Not only was 2019 wetter than normal, but 2020 has been wetter than normal as well. You can see the standing water in low spots in the backyard in this photo, taken on March 18. The soil is saturated, with more rain to come later this week. Fortunately the vegetable garden itself (on the other side of the fence) has enough of a slope that water does not puddle on it.
In my previous post I described how I asked last year’s garden if I can use my urine as a source of nitrogen. With the caveats that I mentioned, the garden seems to have answered in the affirmative. Thus I’ll use urine on all the vegetable and grain beds this year to replace cottonseed meal and assess the effects that it has. But this isn’t the only question I’ll ask the garden to answer in 2020. Read on to learn what else I’m asking the garden, and why.
Nitrogen isn’t the only nutrient that I’ve needed to import in order to re-mineralize the soil in my garden. In this post from 2019, I discussed the results of asking the garden if the wood ashes left over from burning wood in our wood stove can be used to replace, in full or in part, the materials I purchased to supply calcium (Ca), phosphorus (P), and potassium (K). This experiment was done on a single bed, the bed in which I planted garlic and potato onions in autumn 2018. For this bed I added enough wood ashes to correct the entire deficiency in K and about 1/3 of the deficiency in P, which also supplied an excess of Ca and magnesium (Mg). After harvesting the garlic and potato onions in June 2019, I sent in a sample of the soil in this bed for analysis, in order to learn if using that large an amount of wood ashes (about 7 pounds for the 100 square foot bed) had brought that bed out of balance with the rest of the garden. The table below gives the analysis of nutrient deficiencies in the garden when the re-mineralization program was begun (spring 2013); in all the beds except the allium bed in spring 2019; in the allium bed in July 2019, after the allium harvest; and in all the beds except the allium bed and the bed that I ran out of time to plant in 2019 (spring 2020, from a sample I took on March 11).
Let’s look at the results in detail. TCEC means total cation exchange capacity: how well the soil can hold onto cations until the plants growing in it need them. The cations are everything from calcium (Ca) down in the table and are stored on the clay fraction of the soil. Steve Solomon says that a soil with a TCEC of 10 or more can hold onto sufficient cations to supply the plants’ needs for an entire growing season. Less than that means that the gardener should consider adding more of the re-mineralization mix about halfway through the growing season. Although the TCEC of my soil is less than 10, I have not done this, so I may not be obtaining as high yields as I could. I do, however, get decent yields while using less of the sources of the nutrients.
The TCEC of the allium bed may be somewhat higher than that of the rest of the garden in 2019, but as I discussed in this post, there is enough uncertainty about the precision and accuracy of the test to make any firm statement inadvisable. The same uncertainty affects the organic matter percentage. pH measurements have higher precision and accuracy, and the change in pH in the allium bed compared to the rest of the garden is in the direction I expect for adding wood ashes, which raise the pH. Fortunately it did not raise it over 7 even for the high amount of wood ashes I used, since vegetables generally prefer a soil with a pH in the range of 6 to 7. Since we receive rain during the growing season, the acidic rain will help to neutralize the high pH wood ashes. Those of you who live in arid or semi-arid areas or who experience dry growing seasons (anywhere west of about the 100th parallel of longitude in the US) will need to check with your state extension service or local gardening organization to learn if you can safely add wood ashes to your soil and if so, the maximum amount you can add each season. Based on these results, I will feel comfortable in adding as much as five pounds or so of wood ashes to any bed which does not already have an excess of calcium, to correct, in whole or in part, deficiencies of Ca, K, Mg, and/or P.
Now consider the 2020 results compared to the 2013 and 2019 results. There have certainly been changes, but they don’t appear to be consistent. The excesses of P and K that I was so pleased about in 2019 have swung over to deficiencies. What, if anything, can I learn about how the re-mineralization project affects the soil over time?
First, plants take up these nutrients from the soil to form their bodies. When I harvest the plants, I remove and Mike and I eat those nutrients. If there isn’t another source that replenishes the lost nutrients, over time the soil continues to lose them until it no longer can support plant growth.
