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Most lawns only need aeration when compacted soil becomes a problem. They don't need to be aerated twice a year. Soil compaction can become a problem if your lawn receives a lot of foot or vehicle traffic. The problem is likely to arise sooner if you have a soil with lots of clay. Soils that are sandy and richer in organic content do not compact as easily, but they too can become compacted if the traffic is heavy enough. If you have problems with nightcrawler mounds and irrigation water seems to run off rather than percolate into the soil, compaction may be the problem and aeration may be called for. If you cannot shove a large screwdriver into the soil on a relatively dry day, aeration is probably warranted. Have your lawn aerated only in the fall. Spring aeration exposes a lot of bare soil to warm season weed seeds like crabgrass. For best results, follow the aeration with a light topdressing of compost, spread no thicker than 3/8" thick (1/4" or less on hybrid bermuda lawns).