Mobile nutrients are nitrogen in the form of nitrate, phosphorus (P) in the form of phosphate, potassium (K), magnesium (Mg), chlorine (Cl), zinc (Zn) and molybdene (Mo).
Calcium (Ca), sulfur (S), iron (Fe), boron (B) and copper (Cu) are immobile.
If there is a deficiency of mobile nutrients, the symptoms are first seen in the older leaves, as the nutrients are transported to new growth from there. Deficiencies of immobile nutrients first appear in the new growth as the plant was unable to take up sufficient amounts to transport them to the new shoots.
Mobilty or immobility is always relative, plants can transport immobile nutrients to other areas by making use of chelators. A deficiency of these nutrients can be amended by foliar fertilization.
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What does it mean for an element to be mobile or immobile in plants?
Generally speaking, mobile elements are those that can be moved from older to newer tissue in the plant, while immobile ones cannot be. For example, calcium is incorporated into the cell wall, so it is immobile; it cannot be relocated later.
Plants with a deficiency of mobile elements (e.g. nitrogen) will exhibit damage to older tissues as the nutrients are moved to new growth. Conversely, plants lacking immobile elements (e.g. calcium) will exhibit stunted new growth while the older tissues stay healthy. Some elements (e.g. sulphur) fall somewhere between these two extremes.
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A lot of nutrients are wrongly applied or completely wasted because they never reach their target. It is so important to understand how nutrients move within plants in order to make sure that soil- or foliar-applied nutrients are taken up and moved within the plant to the part of the plants where they will be used. Knowing how nutrients move also can help us apply nutrients at the right time during the season.
Which nutrients are non-mobile?
Eighteen mineral nutrients including Carbon, Hydrogen and Oxygen are commonly described as essential for plants. Many scientists have added silicon (Si) as an essential element to their list. Of the elements that are essential to crop production and applied to crops through the use of fertilizers, only 7 (N, P, K, Mg, Na, Cl, and S) are mobile in all crops. There are 9 nutrients which are limited in how they move in most crop plants.
Phloem mobile or not?
Surprisingly all nutrients move in the xylem. When we’re talking about non-mobile nutrients we’re really talking about the nutrients that won’t move in the phloem. The xylem is for carrying water and nutrients from the roots upward toward the leaves. The phloem is for carrying sugar, other metabolites and nutrients either upward or downward in a plant (depending on time of day). Phloem mobility is the key for a nutrient to be redistributed within the plant once it is taken up. It is also a key to successful movement of nutrients applied to the foliage. Zinc, for example is needed mostly in the growing points and in newly expanding leaves. Most foliar applied Zinc hits the older leaves where it is not needed. Since it doesn’t move well in the phloem, most of it stays in the leaves and never hits the target.
Any nutrient that is sufficient in the soil will be carried up to the leaves in sufficient quantities. The non-mobile nutrients are the problem. If, for even a short time, the soil is not able to supply what the plant requires, rapidly growing plant parts quickly become deficient in the non-mobile nutrients because they cannot be moved from older leaves, roots, stems or wood to the plant part in need. A classic example is Calcium (non-mobile) deficiency symptoms in supposedly Calcium-rich soils. Blossom rot on tomatoes, bitter pit in apples and hollow heart on potatoes are often aggravated by a lack of soil moisture during hot weather. The lack in soil moisture reduces the amount of water and Calcium moving up the plant. The little bit of remaining Calcium tends to move toward leaves, not toward the fruit (xylem movement is mainly toward leaves, phloem movement is mainly toward fruit).
Some important exceptions
Non-mobile nutrients are stored in the leaves by specific binding proteins. Some plant growth regulators and biostimulants can down regulate the genes for these proteins and cause these nutrients to become mobile.