Six Secrets to Soil Test Success


Soil testing is considered an essential strategy to avoid driving blind in managing your all-important crop nutrition. Your production and profitability are often determined by nutritional constraints and there is no place for guesswork. A good soil test is your most important starting point.

What is a “Good Soil Test”

A good soil test should measure the 14 essential minerals and it should also include base saturation percentages. “Base saturation” refers to the percentage of the major cations in the soil (or bases) that are attached to the clay component of your soil. Clay is negatively charged, so it has velcro-like sites that attract positively charged minerals to help store them in the soil. These “bases” include calcium, magnesium, potassium, sodium, and the non-nutrient, acid-forming mineral, hydrogen. It has been determined that if you can achieve an ideal balance of these minerals in your specific soil type and relative to your specific crop, then you can maximize production and minimize problems.

A good soil test should provide the ideal percentages of these bases attached to the clay in your soil. In many soils, that “ideal” might involve 68% calcium, 12% magnesium, 3 – 5% potassium, and less than 1.5% sodium. An ideal cation balance would also involve 10% hydrogen because this amount of the acidifying mineral will provide an ideal soil pH of 6.3. This is the pH at which most minerals are most available, so it is worth working toward.

Working in over 50 countries with our agronomy team we have found that it is very productive to work toward achieving certain mineral ratios. There are six of these key ratios and, if you use these as a guideline, it can be super productive. The idea is to work toward improving each of these ratios with each yearly soil test. If you can tick off your improvements in all six ratios each year, you are on the right track and you will have no doubt of this because it will be reflected in your bank balance. So what are these key ratios?

The Six Ratios That Determine Success

1) The calcium to magnesium ratio is the single, most important of these ratios.

This ratio determines gas exchange or the breathing capacity of your soil. The better a soil can take in oxygen and then release CO2 for photosynthesis (gas exchange), the better your production. A soil without breath is like an animal nearing death and the Ca: Mg ratio governs this process. It could be argued that oxygen is the most important element for plant growth.

Here is how the Ca: Mg ratio determines oxygen availability in the soil. It is all about something called the ionic radius. This simply refers to the size of the mineral ion. Calcium is a (relatively) large ion with two positive charges. Think of calcium as a beach ball with a positive charge on either side. The positive charges are attracted, like nails to a magnet, to the negatively charged particles of clay in the soil (clay colloids). The beach ball attaches to clay particles on each side and holds them together as stable soil aggregates with air-space (pores) in between. This is called flocculation, which enables all-important oxygen to diffuse from the atmosphere into the soil.

By contrast, magnesium is a golf ball, which also attaches to clay colloids on either side with the two positive charges. However, instead of holding the particles together as stable aggregates with pore spaces in between, the much smaller magnesium ions pull them closer together. In fact, the higher the magnesium in your soil, the tighter it becomes, and the less it can breathe. A high magnesium soil does not favor a microbial workforce that is dependent on oxygen. At this point you might be thinking, “forget about the magnesium then, let’s open her up and reap the benefits of breathing soil!” Unfortunately, this is not how it works. Magnesium is the lifeblood of chlorophyll, which houses the sugar factories that produce glucose, the key energy source for plants (and most organisms). As such, it is a hugely important mineral that must not be ignored.

Soil breath is all about achieving the optimum ratio between calcium and magnesium in your soil and this, in turn, depends on the CEC of your soil. CEC is a measure of the clay component of the soil. A sandy soil might have a CEC of 4, while a heavy clay soil might have a CEC of 40. In heavy clay soil, you need more calcium to help push apart the high clay component. Here, the ideal Ca: Mg ratio might be 7:1. Conversely, in the sandy soil, you might need a Ca: Mg ratio of just 3:1, because you need more magnesium to help create structure in a soil where there is none. In general, the closer you can move your particular Ca: Mg ratio towards ” ideal” for your soil type, the better you will do in your growing enterprise.

2) The second most important ratio is the potassium to magnesium ratio.

This is a key ratio that was discovered at NTS and I am quite proud of this fact. When comparing thousands of soil tests to associated leaf tests, over many years, I noted that whenever we achieved equal parts per million of magnesium and potassium, we increased the uptake of both minerals into the leaf. Not only did we maximize uptake of these minerals, but there was also an associated positive impact upon the uptake of phosphorus. This is no small thing because phosphorus is one of the essential minerals for photosynthesis and the most critical mineral for plant immunity. Hence, this ratio directly influences plant resilience, creating a reduced need for chemical intervention and less stress and more fun in farming.

The reasoning behind this ratio derives from the idea that “no mineral is an island”. Every mineral affects the uptake of other minerals positively or negatively. In this case, too much magnesium inhibits the uptake of potassium and vice versa. If we get the ratio right, there is no inhibition and both minerals flow into the plant unimpeded. Interestingly, both of these minerals also impact phosphorus. Potassium is a phosphate antagonist if it is oversupplied, while magnesium is a phosphate synergist, supporting the uptake of P. If we balance these two minerals, phosphate flows into the plant (as evidenced by a leaf test) and plant production and resilience are enhanced. The goal here, again, is to work toward achieving equal parts per million of potassium and magnesium on your soil test – i.e., a 1:1 K: Mg ratio – and you will see the benefits.

3) The third key ratio is the phosphorus to sulfur ratio.

