Chemical compounds required for life are termed nutrients. Higher (vascular) plants require only inorganic nutrients. This separates vascular plants from all animals and many microorganisms that need to feed on organic nutrients to survive. An element is considered “essential” to plant growth if it is directly involved in the metabolism of the plant and is required for the completion of its life cycle. There are 17 essential plant nutrients, nine are macronutrients and eight are micronutrients (Table 1). Macronutrients or major elements are those required in relatively large quantities (>1,000 ppm) and include nitrogen (N), phosphorus (P), potassium (K), sulfur (S), carbon (C), hydrogen (H) and oxygen (O). Calcium (Ca) and magnesium (Mg) are also major elements; however, they are often required in moderate amounts relative to N, P, K, S, C, H and O and therefore may be referred to as secondary macronutrients. Micronutrients or minor elements are required in relatively smaller quantities (<1000 ppm) and include iron (Fe), manganese (Mn), zinc (Zn), boron (B), molybdenum (Mo), copper (Cu), chlorine (Cl) and nickel (Ni). It has been shown repeatedly that species differ in their abilities to accumulate various elements. For example, leaves of flowering dogwood and white oak contain over twice as much calcium as leaves of loblolly pine growing in the same soil (Kramer and Kozlowski 1979). Because of differences in ability to absorb and translocate minerals, woody plants of various species and even individuals of the same species react differently to nutrient deficiencies.
Essential or non-essential?
More than half the elements in the periodic table have been found in plants, and it seems probable that every element occurring in the root environment could be plant absorbed (Kramer and Kozlowski, 1979). However, many of these elements such as iodine (I), bromine (Br), fluorine (F), aluminum (Al), chromium (Cr), selenium (Se), lead (Pb) and cadmium (Cd) are not only non-essential they are generally detrimental. The role of some elements such as silicon (Si) are only beginning to be understood in plants. Although Si is the second most abundant element in the earth’s crust, it is not a plant essential nutrient. Of the terrestrial vascular plants, only horsetails have been shown to need Si. However, Si has been shown to be beneficial for increased disease resistance to fungal pathogens and in amelioration of abiotic stresses (Marschner, 2006). Epstein (1994) noted no other element considered non-essential to plants is found consistently in such high amounts in plants.
|To view Table 1 click the image above.|
Nickel is an element only recently shown to be essential for plant growth. The requirements of plants for Ni are lower than that for any other essential nutrient. The second least required micronutrient is Mo. Almost all soils have enough Ni and Mo to support plant growth, but under some conditions, Ni and Mo deficiencies can occur. Nickel deficiencies in the nursery and on rare occasions in the landscape (Fulbright and Cregg, 2007) are found with river birch (Betula nigra). Deficiencies in Mo are generally more common in nitrogen fixing plants (Table 1). In legumes such as Robinia pseudoacacia, Cercis canadensis, Maackia amurensis, and Gleditsia triacanthos and non-legumes such as Alnus sp. and Betula sp. (that fix nitrogen via a bacteria called Frankia), Hippophae rhamnoides, Elaeagnus angustifolia and Shepherdia canadensis, Mo is required in large quantities, particularly in the nodules (Table 1).
Ni deficiency of river birch may have been induced via excessive additions of Mn and Zn in nursery container production in the 90s, similar to what had been seen in pecans in the 80s (Ruter, 2004). Also, in the late 80s it had been recommended that superphosphate be dropped from container substrates due to its solubility and problems with eutrophication of waters. Nickel is known to be a contaminant in phosphate fertilizers and by dropping superphosphate, inadvertently the only source of Ni to the plants was also dropped (Ruter, 2003). Small, wrinkled, often darker green in color, commonly cupped leaves with necrotic margins are symptoms of Ni deficiencies in container-grown river birch. New growth also has severely shortened internodes which gives a witches-broom appearance. The plants also appear stunted and may appear to have been “sheared” into their stunted form. One branch may be affected on the plant and other branches will be normal. Mouse ear of birch (leaf curl, little leaf, squirrel ear) has been used to describe the symptoms of Ni deficiency on this species.
In forest communities, nutrients are in balance, active microorganism populations exist, the soil pH is suited to the plant community and the soil structure is undisturbed. In this environment the addition of supplemental nutrients or fertilizers are not required. In the urban environment; however, soils are often less than desirable and fertilizer may increase availability to nutrients that would otherwise be inaccessible to the plants.The primary difference between forest soils and urban soils is lack of organic matter found in the urban sites. Organic matter is the vast array of carbon compounds in soil. Originally created by plants, microbes and other organisms, these compounds play a variety of roles in nutrient, water, and biological cycles (University of Minnesota, 2002). Many urban landscape plantings will occur on disturbed sites where A or B horizons no longer exist. Plants are placed into subsoil or combinations of construction debris and subsoil. Nursery sites, although not as dire as their landscape counterparts, may also be deficient in organic matter (OM); especially where OM replacement programs have not been practiced and soils have been cropped consecutively or cultivated for more than one decade.
It is important to note that although fertilizers are added to landscape and nursery sites, their addition may have no beneficial impact. Without proper OM content in the soil, the nutrient holding capacity of soil [cation exchange capacity (CEC)] is severely reduced. Also, the pool of nutrients and the chelates that bind the nutrients to prevent their becoming permanently unavailable are all significantly diminished. OM also serves as food for soil organisms from bacteria to worms that further hold on to nutrients and release them in forms available to plants. This explains why several studies in the past 10 years support that top dress applications of nitrogen fertilizers in landscape or nursery field trees provided no benefits in growth versus no fertilizer (Day and Harris, 2007; Harris et al., 2008; Robbins, 2006). These previous studies also stated that N fertilizer did not speed establishment, increase shoot extension or leaf nitrogen (Day and Harris, 2007). If the soil is too low in OM, no amount of N will solve the problem of poor growth.
