Macropores and micropores are the spaces found between particles of soil, much like the holes seen in a sponge. Like a sponge, those holes can hang onto water, or they can be filled with air. They also provide habitat for important soil microorganisms.
Moving through space
Macropores and micropores are critical to the movement of air, water, and roots through the soil. While both micropores and macropores can hold air and water, their sizes play different roles in plant and soil health.
Being larger, and normally found between soil aggregates, macropores allow water to drain away through gravity and are often filled with air and soil microorganisms. Because of this, macropores determine a soil’s aeration and porosity. Insufficient macropores can mean compacted soil and drainage problems.
Micropores, often found between and within soil aggregates, are so small that surface tension holds water in place. Instead of draining away, water moves through micropores only when suction is created by thirsty roots. This determines a soil’s water holding capacity.
If you want to get really picky, there are also mesopores, ultramicropores, and cryptopores. Cryptopores are so small (<0.1 μm), most organisms cannot enter, preventing decomposition, and water is held too tightly for plants to use. Ultramicropores are 0.1-30 μm and tend to be populated by microorganisms. And mesospores are larger than macropores at 30 μm–75 μm. Mesospores are filled with easily accessible water at field capacity, providing plants with plenty of water. [Field capacity is the amount of water found in soil after the excess has drained away.] Most of us, however, don’t need to go into that much detail, so we will stick with macropores and micropores.
Too many, or not enough?
Sandy soil can have so many macropores and micropores that water and nutrients simply leach away. Heavy clay soil, at the opposite end of the spectrum, has more micropores, so water and nutrients are held tightly. Loamy soil, in the middle, provides a healthy balance of micropores and macropores within the soil structure.
One common mistake people make, when trying to improve the structure of clay soil is to incorporate sand. It sounds right, but it’s not. The tiny clay particles fill the spaces between sand particles, creating an even denser soil, frequently referred to as concrete.
To improve soil structure, the best methods are to regularly incorporate organic matter and to apply 3 to 4 inches of coarse wood chips as mulch to unplanted areas. The wood chips will, over 2 or 3 years, break down, adding organic material that helps create a range of aggregate sizes, with plenty of macropores and micropores.
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