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A very high or very low pH will affect the plant’s ability to take up nutrients, even if the nutrients are present in high concentrations. This is called nutrient block-out and will cause the plants to show signs of nutrient deficiencies. It is important to note that not all plants have the same pH preference, but the ideal range is between 5.0 and 7.0. Very few plants can tolerate a pH higher. If your pH is too high or too low, the first thing that you should do is correct it (see chapter 2: water quality). Some issues associated with incorrect pH include:

• Toxic Sodium levels: Alkaline soil (high pH) collects salt and sodium carbonates, which affect a plant's ability to develop roots. Stunted plant roots have difficulty absorbing nutrients and water.

• Mineral deficiencies: Iron and manganese react in highly alkaline soil, changing into forms that make them unavailable for plant use. Plants with insufficient iron and manganese produce fewer and poorer crops.

• Inaccessible Phosphorous: With high pH, the phosphorous (P) in soil becomes an insoluble solid, which is unusable to plants. In order for P to be available for plants, soil pH needs to be in the range 6.0 to 7.5. If pH is lower than 6, P starts forming insoluble compounds with iron (Fe) and aluminium (Al) and if pH is higher than 7.5 P starts forming insoluble compounds with calcium (Ca).

Macronutrients

Nitrogen (N)

• Nitrogen is part of all living cells and is a necessary part of all proteins, enzymes and metabolic processes involved in the synthesis and transfer of energy.

• Nitrogen is part of chlorophyll, the green pigment of the plant that is responsible for photosynthesis.

• Helps leaf and stem growth, increasing seed and food production.

Deficiency signs: Yellowing of old leaves; new leaves and stem often pale green.

Phosphorus (P)

• Phosphorus is also an essential part of the process of photosynthesis. • Encourages root growth and germination.

• Involved in the formation of all oils and sugars.

• It aids with the transformation of solar energy into chemical energy.

Deficiency signs: Leaf tips look burnt, followed by older leaves turning a dark green or reddish-purple.

Potassium (K)

• Aids with the production and transportation of sugars, building proteins, ripening of fruit and reduces diseases.

Deficiency signs: Older leaves may wilt and look scorched. Interveinal chlorosis begins at the base, scorching inward from leaf margins.

Calcium (Ca)

• Is an essential part of plant cell walls which strengthen the plant, it also contributes to root development, primarily that of the root tips.

Deficiency signs: New leaves (top of plant) are distorted or irregularly shaped. Causes blossom end rot.

Magnesium (Mg)

• Magnesium is part of chlorophyll and essential for photosynthesis.

• Activates many plant enzymes needed for growth and a healthy leaf structure.

Deficiency signs: Older leaves turn yellow at edge leaving a green arrowhead shape in the centre of the leaf.

Sulphur (S)

• Essential for production of protein, chlorophyll, enzymes and vitamins. • Improves root growth and seed production.

• Helps with vigorous plant growth and resistance to cold.

Iron: Young leaves are

yellow/white with green veins. Mature leaves are normal.

Calcium: New leaves misshapen or stunted. Existing leaves remain green.

Nitrogen: Upper leaves light green, lower leaves yellow, bottom leaves yellow and shrivelled.

Phosphate: Leaves darker than normal. Loss of leaves.

Potassium: Yellowing at tips and edges, especially in young leaves. Dead or yellow patches or spots develop on leaves.

Manganese: Yellow spots and/or elongated holes between plant veins.

Magnesium: Lower leaves turn yellow from inwards. Veins remain green.

Micronutrients

Boron (B)  Aids production of sugar and carbohydrates.

• Helps in the use of nutrients and regulates other nutrients • Essential for seed and fruit development

Deficiency signs: Terminal buds die, witches’ brooms form. Copper (Cu)  Important for reproductive growth.

• Aids in root metabolism and helps in the utilization of proteins. Deficiency signs: Leaves are dark green; plant is stunted.

Iron (Fe)  Essential for the formation of chlorophyll.

Deficiency signs: Yellowing occurs between the veins of young leaves.

Manganese (Mn)  Functions with enzyme systems involved in breakdown of carbohydrates, and nitrogen metabolism.

Deficiency signs: Yellowing occurs between the veins of young leaves. Pattern is not as distinct as with iron. Reduction in size of plant parts (leaves, shoots, fruit). Dead patches. Molybdenum (Mo)  Helps in the use of nitrogen.

Deficiency signs: General yellowing of older leaves (bottom of plant). The rest of the plant is often light green.

Zinc (Zn)  Essential for the transformation of carbohydrates. • Regulates consumption of sugar.

• It is part of the enzyme systems which regulate plant growth.

Deficiency signs: Terminal leaves may be rosetted, and yellowing occurs between the veins of the new leaves.

In aquaponic systems, the best way to ensure that plants do not suffer from nutrient deficiencies is to maintain the correct pH (7-7.5), and to feed the fish a diet containing a full nutrient spectrum such as soldier fly larvae and duckweed. If plants still show nutrient deficiencies then it will be necessary to add the missing nutrients. This may be achieved either inorganically (for example, phosphorous can be added in the form of phosphoric acid (used to lower pH), and iron can be added in the form of chelated iron); or organically in the form of a foliar feed, compost tea or worm castings (see below). It is also possible to use inorganic, micronutrient-laden plant fertilizer, but first check that it contains no ingredients that could be harmful to the fish, and add it gradually to the system.