The primary study objective was to quantify the effect of cover crops and amendments on soil fertility, potato yield and potato pests.Two cover crop studies were conducted at IREC — a study begun in 2014 that evaluated mid-summer cover crops and a study begun in 2016 that evaluated covercrops planted in spring, mid-summer and fall. Cover crop planting times and species were selected to fit local cropping systems and to maximize biomass production under local growing conditions. For example, planting cover crops in mid-summer is desirable for producers growing a grain hay crop because the mid-summer planting occurs shortly after hay harvest, which allows producers to generate crop income. A mid-summer planting also allows cover crop growth during the warm temperatures of summer and early fall. Planting cover crops in the spring is a good fit for producers with limited water availability because it takes advantage of stored winter soil moisture and cool, wet weather conditions during establishment. Planting in the fall is a good fit for producers who want to grow a full-season cash crop, such as hard red wheat, because fall planting allows them to plant after cash crop harvest. Fall planting is also desirable because the cover crop can prevent soil erosion during winter and early spring. In both studies, hydroponic rack potatoes were planted the year after cover crops were grown. Cover crop species included cool-season and warm-season species, seeded alone and in mixes. Cover crop species were selected based on their previous success in the local area or on previous research documenting success under similar growing conditions. A list of species evaluated is shown in table 1.
Cover crops were drill-seeded into a disked, packed seedbed using a drill cone planter with drill rows spaced 6 inches apart. Cover crop plant density was estimated using visual plant counts within a central rectangle in each plot, measuring 5 feet by 10 feet, when plants were 3 to 5 inches tall. Cover crops were grown under sprinkler irrigation, without synthetic fertilizer or pesticides. They were managed as a green manure by flail-mowing and disk-incorporating above ground biomass at early flowering. Cover crop biomass in each plot was estimated from a quadrat of 5 feet by 10 feet. An above ground biomass subsample was sent to a laboratory to estimate total nitrogen content in cover crop biomass. An untreated fallow treatment and a urea treatment were included in all trials for comparison purposes. The fallow treatment for spring cover crops was fallowed for 12 months before potato planting; the fallow treatment for mid-summer cover crops was fallowed, after harvest of the barley hay crop, for 8.5 months before potato planting; and the fallow treatment for fall cover crops and several amendments was fallowed, after harvest of the barley grain crop, for 6.5 months before potato planting. All fallow treatments, after weed suppression ratings were taken, were hand-weeded to prevent excessive weed growth and weed seed production. Planting of the spring cover crop occurred in midApril. Mid-summer plantings occurred in late July, after a spring barley hay crop was grown. The fall cover crop planting occurred in mid-September, also after a spring barley grain crop was grown. Cover crops were incorporated into the soil at 50% flowering — 71 to 77 days after planting for the spring planting, 70 to 76 days after planting for mid-summer plantings and 230 days after planting for the fall planting.
Fall-planted cover crops did not reach the flowering stage before incorporation. The reason for early termination of fall-planted cover crops was to allow 4 weeks between cover crop incorporation and potato planting and thus enable cover crop decomposition and prevent a green bridge. Total applied water for irrigated cover crop trials was 12 inches for the spring planting, 6 to 8 inches for midsummer plantings and 3.5 inches for the fall planting. Cover crop vigor was determined by visually evaluating plant canopy cover and height in the plot area, with a vigor score of 10 equal to the most vigorous growth and 1 equal to bare ground. Weed suppression ratings were determined by visually evaluating the density and height of weeds in each plot. A weed suppression rating equal to 10 represented zero weeds in the plot and 1 was equal to weed density and height similar to the unplanted bare-ground control. Weed suppression ratings were taken when weeds and cover crops were 6 to 10 inches tall. Weed biomass was measured in each plot at the time of cover crop harvest by hand-separating cover crop and weed plant material derived from the quadrat sample.Two amendment studies were conducted at IREC. One study evaluated fall-applied amendments in 2014 and another study evaluated amendments applied in fall 2016 and spring 2017. Amendments were applied by hand and disk-incorporated into the soil — in midSeptember for fall applications and in late April for spring applications. The tested organic amendments included chicken manure, steer manure, composted chicken manure and a compost mix using green waste and cow manure.
