Porosity

Recent heavy rains have brought much needed water to the drought stricken San Jose, California, but one of my fruit trees was not happy about it. Take a look at the photo below to see how water pooled around the tree.

Standing water around nectarine tree

The drainage around trees with irrigation rings was fine. I can only assume that it was the cement curb around the tree that was at least partially to blame for flooding my poor little nectarine tree. At the same time, the drainage pattern was so significantly different, that I will have to explore other possible causes and remedy them. Rainwater generally moves down, due to gravity, and sideways, toward drier areas. The curb prevents that sideways movement. All that standing water can lead to crown rot, root rot, and many other fungal diseases.

What is porosity?

Porosity, or permeability, in the garden refers to the ability of air and water to move through tiny pockets in the soil. These tiny spaces are called macropores and micropores, depending on their size. Soil that is rich in organic material tends to have a variety of macropores and micropores that improve its porosity. Porous soil allows roots to reach out freely to find water and nutrients. In my yard, we tend to have heavy clay soil that is made up of very tiny particles that leave few spaces in between. Porosity is measured as a percentage of spaces compared to the soil around them.
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What happens when it rains?

When rain starts to fall, or the sprinklers kick in, the soil is initially hydrophobic, causing runoff and urban drool. This is because the water is repelled, the same way a dry sponge allows water to run off the top, rather than being absorbed. Now, we all know that sponges are very porous. They have lots of holes that can hold water. That’s why we use them! 

Once the soil becomes damp, like a sponge, it can then hold a surprising amount of water. When all the pores are full, gravity then pulls the water downward into groundwater, where it is taken to creeks, lakes and oceans. That’s why it is so important to not overuse fertilizers, pesticides, fungicides or herbicides. All those chemicals leach into our water supply!

Porosity and plant health

As plants become saturated with water, tiny pores, called stoma, open wide and the plant starts panting in a process called evapotranspiration. Just as we can see steam when we breathe on chilly days, plants exhale moisture along with other gases. When there is no water to be found, and the plant risks the other side of water stress, the stoma close, to hang on to every bit of moisture possible. When the ground gets muddy, whether from too much rain or over-watering, roots cannot breathe and the plant can drown. In the case of my nectarine, I used plastic tubing to redirect the standing water away from the tree. That’s not something I want to do every time it rains, so I will use these methods to improve the porosity around the nectarine tree:

• Aeration - Using my soil test rod, I will remove cylinders of soil from around the tree.
• Cover crops - I will plant a deep-rooted cover crop to help break up the soil.
• Compost - Top-dressing under my tree with compost will add organic material to the soil. I will have to check my soil test results to see if compost would be a good idea, or if mulch would be better. Too many nutrients are just as bad as not enough. Since I raise chickens, my compost has a lot of nutrients!
• Mulch - I will apply a thick layer of free arborist chips to encourage worms and soil microorganisms. This is the best way to improve the porosity (and health) of any type soil.

If you see standing water after a heavy rain, these tips can help you improve the porosity of your soil and the health of your plants.

UPDATE (1/10/2017) After heavy rains, I was very happy to see that the soil around my nectarine tree is draining very nicely. It really is amazing how effective just a little mulch, compost, and cover crop treatment can change an area for the better!