Wednesday, April 24, 2019

2019 garden science: homegrown nitrogen

Dogwood blooms, trees leaf out: spring!

In the last post, I discussed how the 2018 garden responded to my poorly phrased question about using wood ashes in place of some of the purchased soil amendments that I have used in past years to balance the soil, following the program in Steve Solomon’s book The Intelligent Gardener. I also used wood ashes in a few other beds in 2018. Of these, the most interesting story features the garlic and potato onion bed. For this bed I added enough wood ashes to provide all the potassium needed for 2018 (about 7 pounds of ashes for the 100 square foot bed area). Besides all of the potassium, this amount of wood ashes supplied a little less than 1/3 of the phosphorus to add, and far more calcium and magnesium than the results suggested was needed to balance the soil. Although excess magnesium (compared to calcium) is not desirable, I think there will be enough calcium added to keep the ratio in the desired range. I plan to test the soil in this bed after the onion and garlic harvest is completed in June, to learn more about the effect on the soil of adding such a large amount of wood ash – which was still within the Missouri Extension’s recommendation of 5 to 10 pounds of wood ash added per 100 square feet. And I’m eager to learn how much food I harvest! So far the plants look strong, with excellent winter survival.
 The allium bed: garlic in front, potato onions behind

While I worked on the previous post, and recalling that I hadn’t taken a soil sample in late autumn as I usually do, I decided to take a soil sample before planting most of this year’s garden and compare it to the previous several years of soil samples. Below is the spreadsheet of nutrient deficits for each year I’ve done soil sampling (the same one I put into the previous post), along with three other soil parameters that help to decipher some of the characteristics of soil. 

 Well, even if your eyes are glazing over, mine sure didn’t when I saw the spring 2019 soil results compared to those of previous years. In fact, I just about got up out of my chair and cheered! Read on to find out why. (For extra credit, take a moment to try to figure out why I was cheering before you go on. I’ll even give you a hint: it’s in the nutrient part of the spreadsheet and refers to a discussion about the soil in the previous post.)

First, before going on to the nutrients, let’s look more closely at the soil characteristics. Of these, pH and organic matter percent (OM%) are probably the ones you are most familiar with. The pH indicates soil acidity or alkalinity; a pH less than 7 is acid, 7 is neutral, and higher than 7 is alkaline. Most garden vegetables prefer soil pH to be in the range of 6.0 to 7.0, though potatoes do well at a slightly more acid pH. The pH of my soil has always been in the 6.0-7.0 range.

After I published the previous post, Chris asked me about what he interpreted as a reduction in organic matter level over the years. This gave me the chance to do something I should have done years ago: to think about the precision in measuring the OM%, rather than assuming the precision based on the number of significant figures in the reported result. Chris assumed, as I had, that by reporting the result for organic matter to hundredths of a percent, Logan Labs is implying that the precision of the test is in hundredths of a percent. (Precision is the degree of closeness of the results from multiple tests run on the same sample.) But when I re-read Erica Reinheimer’s and Steve Solomon’s 2014 revision to the worksheets presented in Solomon’s book, I learned that, in their words, “…OM% test results can vary widely, like 2X, when we send the same soil sample to the same lab.” 2X means a factor of 2: in other words, the 2019 OM% that Logan reported to me might have been as low as 1.5% or as high as 6% across different samples had I sent them multiple soil samples from the same garden!

Going back to the organic matter results across the years and keeping this low precision in mind, I notice that they are all in the 3-4% range, which Solomon suggests is about what can be expected in the US south of the Mason-Dixon line (about 39 degrees 43 minutes north). I’m about a half degree south of this line and our summers certainly quality as hot and humid. Thus I interpret the organic matter percent as remaining roughly constant over the years, to the precision of the test method. Since I’ve been adding the same volume of compost over the years, the rough equality of OM% makes sense. Unless I start making more compost so I can add more (or start making higher quality compost) and as long as I continue using Solomon’s method, the garden will likely continue to be tested at about 3-4% organic matter.

(Aside: in science, the precision of a test isn’t the same as its accuracy. Accuracy is how close the test comes to reporting the true value of what it purports to test. For the moment I’m assuming that the accuracy of the test for organic matter is good enough to fall within the precision that Reimheimer and Solomon suggest I can expect. But I don’t know if anyone knows how accurate it actually is.)

