For proper growth and function plants require a limited set of chemical elements (Table 5.1). Most of the plant body mass is water (~90%). If fresh plants are dried (in an oven at 70 oC) all liquid water is removed and the fresh weight (fresh biomass) of plants is reduced to plant dry weight (dry biomass is ~10% of fresh biomass). A mass analysis of the plant dry weight shows that, depending on plant species, ~96% of the plant dry weight is made up of the structural elements Carbon, Oxygen and Hydrogen (Figure 5.2). Only 4% of the plant dry weight exists of other nutrients. However, the absolute amount is not representative for the importance of the nutrient. For example plants need very small amount of zinc (Table 5.1), but in the absence of zinc plants will not grow. Note, however, that supply of extremely high concentration of nutrients can also reduce growth rate or even kill the plant.

The amount of nutrients available to the plant will determine the plant form (morphology), functioning and rate of biomass increase. Regarding plant nutrient status in relation to growth speed one can roughly distinguish three situation: deficiency, adequate levels and toxic levels (Figure 5.2). For example, increasing the nitrogen dose in the deficiency state, will result in an increasing relative growth rate, producing longer flaccid dark green plants with many leafs. Plants that are nutrient deficient or contain toxic levels are generally small compact stressed plants with physiogenic disease symptoms, depending on the nutrient in question. In professional horticulture the optimal amount of plant available nutrients are based on research and practical experience. The amount of nutrients that plants need depends on the species, variety, growth stage and environmental conditions. In other words for each specific plant variety under given cultivation conditions at a given developmental stage there is a required recipe for optimal growth. In practice each grower has a basic recipe per plant variety e.g. in de Kreij et al (1999) . This recipe is adjusted during the cultivation period depending on environmental conditions and the plant growth stage. To further optimise, i.e. fit the plant available nutrients to the plant requirements, growers analyse the element composition of drain water, substrate water, plant sap and dried plant material. This is in many cases required because the basic recipe and recipe adjustments are estimates of netto nutrient uptake activities. The analysis can verify whether the provided amount of nutrients is optimal. This verification is especially required when new varieties or new growth environments are introduced. Another reason that these tests are needed is that salts will accumulate in the substrate. This can cause local increase in osmotic potential, toxic salt levels and/or deviations in pH. The amount of one ion can, furthermore, influence the uptake of another ion. For example, an excess of potassium will hinder the uptake of calcium and magnesium. Therefore, the ratio’s (relative amounts) between the nutrients is very important. The absolute amount, the number of ions per litre of nutrient solution, determines the osmotic potential of the nutrient solution. Because most plant nutrients are ions the osmotic potential can be estimated using the electric conductivity (ECs in Siemens per meter often mS·cm-1) of the nutrient solution. Electric conductivity is, moreover, a functional measure because nutrient uptake is for a large part depend on voltage differences between the inside and outside of the cell membrane. Together the ratio of the essential elements, the acidity (pH) and electric conductivity (ECs) of the nutrient solution determine the amount of nutrients that is available for uptake. The actual nutrient uptake is determined by the plant growth stage, internal plant nutrient status and environmental factors.

Nutrient

Element

Uptake form

Element mass %

Function of each element in higher plants

Essential macro elements

 

  Carbon

C

CO2 / HCO3-

45

Major component of plant's organic compounds

  Oxygen

O

O2

45

Major component of plant's organic compounds

  Hydrogen

H

H2O

6.0

Major component of plant's organic compounds

  Nitrogen

N

NO3- / NH4+

1.5

Component of nucleic acids, proteins, hormones, chlorophyll, coenzymes

  Phosphorus

P

H2PO4- / HPO42-

0.2

Cofactor that functions in protein synthesis; major solute functioning in water balance; operation of stomata

  Potassium

K

K+

1.0

Formation and stability of cell walls; maintenance of membrane structure and permeability; activates some enzymes; regulates many responses of cells to stimuli

  Calcium

Ca

Ca2+

0.5

Component of chlorophyll; co-factor in many enzymes

  Magnesium

Mg

Mg2+

0.2

Component of nucleic acids, phospholipids, ATP. several coenzymes

  Sulphur

S

SO42- / SO2

0.1

Component of proteins, coenzymes

Essential micro elements


  Chlorine

Cl

Cl-

0.01

Required for water-splitting step of photosynthesis; functions in water balance

  Iron

Fe

Fe2+ / Fe3+

0.01

Component of cytochromes; cofactor in many enzymes

  Manganese

Mn

Mn2+

0.005

Active in formation of amino acids; cofactor in some enzymes; required for water-splitting step of photosynthesis

  Boron

B

H2BO3-

0.002

Cofactor in chlorophyll synthesis; may be involved in carbohydrate transport and nucleic acid synthesis; role in cell wall functioning

  Zinc

Zn

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

Ni

Ni2+

0.001

Cofactor for an enzyme functioning in nitrogen metabolism

  Molybdeen

Mo

MoO42-

0.0001

Essential for mutualistic relationship with nitrogen-fixing bacteria; cofactor in nitrate reduction

Growth beneficial elements


  Sodium

Na

Na+


Growth stimulant in low dose and essential for some species

  Silicium

Si

Si(OH)4


Mitigating of abiotic and biotic stress, i.e. salt, water, pests and diseases.

  Cobalt

Co

Co2+


Factor in legume nodule functioning

  Selenium

Se

(H)SeOxx-


Higher seed set in Brasica Napa, delayed senescence in lettuce, better UV resistance and growth promotion in ryegrass. Aside from these examples there are not many reports on the effects of Se on higher plants.

Table 5.1: A generalisation of essential and beneficial plant nutrients. The last column gives a very rough approximation of plant mass% (dry weight) of each nutrient. (Based on Marschner 2012)

Figure 5.2: Schematic presentation of the relation between growth and nutrient uptake. Note that the amount of supplied nutrients does not equal the nutrient uptake. The nutrient uptake also depends on pH, EC, temperature and other environmental factors.


References

de Kreij C, Voogt W, van den Bos AL, Baas R. 1999. Bemestingsadviesbasis substraten. Naaldwijk: Proefstation voor Bloemisterij en Glasgroente.

Marschner P. 2012. Marschner ’ s Mineral Nutrition of Higher Plants (Third Edition). Universitat of Hohenheim, Germany: Academic Press. DOI: 10.1016/B978-0-12-384905-2.X0001-5