Nature has many different ways to keep nutrients cycling through the air, water, and soil; if you’re curious, you can find descriptions in ecology textbooks. However, if the cycle for any particular nutrient cannot supply enough of it to replace what I remove via the harvest, then that nutrient will, over time, become deficient. This is the bane of annual agriculture, and traditional vegetable gardening as well. Nature cannot re-supply all of the nutrients we remove fast enough to continue to grow annual plants on the same plot for years in a row. Some nutrients will go deficient and need to be replenished, as I am doing by re-mineralization. Taking soil samples and having them analyzed, then tailoring a re-mineralization to add just what the soil needs, avoids adding excess nutrients, which can cause as much harm as not enough of them. While I hope that over time I can get some cycling of nutrients from the compost pile back into the soil as the weeds I put into them become better balanced, I don’t expect to drop all re-mineralization. To the extent that I can partially close the cycles by using on-site resources like urine and wood ashes, I will do that. It’s probably the best I can hope for, though I would not mind being proven wrong.
Second, I don’t have a good feel for why particular nutrients change in particular directions over time. Possibly an ecologist could explain it, but I have no formal training and not enough informal reading in the field. Among other things, the apparent excess of Ca in 2020 stumps me. Calcium tends to dissolve into the soil water and move with it down into the groundwater, thus being lost to the garden and its plants. Considering the excessive rain we had last year and have had so far this year, I would have expected more than the usual amount of Ca to be lost to the garden and therefore to see a deficiency this year. This is a common frustration in scientific research, just something we garden scientists have to keep in mind as we try to understand what our gardens are telling us – and a good excuse to spend some time with textbooks on ecology or agronomy.
The question now becomes, can I use wood ashes to add some or all of any of the deficient nutrients in 2020?
Since trees take up the same range of nutrients from the soil as do vegetable plants, wood is a potential source of nutrients for re-mineralization. Those of you who add woody mulch to your gardens are at least partially closing the nutrient cycles by doing so. I don’t have a convenient source of woody mulch that I trust to not contain systemic herbicides. Since I have the wood ashes and would prefer to use them rather than landfill them, wood ashes it is.
Wood ashes have a variable composition. A Missouri Extension publication on using wood ashes in the garden indicates that wood ashes contain, by weight, about 1% P, about 5% K, and about 25% Ca. It didn’t mention Mg, but a brief web search brought up an article analyzing the elemental composition of certain hardwoods from forests in England, which indicated that the Mg level in these hardwoods is about 10% of the Ca level. Thus wood ashes are roughly 3% Mg.
In 2020 the soil is deficient in P, Mg, and K, and in excess in Ca. I would have to add about 3 pounds of wood ashes to each bed to correct the entire K deficiency. With an excess of Ca already, this does not strike me as a wise move. So I will add potassium sulfate to correct the K deficiency, which also adds more than enough S to correct the S deficiency. I had hoped to not have to use this soil amendment as it is depleting, but perhaps some years it will not be needed, as it was not in 2019. That would be preferable to adding it every year.
I can add a smaller amount of wood ashes to correct for the Mg deficiency. The Acid Soil Worksheet indicates that I should only add 10% of the amount needed to correct the deficiency this year. Adding more risks getting the Ca:Mg ratio out of whack, which among other things makes for too-sticky soil. I can add about 5 ounces of wood ashes to each bed to correct for 10% of the Mg deficiency without adding more Ca than I am comfortable doing. This also adds a small amount of P and K, but not enough to correct these deficiencies.
Last year I was very happy that the soil had an excess in P, because sources of phosphate are depleting. This year, while I do need to correct a deficiency in P, at least it is less than it has been in any other year with a deficiency. I’ll correct it by using Tennessee brown rock, which has about half as much P as rock phosphate and comes from the washing piles left behind from extracting superphosphate from high-grade ore about 100 years ago.
One other question I’m asking the garden this year stems from my continuing interest in the possibility of increasing the TCEC of the garden soil. Fedco is offering for the first time this year a product called Hum-Amend Max which is touted as doing just this. Given the uncertainty in the precision and accuracy of the test for TCEC, rather than adding it to only one bed and not to the others, I will add it to every bed in 2020 and see if it changed the TCEC enough to notice in 2021. I’ll also observe the garden as I usually do with an eye to noting differences between this year and past years. This may be a one-time addition (Fedco’s write-up indicates that at least part of the formulation is intended to have long-term effects), but I will wait to see the results from this year’s test before deciding if I should add any more in future years.
So that’s what I’m asking the garden in 2020. I wish all of you the best in your own projects! Meet you here again in April.