Do Not Dig Chipper Materials into the Ground
Question from Harry: I have heavy clay soil. When it gets wet, it sticks to my shoes. Recently I got a load of chipper materials from a local tree-pruner. I have been wheelbarrowing it around and spreading it on paths and beds and around my fruit trees. Would it be okay to dig it into the ground to lighten the soil in my vegetable garden before planting planting vegetables?
Answer from Pat: No. Do not dig chipper materials into the ground until they are fully rotted, which usually takes a year or two and often even longer. Chipper materials from tree trimmers make excellent organic mulch for covering bare garden soil, but all raw woody materials, such as sawdust, wood chips, or shredded wood, will rob nitrogen from the soil in order to rot. If you dig chipper materials into the soil of your vegetable garden it could kill your vegetables or at least give them yellow leaves.
Here is a list of organic substances that you can safely dig directly into garden soil:
- commercial organic soil amendment intended as planting mix
- bean straw
- apple or grape pomace
- homemade well-rotted compost
- fully composted wood shavings
- grass clippings
- green manure
- ground carrot, apple and vegetable fibers
- kelp and seaweed,
- leafmold
- aged horse or cow manure
- rabbit manure
- rotted hay
- sludge
- wet and sloppy vegetable and fruit leavings from your kitchen or supermarket
After chipper materials have been lying on top of the ground as mulch for a year or two, during which they have been frequently moistened by rain or sprinklers, most of the woody product will have absorbed enough nitrogen from the air in order to rot and will thus become like compost and can be incorporated into garden soil. This is the time to add more fresh mulch on top. Even after a year or two you may find that some big chips of wood or leaves have not composted enough to be combined with soil. Rake these bits aside and let them sit longer with your next load of mulch until they too have fallen apart and become indistinguishable from soil. Then you can safely dig them into garden soil to build its humus content.
For more on this subject, please see page 22 in my new organic gardening book, lower left hand column. Please also see the chart on pages 28 to 30 for an extensive list of organic soil amendments and an explanation of how to use each one. This chart will tell you which substances you can mix into soil and which should not be mixed into soil without adding nitrogen. It also tells you how much nitrogen to use.
Another chart on generic fertilizers was accidentally omitted from this printing but will be included in the next printing. (See this website under Fertilizers for that chart.) I suggest you download it, print it, and stick it in the back of your book. And thanks for writing me to ask such a good question.
I was reading early on, about Kugelkulture, where the wood (not necessaryly chipped ) is deeply dug into the soil and even covered with a lot of soil…..Yes, will use a lot of Nitrogen in the
process , but I did understand that is a good method. So, my
question is: why one con’t mix the eucaliptus wood chips with soil and continuously add Nitrogen in different forms?
I had never heard of Kugelkulture or Hugelkulture until you mentioned it but today almost everything can be found on the Internet. I looked there and now I understand and am intrigued. Thank you for bringing this German gardening system to my attention. I have often hiked in forests in rainy areas and seen whole tree trunks rotting with new ones springing out of the top. Glorious rot! During the rotting process the tree trunk would need to take nitrogen from the air but then once rotted, it gives all that nitrogen back and thus the whole forest survives on its own residue. (For others reading your comment, here is an interesting description:http://www.richsoil.com/hugelkultur/.) Unfortunately in a dry Mediterranean climate, where the temperature is mild year-round and rainfall is sparse and largely limited to the fall and winter months, wood—and especially eucalyptus wood— does not rot rapidly. This is basically why our native soils are so thin and lacking in nitrogen. I had a log in my garden when my husband and I first built our home. It was a huge felled eucalyptus, but it fitted perfectly into our landscape and helped to support a bank. Our daughters and their friends played on it when they were children. That was over fifty years ago. A few parts of that enormous tree trunk are still visible today sixty years after I first saw it and still not completely rotted after this many years. And who knows how many years it had lain on the ground prior to that? At least ten years is my guess. Covering it with soil would not have speeded the process much since the back of the trunk had always been covered in soil. Kugelkulture is a great idea and is in many ways environmentally sensitive, including sequestration of carbon. But this method of making high garden beds with wood buried inside it is far more practical and workable in a cold climate with rainfall year round than in a dry Mediterranean climate such as ours. If you were to pour on nitrogen, such as sulfate of ammonia, onto the wood to get the effect and also irrigate the mound constantly, then you would be ruining the basic concept. KugelKulture is an environmentally sensitive gardening method which is using the nitrogen in rotted wood to grow plants and also saving carbon from getting into the atmosphere while causing no ground pollution from runoff. The information I read also talks about no need for irrigation, something that would work fine in Germany but would never work in our dry climate and Santa Ana winds. Nonetheless, thanks for bringing it to my attention.