Some science for the gardener

This is Section 3 of the Gardening Seminar:Start your own garden. Comments or questions are appreciated. You can also write to John Eagles.

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Photosynthesis

Diagram of the process of photosynthesis

Main article: Photosynthesis

All green plants can be seen as little factories that produce energy and food for all other living beings. The basic elements they need for this are water, air and sunlight. The process that runs in these countless factories is called 'photosynthesis.'

Photosynthesis takes place in a green pigment in plants, so-called chlorophyll, which absorbs sunlight. Plants are green because plants cannot absorb the green light that comes from the sun, though they absorb blue and red light. The green light is reflected back and makes the plant appear green.

Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. It is the magical process that makes all our gardening and farming worthwhile. Air (carbondioxide) and water and sunlight are transformed into energy (sugar) and the structural element (carbon) for cells and oxygen to breathe.

Many plants survive in low light conditions. But when a plant is denied enough light, it attempts to grow taller in order to find light. The plant is taller but also weaker.

Photosynthesis is vital for all life on Earth that needs oxygen.

1. Photosynthesis maintains normal levels of oxygen in the atmosphere
2. It is the source of energy for building up the cells of an organism in which photosynthesis occurs, or as a source of food, it gives this energy to other organisms. For example, humans and animals eat plants.
   

Nitrogen cycle

The nitrogen cycle is rather complex, as this diagram of the flow of nitrogen through the environment shows. The importance of bacteria in the cycle is a key element in the cycle, providing different forms of nitrogen compounds assimilable by higher organisms.

Main article: Nitrogen cycle

The diagram to the right shows how nitrogen gas that is present in the atmosphere goes into a cycle that makes nitrogen available to plants.

• Nitrogen from the air is fixed by bacteria that live as nodules on roots of legumes and some other plant species. These nitrogen-fixing bacteria give nitrogen to the plants on which they live, and when these plants die, the nitrogen remains in the soil.
• Nitrogen from the air is fixed by nitrogen-fixing soil bacteria. They transform nitrogen to ammonium (NH4⁺). This process is called ammonification.
• Animals drop manure into the soil. Animals and plants die and manure and dead animals and plants are decomposed by aerobic and anaerobic fungi and bacteria. Also through this process, ammonium (NH4⁺) is formed.
• Nitrifying bacteria transform ammonium (NH4⁺) first into nitrites (NO2^-) and then into nitrates (NO3^-) in a process called nitrification.
• Plants can assimilate nitrates (NO3^-). Nitrates are also affected by denitrifying bacteria that bring nitrogen gas back into the atmosphere in a process called denitrification.
  

pH

Main article: PH

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