Why is my St Augustine grass yellow?

I will give two answers here. One the quick answer that applies here in Austin, Texas. And a second answer for those not in Austin, or for those who read this in the future and are trying to figure out what is wrong with their lawn.

Quick Answer

Our soil composition is primarily that of a heavy clay. Clay soils have the unique property of absorbing water slowly, and releasing it even slower. This phenomenon is primarily caused by the size of the pore space between the soil particles. (it is very small) This leaves very little room for gases, that must be present, for respiration to occur. Without respiration, any plant will die.

Simply put, your lawn cant breathe!

To really understand what is causing the yellowing of your St. Augustine grass, you must first understand a little about plant biology. First, the green color that you see on all plants is from chlorophyll. (I hope you already knew that)

Chlorophyll is a green pigment, that makes it possible for plants to convert carbon dioxide and water, in the presence of sunlight, into oxygen and glucose. During this process of photosynthesis, chlorophyll produces energy, in the form of sugary carbohydrates, which will power all of the plant's essential growth and development. This amazing molecule also gives plants their distinctive green color. Chlorophyll is produced in the plant, during a process called respiration. (see below)

We can understand chlorophyll as fulfilling the same operation as our digestive system. They both take raw materials and convert them into useable sources of energy in the form of glucose. Energy it doesn't immediately need it stores as starch for later use.

Technically, the chemical reaction of photosynthesis uses an equal number of carbon dioxide and water molecules to create the same number of oxygen and glucose molecules. This is why we say plants "breathe in" carbon dioxide and "breath out" oxygen. This gas exchange takes place on the bottom sides of the foliage. The reaction only occurs if both chlorophyll and sunlight are present. As a photoreceptor, chlorophyll is sensitive to light of a certain spectrum that's emitted by sunlight. This means it can capture the energy of visible light, mostly red and blue. Then it reflects the unneeded green light, giving plants their color. Plants use up to 90% of the light that strikes them, whereas commercial solar panels use less than 30%.

So, Photosynthesis is the production of energy that can to be used at a later time. This production takes place primarily in the foliage.

Respiration is the process in which plants use this stored energy. In fact, you could say respiration is the opposite of photosynthesis. Respiration begins in the root system. In this process, the plant actually takes in small amounts of oxygen and expels small amounts of carbon dioxide. This gas exchange takes place in the root zone and is necessary for this process to occur.

A secondary problem associated with soggy/compacted soils, is the lack of aerobic bacteria. As the name suggests, this bacterium needs oxygen to survive. Aerobic bacteria is what actually breaks down organic matter, in the soil, releasing nitrogen. We often times are convinced that there is more than one kind of Nitrogen that we fertilize our plants with. This of course is not true. Back in grade school you probably learned that Nitrogen was an element, that it was inorganic, and that was either in one of two states: liquid (frozen) or gas.

That's right, your plants don't actually feed off that expensive organic cow poop you have been shoveling on your yard and they don't feed from granular Urea fertilizer either. What you say? How can that be? That's not what the guy who sold me that poop said?

The answer is in the previous paragraphs. Aerobic bacteria feeds from carbon and releases nitrogen in the process. Technically, your lawns (and all plants) produce their own food during photosynthesis. The Nitrogen that is released during decomposition of organic matter (by aerobic bacteria) is only one of the materials used in this process. Without nitrogen, photosynthesis cannot occur. Oh, so Nitrogen is used during photosynthesis, you say? No, not exactly. Nitrogen is used in the production of proteins, DNA, and chlorophyll. Proteins are used for cellular production and expansion, DNA is the hereditary chemical in all organic life and Chlorophyll is the primary component to photosynthesis. And as you know by now, (I hope) without chlorophyll your plants cant be green.

To summarize the above

• Plants must have food to survive.
• Plants produce their own food during photosynthesis. (sugars)
• Plants store excess energy produced during photosynthesis as starches.
• Plants convert starches back to sugars to be used for energy in a process called respiration.
• Photosynthesis cannot occur with out Chlorophyll.
• Green, plant color comes for chlorophyll.
• Chlorophyll cannot be produced with out Nitrogen.
• Nitrogen cannot be produced with out aerobic bacteria.
• Aerobic Bacteria cannot live in compacted/saturated soils.
• Respiration requires the exchange of gases from the root system. (Carbon Dioxide and Oxygen)
• Respiration cannot occur in waterlogged soils because water has displaced the gases.

What Can I Do and Other Common Questions?

• Will adding iron or Ironite make my lawn green? No
  Why? If your lawn is located in Central Texas, most likely you have an over abundance of Iron already present in the soil. You have a drainage problem. Fix it!
• Why did/does my lawn turn a darker green after it rains?
  Iron, like Nitrogen is also required for chlorophyll production. Because our soil is alkaline, the Iron is insoluble. (held in suspension in the soil, unable to be used by the plants) Rainfall is acidic. Acidic rain lowers the pH of the soil, allowing the iron to become soluble for a short period of time, which results in greater chlorophyll production, and therefore a darker green color in the lawn. Assuming there are no other stress related problems.
• Will fertilization help my lawn?
  No, in fact it can make it worse. If you add a fertilizer to saturated soils, and there is enough aerobic bacteria to break down the carbon and release Nitrogen; you risk a very severe attack by various fungal pathogens.
• What can I do? If anything....
  An aeration will give the fastest relief. Aerate again, even if you have already had an aeration this year. The amount of rainfall we had, will close any aeration holes that may have been mechanically made in the past. Increasing the volume of air to the root zone will give immediate results. Bottom line is that we normally don't have this much rain. In fact, in recorded history, we have never had this much rain in this short of a period of time. An aeration is about all you can do with out serious soil profile modification. I would not recommend soil profile modification, as this is very expensive, destructive, and as I said before... "Most rain in the shortest period of time, in recorded history" It is unlikely that we will have this much rain again, any time soon.

Other issues that can cause a yellow lawn:

Grey leaf spot

Brown Patch

Chinch Bug