Plots that had been planted with field peas and vetches in mid-summer were associated with lower flag leaf nitrogen than plots that had received spring plantings of field peas and vetches. Grass cover crops were associated with the lowest leaf nitrogen, suggesting that the ability of grass decomposition to tie up nitrogen can be persistent. Visual growth differences were apparent throughout the winter wheat growing season; wheat in spring field peas and vetch cover crop treatments were taller and much greener than other treatments. This suggests that nitrogen release from legume cover crops can continue for more than 1 year and can potentially have cumulative effects in crop rotations.Chicken manure amendments were the most effective fall-applied amendments for increasing soil nitrate levels at potato planting . Soil nitrate at potato planting in soil amended with chicken manure was greater than 75 pounds of nitrogen per acre , similar to levels in plots treated with field peas and vetches. Potato petiole nitrate levels for plots amended with chicken manure were over 19,000 ppm at early tuber bulking, similar to levels produced by many field peas and vetches. Potato petiole nitrate at early bulking for blood meal and soy meal amendments was similar to levels associated with both chicken manure and 150 pounds per acre of urea fertilizer . Green waste compost applied at all rates, as well as composted steer manure, led to lower soil nitrate at potato planting than did chicken manure, and these amendments did not increase soil nitrate at potato planting compared to the fallow treatment . Green waste compost and steer manure did not increase potato petiole nitrate at early bulking and vine maturity compared to the fallow treatment, flood drain table suggesting that nitrogen in these amendments mineralized too slowly for a single application to benefit a potato crop .

Potato establishment and early season vigor did not differ significantly among treatments, but differences in potato vigor were significant at row closure and tuber initiation . Treatments producing high potato petiole nitrate produced taller, greener potato plants than did treatments producing low potato petiole nitrate. Russet Norkotah total potato yield, average tuber size and cull yield were influenced by cover crops and amendments while Yukon Gold potato yield was similar for most treatments . This trend was not surprising given that Russet Norkotah is more responsive to nitrogen fertilizer than Yukon Gold. For Russet Norkotah, vetch species , chicken manures, steer manure, blood meal and soil protein fertilizer produced higher total potato yields than did the untreated fallow. These treatments, along with five field pea varieties, resulted in a larger average tuber size than did the untreated fallow . Total yield for the treatment with 150 pounds per acre of urea fertilizer was similar to that produced with vetches, chicken manures and blood meal, suggesting that soil nitrogen availability was a primary factor in increasing potato yield . Nitrogen’s important role is also supported by a strong positive correlation between total Russet Norkotah potato yield and potato petiole nitrate at early bulking. The r value for this correlation equaled 0.656 when Russet Norkotah and Yukon Gold data were combined. The only treatment-related effect on total Yukon Gold potato yield was that cover-cropping with spring wheat and fall triticale produced lower total yield than did cover-cropping with legumes . Grass cover crop treatments led to numerically lower soil nitrogen at planting and lower potato petiole nitrate at early bulking, compared to the untreated fallow .

This suggests that the low potato yield following grass cover crops could be due to nitrogen immobilization during potato growth and development. Cover crop and amendment treatments did not cause a substantial increase in tubers with knobs or growth cracks in either Russet Norkotah or Yukon Gold , but the percentage of cull potatoes based on total yield for Russet Norkotah differed among treatments . Both chicken manure treatments, as well as blood meal and soy protein, resulted in higher percentages of culls than did the untreated fallow. An increase in cull percentage often occurs as total yield increases, but Perfect Organic Blend chicken manure also produced a higher percentage of culls than did the treatment with 150 pounds per acre of urea fertilizer. All cover crop treatments led to a percentage of culls similar to or lower than was associated with the treatment with 150 pounds per acre of urea fertilizer . Yukon Gold was chosen for the 2017 trials because Rhizoctonia black scurf and black dot tuber blemish, common problems for organic potato growers, are easy to see on yellow varieties. The severity of black scurf and black dot did not differ according to cover crop species, but in potatoes grown after spring-planted cover crops , 27% exhibited black scurf — compared to 13% in potatoes grown after mid-summer and fall plantings of cover crops. On the other hand, spring plantings of cover crops led to lower black dot severity on tubers than did mid-summer plantings .Economic issues play a major role in the feasibility of using legume cover crops to boost soil nitrogen in a crop rotation. Organic growers must consider the opportunity cost involved in growing cover crops instead of a cash crop as well as the cost of applying an amendment such as chicken manure. The economic analysis required to weigh all benefits and lost opportunity costs is complex, and beyond the scope of this study, but a comparison of monetary costs shows that cover crop production is more expensive than synthetic fertilizer, similar to applying chicken manure and less expensive than applying blood meal and soy meal.

