5

I'm doing raised bed gardening (wooden boxes 6'x2'x28"deep). Excellent drainage through underlying gravel into natural earth.

Question: I'm using a mix of 50% local dirt and 50% compost (home made). Moisture retention seems fine but not soppy. Is this mix correct? The reason I ask is because last year I used only commercial raised bed mix and it retained hardly any moisture.

Thank you.

  • 1
    What's "decomposed granite"? – Graham Chiu Apr 19 '18 at 5:50
  • Do the wooden boxes have a bottom, or are they open to the ground beneath? And when you say '50% compost', what type of compost? compost you've made yourself? – Bamboo Apr 19 '18 at 10:09
  • The boxes have a hardware cloth bottom covered by shade cloth. They sit directly on the gravel. The compost is homemade. – Neil Apr 20 '18 at 14:13
1

When you say 'retains hardly any moisture,' I picture that within a day after watering the surface is crumbly dry and if you push your finger down a couple inches it's, well, cool but definitely not wet. If these assumptions are wrong, so will be this reply.

It sounds like the mix isn't the issue. Your soil this year sounds top-notch. It also may well be not much different from what you bought last year. When you water amply, the soil naturally drains to the point that, picking a rather arbitrary number from a wide range, 10% of its depth is water - say 2-3" depth equivalent. See http://www.fao.org/docrep/r4082e/r4082e03.htm for a good if long description if you are curious. The drainage is a good thing. Soil has and needs lots of air pores as well as room for the dirt particles. With the excellent drainage you provide and good loose growing soil, this will happen within a couple hours after watering, or sooner than if the garden were at grade.

The water goes away quickly. Vegetables drink heavily, many of them > 1" / week. The surface dries out quickly from sun, and most vegetables are grown in full sun. In my dry climate, I irrigate my vegetables every day. In midsummer if I watered less often than every other day given no rain, it would hamper the plants' growth.

The more clay in the soil, the more water it will retain. But high clay content in most other ways is not an optimal growing medium. Still, if you want more water retention, and depending on the texture of your native soil, you may be able to use more dirt and less compost.

1

The nature of "local soil" and "home made compost" can mean a lot of things, so it is hard to answer this question precisely. However, it seems like you've got a great start on contents for the raised beds this year. Well-made compost is a general problem solver regardless of what soil you started with.

  • If you notice problems with moisture retention you can consider a few remedies. Amending the soil with coconut coir or sphagnum moss will help improve moisture retention. You can reduce evaporation by adding a mulch to the top of the beds.
  • It's always a good idea to get a soil test and they are fairly cheap compared to spending lots of time and money amending the soil in ways that may not be necessary.
  • I suggest for next year that you make smaller adjustments: adding 50% compost is a big change to what's going on. I tend to follow the rule of adding 3-5 inches of compost in a first year of building a bed and then 1-2 inches of compost in subsequent years.
  • The needs of individual plants vary widely, so the mix you've got might not be ideal for certain plants (e.g. carrots like sand or loam while cabbage is happy in clay).
1

Water binds to clay particles, so soil with clay in locks in moisture which then does not easily evaporate but which is accessible by plants' roots. If your local soil has some clay in its structure then that would be a vast improvement on pure organic matter in terms of moisture retention

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.