Sawdust results in Garden
I tilled about 5 tons of fine dense sawdust into my 3,000 square foot garden this spring, and added 100 lbs. of urea 46% nitrogen throughout the summer. I thought I would pass along the results.
The soil has changed from clay-like to very well-drained and friable, almost like a 50/50 ground sphagnum peat moss/ soil mixture.
The soil would not support most crops this summer. The plants turned yellow from nitrogen deficiency, even with added nitrogen fertilizer, and the soil dried out quickly, even with irrigation, because the sawdust repelled water.
My fall crops, including Brussels sprouts, broccoli, carrots, lettuce, spinach and cabbage, have done very well (better than ever before) in the now partially composted sawdust/soil mixture. The soil stays moist but well drained and no additional nitrogen fertilizer was needed. It now drains so well I can even till the soil 2-3 days after a heavy rain! The soil remains loose and friable rather than clumping up.
In the 1,000 square foot area to be used for blueberries I also added 50 lbs. of sulfur. A soil test performed by Penn State produced the following results:
Ph. 4.8 (Optimum) - Was 6.5 in the spring.
Phosphate - 966 lbs. per acre (above optimum)
Potash - 991 lb./A, Slightly above optimum
Magnesium - 953 lb./A, Exceeds crop needs
Calcium - 9,095 lb./A, Exceeds crop needs
Cation Exchange Capacity - 24.2
Organic Matter - 12.8%
A soil test last spring, before adding the sawdust, found calcium and phosphate levels to be below optimum, potash to be optimum, and magnesium to be slightly above optimum. It appears that the sawdust added a large amount of phosphate and calcium to the soil, as well as adding lesser amounts of other nutrients, doubled the CEC from 12 to 24 and more than tripled the organic matter content.
Would I do this again? Absolutely - a short term setback for a long term gain! After having "clay" soil gardens at three different homes, gardening is now much more enjoyable. I no longer have to wait for a week or more of dry weather before tilling (I do far less tilling now - one pass does it). I can pull weeds roots-and-all rather then having the weeds snap off at the surface and regrow. I no longer have to wade through the mud when harvesting crops.