Category Archives: soilfoodweb

a soilfoodweb assignment

I’ve been away in Lismore studying soil biology, while I was up there I was required to complete an assignment which I’m reproducing here. I’ll caveat by saying that I was only half way through the course when this was done, so there are parts I’d write a little differently with the knowledge I have now, but I’ll leave it as I wrote it, and when its assessed I’ll share the results too. Feel free to (stop laughing any time) comment, correct the not so deliberate mistakes, or make helpful suggestions either via the blog by email or if you’re one of my fellow students via our google group if you prefer. And if you’re visiting Katoomba sometime, I’d be delighted if you’d come take a look, even grab a soil sample and see how we got on.

Nutrient Cycling in a Chosen ecosystem

The ecosystem Iʼve chosen is a heritage apple tree walk in Katoomba. It sits in a community garden around 1000m above sea level in the blue mountains, which is a slightly cool temperate climate. The garden lies north of the town in the basin of a water catchment area. The apple trees are sited on the high side fringe of a riparian zone prone to flooding during large rain events.

History & conditions

The site was running horses for 22 years previously, and may well still be suffering from compaction. Parent material in this area is old sandstone, and soil is generally considered poor. The original soil material was sandy and tested a pH of 9.5-11 around ʻ92/3 when the site became a community garden. A selection of 3-4 year old apple trees planted in ʼ96, one of each different variety, coxʼs pippin, gravenstein etc. At this time, cover crops of clover lucerne, daisies, and then chives, strawberries, leeks, parsley and comfrey were planted but most of these have not survived. Cow and chicken manure have been added at various times. Some greater celandine has recently spread rapidly where calendula were planted at the south end, under tree canopies and continues to spread. A visibly thriving population of dandelions attest to remaining compaction sites around the gardens, although the particular area of interest has a dominant understory of comfrey. Little else is growing under the trees, raspberries were planted 2 years ago on the east around the drip line, and some mint has taken up residence to the west, shady side. Annuals including chickweed spring up in patches within the canopy of the trees. Itʼs definitely on the damp side and snails seem to be quite happy hiding up there in summer. The pathway between the trees is now mulched with woodchips. The trees are planted close together. One tree looks quite sick, not only did it not produce any fruit this year, but has little leaf or new growth. Other trees have not been so drastically affected but are also beset by pests, codlin moth, wasps, borers, scab, black spot and other as yet unidentified fungal problems. There are also known to be rats in the area. No nutritional deficiencies have been spotted through leaf discoloration or deformity.
The apple trees are surrounded by other fruit and nut trees including a plum within a metre which which is also sick. 10m away are chestnuts, walnuts, hazel bushes as well as a large pine which seem to be doing ok for leaf but not fruit. Broadly speaking then weʼre looking at decidous or transitional forest[1] However with only two dominant species in the immediate vicinity weʼre looking at low supporting plant community diversity. At the same time, the system is visibly distressed. So, slightly reduced oxygen capacity at our elevation, generous rainfall, potential remaining compaction areas in the soil, along with a large volume of falling fruit that isnʼt cleared by animals may be creating anaerobic conditions. If the compaction is widespread then a layer of water may be settling on top. This would not only create anaerobic conditions at the depth it occurs but could also prevent microbial life moving deeper through the soil layers or connecting, interact and make exchanges with those that might normally transport and replenish elemental material from the parent material/base rock.
If the system was functioning well, plants in this ecosystem will be getting all the nutrients they need from the life in the soil. ʻThere is no soil (even) in Australia that does not contain all the nutrients required to grow healthy plantsʼ[2]. So itʼs not about what nutrients are found in the soil directly where the plants roots are, but whether youʼre plant is able to access a wide tradign network which is able to source what it needs. How does it do this? Be it an annual plant or a rainforest tree, the plant can only make certain foods for itself, and for everything else, thereʼs an extended network which the plant trades with for its nutritional needs. We all know that plants convert the energy from the sun and carbon dioxide to create sugars and proteins in the form of pollens, and nectars in above ground parts. The plant in turn feeds a variety of insects and birds, as well as some microorganisms on leaves and other surfaces, and for this payment the trees are pollinated, and left bird droppings high in phosphorus, bacteria, as well as other microorganisms including pathogens. These land on our orchard floor, to join with other creaturesʼ manures, leaf litter, fallen branches and other plant residues. Eventually it will meet up with and be torn, chewed spit out, pooped out, swum in, slid on and eaten again by the biggest population of nutrient cycling microorganisms in this system, right there in the soil. Its here we say hello to bacteria, fungi, protozoa, nematodes, micrarthropods, predatory nematodes, earthworms and their predators.
Weʼre looking for high biological activity around the root zone or rhizosphere to keep the nutrients flowing, and allow them to be cycled quickly around our forest system. So while we may not have the highest levels of available minerals from the base rock in the soil, weʼre interested in how much life in the soil, and through them, the diversity and quantity of nutrients are made available to the plants we want to grow, as well as understanding that the organisms that are around are supportive of our (primarily apple) production objectives, or not.

