The Influence of Winter Cover Crops and Nitrogen Fertilizer Rates on Nitrogen Uptake and Yield Response in Organic Sweet Potato (Ipomoea Batatas) Production

摘要

Two studies were conducted to evaluate nutrient dynamics in the soil, plant tissue, and yield responses in sweet potato production. The first study evaluated the use of polyhalite as a source of potassium fertilizer for sweet potato compared to the more commonly used Muriate of Potash (MOP). The rates for MOP and polyhalite were 0, 32.5, 62, 126, and 188 kg K ha-1 . The other treatments also included a control with no added fertilizers, a control for nitrogen, and a control for nitrogen and sulfur. A MOP and Ca, Mg, S treatment was included to examine the impact of these nutrients within polyhalite. A regression analysis showed a statistically significant quadratic response to fertilizer application regardless of source. The distributions of grades among treatments varied. Higher rates of polyhalite treatments produced a higher proportion by total weight of jumbos and a smaller proportion of canners when compared to the control. Soil samples showed that MOP initially released a higher amount of potassium than polyhalite. However, the amount of potassium sustained in the soil by the end of the season was equal between treatments when initial K levels were low. These findings suggest that polyhalite can be a useful source of potassium and its use will result in comparable yield responses to the MOP. The second study evaluates three different cover crops for their effectiveness at managing nutrients for organic sweet potato production. The cover crops included were cereal rye (Secale cereale), crimson clover (Trifolium incarnatum), and Austrian winter pea (Pisum sativum). Over-winter fallow plots were also used to compare cover crop treatments with sodium nitrate applied at 0, 20, 40, 60, 80, and 120 kg N ha-1, allowing a fertilizer equivalency estimate. Soil data showed that there were no differences in N among any of the cover crops compared to the control, likely due to extensive environmental losses of N during decomposition of cover crops before sweet potatoes were planted. Root nitrogen concentrations from the Austrian winter pea were observed to store leaf and root nitrogen at rates equivalent to the 25% recommended nitrogen fertilizer rate (20 kg N ha-1 ). However, the cover crops cereal rye and crimson clover, stored sweet potato N equivalent to the control. Marketable yield showed statistically significant differences among the three cover crop treatments and the nitrogen controls. The Austrian winter pea and the 60 kg N ha-1 plots produced greater yields than did the plots receiving the 80 and 120 N ha-1 . These findings indicate that higher rates of nitrogen application cause a steep decline in yields, potentially due to increased partitioning of biomass to aboveground biomass and away from root production.