This post is about dealing with excess minerals or nutrients in the soil. Dealing with excess tomatoes or zucchini is a different problem.
Adding deficient minerals to the soil is relatively straightforward and OrganiCalc is a great aid in deciding just how much of what to add. In soils with a low pH, deficiency of various soil minerals is usually the main issue. However in some soils the main issue may be an excess of minerals. This is harder to deal with. The first rule of mineral application is avoid creating excesses.
The trouble with soil mineral excesses is that they may block the uptake of other minerals into the plant. We talk about soil mineral balance and this is where it comes into play. Excesses of one mineral can cause deficiencies of another in the plant, despite there being plenty of the deficient mineral in the soil. (High or low pH can also block mineral availability, but that’s a different issue. See mineral availability vs pH.)
Although mineral excesses are to be avoided, that doesn’t mean you can’t still grow a good crop in their presence, so read on.
Besides blocking the uptake of some minerals, an excess of certain minerals can stimulate the requirement for other minerals in the plant. The classic chart showing mineral interactions is known as Mulder’s chart (below). The excess that results in blockage of uptake of another mineral is referred to as “antagonism”. The excess that results in an increased need for another nutrient in the plant is referred to as “stimulation”.
The trouble with Mulder’s chart is that it doesn’t tell you what the most likely interactions are, or what to do about them. For that, we’ve created a table with information mainly gleaned from references 1 and 2.
| Nitrogen | Increase in insects and disease. It may cause an iron deficiency in the plant. Nitrate pollution of runoff. Excess nitrogen can result in a potassium and/or boron deficiency. | Organic matter with C:N ratio higher than 15-20 will tie up excess nitrogen. Additions of calcium, sulfur or potassium can also help reduce excess N. |
| Sulfur | Elemental sulfur needs the presence of Thiobacillus bacteria to break down into sulfates and lower soil pH. It should not be added if these bacteria are not present. Sulfur in the sulfate form easily leaches and makes many other minerals soluble, so they may be leached. This includes sodium, potassium, magnesium, manganese, copper, zinc, cobalt. | Only add elemental sulfur to lower soil pH, and then only at a maximum of 300 lbs/acre, preferably 100 lbs/acre. |
| Phosphorus | Can make zinc and iron deficiencies worse if these are already deficient in the soil. An excess can also impact calcium, manganese and boron if these are deficient. | Increase the level of the deficient mineral in the soil (get a soil test) and try foliar feeding it to look for a response. |
| Calcium | An excess can inhibit magnesium and/or potassium uptake. | Increase soil levels of the deficient minerals. |
| Magnesium | An excess interferes with the uptake of calcium and potassium in the plant. It tightens soils containing clay. Surprisingly an excess of magnesium can suppress uptake of magnesium in the plant. | Look for deficiency symptoms of the cations in the plant and try a foliar feeding of the deficient cation, checking for a response. It is difficult to correct soil excesses so it is better to avoid creating them. Soils naturally high in magnesium are best treated by making sure there is adequate calcium and potassium in the soil. |
| Potassium | Soils can have huge amounts of potassium locked up in mineral form and unavailable to the plants. The potassium on the soil colloid and in the soil solution is the part that plants can access. An excess can inhibit the uptake of calcium and magnesium in the plants as well as manganese, boron and zinc. | Soluble potassium is easily leached. Gypsum applications along with leaching will remove potassium. |
| Sodium | Toxicity symptoms in susceptible plants include tip and margin burn in leaves. Sodiom interferes with the uptake of calcium,, magnesium and potassium. An excess can destroy the soil structure of clay soils by clogging the pores and stopping drainage. | Sodium excesses are usually due to saline irrigation water and the water should be treated in order to prevent soil damage. Gypsum applications can help counteract the problem of clogged soils, as can eliminating drainage problems. If the problem is caused by runoff grom salted roads ro sidewalks this can be flushed with water. |
| Chloride | Can cause tip burn on leaves, especially those susceptible to salt damage. | Usually caused by poor irrigation water. If the water cannot be treated, avoid planting crops that are susceptible to salt damage. It can also be caused by salt runoff from salting roads or sidewalks. |
| Boron | Boron toxicity can cause leaf death, starting with necrotic spots on older leaves. | Boron works in concert with calcium and is moved from the roots to the leaves through the xylem. If calcium levels are inadequate, boron can become toxic. Some crops are more susceptible to boron toxicity than others. It is best to check boron levels with a saturated paste test alongside a soil test, especially if toxicity is suspected. However, boron deficiency can really limit plant growth, so it's best to test, not guess. |
| Iron | Can cause phosphorus tie-up in low pH situations or if over-applied. | Most mineral soils contain iron in adequate amounts and sometimes in huge quantities (e.g. red soils). Not much can be done to change that. |
| Manganese | Manganese becomes extremely available at low pH below pH 5.5. Symptoms include brown specks on the leaves and browning of the stalks. | Raise the soil pH unless you're growing a low pH crop like blueberries. |
| Copper | Toxicity can occur from overuse of the fungicide, copper sulfate, or other forms of copper. It is toxic to all soil ecological systems. An excess can block the uptake of the other metals, iron, manganese and zinc. | The use of fungicides should be targeted at getting the copper into the plant rather than on the leaves. A program to eliminate their use should be put in place. |
| Zinc | An excess of zinc can make a phosphorus deficiency worse. An excess can block the uptake of the other metals, iron, manganese and copper. |
We can take a couple of different approaches when dealing with excesses, depending on which mineral is excess.
- In the presence of excesses, maintaining biological activity becomes even more important. Microbes can turn the blocked minerals into a form that plants can assimilate rather than relying on the plant to absorb them directly. It is always important that soil microbes have plenty of food sources and good conditions to thrive but even more so if you are dealing with excesses.
- If the excess mineral is mobile in the soil (see this page for which minerals are), it can be driven deeper in the soil by leaching with water. All minerals that form sulfates are mobile in the soil in the presence of sulfates. So they may all be leached with the addition of sulfur or sulfates. This double edged sword means that not only the minerals that are in excess, but all minerals that form sulfates may and will be leached.
- We can use the cation displacement hierarchy Ca > Mg > K > Na on the soil colloid (see this page for more details). This is something of a zero sum game and doesn’t really describe everything that is going on, but on the whole the principle works. If we add calcium, then magnesium, potassium and/or sodium will be displaced (and leached out if there is enough water). If we add potassium, only sodium will be leached out. The entire hierarchy is Al+++ > H+ > Ca++ > Mg++ > K+ > Na+. To explain, H+ represent acidity, H+ ions. If we make a soil more acid, then calcium, magnesium, potassium and/or sodium will be displaced off the soil colloid. And if we add aluminum we might even raise the pH by displacing H+ ions, but also at the expense of Ca, Mg, K and Na.
- Another technique for dealing with excesses and deficiencies in the plant is to run brix tests. Using a refractometer, measure the brix of your plants. Apply a foliar spray to some but not all of the plants. Measure the brix at the same time the next day. If the brix went up on the foliar sprayed plants relative to the controls, you have discovered a way to supply a missing nutrient. Don’t be tempted to use visual observation instead of brix since it is easy to fool the eye. For instance, adding foliar nitrogen almost always makes the plants look better, but it can decrease the weight and nutrient density of the harvest.
Neil Kinsey presented some of the most common practical interactions in table form in Acres USA magazine in 2013.
References
- The Farm As Ecosystem, by Jerry Brunetti, 2014, Acres USA
- A Grower’s Guide for Balancing Soils, by William McKibben, 2021, Acres USA
