KNOWING NUTRIENT MOBILITY IS HELPFUL IN DIAGNOSING PLANT NUTRIENT DEFICIENCY

Nutrients important for plant growth vary in their ability to move within the plant. Knowing how they move can be helpful when diagnosing deficiency problems.

Seventeen elements have been identified as vital to plant growth. Three elements, carbon, hydrogen and oxygen, are non-minerals and the other 14 (Table 1) are minerals. Carbon and oxygen enter plants through leaves as carbon dioxide. Oxygen also enters plants with hydrogen through roots as water. The other 14 must be dissolved in soil water and enter the plant as roots take up water.

Mineral elements can further be divided into primary or secondary macronutrients and micronutrients. Macronutrients are those needed in relatively large amounts while micronutrients, as their name implies, are needed in small amounts. However, a deficiency in any vital element can seriously inhibit plant development.

Table 1. The 14 elements essential for plant growth and their mobility and role within the plant.
Macronutrients	Symbol	Mobile in plant	Role in plant
Primary
Nitrogen	N	Yes	Formation of amino acids, vitamins and proteins; cell division
Phosphorous	P	Yes	Energy storage and transfer; cell growth; root and seed formation and growth; winter hardiness; water use
Potassium	K	Yes	Carbohydrate metabolism, breakdown and translocation; water efficiency; fruit formation; winter hardiness; disease resistance
Secondary
Calcium	Ca	No	Cell division and formation; nitrogen metabolism; translocation; fruit set
Magnesium	Mg	Yes	Chlorophyll production; phosphorus mobility; iron utilization; fruit maturation
Sulfur	S	No	Amino acids formation; enzyme and vitamin development; seed production; chlorophyll formation
Micronutrients
Boron	B	No	Pollen grain germination and tube growth; seed and cell wall formation; maturity promotion; sugar translocation
Chlorine	Cl	Yes	Role not well understood
Copper	Cu	No	Metabolic catalyst; functions in photosynthesis and reproduction; increases sugar; intensifies color; improves flavor
Iron	Fe	No	Chlorophyll formation; oxygen carrier; cell division and growth
Manganese	Mn	No	Involved in enzyme systems; aids chlorophyll synthesis; P and CA availability
Molybdenum	Mo	Yes	Nitrate reductase formation; converts inorganic phosphates to organic
Nickel	Ni	Yes	Nitrogen metabolism and fixation; disease tolerance
Zinc	Zn	No	Hormone and enzyme systems; chlorophyll production; carbohydrate, starch and seed formation

Once inside plants, nutrients are transported to where they are needed, typically to growing points. Once incorporated by the plant, some elements can be immobile while others can be remobilized. Immobile elements essentially get locked in place and that is where they stay. Those that can be remobilized can leave their original location and move to areas of greater demand. Knowing which are mobile or immobile is helpful in diagnosing deficiency symptoms.

Since immobile elements do not easily move within the plant, when deficiency symptoms occur they show up in new growth (Photo 1). When mobile elements become limiting, they can be scavenged from older growth and moved to where they are most needed, causing deficiency symptoms in older growth (Photo 2).

Photos 1-2. (Left) Typical deficiency symptoms of a non- mobile nutrient (iron) within the plant. Note newer leaves are more affected. (Right) Typical deficiency symptoms of a mobile nutrient (nitrogen) within the plant. Note older leaves are senescing while younger leaves are still green. Photo credits: Howard F. Schwartz, Colorado State University, Bugwood.org (left) and R.L. Croissant, Bugwood.org (right)

Most nutrient deficiencies need a certain amount of time after growth begins for symptoms to occur. Early growth is often not fast enough or of a great enough volume for symptom expression. Deficiencies are often revealed when the plant is at maximum growth or at other times of high nutrient demand such as fruit development.

According to Michigan State University Extension, nutrient deficiencies can be due to a number of reasons. The most obvious is that the element is not in a high enough level naturally in the soil. Many sand-based soils with high leaching potential are often low in highly soluble nutrients. In some cases, the element is in adequate levels, but unavailable due to pH being too high or too low or the soil temperature being too low for adequate uptake. Other reasons could be too little or too much water or soil compaction. Remember all mineral elements need to come from the soil and if water uptake is interrupted for any reason, so is nutrient uptake.