Just. Add. Compost.

With the abundance of snow and ice recently here in the Portland area, it is hard to imagine just a month or two forward to the time for planting our Spring vegetable gardens!  But that time will come sooner than we know it and our seeds will take to the dark earth and grow into these ever longer days.  

But before I do any planting in my own vegetable garden, I will need to do the important work of preparing my soil and creating an environment in which the things I plant will flourish.  The hardest work I do in my garden each year is, and should be, in preparing the soil for the year ahead.  And though there can be a lot to this process, in many ways it all boils down to one thing: Just Add Compost!

Compost is one of the most important and transformative tools that we have as organic vegetable gardeners.  It is a miraculous substance that has immeasurable benefits for your garden.  So I thought I should take the time to sing it's praises today...

What is compost?

Compost is, quite simply, fully decomposed organic matter.  It is both a substance and the end result of a process of decomposition that every plant goes through at the end of it's life.  This incredible process is accomplished by literally millions of actors.  Innumerable soil microbes like fungi, bacteria, and protozoa work in conjunction with larger pill bugs, earthworms, mites, beetles, flies, and others.  All of these organisms, macro and micro, do the important work of feeding on and breaking down plant matter.  Their waste products make up the nutrient rich humus of finished compost.

All soil is made up of Minerals (sand, silt, and clay), Air, Water, and Organic Matter.  While mineral components of soil are somewhat fixed, levels of soil organic matter can fluctuate significantly based on what we do, what we grow, and how we treat our soil.  And in general, the best soils for growing vegetables (or just about anything, for that matter) are high in organic matter.  So if you follow, compost is pretty important stuff.

What are the benefits of compost in soil?

Improves Soil "Structure":

Soil "Structure" refers to the way that particles of soil bind together.  Soils with good structure or good "tilth" are crumbly and well aggregated with lots of pore spaces for water, air, roots, and microbes.  Soils with bad structure are compacted and poorly joined together.  The higher the level of organic matter in your soil, the better structure your soil will have.  Soil structure is important for erosion control, root and water penetration, and plant and microbe respiration, among other things.

Improves moisture retention/drainage:

Very sandy soils typically have problems retaining moisture and need to be watered quite frequently for optimum plant health.  Clay soils, on the other hand, have issues with drainage and can become waterlogged during rainy periods.  Both of these scenarios present problems for our vegetables, and compost is the great stabilizer for both situations!  The porous yet well aggregated quality of compost will help sandy soils hold onto valuable water resources and help heavy clay soils drain more quickly!

Increases population/diversity of soil microbes:

Compost is teeming with life.  It is full of bacteria and fungi that are necessary to the health of your garden.  Even though we never see these microscopic populations, the work they do consuming dead and decaying plant material adds humus to your soil and makes nutrients available to your vegetables!   They also protect your plants from many pests and diseases.  So don't neglect the microbes!  Adding compost will maintain a healthy population of this all important army of garden helpers.

Increases nutrient retention/availability for plants:

Compost is an incredibly rich and stable source of plant nutrients.  And unlike many fertilizers, both chemical and organic, compost releases these nutrients slowly over time.  Many plant nutrients are also water soluble, so even adding them in the form of organic fertilizer doesn't guarantee that they will all get to your plants.  Think of the compost you add to your soil like a savings account of nutrients.  If you add a little every year your vegetables will have a steady supply of the things they need to thrive.

Maintains optimum pH:

pH is extremely important in your plants' abilities to take up and make use of the nutrients that are present in your soil.  Most compost has a pH of between 6.5 and 8, an optimum range for your vegetables.  So if you have soil that is too acidic or basic, adding compost will help buffer your pH!

Should I buy compost or make my own?

The simple answer is, you should make your own!  If you are an avid gardener, then you are probably used to having an abundance of plant material at different times throughout the year, whether pruning perennials, mowing grass, raking up leaves in the fall, or pulling out vegetables that have finished their life, your garden is in a continuous state of both growth and decay.  So gathering the spoils of your garden and letting them compost in one place is the best way to capture and recirculate the valuable nutrients, microbes, and organic matter that you have generated!

