Okay, this post is going to be long, and I'm sorry about that, but I promise it will be absolutely worth it! During the winter I had the pleasure of reading two fascinating and wonderful books about soil nutrition and its impact on our health, and I've got to tell you why my approach to growing food will never be the same again.
The first book, which I just happened upon while idly browsing for free Kindle books one day, was Beyond Organic: Growing for Maximum Nutrition by Dr Jana Bogs. It kicks off with an analysis showing how vastly different the levels of vitamins and minerals could be in vegetables grown in different soils across the US: "not by just 10 or 20 percent, or even a two-fold difference..." but "...as great as 1,938ppm (parts per million) iron in some tomatoes as opposed to only 1ppm in other tomatoes of the same variety. This is nearly a 2000-fold difference!" Not rocket science, is it, to work out that more nutritious soils would grow more nutritious veggies? But I'd always assumed that a carrot was a carrot was a carrot, and any variations would be so small as to be irrelevant.
It's funny the way we often just go on doing things the way we were taught, without really thinking about them. For years I've believed it when I was told that all my soil needed was as much compost as I could make, plus a bit of horse manure or fish, blood and bone for a boost, and some lime every few years to keep the pH level where it should be.
Nutrition levels in our food have dropped off by around 75% in the last hundred years or so. Concerns about depleted soils and nutrition deficiency leading to poor health in Americans were first brought to the US Senate way back in 1936. Between 1948 and 1991, Australia's 'Commonwealth Scientific and Industrial Research Organization' recorded an 80% drop in vitamin C in apples, an 82% drop in magnesium in broccoli, a 75% drop in iron in potatoes and an 89% drop in calcium in potatoes, and a similar American study showed that roughly half the vitamins in vegetables had significantly decreased between 1950 and 1999. An analysis on canned peaches over a hundred years old found that they contained more vitamins, even after a hundred years in a can, than today's canned peaches do! We'd need to eat a huge amount of today's fruit and veg to get the same nutrients we would have got from our 'five-a-day' a hundred or a thousand years ago. Is it any wonder we're all fat and sick??
Dr Bogs explains some of the complex interplay between nutrients in plants: minerals taken up by their roots activate enzymes which create the proteins, fats and vitamins we need. Though plants need only around 20 nutrients to survive (some say up to 42), if given a fuller range of nutrients they are capable of generating all the 50+ (some say up to 90) nutrients that we humans need. In other words, a vegetable might have everything it needs to reach maturity looking and tasting good, but that doesn't mean it has reached its full potential by producing all the nutrition it can, and should, contain: plants pass on a certain amount of nutrition from the soil (iron, copper, magnesium, calcium) but they're also little factories producing fatty acids, proteins and vitamins, as long as they have the building blocks to do so. You might have plied your plot with N:P:K fertilisers, but have you ever wondered whether your veggies get enough zinc, cobalt, boron, molybdenum or selenium? I hadn't!
The second book, The Intelligent Gardener: Growing Nutrient Dense Food by Steve Solomon, was even more eye-opening and exciting, not to mention more practical, offering a full hands-on approach to finding out what your soil needs and making it the best it can be, and I have to thank Douglas of @SweetPeaSalads for recommending it to me! This book busts some myths about the usual organic approach to feeding the soil, and offers a really good and easy-to-understand lesson in soil science. Solomon speaks from a broad range of experience from gardening on a variety of soils in several different parts of the world, and tells of healthier, tastier plants, a complete halt to fungal disease, and dramatic improvements in his own health when the soil on which he grows his food is properly nutritionally balanced. He also keeps a free online library - The Soil and Health Library - full of books, papers and studies on holistic agriculture, the connections between soil and health, and more, and he draws heavily on the works in this library to explain his theory and method.
He particularly highlights a study published in 1939 by one Weston Price called Nutrition and Physical Degeneration. Price visited remote communities around the world; tribes and isolated villages that as yet had no access to the mass-produced "foods of civilization", but survived on what they hunted, foraged or grew on a small scale locally with old methods. Despite his subjects having a wide variety of colours and shapes, of diets and of lifestyles, across the board their health was far, far better and when analysed, their foods were found to contain a huge amount more nutrition; for example, the primitive people's diets contained at least ten times more vitamins A and D than the American diet even back in the 1930s! Steve Solomon believes that the ideal human diet "has more to do with the soil food comes from than which foods are chosen"; some of these people ate no animal products while others ate virtually no vegetables, and others subsisted on just fish and oats supplemented with a few leafy greens. You can read Solomon's in-depth review of Price's book here, and I highly recommend it.