Bloodmeal and soy meal were broadcast-applied and incorporated using a Lilliston cultivator after bed preparation and before planting. These two amendments were included to represent organic alternatives to quick-release synthetic nitrogen fertilizers such as urea. Amendment application rates were based on the products’ moisture and nitrogen content, with the goal of applying 150 pounds of nitrogen per acre . Amendment application rates ranged from 1,100 pounds per acre for blood meal with 13% nitrogen to 10,000 pounds per acre for compost with 1.5% nitrogen. The nitrogen mineralization rates for the amendments varied and were not controlled in the experiment.Potatoes were planted over areas treated with cover crops, amendments and combinations of cover crops and amendments. Potatoes were also planted over areas treated with urea fertilizer and over untreated fallow areas. Planting occurred in the spring, without the use of synthetic fertilizers or pesticides. Preplant soil samples were taken at potato planting to confirm that supplies of phosphorus, potassium, sulfur and calcium were adequate to avoid deficiencies; all soil tests showed adequate nutrient levels according to University of California guidelines . Potato row spacing was 36 inches and seed spacing was 10 inches. The Russet Norkotah potato variety was evaluated in 2015 and the Yukon Gold variety was evaluated in 2017. Soil samples were collected from each plot shortly before planting to determine nitrate available at preplanting, as well as available ammonium and total nitrogen. Plot size was 12 feet by 40 feet; all sampling occurred in a middle area, measuring 6 feet by 30 feet, to avoid edge effects. The soil type at IREC is a Tulebasin mucky silty clay loam with 4.5% organic matter. To meet crop evapotranspiration needs, potatoes were irrigated with solid-set irrigation that entailed use of soil moisture monitors and an on-site CIMIS weather station. Crop vigor was monitored multiple times during the growing season by visually evaluating plant canopy cover, height and color over the plot area, with a vigor score of 10 equal to plants in the plot with the highest canopy cover and a dark-green color and 0 equal to short, senesced, yellow plants. Petiole nitrogen was measured at early tuber bulking and at crop maturity. Potatoes from each plot were mechanically harvested and graded to determine firesh-market tuber yield and tuber quality. Potatoes were graded by counting all potatoes in each plot and mechanically sorting them by weight into five size classes based on U.S. grade and carton classes. Tuber quality was determined by counting and weighing all cull tubers that displayed rot, greening, knobs, growth cracks, irregular shape and irregular skin appearance. A 10-tuber subsample from each plot was evaluated for internal deffects including hollow heart, brown spot bruise, rolling grow tables vascular discoloration and specific gravity.Cover crop establishment in all trials was successful. Plant densities were measured at or above 80% of the seeding rate , with two exceptions — a crop of cowpeas seeded in mid-summer and a crop of spring-seeded arugula . Low plant density for spring arugula was probably due to planting too deep. Arugula requires a shallow seeding depth of less than 0.5 inch. Subsequent seedlings of arugula at the correct seeding depth produced plant density higher than 80%.
Spring wheat, fall triticale, woollypod vetch, field peas, spring mustard and oilseed radish displayed rapid growth, high vigor and high weed suppression . Mixes of mustards and field peas or vetch, in 50/50 proportions, also had high vigor and high weed suppression. Spring-seeded arugula exhibited lower vigor and weed suppression than the other spring cover crops, likely due to the stand problems associated with excessively deep seeding. Oilseed radish, mustards and grasses planted in mid-summer, after a spring barley crop, exhibited lower vigor and biomass than spring plantings . This effect was caused by a deficiency of plant-available nitrogen at planting; the mustards, radish and grasses had low nitrate in plant tissue during the early season and a low percentage of nitrogen biomass at harvest compared to spring plantings . Nitrate nitrogen in the top 10 inches of fallow plots averaged 17 parts per million at the spring planting and below 5 ppm at the mid-summer and fall plantings. These nitrate concentrations respectively correspond to approximately 28 and 8 pounds of nitrogen per acre in the top 10 inches of the profile. Many growers express interest in growing a spring barley or wheat crop for revenue before planting cover crops, but these results clearly show that adequate mineralized soil nitrogen is needed for non-legume cover crops to flourish. The idea that legumes might contribute nitrogen to non-legume cover crops in a mixed planting was not supported, as mustard, radish and grass grown in a mix with field peas and vetches had vigor and biomass similar to the single-species planting; the mix was instead dominated by field peas, which fixed their own nitrogen but did not share it with other species.Field pea and vetch green manures contributed substantial nitrogen to the system, adding over 150 pounds — and in many cases over 200 pounds — of nitrogen per acre from above ground biomass . The highest nitrogen contributor was spring-planted “flex” field peas, which added 306 pounds of nitrogen per acre. Berseem clover and cowpeas contributed less than 70 pounds of nitrogen per acre because Tulelake’s short growing season was too cold for these species to reach maturity before frost. Several grass and mustard cover crops produced significant biomass, but their nitrogen content was less than half of that produced by most legume species . More than 150 pounds of nitrogen per acre were contributed by 50/50 mixes of legumes and either grass or mustard. Mineralized nitrogen at the time of potato planting was correlated to added nitrogen from cover crops , suggesting that little nitrogen was lost to leaching or denitrification over the winter. Mineralized nitrogen in the top 10 inches of soil for most field peas and vetches was more than double that for non-legume cover crops. Mineralized nitrogen at potato planting, in treatments that involved haying field peas’ above ground biomass and removing it from the field , was no different from fallow treatments. This is consistent with other studies demonstrating that above ground biomass contains most of the nitrogen in legume cover crops. Mineralized nitrogen at potato planting in fallow treatments averaged 55 pounds of nitrogen per acre for spring fallow, 48 pounds per acre for mid-summer fallow and 43 pounds for fall fallow. Mustard, radish and sorghum-sudangrass resulted in mineralized nitrogen similar to that of fallow treatments, suggesting these cover crops had a neutral effect on soil nitrogen . Spring wheat and fall triticale resulted in lower mineralized nitrogen at potato planting than was measured in fallow treatments, likely because decomposition of grass residue tied up available nitrogen. Delayed release of nitrogen in potatoes is problematic because potatoes require adequate nitrogen in the early season for vegetative growth and tuber initiation. Potato petiole nitrate at early bulking was used to evaluate in-season nitrogen availability. Legume cover crops resulted in much higher potato petiole nitrate at early bulking than did grasses; petiole nitrate for treatments with field peas and vetches was similar to petiole nitrate produced in conventional fertilizer controls . When comparing potato petiole nitrate in cover crop treatments to that in fallow treatments, legumes were higher, mustards were similar and grasses were lower . One year after growing potatoes , flag leaf nitrogen in winter wheat was higher in plots that had received spring vetch and field pea treatments than in fertilizer controls and fallow treatments .