What is the best way to build soil fertility? In my previous post I considered the use of a microbial tea as a soil supplement and described the results of my first Citizen’s Science experiment with broad beans. Here I want to expand on this topic and share my findings from additional experiments with tomatoes and sweetcorn. As before, I determined that the overall goal of my experiment was to understand the impact of the microbial tea on crop yield, plant health and root quality.
In the case of the tomatoes I chose a variety I have had success with in the past (Gardener’s Delight), growing plants in pots in my greenhouse. Seedlings were potted into a 1 to 1 mix of soil and home made compost. Two plants were treated with a root soak solution of microbial tea upon transplanting and two control plants were just soaked with rain water. For the rest of the season the plants were treated identically, feeding occasionally with a solution of fermented kelp.
For the sweetcorn I chose to grow two blocks, of ten plants each, in the same bed separated by a companion row of squash plants. One block of plants was treated with a root soak of microbial tea at transplanting and the other control block of plants was just soaked in rain water. The whole bed had been mulched over winter with straw and treated with a home made compost tea in the spring ahead of planting.
So what were the results like? Was there any visible or measurable difference between plants treated with microbial tea and the control groups? For the tomatoes the plants looked very similar, with healthy leaves, 3-4 fruiting trusses on each plant and a moderate level of fruit set. The yield of tomatoes per plant was slightly higher for the control group compared to the test group (see plot below). In terms of quality of the fruit there was no discernible difference in taste and the BRIX values (see previous post for info on BRIX) were also very similar (6-10% for control; 7-10% for test) and consistent with typical values for cherry tomatoes. The one final thing to consider was the plant roots and whether there were any obvious differences. As you can see from the photo below, the roots looked pretty similar.
Up to now the results were looking consistent with those for the broad beans, described previously, but would the sweetcorn be any different? In some ways, yes. The two blocks of plants looked visually quite different (see photo below) with the test plants being much smaller. It is possible that this may, in part, have been due to this block being closer to a hedge but I have not observed such a dramatic effect on previous crops grown in this bed.
In terms of sweetcorn yield the control group also performed better with a 23% higher yield per plant than the test group (see plot). The corn cobs were generally much larger and better developed (see photo). The corn from the control plants was also tastier with a higher BRIX value (26-30%) than the test corn (23-25%). Overall the yield of sweetcorn was not that impressive. I’ve had better crops in the past when I’ve pre-composted the bed; something I will pay more attention to next year!
The final measure was to look for any difference in the roots of the sweetcorn plants. As you can see from the image below, the roots of the control plant were more extensive than those of the test plant. Interestingly a good portion of the roots in both cases were coated in soil (rhizosheath – see Box 1) which indicates the presence of mycorrhizae.
Box 1 – What can the roots tell you? – Rhizosheaths are coatings of soil particles that cling to plant roots, making roots brown instead of white. They are a sign of biological/microbial activity in the rhizosphere (root zone). Soil particles are bound to the roots by biotic glues, secreted by microorganisms. This is aggregation in action and therefore indicates the formation of good soil structure and healthy plants (note – some species do not form rhizosheaths, such as cultivars in the brassica, allium & asparagus family). To find out more about rhizosheath analysis visit Vidacycle.
I decided to take a closer look at the roots. Using a macro lens on my phone I was able to see a web of filaments around the root which was holding clumps of soil in place but it was not very clear. I turned to the microscope to get a closer a look and what I saw was truly amazing and quite beautiful.
Instead of staining the roots, as is typically done for this kind of analysis (see image in Box 2), I looked at the root filament (soil and all) under natural light conditions (see images below). In both roots from control and test plants I could clearly see mycorrhizal filaments entwined with soil particles. From these examples it appears that the mesh of filaments was more extensive on the control roots but this is highly subjective. What is clear is that both sets of plants had formed mycorrhizal relationships and that the plants did not need to be inoculated with a microbial tea to achieve this.
Box 2 – What is actually in the microbial tea? According to the supplier’s label the tea contained a mix of Endo Mycorrhizae, Trichoderma, Bacilli, Pseudomonas, nitrogen fixing bacteria, humic acids, amino acids, enzymes, yeasts, protein, carbohydrates and seaweed extract. However there were no details on which exact species of mycorrhizae, whether there were spores or auxiliary cells and in what quantities. I had a go at analysing a sample of the microbial tea using my microscope. See the images here for some of my findings. I am no expert on this so I was not able to form a clear opinion on the quality of the sample; my assignments are tentative based on similar images I found online. If you want to dig deeper into this topic then I can highly recommend the University of Kansas INVAM site as they are the real experts.
I think on balance, my results from all three experiments suggest that there was no real benefit from applying the commercial microbial tea to my plants. In the case of the sweetcorn it may even have been detrimental but I can’t be sure. As I said in the first post on this topic, it is useful to consider the advice from Jeff Lowenfells & Wayne Lewis in Teaming with Microbes. Most vegetables, annuals, grasses, shrubs and perennials form symbiotic relationships with Endo Mycorrhizal fungi which provides the plants with a valuable source of nutrients. Growing methods such as tilling (digging) and soil sterilisation disrupt this relationship. So in cases where your soil is recovering from tillage or when you are planting seeds into sterilised potting composts it may be beneficial to inoculate your plants. Otherwise you are probably better off not tilling, leaving the roots of previous crops in the ground and adding good quality home-made composts and mulches to provide everything your plants need – that’s what I will be doing – Happy Growing!
Heather Comina 