Introduction

Through farmer and researcher discussions, the decision was made to focus on nitrogen in the potato growing system.  Farmers were looking for a recommendation, so the researchers decided that in order to give any type of recommendation, measurements would need to be made first.  The measurements were:

• Routine soil analysis of macro and micro nutrients, organic matter content,  and pH
• Soil nitrate-N levels during the prior to and during the growing season
• Incubation of soil for potentially mineralizable N
• Plant bioassay of N mineralization in unfertilized plots on-farm, plus calculated yield
• Plant N uptake (2006 season only), plus calculated yield
• Potato petiole-N
• Irrigation water N

Year 1 (2006) Summary

On-farm data on nitrogen and yield was collected in 2006, including: pre-plant soil testing, laboratory soil incubations, potato petiole nutrient analysis, potato nitrogen uptake from on-farm fertilized and unfertilized plots, end of season 3-ft soil nitrate sampling, irrigation water nitrate, and fresh tuber yield.

Pre-plant soil testing

Eleven farms had complete soil nutrient testing prior to the growing season. Results and interpretations were distributed to the farms. None of the results were unusual for organic farms with compost-based nutrient management.

Laboratory soil incubations

Soil was collected for laboratory incubations in spring prior to planting and again in the summer. Approximately 500 g soil on a dry weight basis was added to Ziploc bags and then incubated at 22 C for 9 weeks. Soil was subsampled in three week intervals and extracted with 2M KCl and analyzed for nitrate nitrogen. The results suggest that the mineralization rates of organic farms in our study groups are high, with an average of 0.8 mg nitrate-N/ kg soil/ day at 22 C from the spring sample.

Potato Petiole Values

Potato petioles were collected at most farms during tuber growth. Our 2006 petiole nitrate-N values were below published recommended ranges for Russet Burbank potatoes grown in the Columbia Basin.

Crop Nitrogen Uptake

Crop nitrogen uptake (vines + tubers) was measured weekly during tuber growth to determine typical N uptake rates using normal grower fertilization practices. During mid-season (40 to 100 days after planting), crop N uptake averaged 2 lb N per acre per day. Six farms also participated in an unfertilized N uptake study, in which a field plot was marked off before planting and no compost or other fertilizers were added. Crop N uptake in the absence of current season fertilization ranged from 100 to 200 lb N per acre.

At harvest, soil samples down to three feet in 1-ft increments were collected from most of the farms. Most fields had low to medium levels of nitrate-N at harvest, suggesting that overall N supply from was not excessive.

Tuber Yields at our farms ranged from 15 to 25 tons/acre for fresh tuber yield, based on 36 inch spacing between hills.

Summary of N management findings

In this season the project quantified plant uptake, yield, nitrogen mineralization, petiole nitrate values and soil nitrate testing at different times during the growing season. This information was new to both farmers and researchers. The largest source of nitrogen for the potato crop appears to come from in-season mineralization of N from decaying soil organic matter.