The root zone or rhizosphere is generally considered to be about 2cm from the surface of the roots but in reality this varies considerably and the plantsʼ influences can be much further afield. Not only do organisms feed on nutrients as far away as the bedrock (which in our case isnʼt more than a metre or so down) to exchange with the plant that it can not directly source for itself, but provided they exist in sufficient number and in ratios appropriate to the type of plant, they provide all manner of other services from cleaning away discarded root matter and provide a protective barrier against infection from predatory organisms. Its little wonder that trees can send a whopping 80% of their energy into roots*, 50% of that being released as simple sugars, proteins & carbohydrate exudates to trade with all this microbiology.

Our system is struggling a bit, life in the soil may not be getting what it needs, can not give the trees what they need, and their existence becomes unstable and sustainable. The greater the deficiencies or imbalances, the more likely access natures decomposers have to identify our trees for demolition, and those we call the bad guys will move in, to remove these unhealthy plants. Really theyʼre just doing their job. If the plants and trees weʼve sited here canʼt cope in the biological environment, theyʼre pretty much be recycled themselves, perhaps to be cleared away for earlier succession plants will have ideal conditions to germinate and slowly the system will establish a new developmental path. If we leave it alone that is. Not a chance, we want to help the apple trees and give them a good chance to thrive. My motives are not noble, for where else am I going to get a Coxʼs orange pippin in Australia?

How should this system be working?

The trees and plants put out exudates which aerobic bacteria and fungi take up. Theyʼre then able produce ʻa matrix of sugars, proteins and DNAʼ[3], we just call bacterial slime. Bacteria can move around using this slime, and even stick themselves to food they find, further helping to aggregate organic materials in the soil. nitrogen available as nitrates NO3, a form of nitrogen useful to our annual plants in the system, in this case the dandelions on the edges, chickweed and other pioneering annual weedy species. Since we want a perennial system, weʼre not that thrilled that weʼre supporting these annuals in our system. Rather than pulling these weedy plants out, however, we need to consider what theyʼre telling us about this system, and how we might improve conditions. Annuals thrive in bacterially dominant soil conditions, which operate in more alkaline conditions, where pH is above 7. Originally the soil in this area tested with a very alkaline 9.5-11, and more recently tested around 7. Later succession plants, like our perennial comfrey understory and apple trees have more relationships with fungi. Theyʼre best able to use nitrogen in the form of NH4, luckily then that our decomposers and predators protozoa, nematodes and microarthropods mineralize the available nitrates NO3 into NH4. In our perennial system then, weʼd prefer to be fungally dominant and have more ammonium available which our perennials do better with, in more acidic conditions, more like pH 5.5-7, but weʼre still seeing those annuals and the trees arenʼt especially happy…
So, what could be happening? The trees do seem to be getting sufficient nutrients to produce at leaf and set some fruit, although weʼve no data to compare across the years on fruit production here. If all the trees nutrients get to the tree through soil biology does that mean weʼve enough? It may be that there is just enough and if we had more biological activity weʼd have more productive trees. Also producing fruit is seasonal, the tree is not geared towards this all year round. Would the leaves be the first thing to show a sign of deficiency? Or would it already be quite bad by then, what suffers first. leaves, roots, fruits? Its alot of guesswork on my part.
Problems we can see are a lack of perennial plant diversity in the system, some long term compaction, reduced oxygen availability, trees attacked by pests and fungus. These trees dont have enough friends helping them out. We could then be looking at high bacterial conditions, maybe anaerobic not enough beneficial fungi, not enough fungal feeding nematodes and other predators. I suspect we have reasonable numbers of bacteria, where are those nitrates coming from if not via nitrification via Nitrosomonas and Nitrobacter. Who else do we have? Is that the only nitrogen thatʼs geting away (from the perennials), or do we have numbers of anaerobic bacteria too, blowing our nitrogen off as N2? We can have a look at the cilliate count to see if this theory bears out. If its high, and much higher than amoeba and flagelates then we know weʼre in anaerobic country, since ciliates graze on anaerobic bacteria, and operate in lower oxygen conditions than the other two protozoa. We want to make sure the good guys, the aerobic bacterial community are nice and active, and are present in numbers to outcompete the anaerobes. Letʼs have them start to build structure back into those compacted areas. Then we can have a look at our fungi. Whoʼs there?
Before a soil test, onsite observation of the rate of leaf litter breakdown might tell us something. There are certainly alot of fruit that very slowly breakdown under the trees, does that mean toxic alcohol production necessarily? We know we have fungal attack and insect pests appearing, we need to protect the trees, make sure the good fungi are occupying any infection sites and keeping out the bad guys – up the beneficial fungi. The fungal bacterial ratio for our forest system should be somewhere between 50:1 and 10:1 If fungal diseases can only step in when thereʼs not a healthy relationship between the trees and beneficial fungi already established. It looks like our apple trees have few bodyguards keeping the bad guys out. If we had beneficial fungi and a good predatory population active in sufficient numbers weʼd be much less likely to have these bad guys reeking havoc. We need to grow the diversity and numbers of beneficial fungi and predatory populations. So we should provide a diverse range of high carbon foods for them to feed on.

