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Soil Wetting Agents

Last Updated Dec 12, 2012 · Written by Craig Gibson

Are your plants looking dry and stressed, no matter how much watering you do?  You may have soils that have become hydrophobic, in other words, soils that cannot absorb water.  The good news is that there’s something that you can do about it.  Read on to learn more.

What do Soil Wetting Agents do?

The purpose of a soil wetting agent is to overcome water repellence in soils.  This is where the soil actually becomes hydrophobic – instead of water sinking into the soil, it beads on the surface without penetrating. Soils can become hydrophobic if they are dry for long periods of time.

What Soils are Likely to be Hydrophobic?

The soils most likely to be affected are sandy soils, soils that are rich in organic matter and also potting mixes.  When it comes to pot plants, it is possible to think that you are well watering the pot, when actually the water is running down the side of the pot and through the drainage holes without wetting any of the soil.  To see if your soil is hydrophobic, there is a simple test that you can do.  Take some dry soil and place it in a container.  Make a well in the soil and then pour water into the well.  If your soil is hydrophobic, the water will simply sit on the top of the soil.  If the water is absorbed, you do not have a problem.

How Do Soil Wetting Agents Work?

Soil wetting agents are surfactants.  Basically, they are the same as detergents. Their purpose is to overcome waxy organic coatings that can form on the surface of the soil.  These coatings are what repel water.  This coating can occur when organic matter degrades.  By dissolving these coatings, the water is able to soak into the soil.  What soil wetting agents don’t do is change the structure of the soil or improve the soil as such.  As well as hydrophobic soils, the rate of water absorption can also be affected by such factors as compacted soil and large amounts of clay in the soil.  Soil wetting agents cannot solve these problems.

Are Soil Wetting Agents Safe?

For soil wetting agents to work properly, they cannot be too readily biodegradable.  The soil wetting agents that you buy are designed to work for a considerable amount of time but they will degrade eventually, so they do need to be reapplied.  Be careful when applying wetting agents – try to ensure that they do not enter the stormwater system as runoff can affect some marine life.

Using Soil Wetting Agents

Soil wetting agents are available in both liquid and granular forms.  The granular forms are great for potting mixes and they work by putting the wetting agent into small granules of clay or another organic material.  The wetting agent is then leached out when water is applied – either through watering or through rain.  Liquid wetting agents are good for larger areas such as garden beds.  When using wetting agents, always do a test patch before widespread application to ensure that there is no adverse effect on your plants.  Also be careful not to apply wetting agents to foliage, as they can dissolve the waxy coatings that protect the leaves of plants.

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