and elements to survive, and these nutrients are only available to the plant when in an aqueous (dissolved in water) solution. Essential nutrients are not available in their elemental form, and instead are provided as
ions which are positive or negatively charged portions of a compound. For instance, the chemical formula for table salt is NaCl, comprised of sodium (Na) and Chlorine (Cl). When salt is dissolved in water it separates into its positive sodium ions (Na+) and negatively charged chlorine ions (Cl-).
For almost all plant nutrient needs, elements can be provided by various different compounds. For instance, in my post on
blueberries I discussed how nitrogen could be provided in either a nitrate (NO3) form, or an ammonia form (NH4). For nitrates, when dissolved in water for instance calcium nitrate Ca(NO
3)
2 separates into Ca+ and NO
3- ions, and potassium nitrate (KNO
3) separates into its potassium (K+) and nitrate (NO
3-) ions. Nitrogen in its ammonia form, can be provided similarly by compounds such as ammonium nitrate which provides nitrogen in both ammonium (NH
4) and nitrate (NO
3) forms
.
Ions in solution are transported from the roots into the plants through a process called osmosis, where due to a chemical pressure, which causes ions to move from the soil into the plants roots, and then up into its tissues.
Osmosis provides an upwards pressure on nutrients and water into the plant, but at the same time there exists negative downward pressure in the soil keeping water and its dissolved nutrients from moving into the plant. These negative pressures include gravity and the level of undissolved compounds in the soil.
Because of this, if too many soluble undissolved (or dissolved but less so) compounds exist in the soil media, the plant can literally have nutrients and water drawn out of its cells, and at the very least have any water and nutrient made totally unavailable, rendering the media useless. In these cases plants die of thirst and starvation, despite totally being surrounded by food.
Another interesting phenomena exists where the presence of one ionic compound can cause another to be locked out, and less available to the plant.
Finally, pH is an important soil attribute that effects the availability of nutrients, with some ions being more available in a acidic soil, and others being more available in a basic soil.
pH - More Science!
pH is a measurement of the acidity or alkalinity of a soil. In the simplest of terms pH is the amount of willingness a soil has to donate or receive hydrogen ions (H+). A solution or soil that is a donator of H+ is acidic and those that receive are alkaline, or basic. The more H+ ions that are donated and received indicate the strength of the acid or base.
pH is measured on a scale from 1-14, where 1 is extremely acidic and 14 is extremely basic. On this scale 7 the pH of pure water, is neutral. pH is measured on a logarithmic scale where each change of 1 on the pH scale is 10 times more or less able to donate hydrogen ions than the previous score. For instance, a pH of 6, one less than pH neutral 7 is 10x more acidic, and a pH of 5 is 10x more acidic than pH 6, and 100x more acidic than pH 7.
At different pH levels, elements are available to plants in different amounts, which can be seen by this picture;
Soil requirement 3 - Soil Particles, Water and Air
Now, back to our professors story! I promised I'd get here!
Apart from being an interesting metaphor for life the professor's example provides an insight into soil structure! Just as in his jar, filled with particles of different sizes, we can create soils of different particle sizes to help or hinder us in our containerised gardens. This goes directly to our third soil need, the need for air.
If we fill our containers with large particles, such as the professor's golf balls we create a growing media with tonnes of room for air. Every time we put a smaller particle in, we remove some of the space that air can occupy, and fill it with some other sort of media, which can either be beneficial, neutral or harmful.
Too much large pockets of air however can also create a problem. If we planted our plants into golf balls, we would have a large supply of air for our plant's roots, as well as good anchorage, however golf balls do not have a particularly strong ability to hold water and nutrients. As golf balls are non-absorbent there is nowhere for water to be held, except in the places between where the golf balls touch each other, as a thin film that exists between particles.
Obviously, planting in golf balls is probably not something any of us would consider, but more realistic growing media, such as expanded clay pellets or gravel have similar properties, and will provide a very well aerated, but not particularly moisture retentive, environment for growth.
As a general rule, as particle size decreases, water retention increases, and air capacity decreases. So our goal then it to find a soil media that is both water retentive and also hold air.
To achieve this, we can either have particles that hold moisture within their core, and allow air between the particles; or we can have a mixture of large particles that hold water where they touch or within themselves, and allow for pockets of air to exist simultaneously.
In cases where too much water is present in the soil the plant roots literally suffocate and the plant drowns.
Perched Water Tables
A problem we face, pretty much unique to container growing is that the more fine particles in a soil mixture, the more a water table begins to form in the base of the container. This water table, known as perched water, is an area of saturated soil mixture where plant roots cannot survive, because of lack of air.
It is because of this that my plants that I planted in containers using garden soil as a youth failed so dramatically. The soil of the area where I grew up was heavy clay. Heavy clay is comprised of very, very small particles, and despite typically having high nutrient levels, in a container the perched water table is so high that there is little room for roots to grow, and little available air throughout the rest of the soil either, as very little space exists between particles even when not saturated with water.
So What is a Good Soil Mix Then?
Over the last year I have taken an interest in the best types of soil to promote vigorous plant growth. In my search I was eventually ended up at the GardenWeb forums, where I came across the
posts of a forum member called Tapla, or Al. He speaks in depth about much that I have talked of in this post, as well as a lot more, and he suggests using a soil mixture made up of three components; fine pine bark, perlite and peat moss.
The mixture called Tapla's 5-1-1 mix is seven parts in total; 5 parts pine bark, 1 part perlite and 1 part peat moss. For the majority of plants it is wise to add some dolomitic lime to the mix to increase the pH, and to introduce some magnesium into the soil. For plants that love acidic soils, such as blueberries, it is best to leave the lime out.
This mixture should be wet as you mix it, as the peat moss and pine bark can be quite hydrophobic before it is moistened.
Today I made up a big batch of 5-1-1 to pot two new blueberry plants that I purchased today, and to plant out some new Chilean Guava, I have arriving next week.
