Is diluted muriatic acid harmful to grass?

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The short answer is, no, it is not harmful.

If you are having problems with your grass having dead spots, there are many things which can cause these problems. One is a fungus, which can only be treated in the winter when your grass goes dormant. Over-watering or under-watering can also cause problems. To have a rich, thick grass that is lush and deep green you need to fertilize right before a rain storm (to save water). Also most people don't realize that if you water and fertilize too often your grass will become dependent on you and will not grow out the roots that it needs. Always remember Mother Nature will care for your lawn most of the time. To get thick grass you might need to water once a week. Unless there is a drought then twice a week might be good. This will make your grass roots grow down to a depth where it can absorb nutrients and water on its own.

Also, if you like thick lush lawns cut the blades to be 2.5 to 3 inches tall after mowing, mow your lawn with a mulching mower. Never cut more than 1/3 the total height of the blade in any one mowing. With a mulching mower you are recycling the nutrients back into the turf. Less fertilizer use is good. Lawns in the Midwest require an inch of water per week. A slow drizzle is best because the water has time to seep deep into the soil and not run off. If you water quick and light, only moistening the surface of the soil the roots will migrate to the surface where they will then dry out quickly and suffer.

Water turf slow and deep driving the roots down; making the turf more drought resistant.

This method of watering is also helpful for trees and reducing the mats of tree roots at the surface that make it almost impossible to grow lawn.

Most nutrients are gathered by the plants roots in the top 18 inches of soil (standard Midwest soil) where there is the necessary exchange of water and oxygen required to support the organisms that breakdown the organic matter into nutrients the plant can use. Plant roots do go deeper for water. Fescue grasses can be very deep, 3 feet is not unusual.