garden pH

Learning from a Soil Test

28 February, 2021 by amygwh

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?

Soil pH and Garden Success

15 December, 2018 by amygwh

When a garden hasn’t been doing well, one of the first conditions to check is the soil’s pH, its acid-alkaline balance. The pH scale goes from 0 to 14; 7 is the neutral pH, measurements below 7 are acidic, and measurements above 7 are alkaline.

For reference, distilled salad vinegar has a pH in the range of 2 to 2.5 (acidic), and cold-process soap has a pH in the range of 9 to 10 (alkaline).

Soil pH influences how well plants can take in nutrients from the soil, so getting a garden’s soil into the best pH range for your plants will help them be healthier and more productive.

What is less well-known is that correcting a pH that is either too high or too low takes time. Testing soil pH should be done now, or soon, for gardens that have not done well in the past year.

What pH is good for gardens?

Blueberries will be ripe beginning in mid-to-late June. — Blueberry plants prefer a soil pH near 5.

Most garden veggies need a soil pH of between 6 and 7. Where I live, northwest of Atlanta, the natural soil pH is lower than that range — it is naturally acidic. With that acidity in mind, many people here routinely add lime to their lawns and gardens without checking the soil pH first, and end up with a soil pH that is too high.

When I worked at the local Extension office and managed the soil samples that went to the soil lab at University of Georgia (UGA), it was not unusual to see some samples with soil pH around 5, but others with pH higher than 8. Those higher pH soil samples were from lawns and gardens that had been limed pretty much every year.

Once, a friend who decided to plant blueberries in the space where she had originally kept a vegetable garden sent a soil sample in to the lab at UGA. The lab found that the pH of her soil was 7. This pH level was not a surprise because she had been spreading ashes from her fireplace on the garden. Wood ashes are a good source of potassium, but they raise the soil pH.

For many veggies, a soil pH of 7 is fine, but the blueberries she hoped to plant in that space prefer a pH closer to 5. That soil needed some work before it would make a good home for blueberries.

How can I find the pH of my garden soil?

Example of easy to use soil pH test kit for home gardeners.

The most reliable method of finding your garden’s soil pH is to send a soil sample to your state’s soil lab. Call your local county Cooperative Extension office to find out how to take a representative soil sample, how to package it, and the lab fee for this service.

Another option is to purchase a pH test kit at a local garden center. One I have used (pictured nearby) includes a container with color chart for comparison, capsules of pH-indicator-powder to add to the container, and instructions.

The basic instructions for this kit, and others:

put a tiny bit of garden soil into the container
add the indicator powder
add water up to a line near the top
put the lid back on the container, then
mix the parts together by shaking the container until color develops.

You find the pH by comparing the color of your sample to the chart on the front of the container.

Soil sample in pH test kit with water and indicator powder, to read the soil pH. — Sample in pH test kit shows pH of soil in my upper flower bed to be about 6.5.

How to lower a too-high soil pH

The UGA soil lab sent information to my friend about how much aluminum sulfate (the cheapest option) to add in order to bring the pH down to a better level for blueberries. If you know that the pH of your garden soil is too high, but don’t have a lab-recommendation for how to lower it, the amount of elemental sulfur (my preferred option) needed can also be found in a table by Clemson University Extension, in its Changing the pH of your soil publication.

For a soil pH that is only slightly high (about 0.5 higher than the desired level for your plants), one of my county’s former Extension agents used to recommend mixing sphagnum peat into the soil as a quick fix. Iowa State University Extension suggests other amendments that can lower a too-high pH, including “elemental sulfur, aluminum sulfate, iron sulfate, acidifying nitrogen, and organic mulches.”

There is a note, too, that a very high pH (over 8) is hard to bring down. When the local Plant-a-Row-for-the-Hungry garden (one of my volunteer groups/projects) was at a location that had a high-pH soil, we added the recommended amount of sulfur several years in a row before the pH started to come down.

How to raise a too-low soil pH

The Iowa State University publication How to change your soil’s pH includes a table showing how much ground limestone will be needed to raise soil pH. If you have hydrated lime handy, and think it might be a “quicker fix” for a too-low pH, please don’t try it. The risk of raising the pH too high with this product, by accident, is large. Lowering the pH after over-liming is difficult and can take a long time. Use ground limestone instead of quick lime or hydrated lime.

It takes more lime to raise pH in a clay soil than in a sandy soil. In general, according to the table in the Iowa State publication, for 100 square feet of clay-soil garden, it takes 5 to 6 pounds of limestone mixed into the top six inches of soil to raise the pH by 0.5.

When is the best time to adjust the soil pH?

For my friend’s blueberries, the UGA lab report recommended that the bushes shouldn’t be planted until 6 months after applying the sulfur. If she added the sulfur in October, that means blueberry-planting should — for best effect — wait until April. That is pushing the boundary for good planting time for woody plants like blueberries; as spring progresses and the weather warms, newly planted bushes are less likely to do well. They need time in the soil for their root systems to become established before being stressed by the heat of a Georgia summer.

What this means is that soil testing should be done now, if it hasn’t already been done, so adjustments to pH can be made soon enough to benefit plants that will be planted in spring.

What soil pH is best for my plants?

An article by Lewis Hill, published in Robert Rodale’s The Best Gardening Ideas I Know (1983) includes a list of some garden plants and the pH ranges they prefer. I’ve pulled some of the food plants from his list to post here:

pH 4 to 5: blueberry
pH 5 to 5.75: blackberry, grape, parsnip, plum, potato, pumpkin
pH 5.75 to 6.5: bean, citrus fruits, cowpeas, currants, gooseberry, oats, pepper, rutabaga, rye, squash, strawberry, tomato, turnip
pH 6.5 to 7: apple, beet, broccoli, buckwheat, butternut, chicory, chives, cucumber, eggplant, endive, kale, leek, muskmelon, onion, pea, peach, radish, raspberry, rhubarb, spinach, watermelon, wheat
pH 7 to 7.5: alfalfa, asparagus, barley, cabbage, carrot, cauliflower, celery, lettuce, nasturtium, parsley

This list may help other gardeners in planning what to plant where, but it is comforting to remember that plants will still grow and produce in soil that is slightly outside their preferred pH range.

This is lucky, since most of us grow many of these plants mixed all together in our gardens. However, for peak production, planting in soil that is actually the preferred pH for each crop works best.

Succession planting with preferred pH for each crop in mind

When I plant Irish-type potatoes in the spring, I will have added some sphagnum peat to their soil to bring the pH down a bit, but not all the way down to 5. Knowing the pH preferences of other crops helps me know what to plant after the potatoes in that space. For example, cabbages in that space would likely be a total bust, because their pH preference is so much higher (7 to 7.5!).

After potatoes are harvested, a better option than cabbages would be to let a nearby vining crop, like melons, sprawl across that space. Another option would be to plant a crop there that prefers a lower pH range, like beans or cowpeas that thrive at pH 5.75-6.5.

(If this information seems helpful or interesting, please remember to “like” or “share” it. Thank you!)

Learning from a Soil Test

What this all means