It would be impossible to share everything of importance in The Intelligent Gardener here - it's a fabulous and fascinating book - but here are my main take-aways:
Leaching is the factor with the biggest impact on nutrient density. Leaching is what happens when soil becomes saturated with rain, which dissolves a lot of the soil minerals and eventually drains downward carrying those minerals with it so that they end up in the subsoil or offsite altogether. Leaching is worse in climates where evaporation is also low - warmer weather slows leaching - and in climates where there is a lot of heavy rain in a short period of time. It's also a great deal worse where soil is left bare. Some nutrients leach out of soil more easily than others: calcium goes fast, while potassium tends to get left behind. This leads to imbalances.
Loss of soil microlife and organic matter is another major factor that harms nutrient density, since micro-organisms help to release the nutrients from the soil for use by plants, and high-quality humus holds a lot of nutrition in the soil and helps prevent leaching! This isn't generally a big problem for us organic food-gardeners, but industrial farming methods have devastated agricultural soils.
Compost isn't enough! Composting waste from the plot to feed the plot cannot provide all the nutrition the plot needs - unless the plot is already perfectly balanced. In fact, feeding a plot exclusively with compost from the plot will only magnify existing imbalances! There's a sustainability discussion to be had here: feeding your plot with compost alone is a permaculture ideal; a natural recycling process where we make the most of our waste to fulfil another need without importing costly additional materials from elsewhere. But Solomon explains that a soil can virtually sustain itself nutritionally - once it's properly healthy and balanced and cared for.
Remineralisation is the answer, Solomon says. Soils that have not been cared for properly have lost their ability to stay balanced, and their mineral content has been devastated. Solomon's own experiences and research (and Jana Bogs' studies) show that remineralised soil allows crops to reach their full genetic potential, enjoy proper health with much less chance of pest attack and disease, and provide the maximum nutrition for us.
Total Cation Exchange Capacity (TCEC) is a measurement (calculated in a lab) of how much minerals a soil can hold. Clay and humus particles in the soil have a tiny electrical charge that makes positively charged mineral particles in the soil attach to them - a bit like static cling. These positively charged particles are called cations, and include calcium, magnesium, potassium, sodium and others. The clay and humus particles have a set number of 'exchange points' where cations can stick, so a soil with lots of clay and humus has a high TCEC and a lot of these exchange points, and a light sandy soil with very little organic matter has a low TCEC and few exchange points. Some cations cling harder than others: calcium will always take first priority, magnesium second and the others will follow suit if there's space left. Solomon likens the TCEC to the number of shelves in a pantry. Calcium will fill up the shelves first and the others fill up the gaps in order. If the shelves are empty, they just fill up with air - hydrogen actually, from the soil water. As well as from the TCEC, plants can also get nutrition from the soil solution: mineral particles in the soil (or in slow-release fertilisers) breaking down slowly in tiny amounts and dissolving in the soil water. But that soon runs out and needs recharging. The soil solution is like plates on the table. When the plates are empty they can be refilled from the pantry (the TCEC). The bigger the pantry, the longer the meal goes on. Thus while a plant relying on nutrients in the soil solution would require very regular feeding to keep up with its needs, the TCEC buffers plants against nutritional ups and downs: with big reserves, their plates are never empty!
Balance, balance, balance. It's not about having a certain weight or volume of each nutrient in your soil, nor about throwing all the nutrients you can at it; too much of a good thing can lead to overdosing and cause even more problems. Instead, it's all about balance and the interplay between the nutrients.
