Beach-cast fertilizers for cultivation of carrots and radishes on Swedish Gotland : The effect on crop size and nutrient composition

Abstract: In the eutrophicated Baltic, algal blooms with toxin-producing algae and plant overgrowth is a large threat to the marine environment, humans and animals. The use of external nutrient inputs into agriculture, such as mineral fertilizers (NPK), is reaching the Baltic through runoff which causes a constant, linear nutrient flow from land to the sea, which is one of the main contributors to accelerated eutrophication. Beach-cast is rich in nutrients and plant promoting substances and was widely used as fertilizer on Gotland before the introduction of NPK. Nowadays large amounts of beach-cast are washed up on the shores of Gotland and most of it is not used. Using beach-cast as fertilizer could contribute to a decrease in external nutrient input into the Baltic and increase a circular flow of nutrients. This study was a part of a broader project operated by KTH with the purpose of evaluating beach-cast as a potential bioresource. The aim of this study was to examine the effects on nutritional content and crop size from the usage of composted beach-cast as fertilizer in comparison with the NPK fertilizer in outdoor cultivation of radish and carrots on Gotland. After the crops were cultivated and harvested, the mass of the root and haulm of the carrot and radishes was determined along with quantification of the content of carbohydrates, pigments, main vitamins and fatty acids. The result revealed a significant increase in 1) mass for three out of four (for all but the carrot root which borderlines the limit of 95% significance), 2) mass ratio between root/blast for the radish, 3) concentration of beta-carotene, vitamin A and fatty acids for the carrot root and 4) concentration of chlorophyll for both haulm. Other observed differences, although not statistically significant, was an increased carrot root mass, increased in concentration of anthocyanin for radish, unchanged mass ratio between carrot root/blast, increase B3 vitamin concentration for all crops, increased concentration of carbohydrates for carrots and decreased concentration for radish, decrease in fatty acids for carrot haulm and radish root and haulm and finally a decrease in vitamin C for all crops. The use of beach-cast as fertilizer has shown to induce growth of the crops, and the majority of the quality parameters were in the same range or exceeded in comparison with the crops cultivated with NPK. A complementary use of beach-cast fertilizer could thus prove beneficial for farmers and at the same time decrease the external nutrient input to the Baltic.