Justus von Liebig (1803–1873) was a very important person for research on mineral nutrition of plants. Liebig claimed that elements N, S, P, K, Ca, Mg, Si, Na and Fe are essential for plant growth. Although these claims had practical use and boosted agricultural yields, they were reached by observation and speculation rather than by detailed experimentation. At the end of the nineteenth century many experiments on plant nutrients were performed. From these and other extensive investigations we now know that neither the presence nor the concentration of an element in a plant is a criterion for essentiality. There are many different chemical elements in soils and from this soil reserve plants also take up elements that are not required for plant development. Although there is some indication that rare elements, e.g. like scandium (Sc), yttrium (Y), and the lanthanides: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium(Lu), might enhance growth in soil based systems (Hu et al. 2004).
Arnon and Stout (1939) formulated the definition for elements to be essential for plants. This definition has three components:
Figure 3.1 provides an appreciation of the relative amounts of carbon, oxygen, hydrogen and essential minerals in plant. Elements that are known the be essential for growth are also listed in Table 3.1. This table contains a short explanation on the function of each element in plants.
From Figure 3.1 it can been seen that plants require nine macro-elements: Carbon (C), Oxygen (O), Hydrogen (H), Nitrogen (N), Phosphate (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and Sulfur (S), and eight micro-elements: Iron (Fe), Manganese (Mn), Copper (Cu), Borium (B), Zinc (Zn), Molybdenum (Mo), Chlorine (Cl) and Nickel (Ni) although Ni is only required in low amount and not all plants need Ni. All these elements are also required in the human diet and additionally humans need: Fluor (F), Selenium (Se), Chromium (Cr), Iodine (I). There is also a list of “Plant beneficial elements”: Sodium (Na), Silicon (Si), Cobalt (Co), Selenium (Se), Aluminium (Al).
Figure 3.1 Schematic overview of plant body elements in mass percentage (%).
Micro elements | Plant available form | % Mass in dry tissue | Major functions |
Chlorine | Cl- | 0.01% | Required for water-splitting step of photosynthesis; functions in water balance |
Iron | Fe3+ / Fe2+ | 0.01% | Component of cytochromes; cofactor in many enzymes |
Manganese | Mn2+ | 0.005% | Active in formation of amino acids; cofactor in some enzymes; required for water-splitting step of photosynthesis |
Boron | H2BO3- | 0.002% | Cofactor in chlorophyll synthesis; may be involved in carbohydrate transport and nucleic acid synthesis; role in cell wall functioning |
Zinc | Zn2+ | 0.002% | Active in formation of chlorophyll; cofactor in some enzymes |
Copper | Cu+ / Cu2+ | 0.001% | Cofactor in many redox and lignin-biosynthetic enzymes |
Nickel | Ni2+ | 0.001% | Cofactor for an enzyme functioning in nitrogen metabolism |
Molybdenum | MoO42- | 0.0001% | Essential for mutualistic relationship with nitrogen-fixing bacteria; cofactor in nitrate reduction |
Table 3.1 List of essential macro elements, their form of uptake, a rough estimate of dry matter content and a short description of their function in plants.
Plant beneficial elements are not essential according to the definition of Arnon and Stout (1939) but they might enhance plant performance in some cases. Sodium (Na) can turn on plant defences and in many cultivation systems a little Na boosts the plant growth. Sodium is even essential for some plants like halophytes. Silicon (Si) is not essential but more and more growers acknowledge the befits of using silicon in their fertigation plan. Silicon is known to support the plant defence system, increase general stress tolerance e.g.: salt stress, draught, heavy metals. Cobalt (Co) is essential for nodule metabolism in legumes, but as far as we know it is not essential for plant growth itself. Reports of beneficial effects of Selenium (Se) are scares. There is a report on better resistance to UV irradiation (Hartikainen 2005), increase seed set in Brassica rapa by 43% (Lyons et al. 2009) and high concentrations can act as protection against herbivorism. Animals do not eat plants with high Se concentration to prevent Se toxicity which is responsible for certain disorders in animals grazing on native vegetation of seleniferous soils (Brown and Shrift 1982; Miller et al. 1991). Aluminium (Al) is toxic to many micro-organism and through this indirect effect there is a beneficial effect of Al application. In many tea plantations Al fertilisation is used to reduce pathogen growth.