Most rose growers know that growing great roses requires adequate sun, water, and fertilizer, as well as some basic knowledge of pruning and disease prevention. Growers routinely amend the soil in their planting holes with various organic materials like manure, peat moss and compost. They sometimes forget to consider whether or not their particular soil needs to be further amended to adjust its pH. The pH of a soil tells you whether your soil is acid, neutral (7.0) or alkaline. In New England, we typically have acid soil. Very acid soil is enjoyed by holly, rhododendron, mountain laurels and many native plants, but not by roses. Roses prefer a very slightly acid soil pH, ideally about 6.5. References vary on the range that roses will tolerate well, running from 5.5 to 7.1. The main reason that 6.5 is a good pH level for roses (and many other garden ornamentals and vegetables), is that most plant nutrients are widely available at this pH. You may want to check out the “Nutrient Availability” chart in the new Consulting Rosarian Manual (available in our library). You can see that Phosphorus, a vital nutrient for root growth, is almost completely unavailable at a pH less than 5.5. The manual also notes that aluminum can reach toxic levels in plants when the pH is below 5.5, a not uncommon situation in our acid New England soils.
It is important to know the type of soil you have in your garden before adding lime. A clay soil needs three times the lime needed in a sandy soil to achieve the same change in pH. Using the simple guide below, you would need to add 12 oz. of lime per square yard to change the pH in your sandy soil from 5.0to 6.5 (1.5 points change)
Sandy soils - Add 8oz of lime per sq yd to raise the pH one point.
Loam soils - Add 16oz of lime per sq yd to raise the pH one point.
Clay soils - Add 24oz of lime per sq yd to raise the pH one point.
Many soils cannot be classified as just sandy or loam soils. Much of Rhode Island is “sandy-loam”, so your quantity of lime should be averaged between the sandy and loam numbers. Connecticut does have some clay mix soils as well. If you have no idea what type of soil you have, it might be worthwhile to send a sample for testing at least one time, as they will tell you what type you have. The Consulting Rosarian Manual provides a different guideline for adding lime, measuring it more practically by the bush. In a sandy-loam soil they recommend about 8.5 ounces per bush. It should also be noted that the more organic material there is in your soil, the more difficult it is to change your pH. A chart on www.which.net indicated that a loam soil high in organic material would require almost 2.5 times as much lime to move the pH 1 point as one low in organic material.
Changing Soil pH
Changing the pH of your garden soil is a gradual process. It takes several months for ground limestone to become effective. Over-liming, while not common in our acid soil, must be avoided as lowering the pH can be difficult. Applying limestone in the fall, or early spring is recommended. Ground limestone is the most readily available and cheapest material for raising pH. Hydrated lime (added at about 75% of the rate of regular lime) works more quickly. The drawback is that it is more caustic and can kill earthworms and burn new transplants. Dolomitic limestone (also known as Dolo-dust or Mag-Lime) can be used if there is a magnesium deficiency in the soil, but magnesium can easily and cheaply added with Epson salts, so it’s not necessary. It also takes longer to break down in the soil. It is the calcium in these lime products that raises the pH. Rain, which is acidic, leaches out or washes away the calcium in soils, leaving it acid. In drier more desert-like climates, the calcium remains in the soil, sometimes making it extremely alkaline. Some other calcium products that can be used are bone meal, crushed shells or marble chips.
Testing Your Soil
There are a variety of ways to test your soil pH. You can send it to your local soil testing service, in our case the University of Rhode Island Cooperative Extension, and have not only the pH checked, but also nutrient levels. They charge $3 for the pH test, and $8 for both the pH and nutrient levels. Information on collecting samples, an order form and mailing instructions are available at: http://www.uri.edu/ce/factsheets/sheets/soiltest.html or call the URI CE Hotline at 1-800-448-1011. You can test your own soil pH by using test strips or a pH meter. Strips can be accurate, but some varieties can be difficult to read. The better ones have 2 or 3 blocks of color on each strip to help you read them with more accuracy. A pH meter ranges in price from around $15 to many hundreds, the least expensive ones do not match the test strips in accuracy. Both with strips and meters, following instructions exactly is the key to good results. You may need distilled water for mixing, again, check the directions. Meters have electrodes that typically require special cleaning after use, and the machine itself will need some calibration before each day’s use.
Take time to collect a good sample, whether you are sending it out or checking it yourself. Remove mulch and organic debris from the top of the soil. Take a thin slice of soil down to about 10’. Put it in a clean bucket and repeat in various parts of the bed, mixing together the sample. Take separate samples in areas where the roses do not seem to be doing well or in widely separate parts of the yard. Make sure your shovel and pail are clean and do not use brass or galvanized tools, which can add copper or zinc to the sample. If you are sending it out, follow directions for packaging. Some labs want the sample dried and sent in a cardboard box. Others like URI want it in a Ziploc bag after air drying.
In closing, remember that all of a plants nutrients come to itthrough its surrounding soil. A loose well drained soil at the proper pH is the key to getting those nutrients to your rose.