The best answer for when add lime to soil is fall, about 6 to 12 months before you plan to plant. Lime reacts slow. It needs rain, cold, and time to work through the root zone. If you wait until spring to apply it you won't see the full effect until halfway through your growing season. Fall gives lime the head start it needs to raise your pH by the time seeds go in the ground.
I tested my veggie beds one September and got back a pH of 5.2 which is too low for most crops. I spread lime that October, worked it into the top 4 inches, and then let winter do the rest. When I retested the following May the pH had climbed to 6.1. That's almost a full point of change in about seven months. The slow pace surprised me at first but the results were solid. My tomatoes and peppers grew much stronger that year than the season before.
Good lime application timing ties back to chemistry. Lime is calcium carbonate in simple terms. When it meets moisture in the soil it starts to break apart. The calcium swaps places with hydrogen ions on soil particles. That swap is what raises your pH. The reaction starts right away but it takes months for lime to spread through the full depth of your root zone. Rain helps push it deeper. Frost and thaw cycles crack the soil and mix it further. That's why a fall application gets better results than a spring one.
NC State Extension puts the max single lime dose at 50 pounds per 1,000 square feet. If your soil test calls for more than that, split it into two rounds spaced 3 to 6 months apart. Dumping too much lime at once can spike your pH higher than you want. I've seen a neighbor push their pH from 5.5 all the way to 7.4 with one heavy pour. That overshoot locked out iron and manganese and it took two years to bring things back down.
Your soil lime treatment choice depends on your magnesium level. If magnesium reads normal on your report, go with calcitic lime. It adds calcium without extra magnesium. If magnesium is low, pick dolomitic lime instead. Dolomite contains both calcium and magnesium so it fixes two problems at once. Check your lab report's magnesium index before you buy anything. The wrong choice could push magnesium too high and throw off your calcium-to-magnesium ratio.
Never add lime based on a guess or on how your plants look. Yellow leaves can mean low nitrogen, iron lockout, or a dozen other things. Only a soil test tells you if the pH is the real problem. Test first, read the numbers, then act. That simple rule has saved me from wasting money and making my soil worse more times than I can count. A $12 test beats a $30 bag of lime you might not even need.
Read the full article: Testing Soil: 7 Steps for Healthy Gardens
