Managing soil mineral ratios

It is essential that farmers understand the importance of mineral balance in their soils, says Graeme Sait, CEO of Nutri-Tech Solutions in Australia. He visited South Africa this year to explain this balance.

The Ca:Mg ratio is the most crucial of all mineral ratios for achieving soil balance, as it determines whether a soil can 'breathe' or not. Efficient gas exchange is a feature of healthy soil.
Photo: FW Archive

Soil health has a profound impact on the sustainability and profitability of any farming business. It also affects the quality, quantity and nutritional value of the food that farmers produce. Many factors contribute to the development and maintenance of a healthy soil. One of the most important involves effective management of the minerals in soil, and achieving this will result in optimum nutrition for utilisation by crops.

“Crop nutrition should never involve guesswork. Informed nutrition is essential to avoid driving blind,” says Graeme Sait, CEO of Nutri-Tech Solutions in Australia. There is a specific balance between soil minerals that determines a soil’s productivity, and farmers should always work with laboratories or consultants that have a key understanding of how to achieve this productive state of balance, he says.

‘Good soil test’
Optimal soil nutrition always begins with what Sait describes as a ‘good soil test’. The results of this test will generate precise figures for methods and products that must be utilised to achieve the ideal balance among soil minerals. Soils, and the crops that grow in them, need precise quantities and ratios of these minerals.

When sending a soil sample for analysis, says Sait, a farmer should request five more mineral-specific tests in addition to the normal soil analysis:

  • Chloride: Chloride (Cl¯) is particularly important for vegetative crops such as pasture grasses. Sea salt is a good source of Cl¯ but can only be applied at a rate of 20kg/ha to 40kg/ha. 
  • Silica: Silica (SiO2), in its soluble form (silicic acid), is missing from most soils. It is extremely important for increasing the cell strength in plants to resist disease and insect attacks. It is also now recognised as a powerful immune elicitor to boost plant resilience. 
  • Molybdenum: Molybdenum (Mo) is needed because it allows access to free nitrogen (N) from the air. It also helps plants convert nitrates into protein in the plant leaf. 
  • Cobalt: Cobalt (Co) supports N-fixing organisms in the soil and on plant roots.
  • Selenium: Selenium (Se) is critical for plant, animal and human health. Applying Se, as indicated by the results of a soil analysis, can also contribute to increased crop yield. Potato production research in Finland revealed that foliar applications of 250 parts per million (ppm) of Se produced a 30% increase in net tuber weight.

The six key mineral ratios
“There are six key mineral ratios in soils that are applicable to the good development of most commonly grown crops, says Sait.  “For more niche crops, such as berries, these key mineral ratios can be adapted to cater for their specific nutrient requirements. If farmers work towards improving all six of these ratios with every soil test, then they’ll see improved soil health, better productivity and increased profitability.”

It is important to understand the mineral storage capacity of soil before working to address these ratios. This is evidenced by what is known as a soil’s cation exchange capacity (CEC). Soils with higher clay content tend to have a higher CEC, allowing for increased mineral storage capacity.

According to Sait, a good soil test should also feature a total exchange capacity (TEC) measurement. TEC, like CEC, measures the levels of the major cations in the soil, namely calcium (Ca), magnesium (Mg), potassium (K), sodium (Na) and sometimes aluminium (Al). However, it also factors in the non-nutrient, acid-forming mineral, hydrogen (H).

“If you haven’t factored in H, and the soil is acidic, then you’ll have a misleading idea of the relevant percentages of each cation,” explains Sait. “If your soil test did not measure TEC, and you are in an acidic, H-packed soil, the percentages of major cations are misleading because you are effectively only measuring half the pie.”

In addition to clay, the other component influencing the mineral storage capacity of a soil is humus. Humus is the only storage system in the soil that holds onto both cations (positively charged atoms) and anions (negatively charged atoms). It is also the only soil component that can hold onto the anions sulphur (S), boron (B) and nitrate-N, all of which are crucial for effective soil fertility.

Ca:Mg ratio
“This is the most important of the six because it determines soil structure and associated gas exchange,” says Sait. ”It effectively determines if a soil can breathe or not. A soil without breath is like an animal nearing death. Oxygen (O) is required for soil microbes and plant roots, while carbon dioxide (CO2) must exit from the soil to be used by the above-ground green parts of a plant for photosynthesis.”

The ideal Ca:Mg ratio varies depending on a soil’s density. In a light, sandy soil, the percentage of Mg must be increased and the relative percentage of Ca reduced, because the Mg is being used to give a little more structure to this very light medium.

It is important to understand that Ca opens up soil (flocculation), while Mg tends to close up and tighten soil, stresses Sait. In a light, sandy soil, the Ca:Mg ratio should be 3:1. In a heavy, clay soil, more Ca is required to open up the soil, so the ratio should be 7:1 in favour of Ca.

