Dear Ruth,

What is the difference between different kinds of lime available for garden application? I have some instructions that say “Do not use slaked lime”, but I
don’t know what that means.

I asked the local garden center, and they told me their lime was dolomitic. What is that? Is dolomitic lime also slaked? Please help!

-Anna, (Asheville, NC)

 

Dear Anna,

I’ll do my best to keep it simple, and I invite soil scientists in the neighborhood to send us additional comments on this important topic.

There are two basic types of lime available:

-Calcium Carbonate, called calcitic limestone (CaCO3)

-Calcium Magnesium Carbonate, called dolomitic limestone [CaMg(CO3)2].

If calcium carbonate is heated, the carbon dioxide portion escapes and the result is called burnt-lime or quick-lime (CaO). If the burnt-lime is combined with water, hydrated-lime [Ca(OH)2] or slaked-lime is produced.

Limestone alters the pH of the soil and provides nutrients to plant life. Ground limestone, either calcitic or dolomitic, is the most used, most abundant, and generally least expensive form of lime. Certified Organic growers are not allowed by the USDA Organic Rules to use either burnt-lime or hydrated-lime. In your backyard garden, it’s up to you. I figure most of the organic rules are based on reasonable environmental arguments. It is possible that burnt-lime can kill some of the beneficial microbial activity in your soil, and it can also burn plant roots during unfavorable conditions. Hydrated-lime is liable to leach beyond the reach of plant roots becoming unusable to the plant. If you do use either burnt or hydrated lime, extra protection for your skin and eyes is required. Maybe that’s why your instructions stated, “Do not use slaked lime.”

Why is lime so important? Primarily, lime sweetens the soil by raising the pH and adjusting the acidity of the soil. Lime can facilitate better nutrient uptake and it’s probably the most economical way to provide additions of calcium or magnesium. It’s a fact that proper pH is extremely important to optimal plant health and maximum yield in your garden. Lime can also benefit the structure of both clay and sandy soils. Lime should be evenly distributed over the garden and well incorporated into the soil, as it doesn’t move around much.

Most vegetables prefer a pH range of 6.0 to 6.5, but this varies by region, and by crop. Beans and peas like more lime than other veggies. Potatoes don’t really like lime, and a number of plants prefer acid soil, such as blueberries, hollies, and rhododendrons. Getting a soil test is essential to ensure that your application rate is correct, and not overly sweet. Like Goldilocks… you want it just right.

Guess what? North Carolina is one of the only states that still offers FREE soil testing. Go to your local Cooperative Extension office where they will provide you with a box and instructions. After gathering your soil for testing, put your sample boxes in another box and ship it to Raleigh (you do have to pay the postage). Turn-around times are longer in spring than other seasons, so try to think ahead. Right now their website says it takes 5-7 days for the results to be posted, which seems optimistic for this time of year. Your soil test will tell you exactly how much lime to add, if any. Fall is a great time to get your soil tested, and a great time to apply lime to the soil since lime is somewhat slow-acting. If you don’t have a soil test to go by, and the soil has not been limed in the last 3-4 years, it would probably be safe to apply 50 lbs. per 1,000 sq. ft. of garden. Timing recommendations vary from liming a year ahead, to 4-6 months, to 2-3 months, and to applying just prior to spring planting. The general consensus seems to be to apply lime at least a few months ahead of planting so it has enough time to effectively alter the pH. The finer the particle size of the limestone powder, the faster acting the lime will be.

In general, the soils in Western North Carolina are naturally high in magnesium, so in most cases dolomitic limestone would not be the preferred lime for this area. Many area farmers actually use a high-calcium lime that acts to balance the calcium/magnesium ratio in the soil and results in better nutrient uptake. Gypsum can be used when you want to add calcium without changing the pH. Elemental sulfur is used to acidify the soil when you have an acid-loving plant. Bagged lime is available at local garden centers.

Ok, Anna, I hope you are ready to forge ahead with your project!

Best wishes,

Ruth

Web Resources for further reading: www.soil.ncsu.edu/publications http://hgic.clemson.edu

Ask Ruth © 2013 Ruth Gonzalez & Organic Growers School