Soil Management in our Organic Allotment Garden

It’s harder than you might think!

On the internet, the consensus is that raised beds are one of the easiest ways to garden.  In a less than optimal environment, they can be positioned for the best sun, are easy to weed and water, and provide the kind of loose, well-drained soil that can be hard to find in urban areas.  In a community garden setting, there’s an added benefit - the ability to allow a variety of gardening approaches by different groups or individuals, all in a relatively small space.

Since the summer of 2013, AGG has offered rental beds in a variety of sizes. They’ve been used from season to season by singles and couples, collections of friends/roommates, and parents with children of all ages. 

Our gardeners arrive with a wide range of gardening experience, and with varying ideas about how best to go about growing plants, as well as what plants to grow.  Brussels Sprouts? Sure! Loofahs? Fun! Pole Beans climbing up an 8-foot cone? Trellised watermelons in hammocks? Stinky fish emulsion fertilizer next door to a springtime application of old coffee grounds? We’ve seen (and smelled) it all.

So it should come as no surprise that the management of the soil in our organic raised beds has become somewhat of a challenge.  Some beds have been continuously occupied by the same gardener for years, while other beds were just built last weekend and will be rented by a new member.

In order to thrive, plants need a steady supply of nitrogen (N), phosphorus (P), and potassium (K), and well as trace minerals like zinc, iron, calcium, and many others. These nutrients are depleted over the course of the season and must be replaced regularly. In order to effectively utilize the soil nutrients, plants also need a soil pH in the range of 6.5-7.5.

At AGG, we’ve discovered that our soil pH trends alkaline (over 7.5).  Making an alkaline soil more acid is a trickier process than the other way around, and the process takes time. As a result, we’ve realized our plants will need a little extra help this season, while we work through the issue.

How to begin?

It’s not as simple as picking one soil amendment and adding it universally to all of our 37 beds.  The newly built beds were just filled with fresh topsoil, which we purchased from a local vendor, while most others were only topped up to replace what was lost last season. Some beds hosted beautiful crops of beans last summer (plants that add nitrogen to the soil rather than depleting it), while others were filled with “heavy feeders” like tomatoes and eggplant. Some gardeners have been experimenting with various organic fertilizers, like compost tea, fish emulsion, Black Kow and blood meal. Our garden manager planted nitrogen-fixing cover crops on many, but not all, beds last fall.

Forsyth Extension offers free soil testing beginning in April, so we have collected samples from different beds and are sending them to the state lab. The state doesn’t test for nitrogen, however, since it is such a volatile nutrient.  And many of our gardeners are ready to plant spring crops now. So we’ve purchased our own soil test kit, and have begun the process of testing samples from various beds.

As we’ve tested, one thing has become very clear - the higher pH has allowed our soil phosphorus to bind with other soil elements and build up quite a bit.  Phosphorus is not a soluble element, and once it has combined with other compounds in the soil, it is “locked up” and no longer available to our plants.  But plants need phosphorus to be productive! The problem becomes how to assure our plants a supply of immediately available phosphorus, as opposed to the slower-release, locked-up kind.

The internet is filled with advice about phosphorus management (clearly we are not the only gardeners facing this problem) which mostly advises us to avoid adding high phosphorus amendments to the soil and work to adjust our pH to a more optimal range.  Information on long term soil management in an allotment garden is scarce, so our soil management strategy for this season has been extrapolated from sources aimed at commercial farmers and organic gardeners with in-ground, long-term plots.

Here’s what we’ve arrived at:

1) First, some help for the pH: a longer-term fix would be to purchase a granulated sulfur product and add it to the bed according to the package directions. This can lower pH by a full point over a few months, but requires warm temperatures to be effective. For a faster, but more temporary, fix, water with a dilute solution of apple cider vinegar and water about once a month.

2) For beds that are filled with a blend of older and newer soil, our soil tests show that when the soils are thoroughly blended, soil nitrogen is more than adequate.  Gardeners should be sure to turn and “stir” the bed thoroughly before planting.

3) Beds that are filled with new soil will need no amendments added to the bed as a whole.

4) We are providing our gardeners with Black Kow, Kelp Liquid Fertilizer, and Cal-Mag, a foliar mineral supplement.  Black Kow should be added only to the immediate area where planting is happening, at the rate of about 3 quarts of Black Kow per one square foot, as an NPK boost to seedlings and starts. 

5) As plants become established, we encourage regular foliar feeding with kelp and Cal-Mag, as well as side dressing “fruiting” plants with bone meal as the flowering stage begins.

(We’ll cover the topics of side-dressing and foliar feeding in a blog post later this season.)

6) All of our beds can benefit from the addition of low-nutrient organic matter, such as well-composted leaf mulch (which we have available on site for our gardeners to use). Over time, decomposition helps to lower pH and restore the nutrient balance of the soil, and also provides a home for beneficial bacteria that contribute to soil and plant health.

Gardening is a learning process, even for the most experienced gardeners. We encourage our gardeners to record the soil amendments and fertilizers added to their plots.  It’s helpful to be able to see what worked best from year to year!

We plan to do some additional soil testing at mid-season, and will report on the progress of our soil in another post.  Happy planting, everyone.