Not all compost is made equal. As we all know, organic matter is vital for healthy soil. It improves texture, structure, and moisture retention. Most crucially, it feeds soil life, and it holds anions (negatively-charged mineral particles such as phosphorus, sulphur, nitrogen and boron) in the soil. But Solomon warns against adding too much organic matter and recommends only a 1/4" to 1/2" inch layer per year! Too much compost, he says, can overload the soil and its nutrients can end up just going to waste. He also warns that you never know what you're getting when you buy in compost (or organic waste to compost yourself); it may contain contaminants, or you could be importing unbalanced material that adds to your soil's imbalances without you knowing it. Most crucially, he teaches that much of our home-made compost (and shop-bought compost) is only partially decomposed organic matter, but our goal should be truly mature humus...
Humus and clay, clay and humus. These are the two most important components of soil, and they must both be present in abundance to hold a lot of nutrition. Humus is organic matter that has broken down to a point of stability and will change no further. It stays in the soil, instead of breaking down and disappearing. And while clay holds cations in the soil, humus holds both cations and anions. But great humus needs clay to form properly, so it will only form in your compost heap if you add enough clay to the heap as well. Solomon recommends adding around 5% by volume soil, of which 40% should be clay, to your compost heap. If your soil doesn't contain much clay, he recommends finding some, creating a clay slurry with water in a bucket, and sprinkling it generously between layers! Humus might also form in the ground when you add your partially-decomposed compost to a clayey soil - but when added to a light, low-clay soil, little humus will form, if any, and the organic matter will rot away to nothing, and so light sandy soils have great difficulty building up a useful level of organic matter.
Calcium is arguably the most important mineral in a garden soil, not because plants need a lot of it, but because it is critical for good soil structure: not enough calcium but too much of the other cations (especially magnesium) leads to a closed and airless soil structure in which microlife cannot thrive and other nutrients remain unavailable. Calcium is also the mineral most often lacking, as it's so easily leached away. A soil's TCEC should be around 68% full of calcium.
Magnesium takes second priority after calcium, when it comes to sticking to these exchange points in the soil, so it's the magnesium/calcium balance which will most often cause a problem. Magnesium is present in dolomite lime, often used by organic gardeners, so care must be taken to avoid an excessive build-up. Magnesium should fill around 12% of the soil's TCEC.
Potassium always rolls to the front of the pantry shelves, so if there's an abundance of it, plants use it first instead of reaching to the back for the other cations. When plants have lots of potassium they make lots of sugars and starches, but fewer proteins and vitamins and other more important nutrients. An excess of potassium, therefore, can boost yields, but those yields will be lower in nutrition than they should be, and higher in calories. Potassium can accumulate in soils because it doesn't leach away as much as other cations, and because imported composts made with hay, straw and woody wastes are often very high in it (potassium accumulates in the structural parts of plants; the trunks, barks and stems). I guess endlessly throwing N:P:K fertiliser at a plot might well add to this imbalance too, and presumably potassium is a particularly beneficial yield-booster for commercial growers whose primary concern is the size of the crop. Solomon recommends a soil balance that leaves potassium "just a little bit scarce" - just a few percent of the TCEC - for crops with the highest nutrition.
Sodium. We all know salt isn't good for most plants, but a small amount of sodium in soil is crucial and some crops need it. However, Solomon warns that growers using tapwater to irrigate their crops should be aware of the sodium level in their irrigation water. Annually, twelve inches of water containing 50ppm sodium would bring 200lbs per acre of sodium! There's something I'd never considered before! My local mains water contains an average 17ppm sodium, according to an analysis on my water company's website, but I have no idea how many inches of it I put on my soil... There should be around 1-2% sodium in the TCEC.
Phosphorus is a controversial soil amendment, since it is mined from the ground and we are steadily running out - but even a slight deficiency slows plant growth. Many agricultural soils are severely depleted of phosphorus and heavily dependent on phosphate fertilisers, but a healthy balanced soil can hold on to phosphorus for decades, because phosphorus is held in the soil by humus. If it fails to connect with humus, it soon connects with other minerals and becomes insoluble iron phosphate or calcium phosphate instead, and remains unavailable to plants long-term. Solomon suggests there should be around the same amount of phosphorus as potassium in soil, but that it should be added gradually over a number of years to ensure as little as possible goes to waste. Bonemeal provides a sustainable source, but comes with additional sodium and calcium too.
