Estimating the dry matter production, nitrogen requirements, and yield of organic farm-grown potatoes

Graduation date: 2008As part of a participatory research project, where farmers and Oregon State University researchers collaborated, aspects of potato (Solanum tuberosum L.) growing systems were studied. It was determined through conversations with the farmers that quantification of certain growth parameters of potato was lacking, including dry matter accumulation, crop nitrogen (N) uptake, and yield. In order to better assist the farmers in making fertilization decisions a study of the systems was undertaken.\ud An important aspect in making fertilization recommendations is estimating the contribution of N by the soil in a growing season. This contribution was estimated in two ways. The first was through a laboratory aerobic incubation. Soil was collected in spring, summer, and post-harvest from the participating farms' potato fields in 2006. The N mineralization incubation was conducted for 63 d at 22°C with subsampling to determine NO₃-N every 21 d beginning with Day 0. The second method was by using a plant bioassay approach. Plant biomass, excluding roots, was harvested three to four times during tuber bulking from plots where no current season amendments were added to the soil (zero-N plots). The concentration of N was determined for the plant residues and N uptake estimated. Petiole samples were analyzed for NO₃-N to monitor the relative N status during the season and to determine if there were N deficiencies.\ud Growth measurements of potato were also conducted on "Intensive Farms" in plots where farmers conducted "typical" nutrient management. Weekly plant samples were removed beginning when tubers were 1 to 3 cm in diameter. From these samples dry matter accumulation, crop N uptake, and fresh tuber yield were estimated. Petiole samples were also taken to determine the relative N status of the plant. Yield was also measured for a second group of farm, "All-Farms". This sampling used an "as-is" approach that consisted of hand-digging sections of row and calculating yield per hectare using row spacing found in the sampled field.\ud In the laboratory, net N mineralization rates were approximately 0.4 to 1.3 NO₃-N mg kg⁻¹ d⁻¹ during the 63-day incubation at 22°C for soils collected from 11 farms in the spring with a median of 0.7 NO₃-N mg kg⁻¹ d⁻¹. The N-supplying capacity of the soils is estimated at 120 to 160 kg N ha⁻¹ for 2000 degree days. The uptake of N by the potato in zero-N plots at harvest was 83 to 237 kg N ha⁻¹ confirming the high amounts of N mineralization observed in the laboratory incubation. The soils on these farms mineralize an estimated 3% of total soil N in less than 1400 degree days (base 0°C).\ud On Intensive Farms, total nitrogen uptake was an estimated 145, 190, and 245 kg N ha⁻¹ for Farms 1, 2, and 3. Despite different levels of N uptake, fresh tuber yields on Intensive Farms were similar between farms with 53 Mg ha⁻¹ on Farm 1, 45 Mg ha⁻¹ on Farm 2, and 43 Mg ha⁻¹ on Farm 3. Higher N levels on Farm 3 did not increase yields and total N in shoots at harvest was highest at 150 kg ha⁻¹ verses Farm 1 with 40 kg ha⁻¹. Tuber bulking rates were 0.8, 0.7, and 1.0 Mg ha⁻¹ d⁻¹ for Farms 1, 2, and 3. Delaying harvest could have resulted in higher yields. For tuber yields of 50 Mg ha⁻¹ farmers can expect a total N uptake of approximately 200 kg N ha⁻¹ with 0.9 m between-row spacing. Fresh tuber yields on All-Farms varied by cultivar and location with a median of 19 Mg ha⁻¹ that reflected in-field between-row spacing. At harvest, medium tubers 85 to 227 g (3 to 8 oz) averaged 50% of fresh tuber yield. To accomplish higher yields, row spacing near 1 m is recommended. Petiole N levels were variable during the season with starting values lower than common\ud recommendations and decreasing rapidly as time progressed. We estimate that the soil at these farms supplies approximately 120 to 160 kg N ha⁻¹ and applications of less than 100 kg of plant-available N should support current yields