Soil test - Too much NPK?

I've been unable to find good info on the internet covering soils high in fertilizer and minerals. Three years ago, we sheet mulched our yard with a 50/50 mix of horse manure and straw to a depth of 10", plus 2-4" of compost. We've been growing in this for 2 seasons now, and it's fully decomposed. We have top dressed compost each year, and done some N-heavy liquid fertilizer a few times each summer.

We noticed some weak areas in terms of finding it hard to get good plant growth across the few things we've tried, and so I resolved to get serious about fertilizing and amendment this autumn. To get ready, I got soil tests done at Colorado State (our local Ag program), and the results show very high levels across the board.

Wondering if anyone can give me advice on what to do or not do here? I assume I should hold off on any fertilizers, but wondering if continuing with worm castings and biochar would make sense (was planning to add those this year). Seems like if these levels are causing growth problems, I don't have much of a choice but to wait them out, assuming they will wash out with the rain?

Ph - 7.3
Organic matter - 10%
KCI Nitrate-N - 63ppm (Very high > 50ppm)
Phosphorus - 457ppm (V High > 20)
Potassium - 1087ppm (V High >280)
Calcium - 2729ppm (High >2500)
Magnesium - 434ppm (V High > 200)

The list goes on, but everything is considered High or Very High, except for soluble salts which are Low.

Thanks all!

Worm castings and biochar will be fine.
From; too much of a good thing
Unlike other plant nutrients, phosphorus does not leach in the soil.
This means that too much phosphorus in the soil can build up over the course of several growing seasons.
Excessive phosphorus can occur for several reasons.
Most commonly this issue is caused by repeated use of manures or non-organic fertilizers.
While a surplus of any nutrient may not seem like an issue, lowering phosphorus levels is actually quite important.
Too much phosphorus in the soil can be detrimental to the overall health of the plants.
High phosphorus can cause deficiencies in zinc and iron in the soil, as they quickly become unavailable for use by the plants.
Unfortunately, there are no ways to actively reduce excessive phosphorus in garden soil.
In working to moderate phosphorus levels in the garden, it will be imperative that growers avoid the use of fertilizers that contain phosphorus for several growing seasons .

In order to remove excess nitrogen in soil, you need to bind the nitrogen that is in the soil to something else.
Fortunately, as a gardener, you probably grow many things that bind nitrogen
— in other words, plants. Any plant will use some nitrogen in the soil, but plants like squash, cabbage, broccoli and corn
use up large amounts of nitrogen while growing.
By growing these plants where there is too much nitrogen in soil, the plants will use up the excess nitrogen.
Be aware though, that while they will grow there, plants may look sickly and will not produce many fruits or flowers.
Keep in mind that you are not growing these plants for food purposes, but rather as sponges that will help lower soil nitrogen content.

Many people use mulch in their garden
When you have too much nitrogen in the soil, add mulch that uses lots of Nitrogen, IE Cheap, dyed mulch is generally made from scrap soft woods and these will use higher amounts of nitrogen in the soil as they break down. For this same reason, sawdust can also be used as a mulch

First thing to do is to stop adding fertility.  No worm castings!  Raw biochar will help mop up the excess nitrogen without adding too much P or K.  The excessive P is likely your problem as it is toxic at higher levels.  Grow P hungry crops, then remove the excess biomass and use it as mulch or compost somewhere else that still needs fertility.

To those people who are more familiar with soil nutrients than I am... would it be beneficial to bring in some rather inert "fill"? Subsoil with little organic matter and mix it in, to dilute the phosphorus?

Could you tell what soil you had before you began amending it. The values for NPK manure are very low, except that most folks put in a lot of it, but here are the NPK values for horse manure:

composted horse manure:    NPK   0.95 -  0.3  - 1.50
uncomposted horse manure   NPK  1.5  -  1.4  - 1.75

I think the problem is that you added lots of manure, straw, and compost and then added "some N-heavy liquid fertilizer a few times each summer". I'm guessing that the liquid fertilizer is where the problem came from. I would quit adding any manures, composts, or fertilizers and grow what you'd like to harvest and let that burn off some of those values. Get another soil test in the late winter/earlyspring of 2024 and if necessary adjust what you're growing based on that.

I don't see why you couldn't use straw as a mulch to prevent weed growth but I wouldn't dig that in.

I had to leave to make an appointment, so will continue.

We don't know where you are, nor how big your garden is. Which is why I suggested growing what you want to eat, or look at. We also don't know how far your garden is from your home. Using wood products of any kind except bark can be a problem as it attracts termites. It also prevents seeds from growing. So it may be fine putting it in this part of the garden this year, but then in the future your plans may change. You want to rotate crops but can't put the zucchini over  there because the seeds don't germinate, for instance.

Manure and compost makes a fine fertilizer and also is a wonderful soil amendment. Both can be obtained free for the hauling. Wood products just cause other problems for years.

To give you and idea of how much manure can be use. I was having trouble with early blight on tomatoes so opened a 34x64' garden is what was my lawn with some fruit trees enclosed in 3 foot or so diameter 5' fencing. The soil was raw clay with maybe a half inch of sod. I spread lime and 2 inches of over aged horse manure and dug that in with the sod. Then I spread another inch and dug that in going back and forth in the opposite direction; east/west instead of north/south. When I dug a hole to set out a tomato seedling I might have added another spade full or two. I grew wonderful crops, Potatoes, tomatoes, zucchini, peas, cukes, beans. In the following years I spread another inch or so of manure and dug that in. When I find a spot with a clump of clay I toss and dig in a spade full of manure.

But I never add any fertilizer, to the garden or the lawn. It's my opinion that if you start out with a nice black loam soil you might apply an inch of manure every 5 years or so, along with what compost you can make. But clay and sand is another story, the sand story I never dealt with.

Seems to me that the organic matter while "very high" isn't as high as it should be with NPK, Ca, Mg, etc where they are. And anything over 5 PPM of organic matter is considered very high. I'd say that 5 is a little low in a well taken care of garden. An indication, to me, of where the problem originated from.

Thanks for all the feedback. This garden was originally turf that we hadn't fertilized for years. In 2020 before sheet mulching, I had a soil test done (results are lost of course) that recommended adding nitrogen. I added corn gluten in the amount recommended by the lab before sheet mulching. The garden is 2000 square feet in two sections, within 15 feet of the house.

It sounds from the responses that we have simply over-fertilized through some combination of manure and additional fertilizers. I'll plan to add the biochar, optimize for P-hungry plants in this area of the garden, and otherwise stay off the hard stuff until I can re-test and see normalized levels. We normally mulch with straw over winter as well FWIW.

Thanks again!

That basically said it needs more hydrogen and oxygen rotations too solubize more thier holding well and are present I would say grounds staying too moist or dry that day test was done another day be different result.