Saturday, 21 March 2015

More Geeky Soil Stuff: My Soil Analysis Results!

As I've mentioned before, I resolved to get a proper lab analysis done of my soil this year, to identify any deficiencies and attempt to bring it to - or close to - the ideal mineral balance described in Steve Solomon's book The Intelligent Gardener: Growing Nutrient-Dense Food.

I took not just one but four soil samples. The allotment is perhaps the most important, but I also wanted to test the raised bed soil which gave poor growth last year and the soil of my garden veg bed, which did very well last year but which I suspected of a boron deficiency due to some hollow courgettes. Finally, I thought I might as well send off a sample of FoodSmiles soil as well, mostly out of curiosity.

To take a sample, you're supposed to cut regular-sized-and-shaped slices of soil, six inches deep and an inch or so across each way, from several points on the plot, to get a mix representative of the whole area. Professional soil-testers have a tube-shaped device they use to get a perfect even core of soil each time. I was just working with a spade and trowel... but I think I did good enough for a first-timer! For each sample, I put all the soil 'slices' in a bucket or bowl and let them air-dry near a radiator for a few days, then broke up any big lumps, picked out stones and twigs, mixed it all up really well and bagged up two cups of the soil for sending off.

Sending it off was a faff as well! I could only find one lab in the UK that would do the soil test described in Solomon's book - what's called a Mehlich 3 extraction along with pH, organic matter, total cation exchange capacity and base saturation - and it cost three times more than what it costs in Solomon's recommended lab in the US... So I sent my samples off to the US! I felt a bit guilty about the air miles, but I couldn't have afforded to do it any other way. Samples had to be labelled and double bagged, sealed very securely in a strong envelope with all the seams taped up, and an import permit and special label attached to the outside of the package to declare what was inside and show that I was allowed to send foreign soil to this address!

It must have made it through, because two weeks later, my results came by email, and you can see the full report here if you like. But most of the numbers are fairly meaningless until you take a deeper look; everything swings on the TCEC, which indicates the amount of nutrition a soil can hold, so XX lbs per acre of a nutrient might be way too much for a soil with a low TCEC, but not nearly enough for a soil with a high TCEC. Solomon provides worksheets to help crunch the numbers and figure out exactly what your soil has and what it needs.

A Note on Organics...

The soil amendment 'prescriptions' mentioned here, involving measured amounts of all sorts of mineral compounds, might seem to go against the grain for someone who is concerned with gardening organically and sustainably, and have certainly given me cause to think a bit about 'organic' versus 'chemical' fertilisers. But there are plenty of mineral soil amendments approved for organic use where a need is identified, and just because it's not organic compost or manure, doesn't mean a substance is necessarily harmful in any way. Adding pure mineral supplements carefully chosen according to measured needs is a very far cry from routinely applying synthetic, high-nitrate, petroleum-based mixes (many of which contain heavy metals) at maximum application rates, as is much too often done in modern agriculture; these kinds of chemical fertilisers feed the plants but diminish soil fertility. In his book Solomon spends quite a lot of time discussing the sustainability and impacts of different amendments, and before buying I will check out each one and decide individually whether I want to use it - but even though Solomon's method isn't 100% 'organic' all the time, these amendments are all designed to help the soil and its microlife, which is of key importance in this soil health business!


The allotment soil has a pH of 7.1 - just above neutral which is what most veg-growers will aim for, with a range of 6.0-7.5 commonly recommended. 6.5 might be nearer to ideal, but since different veggies like slightly different conditions it's certainly good enough.

It contains 7.6% organic matter, which will supply more than enough of nitrogen through the growing season as it is digested by soil organisms. 7% is plenty - 4% is about the minimum it needs and 10% is more than a soil can efficiently use. More good news! I'm actually surprised it's this high, since I've never been good at adding organic stuff to the soil. I compost allotment waste and return it to the soil as and when it's ready, but only once in eight years have I bought in manure and compost to spread, and I know some gardeners do it religiously every year.

The TCEC is 13.1, which indicates a good level of nutrient-holding clay and humus in our soil. TCECs in very rich clay soils might be as high as 40 or more, but for the purposes of the analysis it only matters whether our TCEC is below or above 10, and above is a good result.

The main purpose of this exercise is to make sure the major cations - calcium, magnesium, potassium and sodium - are in balance, and a good supply of sulphur, phosphorus and trace nutrients are available. 68% of the TCEC should be calcium, 12% should be magnesium, 3 or 4% percent potassium and 1 or 2% sodium. The allotment soil has an excess of calcium and potassium and a deficiency of magnesium and sodium - a balance of 81%, 10%, 4.5% and 0.3%, so I'll take some action to adjust this balance. It also has very significant sulphur deficiency. Phosphorus is very slightly low but not enough to worry about.

Trace minerals measured are boron, iron, manganese, copper and zinc. In the allotment soil, iron, copper and zinc are plentiful (with more than twice the iron needed and more than four times the zinc!) but boron and manganese are very low, so I'll aim to add more of these.

