Did you have your university or another competent lab do a soil test? If so, then there shouldn't be any question about whether your soil is acidic or alkaline and the need for lime. If you used a home kit, they are notoriously inaccurate. If you didn't test, don't add anything until you do.

When to lime is not so much the issue as is whether your soil needs it. Some basic soils (prior to adding any amendments) are very acidic, some very alkaline, some nearer to neutral. Adding compost helps bring soil nearer neutral but that depends on the base soil and how much has been added and what else has been done.

Lime can be added at any time but needs time to work. If you need it, it should be put down as soon as possible. If you need a lot of lime, it's best to put down no more than 50 pounds per 1000 square feet at one time and then wait several months to add more.

Our instructor back in the day had this "mantra" he tried to pound into our heads: DON'T GUESS. SOIL TEST! Ask for a complete nutrient analysis and recommendations for the crops you're growing. Take samples from multiple places in the garden and mix them together, then take the sample to submit from that mix. If you do it now, you should have plenty of time to add whatever amendments are recommended.

We all tend to get into trouble when we "assume." Without that soil test, that's all you or you fellow master gardener or your boss is doing. Poor results from your peppers can come from many problems, soil chemistry being only one. Too much water, not enough water, too much nitrogen, not enough, too much shade, disease or insects, etc. can cause poor crop results. The need for lime is an easy question to answer and will either help solve the problem or allow you to concentrate on other things that may be the culprit.

I've added a link to Virginia Tech's publication on soil testing for the home gardener. It's very comprehensive and should be helpful.

Good luck with your gardening. I hope you get this solved. It's hard to watch the work and time and money put into a garden not turn out the way you want.

Sandy

Here is a link that might be useful: Soil sampling publication

Composted cow manure will not make soil more acidic. And I'm rather surprised a master gardener would apparently make a connection between applications of a composted manure and increased soil acidity. The pH of any kind of compost will approach neutral - that's one of the characteristics of the composting process. In fact it would take repeated applications of a prescribed soil acidifier, like sulfur, to make any appreciable change in soil pH. NW soils tend to be slightly to moderately acidic but that range tends to suit the largest number of plants. Generally, liming is only recommended for lawns or if your soil tests lower than 5.5-6.0.

But I do agree with Sandy that testing for pH is really the only way to know for sure if liming is necessary. I disagree that home testing kits are unreliable - they are about the ONLY reliable soil testing kits for home use. What most folks mess up with is using tap water rather than distilled water in the solution - distilled water should provide accurate results. And soil pH will vary throughout the garden anyway - it will test more alkaline (or less acidic) closer to the home's foundation or any concrete structure as concrete leaches lime over the course of many years. Check with your local extension office to see if they have a pH probe you can borrow - many do and this is a better indicator of soil pH than the home kit.

But a complete soil test is a great baseline for any gardening activity and you should have one done at least once. Oregon, like several other western states, no longers offers soil tests as part of their extension service program. You will have to send your sample to a private lab for testing. I've attached a link to show you how to take samples and where you can send them.

Here is a link that might be useful: soil testing in Oregon

Tanya,

gardengal48 is correct, that home pH tests are quite accurate, particularly for pH with the litmus method. I use a Mosser Lee "Soil Master" (tablet/solution) test kit for pH and N,P,K with less than 10% variance from lab tests. Correct about distilled water vs tap water, but I find that a probe has a wider pH variance than litmus (properly done). IMO even a swimming pool pH test kit is better than a probe.

But, just knowing what the pH is - is not much help when it comes to determining how much, of what kind of lime to add. She is also correct that multiple applications of compost will tend to neutralize soil pH. But has any elemental sulphur been applied to acidify the soil?

That said, Sandy is quite correct, with regard to a lab test needed for determining pH, that will provide a liming requirement based on pH BUFFER, (capacity of soil to resist pH change, sometimes called reserve acidity) which a home pH test cannot provide, since the testing is based on Cation Exchange Capacity - regarding the degree of Hydrogen (+) and Aluminum (+++) ions available in the soil.

The buffering test is simply an exact 8.0 pH solution that a lab adds to the soil sample incrementally, to determine how much lime is needed, to bring the soil up to 6.5 pH. It can be done at home, but most folks cannot do the math to make that lime quantity determination.

So the real issue is: whether any lime is needed in the first place (what IS the base pH?), and if needed, how much of what kind of lime is necessary (over time) to change it?

pH regulates to what degree nutrients are available for plant uptake, with calcium (CA) and magnesium (MG) being the primary micronutrient data points.
Therefore, no lime determination should be made without a FULL nutritional value analysis too.
A Routine + micronutrient + organic content lab analysis of a soil sample is an accurate basis for such a determination, and is $$ well spent - at least once.

Dolomitic lime is usually the recommended liming agent, with a fineness rating: 5.0% of granules are retained on 2 mm mesh with zero effectiveness. 5.0% of granules passed 2 mm mesh but retained on 0.15 mm with effectiveness of 0.6. 90.0% of granules passed 0.15 mm mesh with effectiveness of 1.0.
Which provides a 95% effective pH change agent, with effective watering-in required in several separate applications. Since you are in Oregon, you may be able to obtain the Nutrite brand of Canadian horticultural lime that I find to be best for my own use.

Now the issue of soil structure (tilth) comes to the fore - with regards to percolation. Should be good, with successive compost amendments - even skipping a year - if such additions were 'substantial' and had a fairly high humus content, since the greater volume of compost added into soil, will decompose to 'nothing' in a year - but the humus content remains for centuries.

Since Tanya is a composting specialist, and has been adding compost to garden soil for several years, it is not likely that pH is the main issue with poor (bell?) pepper yields. Soil testing is needed as a baseline, certainly.

However, Tanya - my 'gut feeling' is leaning toward moisture and/or fungus or nematode issues with the pepper plants, rather than pH. Did the peppers become 'wilted' during the growing season even when water was available? Did the pepper plant roots have any 'knots' or 'galls' on them? What were yields of other veggie harvests like?
What is the watering regime that is being used in the garden, specifically regarding the pepper plants?
What was the watering regime for making compost?

Still questions to be answered, before we can help much more, "sight unseen"...

Robert