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The Science of Watering Vegetables
Watering the vegetable garden is not a simple task. Firstly the needs of the plant must be considered. Then the soil properties must be understood.
1. The plant needs:
a. Water
Water enters the plant at the roots, & is sucked up to the top of the plant.
The water is used:

• Â by every cell in the plant for respiration.
•  to convert sunlight into food. Â
•  for transpiration - when water evaporates through the leaves & is lost.

Transpiration (& water loss) increases:

• in hot weather because water evaporates faster at high temperatures. The plant does compensate by closing the stomata (little holes through which the water is lost) - even so, more water is lost & the plant wilts.
• in windy weather because the evaporated moisture is blown into the atmosphere, away from the plant, allowing more water to evaporate & fill the space. This is rather like washing drying quicker on a windy day.

b. Oxygen
Oxygen is used by every cell in the plant for respiration.
Oxygen enters the plant through the leaves & also through the roots.
The roots take oxygen from the air spaces in the soil. If soil is waterlogged, it is full of water and all the air including oxygen has been squeezed out. So the roots & then the whole plant will die.

2. Soil properties
Soil is made up of lots of tiny mineral particles which hang together to make crumbs. Water clings to the soil particles, like drops hanging on a glass window. In the gaps, between the water coated particles, are pockets of air.

The plant roots wiggle between the crumbs and take up the water and oxygen. The water droplet, or film, around the soil will get thinner as the plant takes up the water, unless more water is added to the soil by rain or irrigation.

There are different textures of soil.
Clay soil is made up of very tiny particles which cling together. It is hard to penetrate & heavy. If it is trodden down it will squash into an even more impenetratable lump. There will be very few spaces for air, & water will not move through it quickly
Sandy soil is made up of large particles. It is light, with lots of air spaces & water rushes through it. Sandy soil will dry out quickly, because the large particles cannot hold onto much water.

The gardener has to get to know how the soil in their garden relates to water. Every garden has its own soil & the gardener has to work with it.

• Is it clay or sand, or what combination of the two?
• How does the garden soil look when it is dry?
• How does the garden soil feel when it is dry? Clay soil often feels cold & damp, even though it is dry. Sandy soil looks dry, when it is actually quite damp.
  Dig down a few inches with your hands, feel the soil and look at it. Then dig down with a spade to a foot. Study the soil, & the surrounding plants.Â
• Do the plants look happy, or are they thirsty. The plants will reflect the water available in the soil.Â

It takes time for the gardener to get to know their own soil characteristics.

Related articles:

Drainage         A rain gauge

Frequency of watering    Soil cap

Where, when and how to water