The prescription to amend this soil is to add, per 100 square feet, 225g manganese sulphate, 120g sea salt, 110g agricultural sulphur, 60g magnesium oxide and 20g borax.

Garden soil

It was extremely difficult to get an accurately representative sample of the garden veg bed as it comprises two very different layers: a few inches of old bagged compost and used growbags on top, and a hard clay just underneath at a depth that varies across the bed... So my confidence in the accuracy of these results is lacking slightly, but I was interested to see them anyway.

The pH is 6.8 - very slightly acid and pretty perfect for growing vegetables. The organic matter was very high - 38% - but perhaps not surprising given the thick layer of old compost. The TCEC is high too at 17.9, probably thanks to the heavy clay.

The cation balance is heavily weighted to calcium and magnesium, with potassium slightly low and sodium very low. Funny how we're told salt is terrible for soil - actually a small amount is very important, but it looks like most of our soils don't even have that much. Sulphur and phosphorus are both significantly low, as are boron, manganese, copper and zinc. The boron deficiency I suspected is shown, but actually it's only very slightly lower than any of the other soils I had tested.

The prescription for this soil is 760g monoammonium phosphate, 414g sea salt, 212g manganese sulphate, 110g sulphur, 40g zinc sulphate, 28g copper sulphate and 20g borax per 100 square feet. I think I might take a cautious approach to making the amendments, considering I'm not confident in the consistency of my soil sample, and just aim to fix the worst of it this time round.

Raised Beds

Now this one was interesting. As I described at length a few posts ago, this soil, bought in to fill my new raised beds about this time last year, gave extremely poor growth last season. I suspected an overabundance of potassium and perhaps a low pH, and added calcium and nitrogen to try to counter those things and help plant growth but it didn't really help.

In fact, this is a calcareous soil with an overabundance of free lime and a high pH which makes trace nutrients unavailable to plants. If I pour vinegar onto the soil, it actually fizzes! This means the test I ordered isn't accurate, as a Mehlich 3 extraction can't test accurately where high lime is present - I should have done the vinegar test first and ordered a different soil test... Doh!

The soil shows a pH of 7.7 but this is likely to be artificially low. There does indeed appear to be a high level of potassium, but the level of lime and high pH is clearly the bigger problem and, save ordering a new soil test and a whole load more mucking about, I think the easiest way forward is to address that first by applying sulphur, which is turned into sulphuric acid by soil bacteria and neutralises the surplus calcium carbonate, turning it into highly soluble gypsum which will then easily leach from the soil. This takes time; a year or more. So in the meantime it will be important to only grow what is happy in these beds; so far garlic, brassicas, lettuces, carrots and beets; and to feed plants with a foliar spray to ensure they get the trace elements they need.

I have real trouble understanding how a soil mix that was obviously so high in manure and woody waste can be so very high in lime, but I guess it looks like my lime application last year may have only made things worse - doh again!


The FoodSmiles soil has a pH of 7.3 - right at the high end of a good range - a TCEC of 15.2, and 7.6% organic matter. Its cation balance is not too far from the target, with small excesses of calcium, magnesium and potassium but a deficiency of sodium. Phosphorus and sulphur are both very low. Like the allotment, it has very high levels of iron and zinc, but it is low in boron, copper and manganese.

Amending this soil with mineral applications would be a whole different ball game, since it is Soil Association certified land and we'd have to show a clear need for every amendment and have it approved in advance - and we've got more than enough to do at the moment anyway! But it hasn't had a lot of organic matter added in recent years and I expect applying manure would bring a beneficial pH change. IF I was going to tackle anything else, the priority would be to amend the sulphur and phosphorus levels, but at the moment the soil is growing crops very well indeed.

Some Thoughts

As I write this, I'm wondering about those high levels of iron and zinc... Solomon warns that tiny flakes of rust or galvanizing from the tool used to take the samples can inflate these levels. I didn't have a steel tool I could do it with but I did clean my trowel thoroughly and make sure nothing looked like it was going to come off it... If I do this again in future I'll be sure to be prepared and use stainless steel instead just in case!

All the soils tested require, according to this method, a significant application of sea salt, which feels a bit wrong but I'm doing my best to trust the method and go with it!

I have high hopes that if I correct the sulphur level on the allotment, I might be able to grow onions and garlic there again; sulphur protects against fungus so perhaps the lack of it allows the white rot to thrive.

My next task, then, is to read up on and source the various minerals needed to amend the allotment soil and garden bed, and sulphur for the raised beds. It's quite a bit of work, all in all, but it's absolutely fascinating and I'm really looking forward to seeing the results!


Mark Willis said...

I think you may have just opened-up "Pandora's Box"! I'm afraid all that science would baffle me.

Nome said...

Aww, it may not be for everyone, sure, but it's been SO interesting learning more about what goes on in soil, and the links between soil and the quality of our food! :-)

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