What information can you give me on using molasses for weed control?

B.A.KansasAnswer: Thank you for your recent request for information from ATTRA, the National Sustainable Agriculture Information Service. I am pleased to provide you with information on the use of molasses for weed control.Molasses is a byproduct from the processing of sugar from sugar cane and sugar beets. It is used as a food additive and in agricultural as a supplement in livestock feed, as a soil amendment, and as a natural herbicide. For agriculture purposes, there tends not to be a difference between cane molasses and beet molasses as both types have the same amount of calories as sugar; about 16 calories per teaspoon (1). The difference found in molasses and to consider for use as a soil amendment and for weed control is whether or not it has been treated with sulfur. Treating molasses with sulfur results in the fortification of iron, calcium, and magnesium. Some farmers feel that applying sulfured molasses is an organic way to add these nutrients to the soil. However, many researchers, such as Dr. Elaine Ingham (www.soilfoodweb.com), have found that sulfured molasses can have significant effects on soil biology. This can have damaging effects when applying molasses for weed control. There are two methods for using molasses for weed control. The first method focuses on applying molasses as a soil amendment that will feed soil organisms. By boosting soil microbial activity, more nitrates are taken up and biomass created. As a result, the soil is healthier which makes it harder for weeds to compete with desired plants. Studies have shown that the amount of molasses applied affects the activity of soil bacteria and fungi. These studies indicate that lower application rates promote more fungi while increased application rates favor bacterial activity (1). However, too much molasses increases the potash content in the soil. This can make calcium unavailable and help weeds thrive. The second method for using molasses for controlling weeds is to apply it directly as an herbicide. This can be done by directly applying the molasses or by adding it to water or a calcium/water mixture. There are two trains of thoughts as to why the calcium mixture works. The first considers using molasses directly on the plants where the weed control is affective through the “burning” of the plants. The other train of thought considers the basis for this practice a modification of soil surface conditions that prevents weeds from germinating (2). The calcium and sugar solution reduces soil crusting and compaction, conditions that weeds flourish in. This practice tends to rely on being applied primarily to the soil surface as a pre-emergent, optimally following the drilling of a crop or at lay by following the last cultivation. There are several recommendations for the amount of molasses to apply. As a feed for soil organisms, it is recommended to apply 2-4 oz. of liquid molasses per gallon of water or 20 lbs. of dry molasses per 1,000 square feet. As an herbicide, 40 lbs. of dry molasses per 1,000 square feet or one cup per gallon of water is suggested. For a calcium/molasses mixture, 2 gallons of liquid calcium and 2 gallons of molasses in 20 gallons of water is needed (2). Repeat applications may be necessary. References:1. Moore, Robert. 2009. Molasses. Rockport, Texas: The Soil Guy. Retrieved July 9, 2010. 2. Diver, Steve. 2005. ATTRA Case Letter 62545. Fayetteville, AR: NCAT.