In this third instalment of the Soil Therapy™ guidelines, we will consider the major soil-sweetening cations, calcium, magnesium and potassium.
Calcium (Ca) – The trucker of all minerals
Calcium is always the first mineral to correct in your soil, because it has so much impact upon other minerals. We often call calcium "the trucker of all minerals", because it directly stimulates the uptake of seven other minerals. It also indirectly affects all mineral uptake, as it is the doorman at the cell membrane, through which all minerals move into the cell. In the soil, calcium serves to open up the soil. This allows the easy entry of all-important oxygen and the exit of CO2 for photosynthesis (gas exchange). Calcium effectively allows your soil to breathe.
In the plant, calcium governs cell strength and associated plant resilience. It also promotes cell division, growth and crop quality. In the absence of calcium you will see an increase in problems like blossom end rot in tomatoes and capsicums. However, poor cell strength will reduce resilience and there will be multiple associated issues.
Your Soil Therapy™ report presents calcium in terms of parts per million (ppm), and as a percentage of base saturation (reported in the table at the bottom of your test). The amount of calcium your soil requires is based upon the amount of clay (the medium for calcium storage) in your soil. We have computed the ideal calcium requirement, specific to your soil, in ppm, and as a percentage of base saturation.
There is no such thing as an ideal level of calcium for all soils. The appropriate amount of calcium for your soil is based upon the amount of clay available for storage. A light, sandy soil can store very little calcium, for example, but a heavy clay soil might have huge storage potential. You have what you have, so it is important to use a soil test service that will tell you the exact amount required to top up your unique fuel tank.
It must always be remembered that too much calcium can sometimes be worse than too little calcium, as an excess can lock up the very same minerals that would otherwise be stimulated by this master mineral.
Calcium is the least mobile of all minerals, which means it is sluggishly delivered into the plant and poorly translocated into fruit. For this reason, there can be considerable benefits in foliar spraying chelated calcium to bypass this poor delivery problem.
High magnesium or potassium can reduce calcium uptake, and nitrogen excesses are similarly restrictive.
A refractometer can be used as a guideline as to calcium availability in your crop. When you look through a refractometer, the colour demarcation on the vertical brix axis indicates the brix level, and should not be sharp and distinct. The goal becomes to ensure that this dividing line is indistinct and "fuzzy". The more diffuse this line, the better your calcium levels.
To reiterate, calcium is the first thing to address in your soil, but you must understand exactly what amount is required. It is the "Goldilocks” mineral, which we should always strive to get "just right".
Magnesium (Mg) – The chlorophyll king
Magnesium is the centrepiece of chlorophyll, the green pigment housed within the sugar factories (chloroplasts) that create glucose, the building block of all life. A magnesium deficiency means substandard photosynthesis, and this will always be costly. Magnesium also stimulates phosphate uptake. A deficiency of magnesium will reduce yield and will also reduce resistance to disease.
The calcium to magnesium ratio is the most important mineral relationship in the soil, because it allows the soil to breathe. This ratio also impacts optimum plant availability of both of these important minerals. An excess of either can seriously affect the uptake of the other. In fact, all of the major cations are powerfully interconnected, and too much of any one of them will impact the uptake of the others. This is why the concept of cation balance is so critically important.
Calcium opens up a soil, while excess magnesium can have the opposite effect. A high magnesium soil is a soil that can't breathe well. In these soils, you will have platform shoes in the wet, as the tight, sticky mud accumulates. These soils form clods when worked, and their poor gas exchange (oxygen in and CO2 out), reduces photosynthetic potential and favours pathogens, which may not require oxygen. High magnesium soils require much more nitrogen because nitrogen fixation, recycling and availability is all compromised in high magnesium soils. You must "earn the right” to reduce nitrogen in these soils by first improving the all-important calcium to magnesium ratio.
Just like calcium, there is no ideal level of Mg for all soils. Again, it is all about the clay component of your soil and the relative potential for magnesium storage. In light soils, you might need 120 ppm, but in a heavy clay that might rise to 700 ppm to over 1000 ppm. Ideal base saturation percentages of magnesium vary between 10% and 20%. 10% is appropriate in a heavy clay soil where we are restricting the amount of the soil tightener to avoid a tight, closed soil that struggles to breathe. 20% magnesium base saturation might be the balance we are seeking in a light, sandy soil because we are needing the soil-structuring, tightening effect of magnesium in a soil that has very little structure.
