I finally sent off a couple of soil samples to the state testing lab. The soil here is new to me (sand, not north Georgia’s red clay), and last summer’s garden was less productive than I had hoped. Getting a baseline set of measurements will boost my understanding of what the soil needs to produce more good food.
The routine test for home gardens provides quite a bit of useful information, but I also paid for a couple of extra bits, the organic matter content and percent nitrogen. One sample that I sent in (labeled Veg) was a composite of two scoops each from seven garden beds. The other (labeled Fruit) was a composite of several scoops taken down the length of a newly-dug 6×30 foot bed that will be home to some berries and other small fruiting plants.
The results came back, and they contained some surprises. Here is the information in table form:
| Component | Veg — 7 garden beds | Fruit — one garden bed |
| pH | 6.4 | 5.4 |
| Phosphorus (P) | 353 lbs/acre (very high) | 104 lbs/acre (high) |
| Potassium (K) | 171 lbs/acre (low) | 39 lbs/acre (very low) |
| Magnesium | 334 lbs/acre (very high) | 79 lbs/acre (high) |
| Zinc | 28.6 lbs/acre (very high) | 22 lbs/acre (very high) |
| Calcium | 2331 lbs/acre | 496 lbs/acre |
| Soluble Salts | 0.1 (low) | 0.1 (low) |
| % Organic Matter | 4.2 | 3.6 |
| % Nitrogen | 0.16 | 0.11 |
What this all means
Seeing the numbers for the two areas side-by-side makes the differences stand out, but I will go over the components measured, one by one, to explain how I am using the information.
pH
For both samples, pH was on the acid end of the spectrum. The pH of 6.4 for the Veg sample is excellent for most vegetables, and no additions of limestone were recommended for that set of garden beds. However, the pH for the Fruit sample, at 5.4, was too low, too acidic, for many fruits. Blueberries will be happy at 5.4, but not much else. The Fruit test results came with a recommendation to add limestone: 75 pounds per 1000 square feet, to bring that pH up a bit.
The surprise here is that I had used a simple, home pH test kit last spring on part of the vegetable garden area. The pH as measured with the kit was at least 7 (neutral) or possibly higher. The pH kit relies on color-matching, and it seemed to me that my sample could have been between the colors for seven and eight.
We did find, though, when digging the beds, a couple of the plastic tags that come with tomato plants, that tell which variety they are. This suggests that a previous owner also had a veggie garden in this area, and they probably added lime to bring up the pH. Last spring, the soil sample for measuring the pH was from just a couple of beds, because it took awhile to get them all dug. It is possible that the new sample, a composite from seven beds, includes a mix of high and low pH areas.
I will need to check the pH of each of the seven planting beds to figure out which have a higher pH and which have a lower pH. Some may need a little limestone to improve the pH.
Phosphorus
Just wow. Neither of my planting areas needs any more phosphorus, possibly for years. These came back High (Fruit) and Very High (Veg). It may seem as though having high nutrient levels would be a good thing, but high levels of phosphorus can interfere with a plant’s ability to take up micronutrients, like zinc, that it may also need for good growth and productivity. The Texas A&M article Phosphorus — Too much and plants may suffer indicates that phosphorus levels above 150 ppm may harm plant growth, and levels above 350 ppm can kill some plants. It may be a miracle that my garden has been able to support any crops at all!
The high phosphorus levels mean that I should avoid composts that include manures, especially chicken manure, since those can add extra phosphorus to the soil.
My fertilizers also need to be low on phosphorus. Most fertilizers are labeled with the percentages of nitrogen, phosphorus, and potassium they contain. These are listed in that order, as N-P-K (nitrogen-phosphorus-potassium). That middle number, for my garden, needs to be as low as possible. The good news here is that one fertilizer that I keep on hand is a fish fertilizer labeled 5-1-1. This product offers plenty of nitrogen with a low amount of phosphorus.
Potassium
Measurements for potassium (K) came back low for both areas.
My 2018 article about potassium sources for organic gardens describes pros and cons of several potassium sources. In addition, my Garden Planner and Notebook contains, on page 12, a list of some commonly available organic fertilizer ingredients and what nutrients each one provides. Looking down the list, I can see that greensand is the one I will want to use.
Wood ashes add potassium but also can raise the soil pH. Since the “soil” here generally seems to have a low pH, that is not a drawback. The issue is more one of finding the wood ashes. Kelp meal would be a possible source to try, except that it can also add phosphorus, which I need to avoid. And, sul-po-mag brings magnesium with it, which the garden also seems to have in abundance.
