Final Report for GNE13-061
Potato is an important crop in the United States and rates third among the agricultural products in terms of production volume. There are over 2700 potato fields in the Northeast United States and potato growers often over apply nitrogen fertilizer to ensure against loss of yield. High mobility of nitrate in the soil profile makes it susceptible to leach to the lower soil levels, resulting in nitrate contamination of water resources and environmental concerns. Regardless of costs of fertilizers, nitrate contaminated water causes serious illness for infants and pregnant woman, which could not be tolerated by public. Selecting appropriate type of cover crops, as nitrogen scavengers, along with supplementary nitrogen management can reduce nitrate leaching and water contamination. Traditionally, farmers plant rye or oat in Massachusetts, which might not meet nitrogen requirements of succeeding potatoes. The objectives of this study were to i) evaluate influence of different cover crop mixtures on minimizing nitrate leachate caused by surplus nitrogen fertilizer application and ii) to study nutrient density and tuber yield of potatoes as affected by cover crop mixtures and nitrogen fertilizer. The research data obtained from this project will form a basis for recommendations for potato growers in Massachusetts to alleviate nitrate leaching by proper cover crop and nitrogen fertilizer management.
Our results indicated that cover crop mixtures, regardless of the species, needed less N fertilization to produce high tuber yields compared to no cover crop treatment. Also, potatoes planted after cover crop mixtures were less weedy and tubers accumulated more nutrients compared to no cover crops plots. Despite the fact that forage radish + winter peas mixture has the potential to accumulate substantial amount of nitrogen in the fall and before winter kill, nitrogen is likely to be leached because of narrower C:N ratio of this mixture. A combination of forage radish with cereals (rye or oats) indicated to be more efficient in recycling nutrients in rotation with potatoes. Especially, forage radish + rye mixture was more efficient in nitrate leaching prevention and weed suppression compared to the other mixtures and no cover crop plots. Potatoes planted after mixtures with forage radish as a component, accumulated more nutrients.
Agriculture in the 21st Century faces multiple challenges. It has to produce more food to feed a growing population and to contribute to overall development in many agriculturally dependent developed and developing countries that need to adopt efficient production methods. For most of human history, the earth’s population has increased at a slow, steady pace. However, in the past 100 years, the number of human beings who need to be fed by farms has increased from 1.5 billion to 7 billion, a trend that indicates the necessity of growing high-yielding crops to cope with emerging hunger and malnutrition that limit human productivity in modern times.
On the other hand, decreasing arable lands and the need for more food has resulted in intensive cropping systems and extensive use of fertilizers to cope with soil fertility problems that are associated with nutrient depletion by crop production. According to the Food and Agriculture Organization of the United Nations (FAO), maize (Zea mays L.), wheat (Triticum aestivum L.), rice (Oryza sativa L.), and potatoes (Solanum tuberosum L.) have been the most consumed food in the world in 2012. It is predicted that by 2020, worldwide demand for potatoes will exceed that of wheat or rice in terms of human consumption. Currently, potato rates third among the world’s agricultural products in production volume and consumption (International Potato Center, 2009). The annual per capita consumption ranges from 55 kg in the most affluent to 11 kg in developing countries. Potato consumption has been a major part of the North American diet since early in the 17th century and as dominant arable crop in the Northeastern USA. Potato (Solanum tuberosum L.) is a carbohydrate-rich source with an important role in feeding people and is of interest because of its very high yields. Potato produces as much protein and about twice as much carbohydrate as grains (Aghighi et al., 2011). Its tuber is economically valuable and is extensively used in feeding people and livestock and in starch production (Aghighi et al., 2011).Potato tuber energy and protein production per unit area is greater than that of wheat or rice because of its high yield (Khajehpour, 2004).
Potatoes are important to industrialized and developing countries as a source of income and are a staple food for the world population. Potatoes are grown under a wide range of locations throughout the world, ranging from 70° latitude in Northern Europe to the equator in the South American Andes Mountains (Cao and Tibbitts, 1991). No other crop can match the potato in its production of food energy and food value per unit area. It is also high in Vitamin C, niacin and Vitamin B6. However, limited availability of land and economic pressures have forced the potato industry to move toward intensive potato production systems with extensive use of mineral fertilizers and increased frequency of potato in crop rotations (Stark and Porter, 2005).
Nitrogen presence in soil organic matter can be an important source of soil fertility for potato production but is not always available at the right time and in sufficient quantity for crop growth (Sinick et al., 2008).Proper nitrogen management is one of the most important factors required to obtain high yields of excellent quality potatoes (Rosen and Bierman, 2008). An adequate early- season nitrogen supply is important to support vegetative growth of potato, but excessive soil nitrogen later in the season will suppress tuber initiation, limit yields, and suppress the specific gravity in some cultivars. Excessive soil nitrogen late in the season can delay maturity of the tubers and result in poor skin set, which harms the tuber quality and storage properties (Rosen and Bierman, 2008).
