Six Secrets to Soil Test Success (Part 1)

Six Secrets to Soil Test Success (Part 1)

Soil testing is considered an essential strategy to avoid driving blind in managing your all-important crop nutrition. Your production and profitability is 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 maximise production and minimise 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 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 favour 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 a 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 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 the 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 maximise 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 is 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 maximising uptake of phosphorus. However, this ratio is also about 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 realised 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 recognise 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 antagonise the other. If we achieve a 1:1 ratio, the uptake of both minerals is optimised.

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 the iron to manganese ratio.

To read Part 2 of this article, please click here.

Sign up to our e-newsletter to receive the latest articles, product updates and exclusive offers from NTS. Every new subscription receives a free digital copy of Graeme Sait's book, 'Nutrition Rules!'. CLICK HERE TO SUBSCRIBE