In this instalment of Soil Therapy™ guidelines, I will continue to highlight the key characteristics and strategies relative to the minerals measured on your soil test. Here, we will consider sodium and sulfur.
Sodium – Friend or Foe?
Most growers think of sodium as an unwanted intruder when, in actual fact, it is a very important mineral for healthy plant growth in halophytes and C4 plants. The "bad boy" tag relates to our mismanagement of the landscape and contamination of underground water with excess sodium. When we strip the trees from the farmscape, the water table rises and salinity is a common outcome. When we pour on the salt-based fertilisers and destroy our soil life, the subsequent decline in soil structure facilitates the trapping of sodium in the root zone and a host of associated problems.
Sodium is a key electrolyte that helps facilitate the thousands of messaging reactions that are part of the electrical life of the plant. It is not an essential mineral, but it can be used in small amounts for opening of stomata and for chlorophyll formation.
A high sodium soil will have poor soil structure, as an excess of this mineral disperses clay colloids, making soil more prone to collapse and erosion. This dispersive effect is the opposite to the stabilising effect of calcium on clay. The calcium ion, with its two positive charges, is a stronger flocculating agent and is able to hold soil particles together in small clusters called aggregates, thus stabilising soil structure. Sodium, with a single charge, can not lock clay colloids together in this manner.
When excess sodium enters the plant, it can expand in the heat and burst cell walls, resulting in the telltale burnt margins that are symptomatic of a sodium overdose.
Ideally, we should always try to keep sodium below 50 ppm. In terms of base saturation percentages, sodium should never exceed 1.5%. The most important consideration here, however, is to ensure that sodium never exceeds potassium in terms of base saturation percentages. The ideal potassium to sodium ratio is 5 parts potassium to 1 part sodium.
However, an interesting phenomenon occurs when the percentage of sodium ions attached to the clay exceeds the percentage of potassium. In this instance, the plant becomes confused. Sodium and potassium are similar sized ions and, for millions of years in nature, there has always been a greater percentage of potassium saturating the clay colloid than sodium. However, in many areas, we have messed up that equation and the plant has not adapted to our mismanagement. When the plant requires potassium, it simply selects the most abundant of these lookalikes from the clay. If sodium is more abundant than potassium, then that is the mineral uptaken. The end result is that our crop unintentionally absorbs an unwanted, unproductive mineral, rather than potassium ("the money mineral"). Sugars are not moved, fruit and seed does not size up and there will be less yield and profit as a result.
If you are deficient in sodium, it must be applied. Sea salt is the very best source, however, pool salt can be purchased inexpensively from your local hardware store to address a sodium deficiency. At this point you may be thinking about the Romans’ use of salt to destroy the food-producing soils of their enemies. Here, we are talking kilograms rather than the tonnes of salt involved in the Roman war crimes.
Sea salt offers more than sodium chloride. It is rich in all 74 minerals, which your soil is often lacking. When the Maldives were engulfed by the ocean following a tsunami, it was thought that this would devastate Maldivian agriculture. In actual fact, they experienced record yields the following season.
We have extracted the full spectrum of minerals from our soils for many decades without giving back. Ocean-based inputs like sea salt, kelp and liquid fish can offer restitution for these withdrawals and the visual response can often be quite impressive. The productive application rates vary from 10 – 50 kg per hectare of sea salt, depending upon the size of your deficiency, and application method. It is always a good strategy to include humic acid to buffer the sodium and increase trace mineral uptake.
If you have too much sodium, there is a two-pronged strategy. The first consideration is to boost potassium to ensure that you have more potassium than sodium saturated on the clay. Secondly, you need to look at ways to remove and buffer the sodium excess. Gypsum (calcium sulfate) can be helpful. Here, the sulfates break free from the calcium and form highly leachable sodium sulfate. The calcium, in turn, helps flocculate the soil to speed the leaching of this newly formed sodium sulfate. This sodium-neutralising effect can be fast-tracked with the use of micronised liquid gypsum (Gyp-Life Organic™). Here, we can fertigate to achieve rapid root zone management of sodium, while maximising delivery of the all-important calcium component.
The second sodium strategy involves buffering and immobilising the excess. Humic acid and soluble silica are the most productive tools for this purpose. Humic acid changes soil structure more rapidly than any other input and this can speed the leaching of unwanted sodium. Silica forms a gel compound with sodium and immobilises this mineral. These two inputs have proven to be our most effective tools when confronted with saline irrigation water. In this instance, potassium silicate is fertigated at 5 – 10 litres per hectare.
