Autumn is a transitional time. Animals are making the last preparations for winter. Squirrels are gathering the last nuts and seeds they can find. The birds have begun migrating. The world around us is transitioning from summer to winter, and we are transitioning with it. I know how it is, once the leaves start falling it’s hard not to start thinking about winter. Preparing for the ski season and the holidays, it is easy to overlook late fall for all of the opportunities it provides. Especially in our gardens. Once the nights start dropping below freezing, and the ditches shut off, it is easy to hang up our shovels until next season, but it is a little too early to put our gardens out of mind. There are many things that you can do for your garden in the fall that will help you out next season. One of the most important of these is soil amendments.
Fall is a great time for soil amendments for a number of reasons. One of these is that through the season your plants have drained the nutrients out of the soil in your garden, so it is important to replenish it before you start to grow new plants in the spring. The other benefit of doing this in the fall vs the spring is that it usually takes a couple months for the nutrients in your compost or fertilizer to fully break down, and become bioavailable. This is because of the relationship between plants and the microbiome of the soil. Before I start talking about the wonderful world of microbes, here are some simple steps to help you amend the soil in your garden.
Testing Your Soil.
If you are serious about this, the best thing you can do for amending your soil in fall is understanding what your soil needs, and the only way to accomplish this is to get your soil tested. There are multiple home soil testers on the market that work all right, but the easiest way to get your soil tested is to reach out to our CSU extension. Here is a link to the PDF form with all of the information, and the application to get your soil tested http://www.soiltestinglab.colostate.edu/documents/soilsample_horticulture.pdf
Not Testing
If you are not testing your soil, this is completely OK. Even if you don’t test your soil, it is safe to say that your soil will have lost some vital nutrients throughout the season. Though you might not specifically know which ones you lost, these can be replaced with compost.
A good rule of thumb to follow for amending your soil in the fall is to add about 1-2 inches of compost to maintain your soils organic matter. If you are looking to build your soil you will want to add closer to 3” of compost.
Once you have added this compost, you do not want to till it in. Tilling is an old farming practice of breaking up the soil to make it easier to work with, but this has been found to disturb the microbiome of the soil, so it is more beneficial to disturb the soil as little as possible. It is a little difficult for us to pull off no till growing in the rocky mountains, so at Mountain Roots we take a minimal tilling approach using a broad fork (https://www.youtube.com/watch?v=V_crd4xGUio) to mix the soil together, but leave as much of the microbiome intact. Once you have mixed in your compost, you want to cover the top of your garden with a couple inches of mulch. This can be hay, stray, leaves, whatever. This mulch is designed to help insulate your garden and reduce erosion over the winter, you will remove it in the Spring, so I recommend using something that can be easily removed.
This was a quick overview of amending your soil in the fall, but to understand why it is more beneficial to amend your soil in the fall vs the spring we need to take a closer look at our soil. A much closer look at one region in particular the rhizosphere.
The rhizosphere is the region of soil that is directly surrounding the roots of a plant. A pretty specific region I know, but the rhizosphere is immensely complex and very important. On one plants root system there can be upwards of 30,000 different species of microbes. We know more about the surface of mars than we do about the rhizosphere, which is problematic, because the rhizosphere is paramount to plants survival. It is these microbes that allow the plants to absorb nutrients from the soil.
The rhizosphere is comprised of billions of different species of microorganisms, but these can be grouped into five basic categories of microbes: bacteria, actinomycetes, fungi, algae and protozoa.
Bacteria
Bacteria by number are the most present microbe in the rhizosphere. Though it is nearly impossible to explain what all of these bacteria are doing for the plant, there are three main functions that these bacteria provide. They fix nitrogen in the soil by oxidizing nitrite (the nitrogen in our atmosphere that is unusable for plants) into nitrates which can be absorbed by the plants. They increase phosphorous levels in the soil, which is an important nutrient for plants, and produce growth hormones for the plants. In exchange for all of this, the plants give the bacteria a home in their roots.
Actinomycetes
Actinomycetes are much less common in the rhizosphere than bacteria, but no less important. Actinomycetes are technically a bacteria, but they behave much more like a fungi. They break down organic matter making it bio available for the plants, and they create antibiotics that the plants use to fight off harmful bacteria.
Fungi
Fungi, are the largest organism in the rhizosphere, and probably the most recognizable. Fungi are decomposers, meaning that they break down organic matter, and this is incredibly important for our plants. The organic material that we add to our compost actually cannot be absorbed by plants. It isn’t until they are broken down by a decomposer that the organic compounds are returned to a simple enough state that they become bioavailable to our plants.
Algae
Algae exist in very small numbers in the rhizosphere, primarily because they need to photosynthesize to generate energy, but there are algae that are found in the soil microbiome. These algae also help add nitrogen back into the soil.
Protozoa
One of the oldest living types of microorganisms protozoa play a relatively small roll in the rhizosphere. Amoeba are a common type of protozoa that many people recognize, and one of the most common protozoa in the rhizosphere. Amoebas are like the wolves of the microbiome. Their role is patrolling the microbiome consuming bacteria.
These five types of organisms comprise much of the rhizosphere, but as I stated this is an immensely complex region of life, and what I have said above barely scratches the surface of it. However, we do know that the rhizosphere is incredibly important for the plants existence. Plants will use 30% of the carbon that they uptake to produce sugars and amino acids to feed these microbes living in their roots.
It can be tempting to relax at the end of a growing season and focus your attention on preserving your harvests, but there is important work that needs to be done to your garden at the end of the season. And if you do, it will set you up for a more successful gardening season next year.