To continue, TCEC stands for total cation exchange capacity. That’s enough wonky-sounding words to make most peoples’ eyelids droop. Allow me to attempt to make some garden-level sense out of it, following the argument in Solomon’s book. Cations means positively-charged ions. In the table of nutrients, all but sulfur and phosphorus occur in the garden as positively-charged ions, and these are adsorbed onto the clay fraction of the soil. Thus TCEC correlates roughly with the amount of clay in the soil: in Solomon’s homey language, clay acts as a pantry from which more positively-charged ions can be drawn as garden plants take them up into their bodies. He suggests that a TCEC of about 10 or more correlates to a well-stocked pantry that doesn’t need any topping-up of more minerals during the growing season. My soil seems to have a TCEC of around 7 to 8, though I don’t know what the precision of the TCEC test method is. I’ve been adding humates to the soil in small quantities in an attempt to raise the TCEC, but so far to no noticeable effect. Solomon suggests adding more minerals after 8 weeks or so to gardens with low TCEC soil, but I have chosen not to do this. It would only be necessary for crops that stay in the ground significantly longer than that, which in my case is just the warm-season crops, and not all of these need it (tomatoes don’t, though it might be helpful for the others, and perhaps especially for greedy corn and disease-prone peppers).

As for sulfur and phosphorus, these occur as negatively-charged ions in soil, in association with organic matter, which acts as their pantry. Increasing the OM% would be one way to make better use of the sources of these nutrients, but as I mentioned in the previous post, without livestock to provide a source of OM beyond the limited amount I obtain from my compost piles and without a pickup truck to haul manure from close-in sources, the only other way I know to increase OM is to take beds out of production to grow green manures for a full growing season. So far I’ve not wanted to take this step, though I do allow winter-annual weeds to play the role of green manure and grow crimson clover on beds that I harvest early enough to allow it to germinate and grow enough to survive winter. Both of these are dug into the soil before growing the next crop.

Now let’s look again at the spreadsheet and pretend that we don’t see the results from the spring 2019 soil test. The soil required additions of phosphorus (the P in N-P-K) every year and sulfur every year but one. I add phosphorus as rock phosphate, which is a mined and refined mineral requiring considerable amounts of energy to prepare and transport. Solomon suggests that most of the phosphate I add isn’t immediately available to the soil, which may explain why it was deficient by about the same amount every year. Wood ashes also supply some phosphorus, but I cannot add enough to resolve the full deficiency without throwing other minerals like potassium out of balance. Sulfur, on the other hand, is available in the form of gypsum, the common name of calcium sulfate. Since the closest gypsum mine is in Kansas and it is needed in much smaller amounts than rock phosphate, I’ve been less concerned about using it. Wood ashes may provide a good substitute source of potassium, which I’ve needed to add most years, pending the results from the bed of onions and garlic and any further testing I may want to do. Calcium is of little concern: besides being included in gypsum and in wood ash, there is a large limestone quarry within 10 miles, thanks to the Missouri of long ago having been at the bottom of a sea. Thus, before the 2019 soil test, I had resigned myself to always needing to apply a source of phosphorus, with most of that needing to come from outside of the yard, while I had a possible alternate source for potassium and calcium at hand in wood ashes should the commercial sources I use become too expensive or difficult to obtain.

Now let’s take a good look at the spring 2019 results. Please direct your attention to phosphorus, and note that it is in excess. Let me repeat that: phosphorus is in excess. That means that for the first time since I began the soil testing program, I don’t need to add the mineral that I had to use the highest weight of, which cost the most to ship and is one of the two mineral sources I was most concerned about becoming dependent on due to the high demands on it. Solomon suggests, on page 142 of his book, that once the phosphorus level builds to that required for the soil and the OM% is high enough, phosphorus may remain sufficient for a decade or two. Since I’m 62 now, the current phosphorus level might be sufficient for the remainder of the time that Mike and I will live on this land as long as I can keep the OM% at its current level or perhaps increase it a bit. And potassium, the other mineral source of biggest concern, is in excess as well. In 2019 all I need to apply is some gypsum (for sulfur and some calcium), oyster-shell lime (for calcium without magnesium, which is in excess), and borax, manganese, and zinc (all of which are added in tiny amounts).

Except for the cottonseed meal that I also add. And here’s where the post is going to get, well, earthy.

