I have been gifted horse manure for my blueberries.

Asked March 25, 2019, 3:13 PM EDT

I have a u-pick farm of about 750 plants. My ph usually runs about 5 to 5.5. I would like to mulch my plants with the finely composted horse manure and wonder if I should add elemental sulphur to the spread. I don't want my ph to increase a lot. I live in the Tillamook area and get quite a lot of rain. If I add sulphur, how much should I use. I usually fertilize with nitrogen and phosphate each year, would I still do that if I use the manure.? Thank you. Jan Crabb

Tillamook County Oregon

8 Responses


Thanks for the question!

You’re right to worry about pH changes with manure compost/mulch. I’m not able to recommend a specific sulfur amendment because we don’t know the pH of the manure and how it might impact soil pH. You need to know the pH (and other components) of the manure before you can make a decision. I’d recommend getting it tested before you do anything else. Also, read this article (https://extension.oregonstate.edu/produce-forage/berries-grapes/organic-soil-amendments-mulches-blueberry-good-bad-ugly) that details some of the other considerations you should be thinking about when adding a compost to your blueberries. EC (salt) is a biggie.

So, if you’re planning on using horse manure as mulch, you can, but there are a few things you need to keep in mind. You’re correct to worry about pH changes, but more importantly is the salt (EC) content of the manure. While manure is a good slow-release nutrient source, you need to be careful to not place it too close to the crown of your blueberry plants as it can burn them. A thin layer (1/2 inch) can provide some nutrients, but weed seeds will germinate very quickly when they land in this type of mulch, which is a big disadvantage (there are already weed seeds in horse manure because horses’ digestive tracts don’t break down weed seeds). So at the least, be sure to top dress any horse manure applications with sawdust for good weed control.

If, at the end of the day, you decide to go ahead with a manure mulch, you get your manure tested and find that the pH will affect your soil pH, there’s a nice section in this publication on using sulfur to lower pH in blueberries. (https://catalog.extension.oregonstate.edu/ec1560)

The BEST option, however, is to skip the manure altogether and go with sawdust mulch.

I tested my composted horse manure pile and it is
PH 6.0
N deficient, K sufficient, P sufficient,
2nd field , east
PH. 5.5 N deficient, K adequate, P adequate.
I tested 2 places in each of my 2 fields.
1st field,west.
PH 6.0 N deficient, K adequate, P adequate

So, if I do dress the blueberries with horse manure, how thick should I put it on and should I add any nitrogen and how much for each plant. They are full of blooms and the leaves are healthy. I can also put shredded fir and willow mulch on the berries. Please respond.

Should I also apply triple phosphate on the berries?

Hi Jan,
Did your soil tests give you information on the salt content of the horse manure? That would be electrical conductivity (EC).

Mulch: the sawdust is probably best, but use the willow mulch, too. You’ll need the mulch to be 4-6 inches deep at the base of the plant and into the drip zone. Mulch tends to decompose at about an inch per year, so keep that in mind.

Triple phosphate: you shouldn’t need that, as your soil P is fine in both sites.

Nitrogen: To figure out how much nitrogen to apply to your plants, you have to know how old they are. Use this table to figure that out. The column on the far right tells you how many ounces per plant you need … it’s really not very much. You’ll divide that number into thirds. You’ll apply one-third now (during bloom), another third in about a month, and then the last third about a month after that. (see attached image).

Sulfur: Your soil pH in the west field (6.0) is high. Adding the horse manure with a pH of 6.0 won’t increase it anymore, but you really need to have a plan in place to lower your soil pH over the next several years. Just plan on doing a little at a time each year – it’s a lot easier to manage a small change in pH than a large one, and it’s easier to lower pH than it is to raise it back up again. Nitrogen fertilizer will decrease your soil pH over time, but it does it slowly. That is totally okay, and eventually, your soil pH will decrease, although it might take 5 or 6 years. It seems weird that your horse manure is low in nitrogen; it all must have leached out. If I were you, I’d just skip the manure; it doesn’t seem like it’s going to add very much in the way of nutritional benefit to your blueberries, and it comes with a nice stockpile of weed seeds that you’ll have to fight later. This way you’ll also avoid potential salt toxicity from the manure. So, to circle back to elemental sulfur, if you want a relatively quick drop in soil pH, you can add 2 to 2.75 pounds per 100ft2 to your west field (the one with a pH of 6.0). I wouldn’t add anything to the east field; the pH is on the upper border of what you want, but it’s okay.

Here is the outcome of the horse manure compost testing for PH and soluble salts. ph is 4.61. as rec'd% soluble salts is 0.04. As Rec'd #/T is 0.86.

Is this safe to spread on the Blueberries. The Ph is okay, but is the % of salt safe?

Thank you so much for your help. Jan Crabb Blueberries and Blooms

Hi Jan,
Yes, your salt concentration is very low, so it should be safe to use on your blueberries. The pH is low, too, so that's really good. The only concern I'd have now is the K content of the manure, but in looking at your results from earlier, it looks like it's fine, too. Happy growing!