What do I need to know about soil fertility in sweet corn production?
Answer: Nitrogen (N) is especially important in sweet corn production, not only for plant growth but also for the production of amino acids that influence flavor and nutrition. Research at Michigan State University showed that 6 percent of the total nitrogen is taken up between germination and the sixth leaf stage, 25 percent from seventh leaf to tassel, 25 percent from tassel to silk and 39 percent during ear development. A common recommendation in conventional production is to apply 50 pounds of nitrogen per acre prior to or at planting, followed by side dressing with 60 to 80 pounds of nitrogen per acre when the plants are 12 to 18 inches tall.The Pre-Sidedress Soil Nitrate Test, also known as the Soil NO3-N Quick Test, can determine the need for any additional nitrogen fertilizer. It is now well established that if the nitrate-nitrogen level in the soil is above a threshold level of 25 ppm when the corn is 6 to 12 inches tall, additional nitrogen fertilizer will not increase yield.Supplemental sidedress nitrogen fertilizers used in organic vegetable production include plant and animal by-products like blood meal, fishmeal and soybean meal, as well as pelletized compost products.Research in Connecticut determined that 100 pounds of nitrogen per acre from commercial fertilizer could produce optimum yields and economic returns for sweet corn. This research is significant because it found the standard rate used by Connecticut farmers, 160 pounds of nitrogen per acre, was too high. In addition, it provides further support for the organic farming practice of raising sweet corn in rotation with forage legumes. For example, it is generally accepted that a healthy stand of hairy vetch can provide around 100 to 125 pounds of nitrogen per acre to a subsequent crop.When legume stands are poor and therefore nitrogen is estimated to be lacking, supplemental composts and organic fertilizers can be applied as necessary. For additional information on estimating nitrogen production and release from cover crops, see ATTRA’s Overview of Cover Crops and Green Manures. Sweet corn does best with a pH of 6.0 to 6.5 and needs moderate to high levels of phosphorus (P) and potassium (K). Rate of application should be determined by soil testing. Rock phosphate, potassium sulfate (mined, untreated source), sulfate of potash-magnesia (commercially available K-Mag) and a limited number of other rock powders may be used in certified organic programs. One problem with rock phosphate is that phosphorus is very slowly available. In cold soils, phosphorus deficiencies indicated by purple-tinged leaves may be apparent. Thus, some growers drill a quickly available source of phosphorus, such as bone meal, at planting to insure readily available phosphorus and a healthy crop stand. Other growers simply delay seeding until the weather and the soil warm up.Growers can apply and incorporate rock mineral fertilizers, manures and bulk composts during field preparation and bedding operations. Growers often make applications in the fall before planting the cover crop. Banding to the side of the row at planting is another option, primarily in combination with organic fertilizers or pelletized and fortified composts.The late eco-farming adviser Don Schriefer advocated foliar feeding, used in combination with a chlorophyll meter, as a yield-enhancing corn-production practice. To illustrate the importance of photosynthate production in the early life of a corn plant, Schriefer emphasized the following facts relating growth phases of corn to yield potential:? The number of rows of corn on the cob will be set five weeks after emergence. Rows usually range from 14 to 18. ? Ear length and number of double ears per plant will be established nine weeks after emergence. While corn is relatively drought tolerant, irrigation increases yields, especially when applied during silking and when ears are filling. If irrigation is not an option and weed management is good, plants might be seeded farther apart to reduce interplant competition.To learn more, consult the ATTRA publication Sweet Corn: Organic Production. It discusses key aspects of producing organic sweet corn including varieties, soil fertility, crop rotations, weed control, insect pest management, diseases, harvesting, post-harvest handling, marketing and production economics.