I am not going to take up the question of how to set up a good compost system in this blog post, but if you are interested in learning more, I recommend checking out the aptly titled, Let It Rot by Stu Campbell, a wonderful resource for learning all about compost, including guidance on composting at home.

If you aren't able to compost at home, buying bags of compost or ordering a delivery of bulk compost are both great approaches as well.  If you are lucky, you live in a place with lots of choices when it comes to buying compost.  If so, it can be overwhelming choosing the right product, so ask at your local nursery what is recommended for organic vegetable gardening.  I find many compost products to have a high proportion of woody materials.  These composts are very slow to break down in soil and I prefer compost that looks and feels more like soil than like barkdust.  Look for compost with a dark color and a pleasant earthy odor.

If you aren't able to compost at home, buying bags of compost or ordering a delivery of bulk compost are both great approaches as well.  If you are lucky, you live in a place with lots of choices when it comes to buying compost.  If so, it can be overwhelming choosing the right product, so ask at your local nursery what is recommended for organic vegetable gardening.  I find many compost products to have a high proportion of woody materials.  These composts are very slow to break down in soil and I prefer compost that looks and feels more like soil than like barkdust.  Look for compost with a dark color and a pleasant earthy odor.

When buying compost, I like products that come from plant materials, but "mushroom composts", "vermicompost", and "composted manures" are also options.  Be sparing in your use of these other composts in your vegetable garden as they can be excessively high in nitrogen or imbalanced in other ways.  Combining a small amount of these other products with vegetable based compost is an excellent approach!

A Few Tips For Composting At Home:

Creating compost at home has the obvious advantages of being free and providing the most sustainable approach to composting, but it is important to follow a few guidelines to make sure that the compost you create provides the maximum benefit to your garden:

• Home scale compost should be moist but not wet!  Too much moisture can cause anaerobic conditions and the growth of pathogens in your compost.
• Don't add animal products to home scale compost or you will attract rats and other rodents!
• Never add unfinished compost to your garden. Fully finished compost should have a pleasant earthy smell and you shouldn't be able to identify any of the original ingredients that you started with.
• Don't add weeds to your compost pile.  Though hot compost can kill weed seeds, achieving these temperatures isn't easy and it is safest to send weeds to the city compost to avoid introducing weeds into your vegetable garden

How To Amend Soil With Compost

So how do you actually go about the work of adding compost to your vegetable garden?  In order to get the greatest benefit in your garden, it is important that compost is "incorporated" and well mixed into the top 6-8" of your soil.  This work of "tilling" the compost in is much better than just laying it on top of the ground and accomplishes several things:  It distributes nutrients, organic matter, and microorganisms throughout the soil, it introduces air into soil that has likely been compacted by winter rain improving drainage, and respiration, and it creates a homogenous mixture where roots and water can travel easily through the soil profile.

In smaller urban gardens, the best way to till compost into your soil is using hand tools.  In my experience, a digging fork is the very best tool for the job.  In larger spaces, the use of a small rototiller will make the work much easier, but has the disadvantage of damaging soil structure, killing microorganism and worms, and creating layers of hardpan.  These adverse effects are especially pronounced when soil is too wet or too dry at the time of tilling.  Even when using hand tools, it is extremely important to work your soil at an ideal moisture!  A good test for soil moisture is to pick up a handful of soil and squeeze it into a ball.  If it won't maintain a ball shape, it is probably too dry.  If it makes a ball, but won't break apart when bounced in your hand, it is probably too wet.

Every Spring I add about 2" of high quality compost and till it into my garden where I have grown vegetables in the past.  I also add a heavy dusting of organic granular fertilizer to add the extra nutrients that will likely be needed to sustain my vegetables through the season.  If you are breaking ground on a new garden, or where soil needs more improvement, you may want to add more like 4-6" of compost to do that extra work of conditioning your soil in year 1.

Understanding the incredible value of compost and how to use it in your garden will serve you for years to come.  So when your soil finally begins to dry out in early Spring, you know what to do:  Spread that compost, till it in, and plant those first seeds.  Your soil and your vegetables will be so happy that you did.  Happy growing, and may Spring come early for us all this year!

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