Conventional potato growers can topdress commercial fertilizer to meet a growing potato crop's need for nitrogen, but certified organic growers don't have that option. Meeting the crop's nitrogen needs during the transition period from conventional to organic can be particularly difficult because growers probably haven't been building up soil nitrogen levels by plowing under green manure corps or applying manure or compost.
In the past, organic growers have been primarily limited to using Chilean nitrate or sodium nitrate, but neither source is considered sustainable. Organic rules limit applications of Chilean nitrate or sodium nitrate to a maximum of 20 percent of the plant's nitrogen needs.
That's why Amber Moore, UI soil scientist was excited to try distillers grains from the ethanol plant at Burley last summer. Pacific Ethanol does not dry its distillers grains which limits marketability to dairies, but organic growers don't mind the wet product.
Moore was pleased to see the local byproduct outperform the mined and imported Chilean nitrate. But, as of press time, certified organic growers had not yet received the okay from the Idaho State Department of Agriculture to apply distillers grains as an approved nitrogen source.
In the meantime, Moore recommends potato growers transitioning from conventional to organic production to apply up to 10 tons per acre of compost in the fall to build up soil fertility, particularly phosphorus and potassium. Planting winter peas as a cover crop is also beneficial. She also recommends soil testing to ensure phosphorus levels don't climb too high.
