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Sulfur and Clay

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From: George Singletary
Ankeny, IA
My sod lawn will be 2 yrs old in August. My home is in a new housing development. I have a heavy clay soil - little top soil. A composite soil sample (3-4" depth) in April '10 had a pH 8.1. I subsequently had sulfur appllied (5 lbs/1,000 sqft) in April and Oct '10. The pH of a composite sample this April was 7.9. I subsequently had sulfur (and gypsum to loosen clay) appllied again at same rate as before. I aerated and overseeded Oct '10 and aerated again April '11. My root system is very shallow. Here's my problem. My lawn is now very thin, maybe 75% the density that existed last fall. Also, the grass is growing VERY slowly. Neighbor has mowed three times already, my lawn is almost ready for first mowing. Is the problem from too much sulfur (low pH) in the shallow root zone? Also, I understand from other posts that my best approach to the clay problem could be aerate and over-dress with organic matter or good top soil. Tough to guess how much material is required for over-dressing. If pulling 4" plugs, what would you guess (bulk vol.) is needed per 1,000 sqft? Thank you very much for your help.

Extension Message
From: Greg Stack
Extension Horticulturist
Extension Web Development
Hi, Trying to correct a poor growing environment for plants after the plants are planted is very tough to do. In this case you started out with a very poor seed bed when the lawn was planted and now the grass is not performing well because it basically has a poor root environment. it would have been nice to add considerable organic material before planting seed, work it in and then plant. Adjusting the pH with sulfur is ok but it too is tough to do by applying to the surface. applying large amounts of sulfur can at times be damageing to small plants and the roots. So it needs to be watered in very heavy after application. Gypsum is also used to help with clay soils but it works best when it also can be worked in. Now about all you can do is to core aerate and then topdress with compost that is raked down into the holes. Coring and topdressing about twice a year will help but it to will take time to show results as you are changing the soil one small core at a time. I really can't say how much topdress you need but application can be done by taking shovel fulls of compost and spreading it thinly on top of the grass and then dragging it in.

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