Phosphate

Hello all, I haven't posted in a while, but I have a question that's been bugging me for days.
I just finished reading the new book by Robert Pavlis called "Compost Science for Gardeners", and in it he has some sections about using too much compost.  He says that too much compost can actually be harmful to plants because of the buildup of phosphorus which makes it difficult for them to take up manganese and iron.  He also says high phosphorus levels are toxic to mycorrhizal fungi.
He also talks specifically about raised beds becoming popular and that people are filling them with compost or 50% compost, which I'm guessing is a direct reference to Mels Mix.  He actually recommends taking the bed apart, removing most of the soil and replacing it.
I have a lot of respect for Robert Pavlis, so reading this really has me questioning what I'm doing.  I've been using the SFG method for over three years now, and I mostly have good harvests, but I've also had several disappointments and now I'm starting to wonder if maybe it's the phosphate causing the issues.  I've done a lot of searching around the net to see if this has been brought up before, but I can't find much at all.
Sorry for the long post... It's just one of those things that has my mind running in circles.

I like the way you wrote your question, and I just copied the heart of it and Messaged it to the SFG Foundation leading members for tomorrow's discussion.

Phosphorus does move slowly through soil and can build up.  Nitrogen on the other hand moves fast and needs to be replenished.  The good thing is that we only use composts as 1/3 of the Mel's Mix.  We only replenish each time we plant with the amount that was used up.  We keep the original height in the beds the same as we started with.

I will get back to this thread when the matter has been discussed.

Big Toad, Quick question. Was one of the concerns about using manure-based compost? We recommend limiting those to 20-25% of the blended compost. The SFG Foundation Messaged discussions have already started with the night owls of the group.

Thanks for the quick reply!
He does mention a difference between commercial manure compost and homemade (mostly plant material) compost.
He says that homemade compost has an NPK of 3-0.5-1.5 and commercial manure compost is about 1-1-1.
I mostly use my own compost with veggie scraps, lawn clippings and manure from our pet rabbit. I've mixed in some Black Kow on the occasions that I was short on compost.
Maybe that is what I was missing in reading that section. Pavlis says plants absorb nutrients in the ratio of 3-1-2, so it seems like my homemade compost would be ok and not build up P or K if that's the case.
The beds I had lower production with last year were newly created, so maybe they just didn't have time to level everything out. My two oldest beds had really good results.

bigtoad wrote:
I mostly use my own compost with veggie scraps, lawn clippings and manure from our pet rabbit.  I've mixed in some Black Kow on the occasions that I was short on compost.
Maybe that is what I was missing in reading that section.  Pavlis says plants absorb nutrients in the ratio of 3-1-2, so it seems like my homemade compost would be ok and not build up P or K if that's the case.

A important point is that if using homemade compost from mostly vegetable scraps, lawn clippings, and vegetable plant trimmings will likely never have too much phosphorus since it was not too high in the soil that raised those plants. High phosphorus mostly comes from chemical fertilizers and commercial manures which comes from animals that are fed crops raised on chemical fertilizers.

A good article on this comes from the University of Minnesota Extension Office (Problems Caused by Too Much Compost), which includes these statements:

Best practices for adding compost and manure

• Avoid excess applications of animal-based composts (composted manure), which tend to be high in phosphorus and calcium. 
  
• Test your soil regularly to make sure you are not over-applying certain nutrients. 
  
• Deep compost mulch systems can be effective but should be used with primarily plant-based composts to avoid the problems described above. 
  
• Do not rely on compost as your primary or only soil building strategy. Use cover crops to build organic matter in your soil, and account for the nutrients provided by both cover crops and compost in your decisions about fertilizer use. 
  
• Make sure that any compost you apply to your farm or garden has fully completed the composting process.
  
  • The compost should resemble the texture of the soil, with no chunks of un-decomposed organic matter.
    
  • It should also be more or less the ambient temperature. Compost that is over 100 degrees F is likely not finished.
    
  
  
  

Thank you for that article OhioGardener, I will definitely check that out. I definitely stay away from commercial fertilizers and sparingly use bagged compost after I started my own bin a few years ago.

This is another article from the Univ of Minnesota.

https://blog-crop-news.extension.umn.edu/2021/07/how-to-avoid-soil-phosphorus-build-up.html?fbclid=IwAR2DMGPLXEDE7KjnZejPZPUk-DpaybuqZmV899OfaS3k7VWTIEtd7r_XINc

The SFG Messenger group has been busy all morning. Build-up of phosphorus has been a topic in agriculture for many years. Some states have guidelines on how frequently it can be applied to a field, like every 5 years.

Summary:
1. Limit manure-based composts, especially poultry manure, to 20% of the total blended composts.
2. During the covid and aftermath of covid, if only a manure-based compost, and maybe mushroom compost or such, was found to start making Mel's Mix, don't add anymore manure-based composts for 2-5 years. Use only plant-based composts.
3. If making homemade composts, limit manure as a green ingredient, especially if it is a poultry based.
4. Worm castings are high in phosphorus and should be limited to 5% of the total compost.

Excellent article, Sanderson. Thank you!
We are still a couple months away from our frost date in zone 6, so this is the time of the year I start second guessing everything I do in the garden haha!