Previous studies which showed no benefit to fertilizing trees went against long-term standard practices, resulting in confusion among growers and landscapers. Some growers even eliminated fertilizers from their operations. This decline in fertilizer use was further promoted by the downturn in the economy and slow sales of caliper trees. Growers felt that reduction in fertilizer use was just “a sign of the times.” However, in part, the no-fertilizer practice was based on incomplete studies. The studies that saw no benefits, explored only soluble agriculture grade fertilizers (SAGFs), not controlled release fertilizers (CRFs). No minor nutrient packages were used with the SAGFs that would have been available with the CRFs. To further support that minors are required, in the only recent study where a fertilizer significantly increased growth, minors were included (Robbins, 2006). Fertilizing oak with a minor package Robbins (2006) used a CRF (Polyon 17-5-11) compared to several SAGFs. The Polyon 17-5-11, which contained Ca, was the only treatment to provide a significant increase for caliper with swamp white oak (Quercus bicolor).
The issue with the elimination of field fertilizing may result in long-term susceptibility of the trees to cold, drought or flooding stress and long-term deficiencies. It is well known that nutrient deficiencies (especially minors) in plants are chronic conditions, not catastrophic, and develop early in the life of the plant. In a 2004 study, we found that fertilizing with N-only fertilizers in field-grown deciduous trees was important for increasing the content of certain minors and reducing visual symptoms of minor deficiencies, even though no significant increase in growth was achieved with the extra N. This study also concluded N was not limiting at 200 pounds per acre SAGFs; however, minors were. In 2009 through 2011, we conducted a field nursery study at two Ohio and two Ontario nurseries and found that CRFs containing minor packages increased caliper growth, extension growth and sped establishment (Mathers et al., 2012). Growth was increased with N fertilizer additions alone; however, to get maximum growth specific minor elements were required depending on species (Mathers et al., 2012). The trial results seem to indicate zinc (Zn) and manganese (Mn) were more important to the growth of red maple (Acer rubrum ‘Red Sunset’) versus Chanticleer pear (Pyrus calleryana ‘Chanticleer’) or red oak (Quercus rubra), and Mg and Ca are more important to oak (Mathers et al., 2012).
Fertilizers can be added as CRFs, SAGFs, slow release fertilizers [sulphur-coated ureas) (SURFs)] or liquid fertilizers. The release method of the fertilizer varies with the type of fertilizer applied and includes osmosis, microbial action, hydrolysis and physical breakdown. Generally when nursery/landscape managers buy fertilizer it is called a complete fertilizer or it contains N-P-K. All the N will be available to the plant; however, the P and K are not. The P will be present as P2O5 and the K as K2O. Therefore the manufacturers’ analysis of the complete fertilizer is not the “actual analysis.”
The conversion to the actual analysis is a very important step in proper fertilizing; however, few are aware of these conversion requirements. In most cases, growers are under applying P and K. The proper balance of N-P-K should also be maintained in all applications and includes ratios of 4-1-2 or a 3-1-2. In our example above our 10-6-4 seems to be not in the correct proportion of N-P-K; however with the conversion, we see it is not so imbalanced. As a second example, if we had a 50 pound bag of a complete fertilizer, analysis of 22-3-8 we would apply 11 pound N, 0.66 pound P, 3.52 pound K, indicating a severe disparity from a 4-1-2 ratio or 3-1-2 ratio and the necessity to purchase a more properly proportioned product or evaluate what other fertilizers have been used on the site to correct the imbalance.
Mo and Ni may be required in the lowest amounts in the plant; however, N is required in the highest amounts due to its use in synthesis of all amino acids and chlorophyll. N is also susceptible to leaching and volatilization and needs to be replaced each year in field production and delivered in a constant method in containers. Sometimes N maybe the only limiting nutrient and is expressed as uniform yellowing beginning with older leaves. Small pale yellow leaves, shortened internodes and stunted growth are also symptoms of N deficiency. When only N is required, still ensure that P and K are present in sufficient quantities in the substrate that the proper portion of nutrients are present.
Landscape and field nursery trees have traditionally been fertilized with high nitrogen fertilizers customarily applied as top dress (surface) or drilled sub-surface at the recommended rates of 220 to 264 pounds per acre (Smith, 1991) or 6 pounds per 1,000 square feet. The applications of soluble fertilizers are normally split (spring and fall) to be completed in late spring on or before mid to late June and mid to late autumn before a normal freeze. A late fall application after a hard freeze or early spring application four to six weeks prior to the beginning of growth could be used instead of one of the split applications. However, these late fall or spring applications were recommended for species with determinate growth with more than one growth flush, such as most deciduous shade trees (Smith, 1986).
Our current recommendations are 100 pounds per acre N applied as a CRF for field and landscape trees. It is essential that the CRFs contain a minor package and that organic matter is added to the soil to improve CEC. More research is required to determine which minors are more important to which woody plant species. Manganese deficiencies in maple are well known. Side by side an oak may have Fe chlorosis and a maple Mn deficiency. Mn deficiency closely follows iron in terms of commonly occurring micronutrient deficiencies. Leaves of Mn deficient plants may develop pale, brownish or purplish spots. The incidence and severity of all minor nutrient deficiencies is completely under researched in woody plants but their roles seem more limiting than the N-P-K fertilizers we traditionally apply.
Hannah Mathers is a professor and state nursery and landscape extension specialist at The Ohio State University, Department of Horticulture and Crop Science; firstname.lastname@example.org.