Soil testing

You can have too much compost

We purchased Henbogle House in the winter of 2000-2001.  That first summer, we had a vegetable garden but most of our time that summer was directed at preparing for our backyard wedding.  (In restrospect, I should have ordered the soil test just to check the lead levels — many soils in the northeast have high lead levels which can be very dangerous, especially to children.)  In 2002, happily married, I was able to concentrate on gardening, and decided that year to get a soil test from the U Maine Soil Testing Service.   I haven’t done one since, bad, I know, but I had pretty good soil and was not really worried about it.  Oh, and, I’m cheap, and didn’t want to spend the $15 –a bad decision as it is cheap in comparison to a wasted year of gardening.  I decided to get a test done this year as I’m enrolled in the U Maine Cooperative Extension Service’s amazing Master Gardener Program, (which, so far, by the way, has been great) and thought I could use the test results to learn more about my soil.

2002 results

2010 results

Over the last 8 years, my amendments have primarily been a couple of loads of composted horse manure in 2004 and 2008, annual use of Fertrell Organic fertilizer in the at planting time and as side dressing for some crops, some gypsum based soil lightener, and as much compost as I can make.  The results? 

The soil pH has become slightly more acidic, but is still within the optimal range.  The phosphorus has skyrocketed, over 7 times the optimal range.  In 2002 the potassium level was high, and the test results recommended discontinuing use of wood ash to reduce the potassium level.  In 2010 the level is 3.7%, just within the optimal range of 3.5-5%.  The magnesium level was low in 2002 and is now slightly lower at 8.2%, below the optimal range of 10-20%.  The calcium level remains high at 88.2%, slightly higher than the 2002 results, and above the optimum range of 60-80%.  All that compost has put me at 9.1%, over the optimal range of 5-8%.  Ooops, maybe there can be too much of a good thing.

This fall I will lime the garden with high magnesium dolomitic lime, hoping to address the pH and the depleted magnesium at the same time.  But what about the phosphorus and the calcium?  What contributed to their greatly increased levels?  Is that a result of the compost, the fertilizer, the soil conditioner, or the 2 loads of composted horse manure?  Gypsum is also known as calcium sulfate, so, I’m wondering if the addition of the gypsum based soil lightener resulted in the increased calcium level.  Perhaps that was an unwise addition as I worked on improving the soil texture; it did seem to improve the drainage, though.  For ever action there is an equal and opposite reaction… wait, that’s physics!

I hope I can learn something about this in my Master Gardener classes.  The soils class I will be attending in early May and I am really looking forward to it.  You can bet I’ll be bringing my soil test results along to ask questions.


5 Responses to “Soil testing”

  1. Lee Says:

    I’ve been researching this exact subject when my wife forwarded me your blog post to read. Here in the Northwest, our local gardening guru is Steve Solomon, and his book “Growing Vegetables West of the Cascades” recommends keeping your organic matter levels between 4 and 5 percent. He says you can do this by adding only 1/4″ of compost per year. Our soils are already high in potassium, and too much compost (especially animal manure based compost) would raise the levels even more. Supposedly, too much phosphorous (such as your problem here) tends to run off but not affect plants, while too much potassium damages their ability to make proteins and vitamins. This leads to vegetables that still look fine but may be severely depleted nutritionally.

    Anyway, I had wondered if his advice was off, since the mechanism wasn’t well explained and any gardening book from Rodale says the exact opposite: add as much compost as possible. However, a few weeks ago we started taking a course on pasture management and it has focused extensively on soil nutrition first. This has certainly made the mechanisms clearer to me, although I’m afraid to try to explain it as I’ll probably get something wrong, and there’s too much bad advice on the Internet already. :)

    I guess I am most puzzled as to why your phosphorous levels have gone up, and why your potassium levels have fallen. Potassium is the primary element in dried plant stalks, so removal of large quantities of dried plant stalks could cause it to fall. However, you added horse manure, which is 3-1-2 on a dry matter basis, so this adds potassium. If you used straw in your composting, it would add even more. Very puzzling. Similarly, I don’t see anything that would explain the raised phosphorous, although knowing the breakdown for your organic fertilizer would be important. The calcium levels can certainly be traced to adding gypsum. I would expect sulfur to be higher for the same reason, although it wasn’t checked in the first test. Compost lowers pH because of microbial activity, so that probably explains the pH change too. And of course, if either of the soil tests didn’t get a good cross section of the soil (15 to 20 samples mixed in a bucket) then they could represent a localized soil state and not a true average.

    If you find explanations for your soil behavior in your Master Gardener classes please post them. I’d be very interested in knowing as well. So far, this article has been the best thing I’ve found on the subject.

    • Ali Says:

      Hi Lee,
      Thanks for this informative comment. Thinking more about this, I suspect the decline in potassium is because we have never added wood ash to the garden, where the previous owners did this regularly. If you look at the recommendations from the first test, it says “excessive potassium, if using wood ash, discontinue.” We have made compost from yard waste, fall leaves, kitchen waste, and hen bedding. The bedding was made up of wood shavings until this past year when we switched to straw, I wonder if that will make a difference? Although we’ve always used straw of hay to mulch in the garden.

      The Fertrell organic fertilizer was Fertrell 3-2-3, so no excessive phosphorus there…. I wish I could remember the name of the brand of soil conditioner! I bought it at a very reputable local nursery on their recommendation.

      In collecting the sample I was pretty careful to get samples from all over the garden, including in my hoophouse. I had just made raised beds in 1/3 of it last year, so the soil from that area was also pretty well mixed.

      It is all so interesting… who knew? If only my college chemistry classes had been this good!

      I’ll be sure to read thoroughly the link and your blog posts on your pasture management class. Thanks again for this comment!

  2. Callie Says:

    I’m too chicken to order in or buy any soil or compost. We had our soil tested and it was ok. So, I have been using compost from our chickens and leaf compost. This year I am making planter beds using the hugelkultur method. Much better than burning everything like our neighbors have been doing. Cough, cough! Hope this all works out ok.

    • Ali Says:

      Callie, why do you say you are too chicken to buy amendments? Are you worried about contaminants? If that is the case, I think you can still safely buy, just avoid compost made with municipal waste — go directly to an organic producer if you can. Although as I learned, you can have too much compost!

      I think the hugelkultur method sounds like a good way to use yard waste. I’m planning on a new bed to grow green for the hens, maybe I’ll try this for the new bed. We had an old rotten tree come down last fall in addition to our giant pile of prunings….

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