Let's talk Sulphur, as it is another one of those mineral nutrients (both for plants and animals) which are somewhat overlooked in modern agriculture. It's also one of the most interesting ones to understand if you are a no-dig gardener, and you'll see why in a minute.
First of all, what do plants do with Sulphur (S)? As it turns out, an awful lot of processes would not take place without it.
For a start, without sufficient S, photosynthesis and carbon fixation (arguably the two most important processes on our planet) are heavily impeded.
Sulphur (as well as Molybdenum) is crucial to allow Rhizobia, Frankia and other soil bacteria to fix atmospheric Nitrogen into Ammonia (which is then converted to nitrates). This process is mediated by the
Nitrogenase enzyme, which bacteria can only produce if there is available S in the soil. Moreover, the nodules in which the bacteria live cannot be formed without sufficient Sulphur. The number one culprit for the lack of nodules in a legume crop is usually Sulphur deficiency!
This is not the only connection between Sulphur and Nitrogen metabolism. Nitrogen taken up from the soil is converted and found in the leaves as nitrates. These are then transformed into proteins which are paramount for the functioning of the plant as well as those who will eat it. If nitrates are not converted into protein quickly enough, there can accumulate in the leaves. This is bad news for the plant, as excess of nitrates has shown to attract sap-sucking insects, but it's also bad news for animals (including humans) as nitrates are toxic to livestock and are a known human carcinogen. The conversion of nitrates into protein is performed by an enzyme called Nitrate-reductase, which is activated by Sulphur.
Sulphur is not only critical for protein content in vegetables, but is also necessary for fatty acid formation in oil crops (olive, rapeseed, sunflower, etc.) Brassicas, Garlic, Onions and Leeks are also very sulphur-hungry. It is also important for the transport of starch and sugar to the root, and this impacts not only the production of tubers and perennials, but also the amount of exudates a plant can share with soil microbes.
From an hormonal point of view, S is pivotal during the reproductive phase of a plant's life, so it can improve flowering and fruiting.
However, its most important role in plants has to do with resilience and immune response.
Sulphur is the constituent of amino-acids such (cysteine, methionine) and vitamins (thiamine, biotin), coenzymes and other compounds that have an important role in making a plant respond to stress. We now know that good S levels not only improve plant productivity in normal conditions but also provide protection under less-than-ideal conditions such as in high salinity, drought, excess temperature or light, toxic metals contamination. Several S-containing compounds also directly act as antioxidants, which are as important in plants as in humans to neutralise cellular stress caused by free radicals.
Some of you will have noticed that 1/3 of all pesticides contain elemental S. Already 3000 years ago, Homer mentioned sulphur use as a pesticide. Korean Natural Farming (KNF) practitioners have recently re-discovered the use of natural soluble S in treating pest infestation and boost immune function.
The reasons and mechanisms behind sulphur's role as enhancer of plant resilience are complex and still only partially understood. It suffices to say that S is paramount for the production of two classes of plant secondary metabolites (PSM): phytoanticinins (such as the compound that makes us cry when we chop onions), and phytoalexins. Both of these act as antipathogens in plant tissues (and are extremely important for human health too!) Alliin and Allicin (both contained in Garlic) are two potent sulphuric phytoanticinins. Plant resistant plants have shown to line their Xylem with elemental Sulphur which is bonded into a phytoalexin. Another phytoalexin you might have heard of are glucosinolates, biofumigant compounds produced by Brassicas, which have the effect of making plants less attractive to insects and more resistant to diseases. As part of a specific type of immune response called Sulphur-induced-resistance (SIR), plants also release sulfuric gases to deter insect attacks.
So, where does Sulphur come from and how do we get in our plants? S is taken up by plants both as soluble sulphate from the soil, or through the stomata as sulfur dioxide. Soil microbes, if present, unlock the Sulphur contained in parent rocks and convert it to sulphate. As it is a negatively charged ion (anion), Sulphate can't stick to clay particles (which are also negatively charged), and therefore can only be stored in stable organic matter, or into the bodys of soil microorganisms. In soils lacking organic matter, sulphate is very easily leached. For this reason, lots of soils in the world are deficient. This didn't use to be a problem in the past, because of destructive acid rains which delivered free S to our soils. Now that acid rains have fortunately been hugely reduced, in the last decade 2/3 of the S stored in our soils has been lost due to poor agricultural practices.
Conversely, no-dig gardening is particularly conducive to good S levels (because it's contained in compost) and retention (because of humus abundance).