According to leaf test data, most crops do not contain the optimum levels of magnesium we are seeking, due to either a lack or an excess of this mineral in the soil (ironically, soil excesses also reduce plant uptake of magnesium). Imbalances of the other major cations, including potassium or sodium, also impact magnesium availability. As with most things in life, it is all about balance. That is the true value of a Soil Therapy™ report, because the parameters for productive balance are clearly delineated and there are suggestions for cost-effective correction of imbalances.
If you are also lacking sulfur and the soil is not acidic, the best tool for removing excess magnesium is gypsum. Gypsum (calcium sulfate) is called the "clay breaker", because the sulfates bond with the magnesium to create magnesium sulfate which is very easily leached from the root profile. This is why magnesium sulfate should never be used as a soil corrective. What is the point of trying to build magnesium with the most leachable form of this mineral? Magnesium carbonate (magnesite) is the preferred soil corrective. Magnesium sulfate can be combined with fulvic acid to produce a very well-absorbed magnesium fulvate that is ideal as a foliar.
Micronised magnesium carbonate, in liquid suspension (Mag-Life Organic™) can be fertigated to effectively address magnesium deficiencies in the root zone.
High sodium, potassium and calcium can depress plant availability of magnesium and, ironically, high magnesium soils can often produce magnesium deficient plants. It is all about balance.
Potassium (K) – The spark plug
This super-mobile mineral does not become part of the cell structure, but rather rushes between cells, triggering multiple processes. It is essentially a spark plug that triggers many reactions.
Potassium is involved in the opening of stomata, the tiny breathing pores that suck up CO2 for photosynthesis. Stem strength is also linked to potassium, as is vegetative growth. However, the most important role of the second most abundant mineral in the plant, is the movement of sugars into fruit, seed, or tubers. This is why we call potassium "the money mineral" – because poor sugar translocation means insipid flavours and smaller seed, fruit or potatoes. You will always suffer less yield and profit if potassium is missing.
You will often experience more disease issues if you mismanage potassium. In the remarkable book (co-edited by Professor Don Huber) called "Mineral Nutrition and Plant Disease", potassium imbalance is shown to be the largest single cause of plant diseases. Most diseases have a mineral link, but potassium is the biggest player. The brown spots (Alternaria) that inevitably arrive on the lower leaves on tomatoes, eggplants, potatoes and capsicums (the Solanaceae family), are very commonly linked to a potassium deficiency in these K-hungry crops.
Potassium is the most mobile of all minerals and it will rush to where it is needed, to size fruit and push shoot growth at the top end of the plant. When it vacates the lower leaves, the sap pH drops in these regions, and this is the calling card for a variety of diseases. Ask yourself where the disease usually appears on your crop and if you note the lower leaf link, you now understand the likely suspect. Measuring potassium in both upper and lower leaves with a potassium meter is a powerhouse strategy for effective management of potassium. There should never be a difference of more that 10% K, between upper and lower leaves. The moment the K in the lower leaves start falling, you need to fertilise with potassium.
Once again, the clay component determines individual requirements in terms of the required ppm for each soil. However, there is a crop-specific base saturation requirement for potassium. Woody crops like vineyards and orchards require around double the amount of potassium in the soil, compared to annual row crops. If you can achieve equal ppm of potassium and magnesium in the soil, then you help ensure good delivery of both minerals into the plant.
The most popular form of potassium in modern agriculture is potassium chloride (muriate of potash). It is also the cheapest form of an otherwise expensive mineral, because it comes out of the ground in this form. However, "natural" does not always mean good. Potassium chloride has a very high salt index, so it can dehydrate root cells and the cells of the myriad of beneficial creatures surrounding that root. Part of the chloride component can also become chlorine, which is what we put in our swimming pools and drinking water to kill single-celled organisms. Unfortunately, our soils are filled with susceptible life forms, so it is best to avoid this biocide. I have always advised growers to switch to potassium sulfate. It is more expensive, but even if you use the same dollar rate, you will always do better with this lower salt index fertiliser that also delivers some (usually) much-needed sulfur.
There are often good gains in foliar spraying potassium in the latter half of the season. There is a big drawdown of potassium to size up fruit and grain, so this supplemental supply through the leaf can really boost yields. In fact we call it the "K kick" in broadacre cropping. In a wheat crop, for example, during the seed filling phase, we might foliar spray 6 kg of potassium sulfate with 250 grams of fulvic acid powder and 50 mL of triacontanol per hectare. The combined response can be something really special, for an investment of less than $20 per hectare.
In the next instalment, we will have a closer look at sodium, sulfur, silicon and boron in relation to your Soil Therapy™ report.
PLEASE NOTE: All prices referenced above are in AUD. Prices valid at time of publication and are subject to change.
To read Part 4 of this feature, please click here.
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