Ca flocculates the soil because it is a large ion with two positive charges that attach to tiny, negatively charged clay particles and pushes them apart. Mg is a small ion that does the opposite. However, Mg is necessary because it is the centrepiece of the chlorophyll molecule, and thus critically important in photosynthesis.

Mg:K ratio
This ratio is the second- most important, according to Sait. “You should aim to achieve equal ppm of both Mg and K on a soil test. When this happens, you have maximum plant availability of both Mg and K, but this equal ratio also stimulates a plant’s phosphorous (P) uptake. In high Mg soils, this equal Mg:K ratio can never be achieved. However, every step in the right direction will be beneficial,” he explains.

P:S ratio
P is known as one of the ‘Big 4’ elements in soil fertility. It is beneficial because it drives all aspects of a plant’s immunity to pests and diseases, and because it aids plants’ manufacturing of sugars during photosynthesis. It is for this reason that the P:S (sulphur) ratio is the third-most important ratio in achieving soil balance, determining the performance of these two key anions.

S is often neglected by farmers, says Sait. Previously, it was obtained from factory smokestacks. However, the resulting acid rain killed forests and waterways across the globe, leading to emissions controls. “We’ve also lost two-thirds of the humus in
our soils that holds onto the highly leachable sulphate anion. I see S-deficient soils everywhere around the world and it can be tremendously beneficial to correct this deficiency.

“You cannot make real protein without S, because two of the amino acids in protein, cysteine and methionine, are made from S. Plant, animal and human immunity is protein-driven. S is required to convert nitrates into protein, so there’s a very strong protein connection. S also protects from internal and external parasites, and is one of the most important minerals in the detoxification system of animals and humans.”

S is also critical for healthy root growth and increased density of chlorophyll in plants. Without chlorophyll plants cannot photosynthesise. As with the Mg:K ratio, the most desirable P:S ratio is 1:1 in terms of ppm. “When we lift soil S levels to match soil P levels, there’s a substantial improvement in soil and plant health,” Sait says.

P:Zn ratio
The P:Zn (zinc) ratio is the fourth-most important ratio. Sait describes Zn as ‘the energy micro-nutrient’ and P as ‘the energy nutrient’. This because the latter is required in every enzymatic reaction that takes place in a plant or animal. It is the building block for adenosine triphosphate (ATP), the ‘battery’ of all life, which fires all enzymic reactions. Zn has a major impact on phosphate, particularly in relation to the production of ATP.

Zn also has several beneficial effects on plant health. Perhaps the greatest of these relates to the fact that Zn is required by plants to produce critically important hormones called auxins. These determine leaf size, so a Zn-deficient plant will have a smaller than optimal solar panel (leaf), and this will affect every aspect of production.

Good leaf growth equates to good photosynthesis that, in turn, leads to good plant productivity. Ideally, says Sait, the P:Zn ratio should be 10:1 in favour of P. “If this 10:1 P:Zn ratio is achieved, there’s maximum performance of both minerals. However, too much of either P or Zn inhibits the other,” warns Sait.

“Never try to achieve this ratio in soils that are high in P because the subsequent Zn excess will tie up other elements in these soils. Often when you have overdone phosphate, you have no choice but to foliar spray Zn on a regular basis, rather than apply it to the soil.”

K:Na ratio
The fifth-most important ratio, according to Sait, is that of K:Na. This ratio is based on base saturation percentages rather than ppm, and is critical for ensuring that K, the second-most abundant mineral in the plant, is available to it. He stresses that, in an analysis of a soil’s base saturation, there should never be a higher percentage of Na than K. Ideally, the K:Na ratio should be 4:1 or 5:1.

“The reason for this is that these two minerals are fairly similar. So a plant can take up Na when there are relatively high levels of it instead of taking up the K. Too much Na in the plant instead of K compromises the plant’s productivity because it can cause cell walls to burst, among other detrimental effects. In addition, you won’t have the required K to size fruit and seed.”

Fe:Mn ratio
The final key mineral ratio is that of iron (Fe) to manganese (Mn). Both minerals are key in fuelling plant immunity.
Achieving the correct ratio is important to ensure that both these minerals can be taken up by a plant. Ideally, this ratio should be between 1,1:1 and 2:1 in favour of Fe. The ideal is to always have a little more Fe than Mn, to insure optimum delivery of both minerals.

“Unfortunately, the uptake of both is seriously affected by glyphosate-based herbicides. Microbiologist Professor Don Huber [former scientist from the Plant Pathology Faculty of Perdue University, Indiana] has demonstrated conclusively that glysphosate kills both Mn-reducing organisms and Fe-reducing organisms, having a serious impact on the delivery of both minerals to the plant.

"In a recent study, he proved that glyphosate usage is directly linked to an increased likelihood of 40 different pest problems.”

Email Graeme Sait at [email protected]. Visit www.nutri-tech.com.au. This presentation was given at a soil health seminar hosted by Zylem, which represents Nutri-Tech Solutions (AUS) and Brookside Laboratories Incorporated (US), in SA. For more information, phone Zylem on 033 347 2893 or email [email protected].