Humic acid is fertigated at 30 litres of DIY liquid humic acid per hectare. This inexpensive homemade liquid humate is created by dissolving NTS Soluble Humate Granules™ in water at a rate of 1 kg to every 10 litres of water. The mixture is stirred vigorously and then left to sit overnight, so that the small insoluble fraction deposits on the bottom of your tank. These insolubles should not be fertigated, as they will clog filters. However, they should also not be discarded once you have drained the fluid. This sludge is a truly remarkable, mineral-dense fertiliser to be added to vegetable gardens or your compost heap.
Sulfur – The Protein Essential
We used to get our sulfur for free. Sulfur dioxide billowed from industrial smokestacks and fell back to earth with the rain. This acid rain damaged forests and waterways and was eventually banned. Many years later, most farmers have yet to recognise that they now need to compensate for this loss of free sulfur. Sulfate sulfur, (SO42-)the plant available form of this mineral, is also easily leached. This anion can only be stored in the soil by attaching to the positively-charged humus colloid. We have lost two-thirds of our humus over the past ten decades, so sulfur has suffered a double whammy. There is no longer a free supply of this mineral and there is now much less of the storage medium in the soil (humus) to retain what we have left.
The single most important role of sulfur relates to protein production. The immune systems of humans, animals, plants and microbes are protein dependent. Protein is made from amino acids, and two of these essential amino acids, cysteine and methionine, are sulfur-based. If you are lacking sulfur, your crop will suffer substandard protein production and your plants, animals and customers will suffer accordingly.
Sulfur seriously affects the palatability of pasture, and it is also a root-boosting mineral for all crops (hence the popularity of side dressing gypsum on potatoes and peanuts).
Sulfur is an acidic mineral that can be used to counter high pH as the sulfate form can help leach high magnesium and sodium.
Sulfur is also a strongly reproductive mineral, so soluble forms (like ammonium sulfate) should be favoured at the business end of the season (from flowering onwards).
Sulfur availability is affected by other anions, particularly phosphorus. It can be a really productive strategy to try to maintain a ratio of 1:1 between phosphorus and sulfur in both the soil and plants (although individual ideal plant levels vary). This can help ensure optimum availability of both minerals. Both of these minerals are key players in plant immunity, so if you can balance them at 1:1 there is an associated increase in resilience.
Sulfur also seems to impart a soil warming effect, hence the popular choice of ammonium sulfate as a starter fertiliser in cold conditions.
We have also found that if you maintain luxury levels of sulfur in your soils, there is much less likelihood of suffering from iron deficiency.
All crops require sulfur, but for some crops it is absolutely essential. These crops include brassicas and all members of the allium family (garlic, onions, leeks, etc). Canola, for example, is a brassica that demands a good supply of sulfur. Biodynamics refers to frilly-leafed crops as plants with a "sulfur gesture", as they are more sulfur dependent than other species. Curly kale is a prime example.
I often wondered why we could apply identical amounts of sulfur to the soil, as gypsum or elemental sulfur, and yet the crop response can differ. Sometimes elemental sulfur can paint the field green more effectively than nitrogen, while the gypsum effect is much less obvious. It seemed counterintuitive, because the sulfate sulfur component of gypsum is immediately plant available, while the sulfur in the yellow granules must first be converted to the sulfate form by specialist organisms in the soil. Several years back, I stumbled across research, which finally explained this phenomenon. When bacteria, like Thiobacillus, are converting elemental sulfur to plant available sulfates, some of the sulfur is converted to gaseous sulfur dioxide. It turns out that the tiny breathing mouths, called stomata, on the underside of the leaf can absorb this sulfur gas. In fact, they breathe it in just like CO2 and the plant response is instant.
Elemental sulfur can also be useful to lower pH in alkaline soils to help increase nutrient availability. Sulfur burners can be used to add sulfur to irrigation water for this purpose.
Most soils that we test in over 50 countries are deficient in sulfur. Soil tests commonly reveal levels of 10 – 15 ppm. This is a serious deficiency, which will always reduce resilience, crop quality and yield.
Become sulfur aware, try to improve your phosphorus to sulfur ratio and reap the many benefits.
In the next instalment, I will discuss silica and boron, as these are two more, often neglected anions that can seriously impact your bottom line.
To go back to Part 1 of this feature, please click here.
To go back to Part 2 of this feature, please click here.
To go back to Part 3 of this feature, please click here.