How much is too much? Toxic levels of essential soil nutrients (ASK Around)

Your soil test results report includes results and recommendations related to five macronutrients and five micronutrients that are all essential for luscious grass and for all kinds of plants to thrive in your yard. (Click here for a quick review of what they are and what they do!) Not having enough of any one of these nutrients will have a clear impact on plant health, since nutrient deficiencies can cause all kinds of symptoms like chlorosis (pale/yellow coloration), brown edges or patches on leaves, shriveling, and stunted growth. SoilKit treatment recommendations are all about adding enough fertilizer to get your soil out of this deficient range and keep it at the sufficient level where your plants have optimal access to the nutrients they need. But what about the other extreme -- are there detrimental effects of having test results that are too high? And can an abundance of an essential plant nutrient even become toxic?

For some elements, the answer is no. For example, it's not generally considered detrimental for your plants to be exposed to high soil levels of nutrients like potassium or calcium (although an excess of one nutrient can interfere with uptake of a less abundant one -- Does the “balance” of nutrients matter or just quantity?). If your test results show that these nutrients are present in the soil above the deficient range, SoilKit will recognize that their levels are sufficient and not include them in seasonal amendment recommendations. All good for this year! However, for some elements, high levels can directly harm your plants:

• High levels of phosphorus (above 150 ppm) can lead to chlorosis and stunted plant growth. This is primarily caused by soil phosphorus blocking the roots from absorbing needed micronutrients such as iron and zinc, so the plant deficiencies of these nutrients.
• Iron toxicity is associated with acidic soils and iron test levels above 300 ppm (depending on the plant). Excessive iron tends to cause stunted root growth and a distinct type of leaf discoloration called bronzing.
• Boron toxicity is tricky because of the narrow Goldilocks range between deficient levels (below 0.5 pm) and toxic levels (2 ppm and above for sensitive plants). Excessive boron levels cause plant leaves to turn yellow/brown, dry up, and eventually fall off prematurely.
• Manganese toxicity can cause yellowing of young leaves or the appearance of necrotic (dead) spots or streaks on older leaves. Excessive manganese levels tend to occur in acidic soils, where test levels over 300 ppm can be dangerous for plants.

Besides the direct impact of soil nutrient toxicities on plant growth, high nutrient levels can also take a toll on the environment and your wallet. If your soil is already sufficient, spending time and money to apply fertilizers full of nitrogen and phosphorus not only won't help your plants but also can contribute to pollution of our water and air. Excess fertilizer nutrients that get washed away by rain and erosion can end up in our waterways, cause toxic algae blooms and destroy aquatic ecosystems. Excess nitrogen in particular can also be released into the atmosphere as nitrous oxide, which is a greenhouse gas three hundred times more powerful than carbon dioxide that also damages Earth's protective ozone layer. It's critical to get your yearly soil test done before applying any fertilizers to check how much the soil actually needs! Matching the amount and timing of fertilizer applications to the requirements of your plants will ensure your efforts don't go to waste.