To step back, the reason I’m adding cottonseed meal is to supply an organic form of nitrogen. Actually, it’s not the cottonseed meal itself that the plants use. The meal needs to be converted into a form of nitrogen, nitrate (a negatively charged ion containing nitrogen and oxygen), that plants can use. Our friends the soil microherd (everything from bacteria up through earthworms) do this by eating and excreting the meal and/or each other, gradually releasing nitrate into the soil where it can enter plant roots. The microherd becomes more active as the weather becomes hotter, which follows the plant growth cycle. And yes, to answer the question that you’re no doubt asking yourself right now, the organic matter I add does the same thing, but it’s a much less potent source of nitrogen than the cottonseed meal is. Manures are also a less potent source of nitrogen; if added at a high enough amount to equal cottonseed meal, they can bring too much salt into the soil.

But there is a homegrown source of nitrogen at hand: urine.

Human urine is one of the most potent sources of nitrogen available to the gardener. According to Carol Steinfeld’s book Liquid Gold: The Lore and Logic of UsingUrine to Grow Plants, the N-P-K value of adult urine is, on average, 11-1-2.5. That’s more nitrogen than everything but bloodmeal and monoammonium phosphate on Solomon’s revised Acid Soil Worksheet. It’s almost double the amount of nitrogen in seed meal. There’s a catch: the N-P-K value of urine is calculated for the solids in urine only, not for liquid urine, which is 95% water, meaning I have to apply a lot more urine in both volume and weight than the cottonseed meal. On the other hand, there’s no shipping charge to making use of my urine, just the schlepping charge of getting it out to my garden.

Steinfeld tells us that an average adult excretes 11 grams of nitrogen in each day’s urine. For a 180 day growing season, that amounts to 4.4 pounds of nitrogen. The cottonseed meal I add to a single 100 square foot bed provides 0.36 pounds of nitrogen to the bed during the same 180 day growing season. This suggests that if I collect and apply all the urine I produce to the garden, I can supply nitrogen to ten 100 square foot beds – which is one more than the nine beds that need it. In theory, at least, I can supply enough nitrogen through my own urine to replace all of the cottonseed meal for the entire garden.

Before going on, let’s consider arguments against applying urine. The first issue is a health issue: urine can carry pathogens which can cause serious diseases. Steinfeld writes that urine, unlike feces, transmits only a few significant diseases: leptospirosis, schistosoma, and salmonella. Of these, she says that the first two are rarely encountered outside of an aquatic tropical environment, and the last is typically inactivated shortly after excretion. I live in the temperate zone,  unfavorable for the first two. The third seems like more of a threat considering that we hear of outbreaks of salmonella from contaminated foods from time to time. But those outbreaks usually arise from high densities of livestock, which is not the case in my yard. Furthermore, I have none of these diseases, so I cannot pass them in my urine – though if I do get sick, I will refrain from using urine until I’m well again.

Joseph Jenkins, the author of The Humanure Handbook, adds yersiniosis to the list of diseases that might be caused by pathogens in urine, and he includes E. coli in the list of pathogens potentially carried by urine, another pathogen that makes for occasional news headlines and is often linked to issues with high-density livestock facilities. But, again, I have neither of these, and I’ll use my urine only when I’m well.

Urine that enters surface waters will fertilize aquatic plants. If too much urine enters the water, aquatic plants may overgrow their habitat and die. The decomposition process requires oxygen; if too many aquatic plants die too fast, decomposition can deplete the water of oxygen that other organisms need, causing die-offs. Steinfeld writes that excess nitrogen in waters, from urine or from other nitrogen sources, can lead to blue baby syndrome. For these reasons, it is imperative to apply urine only to land well above the water table and far enough from any surface waters that it cannot enter them, and only on a scale in which the urine can be absorbed and used before it reaches any underground sources of water. Also, the soil should be well aerated, so that it supports the aerobic microbes that oxidize the urine to nitrates and make it available to plants. Since my garden is near the top of a hill and on very deep and well-drained soil, and because I garden organically to support the microherd, I can fulfill these conditions. If there is any standing water in the garden, I won’t apply urine until the soil can again absorb what I apply. This is very rare, occurring only when heavy rains have saturated the soil and then more rains fall.

To use the urine, I will first collect a day’s worth of it in a 2 quart/2 liter bucket. Then I’ll dilute it with water in a sprinkling can, apply that dilution to a garden bed, and follow it with another full can of water, to ensure that all the urine is absorbed into the soil and rinsed off any part of the plant that we will eat. Moreover, I’ll arrange it so at least one day passes before I harvest anything from a bed I treat in this way.