Here, we are once again concerned with maximizing the uptake of phosphorus. However, this ratio is also about the availability of an often-neglected mineral called sulfur. Many soils are sulfur deficient because two things have changed. Three decades ago, sulfur used to arrive freely in the rain. Then it was realized that sulfur emissions from industry were creating acid rain, which was linked to dying waterways and forests across the globe. The subsequent banning of sulfur emissions meant that farmers no longer received this key mineral for free, and many farmers have yet to recognize this fact. The second change impacting sulfur relates to the loss of humus in our soils. Humus is the sulfur storehouse and we have lost two-thirds of our humus through the ravages of extractive agriculture. Once again, this is a case where excesses of either mineral will antagonize the other. If we achieve a 1:1 ratio, the uptake of both minerals is optimized.

In the second installment of this article, I will discuss three more ratios that can improve production and reduce problems. These will include the phosphorus to zinc ratio, potassium to sodium ratio, and iron to manganese ratio.

Getting the Energy Minerals Right

Phosphorus (P) is called “the energy mineral” because it is the building block for ATP (adenosine triphosphate), which drives every enzymatic reaction. ATP is, in effect, the battery of life, because enzymes drive all biological reactions, and life stops without ATP. Phosphorus is also the major mineral required for plant immunity and the production of glucose from photosynthesis is largely based on phosphate-based enzymes.

Zinc (Zn) is called “the energy micronutrient” because this mineral is required, in the right balance with P, to ensure that phosphate energizes as it should. Zinc is also linked to moisture uptake and the performance of nitrogen-fixing organisms in the root zone. However, most importantly, this critically important trace mineral is required for plant and soil organisms to produce auxins.

Auxins are a group of hormones produced by the plant and beneficial microorganisms (to support their host, the plant), which provide a number of essential benefits. The most important of these relates to leaf size. The leaf is the solar panel that determines photosynthetic performance and zinc governs leaf size. A zinc deficiency spells a substandard leaf, less glucose production, and an inevitable yield reduction. This is why zinc is often considered to offer the best cost to benefit ratio of any trace mineral. A deficiency will always be costly and yet it is relatively inexpensive to address.

The key is to supply both phosphorus and zinc in a ratio that ensures maximum performance of both minerals. In this instance, that ratio is actually more important than the numbers game. Ten parts phosphorus to one part zinc is the proven productive ratio between these two minerals. If you had 30 ppm of P on your soil test, for example, and 3 ppm of Zn, both minerals are technically deficient. However, the ratio between them is correct (10:1). Maintaining that ratio is the key with soil correctives.

It would be very counterproductive if you were to lift zinc levels to the minimum required level of 5 ppm (because it is less costly to address Zn than P) but ignore the phosphate correction. Your perfect ratio is now compromised (it would be reduced to 6:1), which is worse than no action at all. If you have a limited budget, then limit your correction accordingly. However, always ensure that you maintain the all-important 10:1 phosphorus to zinc ratio. When we are talking about the mineral ratios relative to phosphorus, I should clarify the P extraction involved. I am referring specifically to Mehlich 3 extraction, as practiced by EAL Lismore and US lab, Brookside Laboratories.

Balancing The Lookalike Cations

Potassium and sodium are two of the major cations that are stored in the greatest abundance on the clay colloid in our soils. “Base saturation” on your soil test refers to the relative percentages of the base cations, including calcium, magnesium, potassium, sodium, and hydrogen, that are attached to the clay component of your soil. Ideally, we should aim for 3 – 5% saturation of potassium (3% for pasture and broadacre crops and 5% for more intensive horticulture).

However, sodium is required at less than 25% of this rate. Sodium should never exceed 1.5% on the base saturation portion of your soil test, but more importantly, you should never have a higher percentage of sodium than potassium. Should this occur, the plant may have problems differentiating between these lookalike minerals. The plant seems to assume that potassium will naturally be present in higher amounts, so it simply extracts the mineral that is present at a higher percentage at the time. If that is sodium, there is a price to pay. Sodium expands in the heat, bursts cell walls, and you have burnt edges on your leaves. Unfortunately, this is not just a cosmetic issue. The plant no longer has the required amount of potassium to transfer sugars, open stomates, sweeten fruit, size up fruit and grains, strengthen stems, etc, etc. You will suffer yield and quality limitations as a result of this imbalance. The key is to always maintain a higher percentage of potassium than sodium in terms of base saturation. The ideal K: Na ratio may be around 4:1, but the critical thing is to ensure that sodium levels are never higher than potassium.

Ensuring Adequate Supply of the Immune Enhancers

Iron and manganese are essential trace minerals for plant resilience. The plant uses these minerals for many of the compounds it creates to defend itself against marauding microbes and insects. The iron to manganese ratio is the last of our six soil secrets. The goal here is to always ensure that your soil contains more parts per million (ppm) of iron than manganese. However, this ratio should never exceed 2 parts of iron to one part of manganese, or you may induce a manganese deficiency. Iron and manganese are antagonistic to each other when oversupplied, so a manganese excess can also induce an iron deficiency. The key is to achieve the desirable balance, where there is more iron than manganese, but never more than 2:1. If we were to be less specific, the most important thing here is to just make sure that iron is higher than manganese at all times.

Note: It is not always certain that a poor balance of these minerals in the soil will negatively impact the uptake of either because various factors including soil type, environmental conditions, and organic matter levels can also be involved. This caution is actually applicable to all of the six ratios and that is why we always suggest the use of leaf analyses to confirm a potential problem.

In Conclusion

If you can work toward improving these ratios and monitor your success with regular soil testing, the benefits will flow. Your soil will breathe better, biology will work better and resilience, nutrient uptake, production, and profitability will increase. If you can not tick off improvements in at least three of these ratios each year, then you might need to seek a new consultant or improve your own knowledge of mineral balance requirements.

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