Friday, April 8, 2011

My Soil Test Results are In

That was certainly worth the $25. Now I have some real data, and there are some surprises, mainly about how GREAT my soil is! I am never again going to tolerate people saying the soil around here is lousy (like that guy at Lowes the other day trying to sell me a bunch of stuff to fix it). I was careful to send in a sample of soil that I hadn't added much of anything to yet, just to see what I have to start with here, and it turns out that my soil is awesome!

The pH
I suspected my pH would be alkaline because of the limestone, but it only came up "slightly alkaline" at 7.6. (I was thinking it would be something more like 8.) That's not too bad, really. Most plants would do fine on that except for certain ones that really want acid soil, like potatoes and blueberries.

The soil nutrients plants need the most of are nitrogen (N), phophorus (P), and potassium (K). Those are the three numbers you see on all fertilizer packages, the percents of each of those elements in the fertilizer, in that order. For each of the nutrients, A&M gives me my level in ppm, and a "critical level", which it says is "the point at which no additional nutrient is recommended". What I got was:

Nitrogen: 5 ppm
Phosphorus: 78 ppm (CL = 50 ppm)
Potassium: 338 ppm (CL = 175 ppm)

That means the only nutrient I'm short on is N. My P and K are well above the level needed for any crops I would grow. And here I was going easy on the N if I added any fertilizer, because I'm always hearing that adding too much N is bad for fruit and root crops, because it makes them just grow leaves. In reality, that all depends on what your soil already has, and actually I should be adding fertilizers that have high N numbers and the P and K might as well be zero. Adding more P and K is just going to waste. That knowledge alone is worth getting the soil test, because otherwise I would be wasting money on fertilizers I don't need.

Looking at A&M's website on how much N different fruit and vegetable crops need, it ranges from 0 ppm for beans (which can get nitrogen from the air) to 100 ppm for potatoes. I'm also surprised that potatoes need so much nitrogen, because again I was told too much makes them grow all leaves and no tubers. On the other hand, Carol Deppe says potatoes are relatively high in protein, at least compared to other starchy vegetables, and proteins are made of amino acids, which contain nitrogen. (Legumes are high in protein because they can get nitrogen from the air, but any other plants need it from the soil.)

I think I've solved the mystery of my lousy potato yield last year. Turns out they needed a lot more nitrogen! I didn't bother with potatoes this year, but now I know what to do for next year to get a good potato crop.

Other Elements
Just to be sure, I ordered the full soil test that tested ALL the nutrients that matter to plants, not just NPK. They were all above the CL, except for sulfur which was only slightly low at 11 ppm, while the CL is 13 ppm. A&M said, "Available sulfur may be found deeper in soil profile, thus limiting any response to added sulfur." I guess that means it's not low enough to worry about.

As I suspected, my calcium was high, but I had no idea it would be THAT high, at 9,341 ppm, while the CL is 180 ppm. According to commenters on GardenWeb, that could be because the soil sample I sent in had little bits of limestone in it, which were dissolved with A&M's extraction methods and led to that really high number. Since in nature limestone dissolves much more slowly than that, the actual Ca available to plants is probably not quite so high, though I'm sure it's still plentiful.

Organic Matter
My organic matter is 6.34%, which seemed low, until I started Googling on how much organic matter soil is supposed to have, since A&M didn't mention it. In a natural setting at least, organic matter ranges from 1% in a forest soil to >5% in a grassland. Which means that my organic matter is actually on the high side! That's good. And again, this is without me adding any more. Of course, like any good gardener, I'm always adding more organic matter in the form of compost. As I understand it, you can't really have too much, and it's always being used up and has to be replenished anyway. But it's good to know that I'm starting with a healthy amount of OM to begin with.

Soil Improvement Goals
Now with good, hard data, instead of guesses and people trying to sell me stuff, I can concentrate any improvements on what I really need, which seems to be little more than nitrogen and acid. Luckily, there are a lot of good organic soil additives out there that are high in nitrogen and may also be acidic. Planting lots of legumes is also supposed to help, since they fix nitrogen in their roots, and then when they die and decay, they release that nitrogen into the soil. Though I'm not sure how much N legumes just use for themselves, and how much they really put into the soil for other plants to use.

Daniel wonders if having low nitrogen is typical for soils of this area. The native plant community does have a lot of legumes in it, from bluebonnets to rattlebush to mesquite trees. That may be because the soil is just naturally low in nitrogen.

Well, that shouldn't be too hard to fix, and it's good that nitrogen's the only real problem I have to deal with. It will be interesting to get a second soil test in a year or two to see if I've made any difference. That time I will get one of their simpler tests, since my other minerals are fine, and probably won't change much from now on.


  1. This is really interesting stuff.

    AS far as the nitrogen goes, I haven't read anything convincing that says that you can remediate that completely with just legumes and no outside inputs. I think you might be able to use animals (manure) to create a closed system, but even in traditional farming systems nitrogen was brought in one way or another (burying fish, gathering guano or using animals in rotation, if they didn't just slash and burn and move on once the nitrogen is depleted.) I know I've read (somewhere... I need to find references) if you ARE hoping to use the nitrogen input from green manures, it's best to turn them under before they go to seed/produce fruit (so you can't have maximum nitrogen from the favas and eat them too).

  2. That's what I was afraid of. I'll probably go ahead and pick up some bat guano or bloodmeal some time soon.

  3. the nitrogen question is one that attracts a lot of attention.
    I have a relatively large number of animals (6 chickens, two full grown goats) producing manure for my ~700sf garden. In addition, I periodically purchase cottonseed meal as an N booster. Consequently, I have had *ZERO* problems associated with too little nitrogen. If anything, I probably have too much. Of course, I have not done a soil test so i don't really know! I need to do one, like you did.
    Glad to know it was worthwhile and informative.

    Cottonseed meal is a good N source, as is alfalfa meal/pellets. The cottonseed will acidify the soil over time, but has a LOT of N. Alfalfa has a lower N content, but has plant hormones in it that encourage growth. You can get either in 50 lbs sacks at the feedstore for ~$10. At the garden center, you pay exponentially more. I've used both and been quite happy. But, now I have lots of compost so I am not really buying the other stuff anymore.