But the scientist in me would like to do a small-scale test of the power of pee before committing to a half year of liquid gold prospecting. And just as it did in 2018, the three beds of corn, dent corn this year, offer me the opportunity for just such a test. For this test I’ll apply the 2019 re-mineralization mix including cottonseed meal, humates, and kelp meal (for micronutrients) to one of the beds of corn as a control. For another bed, I’ll replace the cottonseed meal with urine, collecting the urine once every 10 days and adding it to the bed as above, and add all the rest of the components in the 2019 re-mineralization. For the third bed, I’ll leave out the kelp meal but use cottonseed meal and all the other components of the 2019 re-mineralization mix. It’s a long way from here to the seacoast, so it would be best if I can avoid using kelp meal in the garden. I’ll observe all the plants in all three beds, noting any differences from bed to bed, and keep the harvest from each bed separate so I can calculate that bed’s yield and compare to that of the other beds and to previous years. And so Year 2 of my multi-year project to reduce the need for added minerals and to source as many of any still needed as possible locally is underway.


  1. Hi Claire,

    Again I am in awe of your analysis. And pee ends up in the soils here via way of the worm farm. Incidentally, the 10 metre leach trench (one of four) from the worm farm stays green all year long - regardless of how hot and dry it has been. The wildlife (especially the wombats) crop the plants that grow there every single day of the year, and I note they have healthy coats with no signs of mange, which is prevalent in some wombat populations in other parts of the mountain range.

    Thanks for the shout out and explanation about the organic matter test results. The quality of your soils are beyond what is considered the "norm" down here, and Phosphorus is in short supply across most of the continent. But to have an excess supply is something that I would only ever dream about and is a serious credit to your ongoing work.

    I hadn't known that about manures and salts. Hmm. My manures end up in the soil too via way of the worm farm. Late this afternoon, I just finished weeding, feeding and pruning the last of the 300 or so fruit trees. It is a big job and this year for feed I've mixed a 50/50 mix of composted woody mulch and a very fine compost (which is a new product that I'm trialling). The mix smells nice. I made the mistake of adding too much mushroom compost (horse manure and bedding straw) to some of the vegetable beds a month or two back and the germination rate plummeted. I suspect the mushroom compost had not been composted as long as I was used to and it may have contained vermicides and perhaps antibiotics. It was not good, but a top dressing of the fine compost (previously mentioned) sorted the problem out.

    Interestingly too, wood ash has been used for dozens of millennia down here as a soil fertilising agent. I assume you have a wood heater or do you burn your garden cuttings? I get wood ash from both sources (the forest drops an unrelenting supply of woody material) and usually spread it through the orchard rather than on the vegetable or berry beds. I assume you apply the wood ash prior to planting, and then mix it in with the top soil? I generally apply it to the soil surface and over the plants and let the rain wash it into the soils. It makes a notable difference to their growth, and where ever there have been fires, the growing season produces 'Fairy Rings' of lush growth.

    Thank you for the explanation of the difference between precision and accuracy and that difference had not occurred to me.

    I'll be very curious to hear how your experiment and control garden beds work out.

    Absolutely fascinating and insightful stuff!



    1. Hi Chris! Thanks for your thoughtful reply.

      We do have a wood heater; one of my previous posts mentions it. This past winter for a combination of reasons we didn't use it much, so I didn't have any spare wood ashes for this year's garden. Just as well I didn't need them this year.

      Mike is in the process of chopping and splitting the scrounged wood that we've accumulated in the past year or so, so we have wood to burn next winter. He's borrowed an electric chainsaw from a friend to cut the wood to length, but he splits the cut lengths with a hand-pumped hydraulic splitter or a maul.

      We burn some of the woody waste from the yard, but I forget to collect it before it gets rained on. Nothing grows on it for a year or two, but eventually some of the perennial "weeds" start to grow on it, including violets and their beautiful blue-purple spring flowers. Violets have both edible flowers and edible leaves; I usually pick some for salads.

      I prefer to apply the wood ashes along with the compost and anything else needed to re-mineralize the soil to the top of a garden bed and then cultivate it in a few inches deep and smooth out the bed before I plant into it. Your method of applying it and top and letting the rain carry the minerals in also works, as you've found.