In this second post about Balancing Soil Minerals I want to look at the soil report, and what it means. Any attempt at balancing the minerals in your soil has to start with what is already present and available to plants, and ultimately to you. In the my last post on balancing soil minerals I posted the results of my soil test. Here is that report again.
Balancing Soil Minerals – Reading the Test Report
As I described in my last post there are two sets of test results here. Both are fairly similar, so I’ll concentrate on the second column of results, the one for my new polytunnel site.
Soil Depth in Inches
The standard depth for analysis is 6 inches, which roughly equates to plough furrow depth. This gives some interesting points of reference. The estimated weight of an acre of soil, 6 inches deep, is 2 million pounds. Therefore when we look at the trace elements at the bottom of the report, which are recorded in parts per million, a level of 1 roughly equates to two pounds of mineral per acre in the top six inches of soil. It makes calculations a bit easier to do later. The pounds per acre figure also roughly equates to grams on a 100 sq ft bed. 100 Sq ft (i.e 5 x 20 feet, or 4 x 25 feet) is the standard size of a bed used in the Grow Biointensive method promoted by John Jeavons. I don’t know if that’s a coincidence, but laying out a smaller growing area into beds of this size makes calculating how much of a mineral to add much easier.
Total Exchange Capacity
I’ll try to explain this as simply as I can. It isn’t neccessary to understand all of it, but it might help. Minerals are held in the soil in a number of ways including as part of the parent material (rock/clay), in the bodies of living or dead plants and animals, dissolved in the soil solution, and loosely held (adsorbed) onto the surface of clay and humus. Minerals/chemicals normally have a positive and a negative charge. An example of this is table salt, which comprises of a positively charged bit, Sodium, and a negatively charged bit, Chlorine. When dissolved in the soil water they can separate. Clay particles in the soil can hold onto the positively charged bits (known as Cations). These are loosely held onto the surface of the particle. Many of the major plant nutrients are positively charged Cations (Calcium, Potassium, Magnesium) etc. Plants can take these from the clay, and exchange them with Hydrogen. Humus has the same ability to hold on to cations, but also anions, the negatively charged bits. The main point of all of this is that the ability of any soil to hold onto minerals, and not have them washed out by rain, depends on the amount of clay and humus that it has. The more clay and humus that the soil contains the greater the potential it has for holding minerals.The terminology for this is Cation Exchange Capacity (CEC). So a soil with very little clay would have a low capacity for holding these cations, whereas a heavy clay soil, with a lot of humus would have a high ability to hold them. To give an analogy, if you think of it as the number of car parking spaces available, with the cations as cars. A sandy soil would be like a high street shop. You’d be lucky to find a single space outside of it, so no cars. A pub would have a small car park, an out of town supermarket would have hundreds of spaces, and then you get to the multi story. This would be the equivalent of a clay soil with lots of humus. Thousands of spaces for cars (minerals) to come and go.
The row titled Total Exchange Capacity is the reported level in my soil. 1 would be a sandy soil with little humus, 10 is the level for a ‘heavy’ soil. My Exchange capacity is over 22. A really large capacity, with masses of potential. If you think of these readings like a plate of food. A soil with a Capacity of one is capable of holding one plate of food (minerals), a heavy soil 10 plates, mine is 22 plates. Not bad.
The next row is soil ph. This is a measurement of the relative acidity/alkalinity of the soil. 1 is very acid, 14 very alkaline, 7 is neutral. My soil is 6.9, which is good. As different minerals are made unavailable at different levels of acidity, a level of 6.4 may be optimum, but I’m pretty happy. It’s worth noting that soils can be strongly alkaline, but low on Calcium, which is possibly the most important plant mineral. Other Cations like Magnesium and Potassium can raise the alkalinity of the soil. This is another reason why you need a proper soil test report before balncing soil minerals, rather than just a ph test.
Organic Matter Content
This may be a bit controversial for mulch gardeners, but you don’t need massive levels of organic matter in the soil. In my climate, a level of 7% is ideal. To put this into perspective, a six inch depth of soil is 150 mm, so 7% of that is 10.5 mm, or just under half an inch. Even if all of your organic matter were to ‘walk away’ each year, you only need to add half an inch of compost a year to maintain a reasonable level of organic matter. This is why I think that mulch/no dig gardening is so wasteful. In order to suppress weeds gardeners are applying 2 inches or more of compost. No need for this level, and it may lead to excessive levels of Nitrates in your food, and an increase in pests and diseases on your plants.
My organic matter % is 9.2 on this site, so I don’t need to add anything. I probably will as I need to do quite a bit of digging to weed, level the site, and form beds wihin the polytunnel. As the increase in oxygen will lead to more organic being ‘consumed’ by soil microbes, I’ll add some to compensate.
Anions are the negatively charged bits of soil minerals. The two measured here are both important for plant growth. I’ll go through what the levels mean in my next post.
Cations are recorded in two places. The first set concentrates on those present in the largest quantities/relevant importance. These are split into Desired Value, Value Found, and Deficit. The value found is the most important part, as there is a range of opinion as to what is the desired level of any mineral. These ranges are not all of the ones that I’ll be working towards. Again, I’ll cover how I’ll work that out in my next post.
Base Saturation Percentage
This line takes the values found in the previous set of readings and puts them into a percentage. This is important as when we are looking to balance soil minerals, we want to create an optimum ratio of minerals, as well as an optimum overall level or quantity. A quick look at this set of readings shows that my soil is low in Magnesium and Sodium. The Magnesium level should probably be raised a bit, but my reading suggests that Magnesium binds the soil more tightly, which wouldn’t be great on my clay.
The most important trace elements are shown here. As a recap on the measurement stuff that I wrote earlier, if you look at the level for Managnese, it is recorded in parts per million (p.p.m). The level is 18 for this column, so that means that I have 18 ppm, which is the equivalent of 36 pounds of Manganese in the top 6 inches of each acre, or 36 grams per 100 sq foot bed.
The Soil Test Report Final Thoughts
As well as being the way to start balancing soil minerals here, it also provides a baseline figure to evaluate how effective my mineral balancing program is going to be. That’s pretty important as I’m likely to try and include stuff like Actively Aerated Compost Tea in my tailor made solution. Subsequent Soil Test Reports will allow me to compare the before and after readings. One of my criticisms of permaculture is that there is a lot of talk about how deep rooted perennial plants and trees will bring minerals up from the subsoil for use by other plants, but I’ve not seen any ‘proof’ that it works. Soil testing would be one way to show whether it works, or is just another good idea in theory, but not in practise. Hopefully somebody will take up the challenge and create a proper trial.
Look Forward to Soil Mineral Balancing (Pt3)
The final post in this sequence will look at how I calculate what the ‘right’ levels of minerals are for my soil, and what I’m going to do to bring those minerals into balance. I hope that the post wasn’t too hard going, and I’m happy to reply to comments if something needs to be explained better.