Why is My Magnesium So High, and Is It a Problem?

When soil analysis results come back, it’s common to see high magnesium levels noted, which often get overlooked in favour of pH, phosphorus (P), and potassium (K) levels. However, high magnesium can cause significant problems in soil health, leading to sticky, poorly draining fields. This mineral can make soil hard when dry and clay-like when wet, and at elevated levels, it can lock up potassium, making it unavailable to plants.

As a general guideline, magnesium levels below 100 ppm are considered deficient, while those exceeding 250 ppm are excessive. It's crucial to interpret these figures carefully; without knowing soil calcium levels and cation exchange capacity (CEC), determining true imbalances is difficult.

One important aspect of magnesium (Mg) to consider is that it affects soil pH more effectively than calcium (Ca)—specifically, about 1.67 times more. This means that a soil can have a pH of 6.5 and still be low in calcium. Calcium is essential for improving soil structure, enabling proper drainage and aeration. Therefore, incorporating calcium lime into your soil can help enhance its structure and restore balance to its nutrient composition.

Historically, magnesium levels were monitored closely due to their impact on livestock health. While deficiencies are now uncommon, especially in sandier soils, excess magnesium is frequently found in heavier soils.

Several factors contribute to high magnesium levels:

• Many lime products lack clear magnesium oxide (MgO) declarations, with magnesian lime containing at least 3% MgO and dolomitic lime containing 12% MgO.
• Certain blended fertilizers and calcium ammonium nitrate (CAN) can include magnesian lime as filler.
• Common fertilizers may also have small amounts of magnesium.
• Livestock feed is often rich in magnesium, especially in spring.
• Magnesium flakes used in livestock water troughs can further increase magnesium levels.

It’s also worth noting that FYM and slurry are significant sources of magnesium in livestock farming. For example, applying 40 t/ha (16.2 t/ac) of FYM can introduce over 72 kg of MgO/ha, often exceeding what crops need in a year.

Over time, this can lead to an accumulation of magnesium and disrupt soil balance. If your farm has high magnesium levels, it’s vital to communicate with suppliers about the magnesium content in lime, fertilizers, and minerals. This ensures that you select the right products for your farm's specific needs.

To achieve sustainable soil health, balance is key. Ensuring that calcium and magnesium levels are optimal will allow your soil to function effectively, promoting healthy biological activity. A comprehensive soil analysis will help you determine whether you need calcium lime or magnesium lime. In most cases, opt for calcium lime unless a specific need for magnesium is identified.

Remember, a good pH level does not guarantee adequate calcium levels. Use a high-purity calcium carbonate — free of magnesium content — to support your soil’s health in the best possible way.

For more information on managing magnesium levels in your soil and ensuring optimal calcium content, contact us or reach out to Jonny McKinney at 07841 341305.