However, an issue that is often overlooked in deep-mulch no-dig systems is the antagonism between S an P (Phosphorus). If you apply thick layers of animal-based compost, chances are you have way more P than what's needed to your plants. That's not disastrous, but it can cause issues. The first one is that it discourages Mycorrhizal fungi, but we are not concerned with them in this post. The second is that P locks S in the soil. Ideally you should have as much S as P in the soil (and around 60ppm). If you are keen on gypsum (calcium sulphate) rather than (or in addition) to compost to open up clayey soils, you probably know that sulphur is instrumental to regulating the Calcium-Magnesium ratio that makes clay soil more breathable and accessible to soil micro-organisms. This is largely because magnesium and sodium sulphates are highly leachable. Sulphur can also be used to lower pH in alkaline soils, and this can help increase nutrient availability.
Let's see how we can apply all of this knowledge in the field. How do we spot Sulphur deficiency, and how do we supply S (both as pest control and as nutritional supplement) naturally and cheaply?
Although highly mobile in its Sulphate form in the soil, as soon as it enters the plant, Sulphur is relatively immobile. For this reason, the plant can't transfer S from older leaves to younger ones. This implies that S deficiency shows up in the young leaves, and it looks pretty much like Nitrogen deficiency: yellowing (chlorosis) or striping of the leaves plus poor growth.
How do we add S to our soils and plants? First of all, if you are already working in a no-dig system, you should have plenty of Sulphur. Watch out for symptoms of deficiency and if they appear, this is most likely due to excessive Phosphorus; in this case, move to a plant-based compost, drastically reduce compost application and move towards a shallow-mulch style of no-dig (1-2cm maximum of compost per year).
Okay, time for some recipes. If you want to supplement your plant leaves with Sulphur to aid flowering, fruiting and boost immune response, there are three liquid inputs that I recommend. The first one is a great nutrient supplement and immune booster, it can be used regularly if needed. I would use the second one to prevent fungal diseases that you have struggled with in the past, and the last one only if you fail to control such problems with the second one.
1) Fermented Garlic Spray. The first one is inspired by the work of Graeme Sait, who studied the most recent research on garlic sprays and suggested that in order to make them effective we should enhance their uptake by fermenting them. So, do not just put garlic in water and let it rot, but crush it (this is really important to produce Allicin) and immerse it in Lactic Acid Bacteria (LAB) which you can either make from milk (search youtube for a tutorial) or use the leftover brine from your sauerkraut. When the pH is around 3.5, you are good to go. Dilute 1:500-1000 and spray on your plants.
2) Boiled Garlic Spray. This is a somewhat simpler version of the previous input. It's inspired by JADAM korean farming, where it's called JHS. It's very simple to make: crush some garlic, boil it in a pressure cooker or a pot for several hours (beware, the vapour is going to be strong) with a lid on. Then strain the solids (they can go in the compost), let the liquid cool and store it in a glass jar. It can be sprayed mixed with a natural soap and diluted 1:50 in water.
3) Water soluble Sulfur. This is a potent natural pesticide made from elemental S. It's made via an ingenious process that JADAM founder Youngsang-Cho came up with to make Sulfur water soluble. It's not the easiest thing to make because it requires some space and it's got to be done safely, but you can also buy it by KNF practitioners (see for instance Dr Forest or BallaghBotanicals in the UK). Dilute 1:500-1000 and mix with a natural soap to increase effect. This can also be mixed with boiled garlic spray.
Finally, let me say something about the importance of Sulphur in human health. I am not qualified to give medical advice and do not believe in recipes that work universally on all human individuals, so I won't venture discussing S in the human body. However, I warmly encourage you to do a bit of research on how important Sulphur compounds (in particular the glutathione peroxidase enzyme complex) can be for liver health, and consequently in the prevention of Diabetes, Cancer, and toxin-induced disorders. Foods that can help with Sulphur uptake are Asparagus, crushed Garlic (no big news!), Brassicas (in particular Kale), Milk-Thistle, Beetroot, eggs, Turmeric (Curcumin-Pepperin), Brazil nuts (for Selenium).
You can find out more about practical mineral nutrition on a garden scale in our course: 20+ ways to nourish your soil and plants.
Special thanks go to Graeme Sait, whose podcast motivated me to study Sulphur in more detail. Lots of the knowledge in this article comes from Graeme's wisdom.
References:
Marschner's Mineral Nutrition of Higher Plants (2011) [link]
Mineral Nutrition and Plant Disease (Huber et al, 2007) [link]
Graeme Sait's Nutrition Farming podcast - Episode: The secrets of Nitrogen's silent sister [link]
The Nature and Structure of Soils (Weil, Brady, 2016) [link]
Peer-reviewed articles on SIR [link]