Gardening: Diagnosing mineral nutrient disorders in your landscape
Just like humans, plants need balanced nutrition to maintain their health. Mineral nutrient disorders can be a problem in some landscape plants, such as iron chlorosis in red maple, river birch, and pin oak. All landscape plants – including annuals, perennials and woody plants – can potentially suffer from mineral nutrient disorders if the mineral nutrients are not available in the correct amounts. Fortunately, advances in soil fertility, plant nutrition, fertilizer technology and plant breeding have drastically reduced the mineral nutrient problems in the landscape.
Keep in mind that plant-mobile elements are nitrogen, phosphorus, potassium magnesium chlorine and molybdenum. The deficiencies of these elements show up on older leaves first. The plant-immobile elements are sulfur, calcium, copper, iron, manganese, nickel and zinc. The deficiencies of plant immobile elements show up on younger leaves first. It is important to note which plant leaves are affected (or affected first).
Not all mineral nutrient disorders, especially nitrogen, phosphorus and potassium deficiencies, are common in the landscape. In nitrogen deficient broadleaf plants, older leaves become generally chlorotic (uniformly yellowish green), while young leaves may retain their typical green color. In other words, both leaf veins and the tissue between veins turn yellow on older leaves. The plant growth is stunted. With severe nitrogen deficiency, the entire plant might turn yellow.
Phosphorus deficiencies in broadleaf plants may be exhibited as dark green leaves with purplish coloration, especially early in the growing season. The reddish-purple coloration appears first on older leaves. Phosphorus deficiency is typically caused by poor phosphorus uptake in spring, since root growth is limited by cold and wet soil. A field corn plant with phosphorus deficiency will have reddish/purplish coloration on older leaves.
Potassium deficiencies are characterized as marginal and interveinal chlorosis (discoloration between the leaf veins), followed by necrosis (cell death). Because potassium is mobile in plants, potassium deficiencies show up on older leaves first. Potassium deficiency resembles the marginal leaf scorching from drought.
Calcium deficiency is rare in landscape plants; however, it is more common in fruits and vegetables. Blossom-end-rot of tomatoes is a type of calcium deficiency. In apples, the deficiency causes bitter pit or cork spot on fruit. It should be noted that calcium deficiency in tomato and apple is not necessarily caused by lack of calcium in soil, but rather poor calcium uptake by the plant roots.
Symptoms of magnesium deficiency in plants include interveinal chlorosis in older leaves, curling of leaves along margins, and marginal yellowing, with a green “Christmas tree” area along mid-rib of leaf. This green “Christmas tree” look is unique: While the rest of the leaf looks chlorotic, the shape of green area is pyramidal and – as the description implies – resembles a Christmas tree.
Plants deficient in sulfur show a pale green coloring of the younger leaves, although in severe cases the entire plant can appear pale green and stunted. Because sulfur is a component of proteins, its deficiencies are similar to those of nitrogen. The difference between sulfur deficiency and nitrogen deficiency is that sulfur is immobile in plants. Hence, new growth suffers first when sulfur levels are too low.
Among all micronutrients, iron and manganese deficiencies are relatively common. Iron chlorosis is a common problem in pin oak and river birch trees. Iron deficiency is characterized by yellowing of leaf tissue between veins and occurs when plants are unable to acquire enough iron from the soil. Because iron is immobile in plants, older basal leaves remain green as young leaves become chlorotic between veins. Iron deficiency can be caused by low iron concentration in the soil or limited availability under alkaline soil conditions. Iron is needed for the production of chlorophyll and without iron, the leaves lose their green color. If uncorrected, affected twigs and branches may die. If severe enough, iron deficiency may lead to the death of the tree.
There are other causes of plant stress that mimic nutrient deficiencies. Drought, stress, root damage and herbicide applications can cause the plants to develop symptoms that mimic nutrient deficiencies. In addition to deficiencies, plants can also develop toxicities such as over-application of fertilizer. Toxicities of mineral nutrients cause ‘salt burn’ or ‘fertilizer burn’.
Soil and tissue tests can be very useful tools when diagnosing nutrient disorders. It is easier to fix a problem when you have an accurate diagnosis. Simply throwing fertilizer on the ground may not solve the problem and may make the problem worse.
References: University of Minnesota Extension, The American Phytopathological Society of St. Paul, MN