Answer: Unless the aged sawdust has begun to turn gray and break down, there's very little difference, says Rosie Lerner, consumer horticulturist at Purdue University in West Lafayette, Indiana. If the aged sawdust has begun to break down, then you don't have to worry about adding supplemental nitrogen to your garden plants. When fresh sawdust is used as mulch around garden plants, the soil will require supplemental nitrogen. That's because microorganisms use the nitrogen in the soil to break down the sawdust. A sure sign of nitrogen deficiency on your plants is older leaves turning light green to bright yellow and eventually dying, she says. If you suspect a nitrogen deficiency problem, add an extra 2 to 3 pounds of nitrogen worked into the soil for every 100 pounds of sawdust used. When mulching with sawdust, apply only a 1- to 2-inch layer, says Lerner. Also, use larger, chipped sawdust that's cut in sawmills or by chain saws instead of fine sawdust cut from carpentry work. Powdery sawdust tends to mat and prevent water and air penetration into the soil.
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