Water repellent soil. Its a real problem for lawn and its easily solved

I see a lot of people come into our store and ask, “why does my lawn have dead patches in it?” While there are many reasons why a lawn can develop brown patches, the number one reason by far is because of soils that repel water.

A water repellent or non-wetting soil is where a soil repels moisture instead of absorbing it, like water on wax. When water lands on non-wetting soil, it doesn't sink in evenly and either pools,  runs off or evaporates rather than penetrating into the soil so no matter how much you water it, the water doesn’t make it to where it’s needed which means your lawn suffers from drought stress. Not only that, since water is the carrier of nutrients to your lawn’s root zone, lawns growing on non-wetting soils are often nutrient deficient, shallow-rooted and susceptible to disease. The irony is, even though your lawn may look like it needs a drink –  more water, more fertiliser and more beetle killer won’t solve a non-wetting soil.

The result is a very dry lawn root zone even though you may be watering every day.  It's a common problem - even the best types of soils don't wet evenly. Some areas absorb water deep into the profile, while other areas in the same patch of lawn completely repel it. Wherever soil repels water, you can be sure that the lawn growing in it will be patchy.  You can literally have a brown section of lawn right next to a green one, as in the diagram above even though you may water the area evenly. In that way, non-wetting soils have a random, patchy effect on your lawn which is why it’s so often confused with beetle damage or fungal disease.

What Causes it?

Non-wetting soils can come about in a number of ways. It's certainly more common during dry spells and during the warmer parts of the year. As is the case with many soil types in Adelaide, when they dry out to a point where there is very little moisture left in them, they are very hard to re-wet. Once they get super dry, they naturally repel water.  It takes a lot of steady rain or irrigation to get them to take up moisture and rain is something we haven't had a lot of in recent months. While it's been incredibly wet in NSW and Queensland, that hasn't been the case here in South Australia. December only saw 1.8mm of rainfall in the metro area. In January we managed 58mm, however, 50mm of that fell in 7 days and surprisingly, in February the total rainfall was a mere 6mm. This means that for the whole of summer, we only experienced one significant rain event which has left our state and our soils very dry. As I mentioned earlier, even if you were to go out and turn your sprinklers on to give your lawn a drink because the soil is so dry, the water will in a lot of cases run off and this is where a lot of people come unstuck. Because you have been watering due to the absence of rain, you automatically think that the dead and dying patches in your lawn can't be a watering issue because your water regularly.  Getting back to the issue, turning on your sprinklers for 20 minutes won't solve this problem. You can test this by watering your lawn as you usually would, then go out with a hand trowel and dig an inspection hole in the middle of a dead or dying patch. If the area is dry then you've identified the problem. This is a lightbulb moment for a lot of people because you make the connection - how can the soil possibly be so dry right after I watered it? and the answer of course that the soil has repelled the water.

Another reason soils repel water is because of the build-up of organic residues. These residues are a waxy like substance that coat the soil particles which in turn cause it to repel water. Organic residues occur when organic material in the soil breaks down over time.  Fungi in the soil can also lead to excessive build-up of organic residues. Non-wetting soils can be particularly bad under Eucalypt trees because of two reasons; the first one being that these trees naturally attract a type of fungi around their roots and secondly because of the sap that falls from their canopy. 

Identifying water repellence.

In a domestic lawn situation, water repellence can be identified in 2 ways.

• The presence of a general patchiness in your lawn, with the soil being dry despite regular watering. This can be an all-over uniform dryness or random patchiness where you have brown patches amongst a green lawn.
• How long it takes for a droplet of water to penetrate the soil surface

Ideally, the soil under your lawn should be uniformly damp down to a depth of at least 50mm after irrigating. You can test if this is the case by digging out a small 100mm core from a green area and another from a brown area in your lawn. (If there are no green areas, just take a single sample from a dead or dying area). Firstly, do a visual check – If the green sample has moist soil and the brown sample is dry, you know you have a non-wetting soil problem since the whole area was doused with the same amount of water. A secondary check would be to apply a drop or two of water to the side of a dry soil sample. If it takes more than 5 seconds for the water to penetrate or the water simply rolls off like wax,  then this would confirm you have a problem.

Statistically speaking, for every 100 core samples of turf we see, 45 of them have water repellent soils. That means that around 45% of all dead or patchy lawns in South Australia are caused primarily by soils that won't absorb water!

How do you treat it?

The best way to treat non-wetting soils in a domestic lawn is with liquid wetting agents (not solid wetting agents). Liquid wetting agents do a couple of things – they help break down the waxy organic residues and they break the surface tension of the water, allowing it to penetrate into dry soils.  It's not just in applying wetting agents that you solve this problem though. - a single application of wetting agent won't deliver any long term results.  Unfortunately, wetting agents don’t last very long in the soil and you will find that in time, they need to be re-applied. A good rule of thumb is during the warm months of the year, reapply every six weeks for best results.   The good news is that they are cheap to buy and available at most hardware stores.  An interesting aside is that there are many types of wetting agents, some lasting longer than others and some having the ability to actually store water in the soil. You definitely get what you pay for – but that’s the topic for another blog!

My Recommendations:

• Wetting Agent: Apply a hose on a wetting agent such as Paul Munns Betta Wet or SST Aquaforce every six weeks from November through to March every year in the following way:
  • Apply wetting agents in the cool part of the day
  • Irrigate your lawn prior to application with at least a 20 minute water for each watering zone
  • Apply the wetting agent
  • Water in well immediately after application with at least 25mm of water. That's about 30 minutes per zone or area with traditional popups or a sprinkler, or 1 hour per zone if you have gear drive sprinklers or mini rotors such as MP rotators or Rvan's 
• Water Retention Agent: Once you have applied a wetting agent, apply a water retention agent such as SST Bi-Agra. Also available in a hose on pack, Bi-Agra is designed to hold water in the soil profile. A wetting agent enables water to get into the soil. A water retention agent ensures it stays there. 

In my opinion, liquid wetting agents are the elixir of life for lawns and garden areas, whether you have non-wetting soil or not. They ensure that any rain or water penetrates evenly. They also reduce wastage of water because they make it possible for the water to penetrate quickly instead of evaporating or running off.

For what it’s worth, if you find yourself with a patchy lawn, the first thing I’d recommend you do is apply a wetting agent!