Nitrogen is crucial for healthy green growth: it's vital for production of the plant protein chlorophyll, which plants absolutely depend on for photosynthesis and energy. Dark leafy greens grown on fully mineralised soil can contain up to 20% protein - as much as beef steak! Nitrogen is released when soil organisms feed on organic matter - annually, every 1% of organic matter existing in a soil will provide 15-25lbs of nitrogen per acre, and to produce a good crop, 100lbs/acre is needed. So a healthy soil with 4-7% organic matter should have plenty of nitrogen available (although if it is hitherto dependent on nitrogen fertilisers, it might need weaning off them first). Solomon also advocates digging in leguminous green manures to provide nitrogen naturally, and points out that a healthy stand of field beans can provide the full 100lbs/acre needed.
Sulphur is a really interesting nutrient which helps plants to form amino acids (proteins) and enzymes, and boosts flavour. While we see sulphur as a fungicide and dust it on our plants as a defence, Solomon suggests that fungal disease is actually a symptom of sulphur deficiency, and describes how the onion root-rot on his plot completely disappeared when he rebalanced his soil. This is great news for me, as I've had to give up growing onions on my allotment thanks to my white rot problem! Sulphur is another anion, held in the soil by humus, and when it dissolves into the soil solution it bonds with cations to become a water soluble sulphate (iron sulphate, calcium sulphate, zinc sulphate etc.) Thus too much sulphur can leach cations from the soil - or it can be used, with care, to deliberately deal with a cation excess.
Micro-nutrients and trace nutrients are vital for maximum plant health and maximum nutrition too, and Solomon reveals the amounts needed for good balance and recommends amendments such as seaweed or Azomite for adding trace elements. Surprisingly, he's not such a fan of rock dust, and his analysis of it makes interesting reading.
pH. Wow, now this is important! pH is NOT the be-all-and-end-all of soil health, but a side-effect of the chemical and microbiological activity in the soil. It's not really as meaningful as we tend to think, and it fluctuates in a much more complex way than most of us know! pH stands for 'potential hydrogen' and is defined as the density of hydrogen atoms in water (or in a solution of a substance, such as soil). All the exchange points in the soil must always be filled up, so if there is no calcium, magnesium or other useful minerals around, hydrogen atoms from the water in the soil will stick instead, and the soil will contain a lot of hydrogen and have a low pH (an acidic soil). A soil rich in cations such as calcium and magnesium, on the other hand, has very little hydrogen and a high pH (an alkaline soil). So the pH of your soil can indicate how nutrient-rich it is - certainly an acid soil is lacking in cations. BUT a soil high in cations (alkaline) may have completely the wrong balance of cations and still benefit from liming, and in fact, the liming might not raise the pH further (since the new calcium ions knock off and replace the excess magnesium, potassium or sodium ions) but allow it to fall because correcting the balance allows the microlife to thrive again and kicks off the normal cycles of nitrate release and so on!
Soil testing is the only way to really know what's going on in a soil, and this is the method Solomon recommends to get the maximum potential out of any food-growing plot. The book explains how to take a soil sample and where to send it, and provides worksheets for readers to interpret the results and work out a perfect prescription of amendments for their soil. He also acknowledges that a soil test and a bunch of calculations won't be for everyone and might be severely over the top in a very small plot - so he offers a generic one-size-fits-all solution too, which he promises will greatly improve virtually all garden soils and increase its benefits year by year. He calls this his Complete Organic Fertiliser, or COF.
The Intelligent Gardener is a garden-changing book - maybe even a life-changing book - and I thoroughly recommend it to all home-growers. It's the kind of book that, once you've read and understood it, you just have to put into action. Steve Solomon believes that "many of our current social problems would also vanish by themselves, if only the mass average health of people were uplifted", and I'm inclined to agree, having seen the enormous variation possible in the nutrient content of our foods, and considering how far the impact of an unhealthy soil can reach into every part of our diets. If I'm going to grow my own food, then I want to do it properly and really get the best out of it. And if brilliant health is within my grasp (and my family's), just by changing what I feed my soil... well, no-one's getting in my way!
I intend to have my soil analysed and to remineralise it with The Intelligent Gardener's guidance, but not this year, with so much else going on. This year I'm trying Steve Solomon's Complete Organic Fertiliser (more on this in a future post), both on the allotment and on my beds at home, and I have high hopes for some excellent results.