Signs You May Need to Fertilize

Plants need 16 elements in order to grow. Plants are provided with Carbon, Oxygen and Hydrogen from water and sunlight leaving 13 other necessary elements. Depending upon soil type and the specific plant there may be deficiencies in the 3 macronutrients (Nitrogen, Phosphorus and Potassium) or some of the secondary nutrients.

Most fertilizers address the macronutrients evidenced by the NPK (representing the chemical symbols for these nutrients) percentages listed on fertilizer packaging. In our area Nitrogen is the most common nutrient deficiency of the home garden. Many times Phosphorous and Potassium will be adequate due to generally high natural levels and build up from previous fertilizing.

As for the secondary nutrients, the silty/clay loam soil in Wisconsin holds nutrients quite well and Calcium, Magnesium and Sulfur are rarely low enough in this soil to cause a problem.
Some of the trace nutrients, such as Iron and/or Manganese, even though only needed in small amounts may be lacking due to high soil pH (i.e. strong alkaline conditions). Most commonly this causes problems for Ornamentals such as Roses and trees such as Pin or White Oak or Birch trees.

Both the type and location of symptoms are important in identifying the responsible deficiency. Types of symptoms are: yellowing, burning/browning, mottling and stunting. Locations to check for symptoms are: terminal bud (the bud that develops at the apex of a stem), youngest leaves/new growth, oldest leaves.

Knowing how nutrients move through plants can help to determine what may be lacking. The macronutrients (Nitrogen, Phosphorus and Potassium) are readily moved from old leaves to new growth. Calcium, Boron, Copper, Zinc and Molybdenum all show little or no movement therefore evidence of deficiency will be noticed on new growth.

Following is a list of general appearance of deficiency by soil-supplied nutrient. Symptoms can vary among plant species and in some plants similar symptoms may be due to environmental conditions, insect damage, chemical burn, mechanical damage or disease. Therefore, it is recommended that a chemical analysis is used to confirm deficiency once a symptom is observed.

Nitrogen (N) - The most dominant symptom of nitrogen deficiency is a yellowing of the leaves, and lack of growth. However, yellowing can also indicate iron deficiency. Nitrogen is the number one burner of plants when applied in excess so knowing the difference between these two is important. The answer to this riddle is that lack of nitrogen shows first in older leaves, whereas iron deficiency shows first in newer leaves.

Potassium (P) - Older leaves and/or leaf edges are yellow or brown; leaves may be chlorotic (yellow) and curl, and may have necrotic (burned/brown) spots. Unlike salt/fertilizer burn it is limited to older leaves. This, of course, can really slow a plant down because all of its older leaves drop prematurely, so the plant has trouble getting the energy benefit out of leaves that it spent a lot of energy growing.

Phosphorus (K) - Phosphorus deficiency causes older leaves to become dark green and have red or purple patches in them. Growth can be stunted and/or the plant can have late flowering/maturing fruits. Fertilizers that add potassium and/or nitrogen almost always add phosphorus so this may be the reason this deficiency is seldom observed.

Calcium (Ca) - Deformed or underdeveloped terminal buds and roots; slow to develop; blossom end rot on tomato and pepper fruit

Magnesium (Mg) - Interveinal chlorosis (yellowing) and or/mottling of older leaves

Sulfur (S) - Light green color of whole plant; New growth turns pale yellow, older growth stays green. Stunts growth.

Iron (Fe) - The most obvious symptom of iron deficiency in plants is commonly called leaf chlorosis. This is where the leaves of the plant turn yellow, but the veins of the leaves stay green. Typically, leaf chlorosis will start at the tips of new growth in the plant and will eventually work its way to older leaves on the plant as the deficiency gets worse. Other signs can include poor growth and leaf loss, but these symptoms will always be coupled with the leaf chlorosis.

Manganese (Mn) - Growth slows. Younger leaves turn pale yellow, often starting between veins. May develop dark or dead spots; frequently occurs on middle leaves first. Leaves, shoots and fruit diminished in size. Failure to bloom.

Boron (B) - Poor stem and root growth. Terminal (end) buds may die. Dieback of shoots or growing points; thickened, wilted or curled leaves; distorted fruit; hollow root vegetables.

Zinc (Zn) - Short internodes; Yellowing between veins of new growth. Terminal (end) leaves may form a rosette; small leaves mis-shapen leaf blades and abnormal root growth.

Copper (Cu) - Bluish- green appearance; plants appear wilted; young leaves die, first may resemble frost injury; summer dieback of terminal shoots of fruit trees; affects flowering and fruit formation

Molybdenum (Mo) - Yellowish or pale green leaves; deformed leaves. Some leaves have a mottled and/or cupped appearance.

Chlorine (Cl) - Extremely rarely deficient in WI