The average cost of bulk urea fertilizer from local suppliers in Northern California in 2018 was $365 per ton, or $60 to supply one acre with 150 pounds of nitrogen . The average cost of bulk dried poultry manure from local suppliers in Northern California was $145 per ton, or $272 dollars to supply one acre with 150 pounds of nitrogen . The cost of bulk blood meal and soy meal represented a nitrogen cost of greater than $3.40 per pound, or over $500 to supply one acre with 150 pounds of nitrogen. The cost of certified organic blood meal, packaged in 50-pound bags, was greater than $7 per pound of nitrogen, or more than $1,000 to supply one acre with 150 pounds of nitrogen. The total cost of field pea and vetch production is estimated at $175 dollars per acre, including the cost of seed, planting, irrigation, management and incorporation .Vetch, field peas, blood meal, soy meal and chicken manure, because they produced potato yields and potato petiole nitrate similar to those produced in plots treated with 150 pounds per acre of urea fertilizer , were feasible alternatives to synthetic fertilizer. Whether organic producers favor cover crops or chicken manure as a nitrogen source depends on several factors, including land availability and the opportunity to grow cash crops. Producers who grow high-value cash crops requiring a full growing season may favor amendments because they can be quickly applied after harvest or before planting. Producers with idle land or with time between cash crops during the growing season may prefer cover crops, as many legumes in this study added over 150 pounds of nitrogen per acre and provided multi-season carry-over of soil nitrogen, and also offer protection from soil erosion. For hay producers, it’s extremely important to leave above ground biomass from legume cover crops in place, instead of haying the residue, because most added nitrogen is contained in legumes’ leaves and shoots rather than their roots. Regardless, both options offer benefits in soil health, and in our study the added nitrogen in both cases broke down into mineralized form in adequate amounts to meet early-season and late-season potato nitrogen needs. The economic benefit of using cover crops and chicken manure is more difficult to justify in conventional potatoes because, in our research, both practices entail higher costs and greater difficulty of application than synthetic fertilizer, which produced similar yields. For organic potato production, using either grass cover crops or a one-time application of compost to increase soil nitrogen is difficult to justify economically. In our research, grow tables 4×8 these treatments had a neutral or negative effect on soil nitrogen compared to fallow treatments. Organic nitrogen in these treatments failed to convert into mineralized form in adequate amounts to increase either potato yield or yield of wheat planted the year after potatoes. Mustard, arugula and radish had a neutral-to-positive effect on potato yield and nitrogen.

Several Brassica species have also been shown to have biofumigation properties, although a reduction in soilborne potato diseases Rhizoctonia solani, Colletotrichum coccodes and Verticillium wilt was not evident in this study. Fallowing for an entire year, starting in spring the year before growing potatoes, is another option that growers with idle land or limited water can consider. In this research, the spring fallow treatment resulted in mineralized nitrogen at potato planting similar to or higher than levels that resulted from the summer fallow and fall fallow treatments . In potatoes, the spring fallow treatment produced petiole nitrate at early bulking similar to that produced by a treatment with 150 pounds per acre of urea fertilizer following barley . The additional nitrogen in the spring fallow treatment was likely related to natural mineralization of soil organic matter, as organic matter in Tulelake soils is naturally high .Rice is one of the most important sources of human energy worldwide and is grown in a wide range of agroecosystems, though paddy systems are the most prevalent . In California, more than 200,000 ha of flooded rice are grown in a waterseeded, continuously flooded system that has successfully suppressed certain nonaquatic weed species such as barn yard grass [Echinochloa crus-galli Beauv.] and bearded sprangletop . Currently, rice growers in California flood fields at the beginning of the growing season and then direct seed pregerminated rice seed into the flooded fields from airplanes. A flood depth of 10 to 15 cm is maintained until approximately 1 mo before harvest, when the field is drained to allow rice harvesting. Repeated use of flooded irrigation in the California rice agroecosystem has since selected for weed species such as late watergrass [Echinochloa oryzicola Vasinger] that are well adapted to the system. In recent years, California has experienced unprecedented drought, with the 2012 to 2014 period being the driest on record . Accordingly, concerns about water usage have increased, particularly for crops like rice that have high water use. Due to the flood irrigation, rice is a visible water user, receiving attention from both the general public and policy makers, and there is increased pressure on rice growers to reduce water use. In California, the only alternative to water seeding currently in use is dry seeding followed byflooding after early postemergent herbicide applications. Recent research, however, indicates that drill seeding into dry soil as practiced in California rice systems does not necessarily reduce crop evapotranspiration, crop coefficient, or irrigation delivery in comparison with the continuously flooded system . A number of alternatives to flood irrigation exist in other rice-growing regions of the world, including an alternate wet and dry system , which reduces water use over the crop growth period through alternating periods of flooding with periods of drying, and saturated soil culture , which reduces water use over the crop growth period by maintaining the soil at the saturation point . Yields in aerobic systems are often lower due to the reduced ability of rice to compete with weeds , and this may be an obstacle to adoption of alternative irrigation systems by growers. In addition to changing the competitive ability of rice with respect to weeds, alternative irrigation systems can shift weed species composition, selecting for some species over others. In California, differences in irrigation during the seedling recruitment period have been shown to shift the emergence of certain weed species when comparing wet- versus dry-seeded systems . In these systems, water seeding favored sedges and broad leaves, whereas dry seeding favored grasses, particularly watergrass and sprangletop species . Later in the season, sedges and grasses dominate over aquatic weeds in saturated, non-flooded soils . For continuously flooded systems, water depth also affects the presence of certain species. Grasses are suppressed by continuous flooding to a depth of at least 5 cm, whereas a deeper flood of about 15 cm suppresses most sedges .