How can we improve conditions?

Letʼs consider this within the seasonal cycle. At this time of year (early winter) the bacteria will be at their least active, so it might be an opportune time to add fungally dominant compost, and let the number of fungi and predators build up over the winter. I attempted a fungally dominant compost hot compost as an experiment three weeks ago, which I hope is being turned in my absence. Now Iʼm hoping to use it for just this purpose in another couple of weeks. I could make an extract of worm juice in the meantime, and lay down some humic acid in the form of a worm castings, along with a variety of high carbon mulch material. Thereʼs also some further research to do. We need to add some more perennials that will work with this system. So, visits to local organic orchards are in order, and some soil sampling there if theyʼll allow it. Are they operating a successful polyculture, what plants grow there? A survey of the successes and failures in the area and perhaps further afield, in the UK and Europe where some of these cultivars come from.
Iʼd like to leave some of this compost I made before, and let this mature over the winter and allow more creatures to move in there too. Come August, It will be interesting to compare if different creatures moved in when the compost is left to when its applied to that soil. Back to the trees, roots are getting active in preparation some weeks before the temperature rises and shoots spring up above ground, so that might be a good time to test and see what life is now present. Did the compost change anything, did it help? Letʼs look again at this under the microscope. Already those roots will be busy sending out exudates to exchange with bacteria and fungi of various kinds in preparation for the growing season. In turn a larger and more diverse soil food web including bacteria and fungi, nematodes, microarthropods, and of course worms increasing activity. Worms in the soil break down organic matter, including bacteria and fungi attached to it, producing humic acid and heaps more bacteria. Once we have shoots and leaves might be a good time to step in with a compost tea treatment, before the flowers form. Once flowers are are bursting forth the trees are diverting their attention to the production of fruit, for a healthier reproductive cycle this year, and Iʼm hoping for some great apples. Iʼm on the look out for a microscope so I can keep testing that soil. Did I say I really need to test the soil first and see whatʼs really going on?! A sample may be on its way up right now… I hope.
Refs:
[1] Edible Forest Gardens vol 1- David Jacke
[2] Elaine Ingham, during ʻ Soil Food Web Interactions and Benefits to Plant Productionsʼ – 2010 Elaine Ingham & Graham Lancaster. Course Presentation
[3] p49 Teaming with Microbes – Jeff Lowenfels and Wayne Lewis More 12 steps SoilFoodWeb Institute and Dr Elaine Ingham (from the SFI website)