Polyanthus Plant Growth Tests (Tom & Jo's Nursery, Spring 2017)

In this post we catch up with the trials growing polyanthus (Primrose) in a growing mix supplied by SoilFixer compared to a commercial compost mix

For those not familiar with SoilFixer's testing program, here's a quick recap: 

• Our goal is help gardeners make “super compost” by adding our Compost Humification Agent (CHA) into their composting heaps and bins. One group is testing CHA.
• We also make our own “super compost” (called SF60) used to improve soil and/or container growing media. A second group is testing SF60 by adding it to soil and/or commercial growing media.
• Lastly we have attempted to make our own growing media and a small number of testers have used this, as in this trial.

(If you're unsure on the difference between growing media, peat and compost when you buy in garden centre, follow this link to container growing mixes)

SoilFixer prepared a potting media (based on colloidal humus and biochar) to compare against the commercial growing media used at the Nursery.  This idea was to improve on last year’s mix used successfully by Mark over at Vertical Veg. We tweaked the recipe by using more biochar granules and less compost. The batch was ready to use late Autumn, so we opted to use polyanthus 'plugs' which were due to be potted-on and grown over-winter ready for sale in early spring.  The plugs were transplanted into 2-inch pots in early December. 50 went into pots filled with the SoilFixer mix and the rest of the stock were potted into the nursery's standard commercial growing media. The pots had consistent temperature (winter greenhouse) and same watering regime.

The trial ended in mid-February. Our thanks to Tom, Louis and Danny for potting-up and looking after the plants.

Results

Below are two photos, one from each group. Clearly one set is good and one very poor!

Hands-up: the poor set of pots are in the mix SoilFixer made. Very disappointing result compared to the results in the vertical veg and our own raised beds crops grown over summer. 

The big question - why?  

Was this due to pH of the mix, too many or too few nutrients or maybe the physical structure (density) of the growing mix we supplied?

We tested samples for EC (easy, but only a broad indicator of total nutrient ions not true NPK). The SF mix was rich – but not that strong to be a cause for concern. We ruled out too little and too much nutrient in mix.

We tested the pH of the pots. As expected the commercial mix was pH 7 (+/10.5). The mix from SoilFixer was more alkaline at 8-8.5. We cross checked the polyanthus variety and as far as we could tell, it was not ultra-sensitive to pH. We ruled out pH as issue. 

We looked at the physical structure. In the pots at the end of the test, the mix was neither wet nor dry and was reasonably friable. However, when watered it did become more sticky. 

Tom (head of Nursery) had already commented that the polyanthus roots were restricted in first weeks. Poor establishment resulted in weak plants and many then succumbed to fungal infection and died-off. The material was not open enough.

We concluded that watering after the plugs had been transplanted had given a dense structure which the delicate roots found hard to penetrate. Poor root establishment then led to weak and susceptible plants.  

Lessons / Learning

• It’s very hard to make a soil-less growing media! Let’s face it, the horticultural industry has and continues to spend millions trying to reformulate growing media and come up with a truly peat free option. 
• On reflection, making a media from just colloidal humus and biochar is not the right way forward. It looks far more viable to use existing good commercial grade growing media and seek to improve by adding 10-20% of SF60.  

You can follow further testing at the testing blog section.