Corny work: A garden science experiment and what I learned from it

Spring!

As part of my work to encourage readers to learn and use the scientific method in practical endeavors like gardening, I discuss and interpret a test I devised to answer a question I asked the garden in 2018. It has it all – the original question, a belated realization that the design was flawed, difficulties experienced when I attempted to perform the experiment and the necessary re-boot, and the results that I got and what little I could learn from them. Read on to find out what I did and what all of us, including me, can learn from it!

For the past several years, I have tested the re-mineralization program described by Steve Solomon in his book The Intelligent Gardener. This post from 2013 describes my garden soil at the beginning of the re-mineralization program and the materials I added to it that spring. Since then I have been taking soil samples and, using the most current worksheet (available here), determining which minerals in my soil are deficient and which are in excess. The figure below shows each year’s excesses and deficits in minerals. The units for the deficits are pounds per acre.

Each year from 2013 through 2017 I added a nitrogen source (seed meal, which soil bacteria eat and excrete as nitrates that the plants can use). The nitrogen source ensures that there is sufficient nitrate in the soil for good plant growth; the compost I add isn’t high in nitrogen and we do not keep livestock whose manure could be used for this purpose. The soil tests showed phosphorus was deficient each year, so I added soft rock phosphate to supply that macronutrient. Most years I have also added a source of sulfur (gypsum), calcium (lime, oyster shell, and/or gypsum), potassium (potassium sulfate), and boron (borax) to remedy deficiencies. When the soil test indicated a deficiency, I added mineral forms of manganese, copper, and zinc.

After the 2017 growing season, with five years of garden data to hand, I had proven to my satisfaction that the re-mineralization program produced much better results than did my attempts to follow the Ecology Action program. The garden data from 2017 shows that the majority of the best yields for each crop and variety occurred in 2013 or later, even though most of the crops were grown at wider spacing than I had grown them before 2013.

Then I looked at the data in the figure above through the fall 2017 soil sample and considered what it told me about the soil and how it had changed over the years. So far I had needed to add multiple minerals to the soil each year, in accordance with the low TCEC of my soil (meaning I could not keep a large supply of those nutrients occurring in positively-charged forms – everything from calcium down in the figure – which is due to the low clay content of my soil). I also needed to add the negatively charged minerals, sulfur and phosphorus, which are associated with the organic matter in the soil. Because we have a hot, humid summer which includes rain during the entire growing season, it is difficult to keep a high level of organic matter in the soil, and thus difficult to hold onto these minerals over time. I would add more compost to raise the organic matter level if I could make more of it, but so far I have not been able to do so.

Since I thought I would need to continue adding some minerals each year unless and until I could raise the TCEC and the organic matter level of my soil, I wondered if there were other substances I could use to re-mineralize the soil that I could source for myself, rather than having to purchase them. While cottonseed meal, soft rock phosphate, gypsum, and potassium sulfate are still readily available and cheap, all of these materials come from someplace else. The rock phosphate and potassium sulfate are both depleting. If natural gas supplies become constrained, so will the supply of the ammonia used for fertilization on commercial non-organic farms, because natural gas is used as the hydrogen source in the Haber-Bosch process that produces ammonia. In that case seed meals may become a substitute nitrogen fertilizer for commercial agriculture, which could prevent me from obtaining it or raise the price past what it makes sense for me to pay.

The first material that came to mind as something I can source was wood ash. Whenever we burn wood in our wood stove we generate wood ashes, which we store in a metal trash can to keep any live sparks that could start a fire away from combustible materials. By spring 2018 the trash can was almost full of wood ashes. In order to be able to put ashes generated in the following winter into the trash can, we needed to do something with the ashes already in it. At about that time, the Missouri Extension published an article on recycling wood ashes as a garden amendment. While their opinion was that it was best used to decrease soil acidity (Missouri soils tend to be acid due to rain during the growing season), they also mentioned that it could be used to supply potassium and phosphorus, and they provided an average analysis of P (phosphorus), K (potassium), and Ca (calcium) for wood ashes: 0.9% P, 5% K, 23% Ca, and 2% Mg (magnesium). With this information I could calculate how much of my soil’s needs could be supplied by wood ashes.

I had another material in excess which contains some nitrogen (N) and P: worm castings from the worm bin. While I use worm castings in the potting soil I make for seedlings and container plants to add N and P, the worms produce more castings than I need. With this material also needing to be made use of, I searched the web for analyses of the nutrient content of worm castings. The best I could determine from the web search, worm castings on average contain about 1% N, 0.5% P, and no K. (Whether or not my castings match this analysis I don’t know; I chose not to pay for an analysis.) Worm castings probably contain some sulfur as well, but I could not find any information on their sulfur content.

Having decided on two materials I could use for part of the re-mineralization formula, I thought about how to determine if using them instead of the purchased amendments would be effective for re-mineralization. If I wanted to do a proper experiment, I would need to include a control area where I used the usual ingredients as well as an experimental area where I tried their substitutes. The best choice was the three beds of popcorn I planned to grow in 2018. Thus, for all of the other beds except those growing beans and peas I used a mix with cottonseed meal, soft rock phosphate, dolomitic limestone (for the Mg as well as Ca), potassium sulfate, and borax for re-mineralization in 2018. For the three corn beds I could use that same mix on one bed as the control, with the other two beds available for the experimental mixes. Then I could compare the appearance of the corn plants as they grew and calculate the yields for each bed separately to determine the effect of substitution of materials.

