Our “grow trials” are proving very popular with local gardeners. In the past, we have trialled different potting mixes and also different chemical and liquid fertilizers. This summer we have trialled some different soil conditioners available to all of our gardeners. We focussed on options that are relatively easy to obtain, but not as mainstream as “off the shelf” products from large hardware chains.
Setting Up the Trial
We purchased some bulk potting mix from Fulton’s, our local garden supplier. Each conditioner was then mixed thoroughly with the potting mix. Six capsicum seedlings per product were transplanted into the potting medium in 13 cm pots. This was all done in a similar method to our previous fertilizer trial.
The plants were placed into our greenhouse, in a staggered pattern to ensure that they all had relatively equal access to light and water. The trial was set up in mid-December 2018 and results collated late February 2019.
Products Tested in our Soil Conditioner Trial
The following products were tested in the trial
The negative control plants were potted up in straight potting medium, with no additional treatment or fertilizers. All treatments were assessed against this control. Soil conditioners judged with a positive impact on growth, had plants that were bigger and/or grew more capsicums than this treatment.
Richgro Organic Fruit and Citrus Fertilizer
Richgro Organic Fruit and Citrus Fertilizer was the best organic fertilizer in our previous fertilizer trial. We included it to enable us to compare the effect of the less traditional treatments. To prepare this treatment we mixed four teaspoons through 6 litres of potting mix.
Cost: $13.37 for 5kg bag
According to their website: “Munash Rockdust is a natural mineral fertiliser, which replaces what has been removed from the soil. It recreates the natural process that occurs in the earth to restore soil to its proper level of health”. Basically it is rocks, crushed up into a powder. We used the directions on the pack and mixed 4 teaspoons through 6 litres of potting mix.
Cost: $24.95 for a 3kg bucket
Green Man Biochar is charcoal made from biomass via pyrolysisis. Biochar is gaining popularity as a soil amendment and has the added bonus of sequestering carbon into the soil. This means less carbon ending up in the atmosphere contributing to global warming. We used the directions on the pack and mixed the whole 300mL pack through 6 litres of potting mix.
Cost: $11.50 (inc shipping) for 300mL bag
At home we use (and recommend) the Hungry Bin Worm Farm from Worm Lovers. In the last trial we tested the worm juice (worm wee) and found that it had no effect on the plants. So this time we’re trialling the worm castings. These were harvested from the bottom of the worm bin and mixed with potting mix at two different rates:
10% Mix = 1 Part Worm castings 9 Parts Potting Mix
50% Mix = Equal volumes of worm castings and potting mix
Cost: Nothing after the initial investment in the worm farm
Chicken manure was harvested from our chicken coop, from underneath the chickens roosting at night sit. It was dried, but fairly fresh. This was mixed with potting mix at two different rates
10% Mix = 1 Chicken Manure 9 Parts Potting Mix
50% Mix = Equal volumes of chicken manure and potting mix
We were supplied with a bucket of spent coffee grounds by our friends at Zouz Café. Coffee grounds are high in nitrogen. We commonly see recommendations on social media that it be added to veggie patches and gardens to improve soil health. This also offers an opportunity to divert waste from landfill. However, for some time now I’ve had a sneaking suspicion that coffee grounds actually have a negative impact on growth. The coffee grounds were mixed with potting mix at two different rates
10% Mix = 1 part Coffee Grounds 9 Parts Potting Mix
50% Mix = Equal volumes of coffee grounds and potting mix
The Results of our Soil Conditioner Trial
The plants were assessed for general health, number of fruit produced, leaf colour and pest attack. Results are tabulated below
|Soil Treatment||# Capsicums set (total for all 6 plants)||# Flowers (total for all 6 plants)||Leaf Colour||Leaf Damage due to Pests||Plant size relative to -VE Control||Comments||Overall Effect of Product on Growth|
|Negative Control||0||0||Light Green||None||N/A||0|
|Richgro Organic||5||0||Light & Dark Green||Yes - Lots||Larger||+++|
|Rockdust||0||0||Very Light Green||No||Slightly Smaller||0/-|
|Biochar||0||0||Very Light Green||No||Smaller||-|
|10% Worm Castings||0||0||Very Light Green||Yes - Little||Slightly Bigger||Lots of extra volunteer plants eg tomatoes||+|
