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February / March 2023

SOIL-WATER POTENTIAL and plants at work

SA Fruit Journal: February / March 2023

***Soils differ in how much water they hold and how readily that water is available to plants. Knowing the physical properties of your soils is essential for optimal irrigation scheduling. By Anna Mouton***

Soil consists of particles with air- and water-containing spaces – called pores – between them. The total volume of all the spaces in a certain volume of soil is called its porosity. Soil is said to be saturated when all the pores are completely filled with water and there is no space to add more water. Porosity, therefore, determines the total amount of water that a given soil can contain.

But porosity is not the whole story. The size of the pores also matters. Large pores allow rapid water and air movement, whereas small pores limit it. Pores that are so big that water runs straight through them are called macropores. Micropores are on the other end of the spectrum – so small that they hold onto water – and mesopores are somewhere in the middle.

Sandy soils are free-draining but hold little water, because they contain mostly macropores. Clay soils hang onto their water because they have mostly micropores.

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