(Why did I not use this same mix on the pea and bean beds? Because peas and beans have bacteria associated with them that supply their nitrogen needs, so I used only as much seed meal on these beds as I needed to get a good mix of the other ingredients. Those were used in the same amounts as for the other beds.)

Here I encountered the first issue in translating theory into practice. The experiment I wanted to do required four beds of popcorn: the control bed, with the same mix as the other beds; one bed with worm castings substituting for the seed meal but all other ingredients the same; one bed with wood ashes substituting for all of the limestone and potassium sulfate and some of the rock phosphate but all other ingredients the same; and one bed with both the wood ashes and the worm castings, with enough of the other ingredients to make up the remaining deficiencies. But I planned to grow only three beds of corn and did not have room to add a fourth bed. Furthermore, one of the three beds was a newly formed bed that had been planted at different times to asparagus and raspberries and, most recently, had contained weeds for a few years. I hadn’t taken a soil sample from that bed so I didn’t know how similar or different it was to the rest of the garden. After much thought, I decided to re-mineralize the reclaimed bed as if it had the same set of deficiencies and excesses as the grassy areas surrounding the beds, which I sample each year as controls along with the garden soil. In that way I could test it against the control. In the remaining bed, I used both the worm castings and the wood ashes and enough of the other ingredients to make up the remaining deficiencies, based on the fall 2017 soil analysis in the figure.

In retrospect, I realize the flaw in my design: if the yield in this bed were different from that of the control bed, I would not know which of the two substitutions was responsible for that change. Rather than trying both wood ashes and worm castings in the remaining bed, I should have chosen to try only the wood ashes. I allowed my desire to find a place for the excess worm castings to overcome my scientific judgement, when I could have added them to the compost pile instead. As my mother would say, live and learn … which you’ll notice in future garden science posts.

Beyond this theory-level flaw, I ran into difficulties I hadn’t anticipated when I attempted to prepare the mix with worm castings and wood ashes. While I usually apply compost and the re-mineralization mix separately and then cultivate the soil a few inches deep to mix them in, the castings were so wet that I thought it would be preferable to mix them with the compost and wood ashes and then spread the mix. However, the mixture proved to clump together unevenly and the mixing step took far too long to be practical.

This is where I should have taken a deep breath and dropped the worm castings out of the experimental design altogether. But, still being determined to include them, I persisted long enough to drop clumps of mixed worm castings and compost over about 1/6 of the bed area before I gave up on the castings as too heavy and wet to work as a practical nitrogen source in garden beds.

That left the wood ashes. Until I worked with them, I didn’t realize how light and fluffy they are; I couldn’t mix them evenly by hand with the rest of the ingredients. In the end, I cast the wood ashes, compost, and re-mineralization mix with seed meal onto the remainder of this bed and worked it into the top few inches of the soil as I normally do. For the part of the bed with the castings, I added the proper amount of wood ashes and remineralization mix without seed meal before cultivating it. After preparing the control and newly formed beds as I mentioned before, I added popcorn seeds to all three beds on the same day and then cared for each bed in the same way. When I harvested the ears of popcorn, I kept the ears from each bed together so that when I shelled and weighed the popcorn, I could calculate the yield, in pounds per 100 square feet, for each bed.

What were the results? First, I didn’t notice any visual differences between the plants in any of the beds until the ears of corn were ready for harvest. By that time, it looked as if there were more and larger ears in the control bed than in either of the other two beds. I didn’t see any difference in pest or disease pressure among the three beds, or any difference in plant color or height except near the end of all three beds that is partially shaded by hazelnut shrubs and is attributable to that shading.

The yields of popcorn I obtained for the three beds were as follows:

Bed 10 (control): 9.0 pounds per 100 square feet

Bed 11 (wood ashes and worm castings): 5.7 pounds per 100 square feet

Bed 12 (mix for lawn areas): 6.1 pounds per 100 square feet

I can’t do any kind of statistical analysis on the data because I didn’t set up the experiment to allow for that. But based on the results I obtained, it appears that the yield of the control bed was higher than that of either of the other two beds. The problem is, I don’t know if the yield of the experimental bed was lower because of the change in the re-mineralization ingredients, or because of the practical difficulties I encountered working with the wet worm castings and the fluffy wood ashes. And if it was the change in ingredients, I don’t know which one was responsible, or if it was an interaction between them. As for the third bed, since this is only its first year being re-mineralized and I don’t know if I used the right mix for its soil, I’m not surprised that it seems to have produced a lower yield.

There you have it: a real person doing a real experiment, experiencing real difficulties and doing her best to understand the message in the results. And it’s something any of you could have done as well as I did; it only required a good grasp of fractions and percentage, and a basic understanding of the scientific method. Tune in to the next post to find out where I’ll take my quest to use home-grown ingredients for re-mineralization in 2019!