Poor growth in compost/soil mix

Asked September 4, 2018, 12:34 AM EDT

We recently replanted our garden space in our front yard. We used 6" of compost/soil mixture over corrugated cardboard. I dug holes through the cardboard so that we could plant transplants. We completed the planting in mid to late May. The only plant that seems to be doing well is a manzanita that supposedly likes poor soil. I sowed 2 packs of california poppy seeds to fill in the space between plantings. The seedlings grew to about 1" tall and then shriveled. Hardly any weeds grow in this soil. We've watered about twice / week through the summer. We had similar problems with the same soil/compost mix and plants in our back yard. We bought 4 yards of a compost/soil mixture from a small business that had great reviews on yelp. The compost was supposedly horse manure mixed with sawdust. This was then mixed with soil, which appeared to me to have both clay and sand. Could there be something in the soil that is inhibiting growth? Should I get the compost/soil mixture tested? If so, what test do you recommend? Or should I try mixing in some other compost? We have another yard that we haven't used. Thanks!

Multnomah County Oregon

5 Responses

Was the compost fully composted when you purchased it? If it was not, it's possible that it was 'composting in place' because the breakdown of sawdust takes a *lot* of nitrogen so it's possible there is significantly less fertility than you think. I would recommend you have a soil test through A&L Western Labs http://www.al-labs-west.com/ I would contact them directly and ask them how they want the sample (they may ask you to take it from the pile that has not been used.) I would also contact the company from which you purchased this mix and discuss your issues with them. It's possible you aren't the only one with an issue.

Thanks for your response! I did talk with the owner of the company today. He was concerned. He didn't know of anyone else that had problems. However, there was a yelp review in July that related a similar issue. The owner said that he'd be willing to drop off a load of compost that I could mix in, if desired. I assume that a soil test may be desired first.

I was looking at: http://www.al-labs-west.com/fee-schedule.php?section=Soil%20Analysis
Which soil test do you recommend? S1B, S1BN, or S2N?

Thanks, Frank

I would recommend S2N. If you can afford it, I think S3C would give you a really good snapshot because it includes key micronutrients. My fear is that there may be some organics (residual pesticide/weedkiller) that could contribute to the issues you are having that none of the standard tests as listed on the schedule won't cover. This is different than 'organic matter'. I would hope that the owner might offer up that type of analysis or investigate the raw materials used in the compost/soil mixture. Even the type of sawdust could be an issue. There are certain trees that are allelopathic (black walnut is the most common one) that produce a chemical that suppresses growth of most plants.

Thanks again! I've attached the compost/soil sample results. The samples came from the compost/soil that has already been spread in front of the house, not from the remaining pile. I mixed 7 samples together to get an average. The sample is very low in Nitrogen, very high in Phosphorus and Potassium. Your idea that the sawdust wasn't fully decomposed sounds plausible. The owner of the company told me that the sawdust came from fir trees. The "soil" component came from a farm in Canby.
If you have recommendations for next steps, please let me know. I very much appreciate your help.
Many Thanks,

I think it depends on what you want to grow now. I'm assuming you are not going to be planting turf, but leaving the soil bare over the winter may wash away anything you use as an amendment, or additional N can allow weeds to make a significant foothold. If you plan on adding any plants/shrubs that are actively growing, you will want to add a slow release N fertilizer in the backfill soil. For a more general approach, you can use a slow-release lawn fertilizer and broadcast it, but you will really have to watch for weeds. If you aren't going to plant anything, you could also broadcast blood or feather meal. However, those two can attract critters (and if you have a dog, it can also make them try to dig in your yard.) The two are the highest organic N sources, but they run 'hot' (especially if you have the sawdust) so they are not recommended for use when planting. I use them in my raised beds in combination with straw and it composts over the winter leaving me with a really good planting medium for vegetables.