5.1.3. Water and nutrients

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 (5.1.3. Water and nutrients-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 (5.1.3. Water and nutrients-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.

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)

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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.