Nutrient Mobility

Plants that are struggling need more fertilizer, right?

It makes sense. Your plants were doing fine, growing and thriving. Then, they started to decline. Leaves lost their color and fell off, stems lost their vigor, and flowers started looking like pale comparisons of their former selves. So, you add fertilizer and the plants look better. For a while. Then the decline returns. You add more fertilizer. And the cycle continues.

While you may think you are feeding your plants a healthy diet, what is more likely to be happening is toxic levels of some nutrients are building up in the soil, causing more complex problems. Excluding disease, insect attack, or improper soil pH, adding more fertilizer makes sense, on the surface. Except when it doesn’t, because sometimes that’s the last thing you should do. While there are certainly cases in which adding an all-purpose (hopefully organic) fertilizer is a good idea, more often than not, nutrient-based problems are more about imbalances and nutrient mobility.

Nutrient imbalances

Before we look at nutrient mobility, we need to understand a little bit about nutrient imbalances. In technical terms, this is called the cation exchange capacity. Put simply, cation exchange capacity (CEC) is a measurement of how many positively charged minerals can be held by the surface of a soil particle. Clay and organic material tends to be negatively charged, while many plant nutrients are positively charged. Mineral ions that are negatively charged are held in suspension, in water. These charged ions act like magnets. If the electrical charges are out of balance, plants cannot absorb the nutrients they need. This is why adding even more of an unnecessary nutrient can make matters worse, rather than better. In some cases, nutrients may already be inside your plant, but not where they are needed. This is where nutrient mobility comes in.

Plant nutrients and mobility

Plants absorb fourteen mineral nutrients from the soil. These nutrients are divided into macronutrients and micronutrients. Plants need large amounts of the macronutrients and small amounts of the micronutrients. Don’t be fooled, however. Just because a small amount is needed, doesn’t mean it is unimportant. As in the case of iron, insufficient levels can cripple a plant’s ability to absorb and use many other nutrients.

​Mobile nutrients

When a nutrient is considered mobile, it moves around inside a plant easily. Nitrogen is a highly mobile nutrient. Nitrogen deficiencies can be seen when nitrogen is pulled from older leaves, causing chlorosis (yellowing), to feed newer leaves. Other highly mobile plant nutrients include phosphorus, potassium, magnesium, chlorine, molybdenum, and nickel. When any of these nutrients become deficient, the symptoms will appear in older growth first.

Immobile nutrients

Other nutrients are not easily moved once they are incorporated into a plant. In most cases, these nutrients stay where they are first dropped off, usually at growing points. This is great for as long as there is a steady supply of those nutrients. It takes a lot of water to move them around inside a plant. This is why blossom end rot isn’t really caused by a simple calcium deficiency. Instead, it is caused by insufficient calcium and insufficient water, which is needed by the plant to move the calcium where it is needed when it is initially absorbed. Other immobile nutrients include sulfur, boron, copper, iron, manganese, and zinc. Deficiencies of immobile nutrients are more likely to appear in newer growth.

What nutrients are in your soil?

I’ve said it before and I’ll say it again: you don’t know what is in your soil without a soil test from a reputable, local lab. I wish I could say that those cute, over-the-counter soil tests were effective, but they aren’t. Not yet, anyway. Find a soil test lab on your side of the Rocky Mountains and send them a sample. That is the ONLY way to know what nutrients are available to your plants. 

What are you feeding your plants?