Magnesium
Measurements for magnesium content for both planting areas came back as high (Fruit) and very high (Veg). No magnesium is needed, apparently, even though epsom salts (a magnesium source) did seem to help last year. I wonder whether the super-high levels of phosphorus made it harder for plants to use the magnesium that was already in the soil
Alternatively, it is possible that the magnesium in the soil was tied up somehow, bonded in a way that made it less available to the plants. Also, if the soil pH is very low, plants are less able to take in nutrients they need, like magnesium, phosphorus, and potassium. So, low pH could have been a factor in some of the Veg beds. (Another reason to check soil pH in each of the seven beds…)
The limestone recommended to bring up the pH in the Fruit area also will add more magnesium to that space, since limestone contains both calcium and magnesium in abundance.
Zinc
Zinc measurements came back high in both planting areas. If all the yards around here are high in zinc, that might explain the success of pecan trees locally. Pecans are big users of zinc. Many yards here have at least one pecan tree, and we saw lots of pecans in the fall. In north Georgia, pecan trees often need supplementation with zinc in order to make good crops, since zinc is less abundant in the soil up there.
Calcium
Calcium level in the Veg sample was a lot higher than the level in the Fruit soil sample, but a value-judgement of whether the levels were low-medium-high-or-very high was not given. A Mississippi State University Extension article includes this bit of information about calcium availability in soils that have low pH, such as in the Fruit sample:
Soils with favorable pH levels are normally not deficient in calcium. Acid soils with calcium contents of 500 pounds per acre or less are deficient for legumes, especially peanuts, alfalfa, clovers, and soybeans. At this level, limited root system crops such as tomatoes, peppers, and cucurbite would also need additional calcium. Soluble calcium is available as the Ca2+ ion and is needed for peanuts at pegging time and for peppers and tomatoes to prevent blossom end rot.
Secondary Plant Nutrients: Calcium, Magnesium and Sulfur ; publication number IS1039, by Larry Oldham, phd, 2019
For my garden areas, this means, again, that checking soil pH for each of the seven beds in the Veg area will be a good thing to do, in case any of these show a super-low pH. Maybe low calcium explains the wimpiness of last year’s green bean plants!
It also provides more support to add the recommended limestone to the Fruit area to improve plant health.
Soluble salts
The low measurements for soluble salts were another big surprise. Here we are, just five blocks from the beach, and the soluble salts are low! I will count this as a blessing. Many garden crops are less productive when soluble salts are high.
Organic matter and percent nitrogen
These measurements are just to provide a little more information about this new (to me) soil. The percentages of organic matter are better than I had expected. At 4.2 and 3.6, I may have to stop calling my yard a sand dune and refer to it instead as a sandy loam.
When working in the garden, it just seems like a lot of sand, but my past 30 years of working in north Georgia’s red clay may be coloring my judgement here. However, in that clay soil, I would want a higher percentage of organic matter, to help break up the clay and improve how it both holds and drains water.
To work on these numbers, to boost them slightly higher, I will need to rely more on cover crops than on composts and manures (because of that phosphorus problem…).
Fertilizers for the year, based on the soil test results
The soil test reports recommended additions of 34-0-0 fertilizer, for the nitrogen, and muriate of potash (0-0-60) for the potassium. These are both conventional fertilizers that are in the form of soluble salts. Muriate of potash is, essentially, potassium chloride. (You may recall from a long-ago science class that table salt is sodium chloride.) These salts dissolve readily in water and are instantly available to plants as soon as they are watered in to the garden.
However, I am an organic gardener. Soluble salts are not part of my game plan. Hence, the decision to use greensand as a potassium source. In addition, I will be adding some Azomite (not listed in the Garden Planner and Notebook), another kind of pulverized rocks, as a source of additional micronutrients and a tiny bit more potassium.
My nitrogen source, selected by looking at page 12 of my Garden Planner and Notebook, could be either feather meal or alfalfa meal. Feather meal smells a lot like dead things, but the odor only lingers for a few days. No local stores carry feather meal, though, which leaves me (thankfully) with the better-smelling alfalfa meal.
I plan to follow Kevin Meehan’s instructions for DIY alfalfa fertilizer, posted on Rodale Institute’s website, which uses alfalfa pellets that can be found at any local feed store. This should be — yet another — interesting adventure, along with the other “special projects” I have listed in my Garden Planner for the year. Wish me luck?