|50% Worm Castings||2||0||Light Green||Yes - Lots||Bigger||Lots of extra volunteer plants eg tomatoes and one pumpkin||++|
|10% Chicken Manure||6||0||Light & Dark Green||Yes - Lots||Bigger||+++|
|50% Chicken Manure||1||6||Very Dark Green||Yes - Lots||Mostly Dead or Smaller||Two plants survived. One is thriving with very dark leaves. One is quite small with very dark leaves. 4 Plants dead||- -|
|10% Coffee Grounds||0||0||Yellow & Very Light Green||None||Tiny - Not much bigger than when planted||All 6 plants alive but struggling||- -|
|50% Coffee Grounds||0||0||Yellow||None||Tiny - Not much bigger than when planted||Two plants dead. 4 x plants struggling (with only 3 leaves between them)||- - -|
As you can see from the image above the Richgro Organic Fruit and Citrus Fertilizer was again one of the best performers with large, healthy plants and five capsicums set on each of the plants. The 50% Worm castings had fantastic growth and two capsicums set. An added bonus of using the worm castings was the germination of many self-sown tomato plants and even a pumpkin plant.
The best performer in our trial (by a narrow margin) was the 10% chicken manure with a total of six capsicums set on the plants.
Not surprisingly, the 50% chicken manure treatment had a negative impact on most of the seedlings. This was likely due to the high nitrogen content burning the roots of the freshly transplanted seedlings. Interestingly, one of the small seedlings survived the initial transplant shock and is now thriving in the conditions. The leaves on this plant are the darkest and it generally looks the healthiest.
Both coffee treatments had very negative impacts on plant growth. In fact, the plants are about the same size as when they were transplanted. This is likely due to caffeine having an allopathic effect. That is, the coffee plants produce caffeine, not to make us high, but so that they can secrete it into the soil to supress growth and competition from other plants. Two plants in the 50% coffee grounds treatment died.
Impacts of the Soil Conditioners on Pest Attack Patterns
Our greenhouse doors and vents are kept wide open to stop plants from cooking in the summer sun. This means that pest insects such as moths, aphids and whitefly can freely come and go (and so can their predators). In early January, we noticed an outbreak of aphids on the capsicum seedlings in the trial. However, most of these aphid infestations occurred on the strongest growing plants.
A fortnight after we had noticed the aphids, we noticed ladybug larvae had turned up in large numbers to clean up the “problem” for us. After this, the plants all bounced back and showed little signs of long-term damage.
Towards the end of January, we noticed that something had started chewing holes in the leaves of some of the plants. These were likely little slugs (or possibly caterpillars).
However, we couldn’t easily locate them (perhaps they had already moved on before we noticed them). Again, there was a strong correlation between the strongest growing plants receiving the most leaf damage from pests. There’s two things worth considering from this insight:
- Just because your plants are being attacked by a pest (or disease) it doesn’t necessarily mean they won’t be as productive as non-attacked plants. We found the most productive plants were the ones that were attacked.
- In my experience, truly healthy plants don’t get attacked by pests. Plants subjected to high levels of nitrogen often go through very fast periods of growth and seem to have a lot of pest issues. Nitrogen can be a bit like junk food for plants. It causes rapid growth, but doesn’t allow the plant time to fully incorporate other minerals such as calcium into cell walls. This leaves the plant tissue a bit weakened and prime fodder for pest bugs.
In summary, the best soil amendments in our trial were worm castings in heavy doses, chicken manure in light doses and Richgro Organic Fruit and Citrus Fertilizer.
We have contacted the manufacturers of the commercially available products included in this trial via email. We invite them to add their responses in the comments section below.
We will be running many more trials like this in the future. Please sign up to our newsletter to be kept up to date on future results.
If you would like further information on soil conditioners, then please sign up to our Science of Edible Gardening Workshop Series. The August workshop (repeated annually) in the series focuses heavily on compost and soil conditioners.