Nitrogen management systems that use inorganic fertilizer and organic sources, such as cover crop residues, could combine the benefits of fertilizer nitrogen with soil organic matter maintenance and carbon sequestration derived from the organic source (Legg and Meisinger, 1982). Another alternative that has been tested on potatoes is to supply nitrogen in synchronization with potato nitrogen crop demand through the use of controlled- release fertilizers that permit a 50% potential reduction in nitrogen applied without reducing tuber yields (Collins, et al., 2007; Shoji, et al., 2001). Included in these new methods and changes in agricultural systems for higher nitrogen practices are sustainable crop rotations, use of cover crops, viable nitrogen rates, synchronization of inputs and nitrogen uptake sinks, fertilizer type, precision agriculture, precision conservation, nitrification inhibitors, nitrogen index, and management zones (Berry et al., 2003).
In recent years, there has been interest in new methods for producing crops of high quality and yield in an environmentally sustainable manner (Delgado 2002). Cover crops are considered to be an appropriate tool to improve potato yield and quality while alleviating environmental concerns. A cover crop is grown mainly to take up available nitrogen in the soil after harvest of the main crop and thereby to reduce leaching losses of nitrogen already in a cropping system (Delgado and Follett, 2010). Cover crops can provide nutrients for the following crop, increase soil organic matter, improve soil structure, reduce soil erosion and soil compaction, increase residue recycling, enhance water availability, suppress weed, reduce soil salinity, increase crop and soil organism diversity, and enhance wildlife habitat. At the same time, they can reduce costs, increase profits and even create new sources of income (Delgado and Follett, 2010).
Cover crops have been promoted as a means of maximizing the efficient use of available nitrogen to subsequent crops in agricultural systems, thereby limiting risks of environmental problems associated with nitrate contamination of surface and ground water while potentially enhancing profitability through a reduced nitrogen fertilizer nitrogen requirement (Decker et al., 1994). Cover crops can accumulate substantial amounts of biomass and potentially available organic nitrogen. However, the full benefit of using cover crops will be dependent on the synchrony between nitrogen mineralization from the cover crop and nitrogen demand of the subsequent crop as well as accurate estimation of supplemental fertilizer nitrogen requirement of the subsequent crop. The quantity of cover crop nitrogen available to a subsequent crop is species–dependent and usually associated with greater availability of nitrogen from legumes than from non-legumes (Dekker et al., 1994; Vyn et al., 1999). Cover crops can recover 150 to 300 kg N ha-1 from the soil profile and return this nitrogen to the surface soil. Cover crops have been reported to increase potato tuber yield and quality (Essah et al, 2012). Vyn et al. (1999) found that cover crops, such as annual ryegrass, oat, oilseed radish, or even red clover, could serve as scavenger crops that can recover residual soil nitrate and potentially cycle it to the following crop.
The objectives of this study were to i) evaluate influence of different cover crop mixtures on minimizing nitrate leachate caused by surplus nitrogen fertilizer application, ii) study nutrient density and tuber yield of potatoes as affected by cover crop mixtures and nitrogen fertilizer and iii) effect of cover crop mixtures and nitrogen fertilization on weed biomass.
This project contained two major phases during 2013-2015 growing seasons. The first phase included land preparation and planting cover crops in fall 2013 and 2014. The second phase included planting potatoes in the spring of 2014 and 2015. Experimental plots were disked, leveled, and prepared for cover crops in mid August-early September 2013 and 2014. Before planting cover crops, soil samples were collected from the experimental plots to test the nutritional background of the soil and fertilizer was applied where deemed necessary.
Before planting the cover crops, nitrate absorbent capsules/collectors were prepared and buried in the soil to monitoring nitrate leaching during cover crop growing period. We used a nitrate absorbent resin with high anion exchange capacity, tightly packed with mesh bags to prevent mixing soil particles with resin. Each collector consisted of cylinder shaped containers with a fine mesh at the bottom and at the top of the containers. Nitrate collectors allowed easy percolation of water through the capsule while prevented soil of being mixed with resins. Each capsule contained 100 g of nitrate absorbent resin and buried below the cover crop root zone. Therefore, the capsules were buried at different soil depth depending on the cover crop species. Cover crop seeds were weighted separately according to each species’ planting density and mixed in small bags for easier application. Afterwards, five cover crop seed mixtures including forage radish (Raphanus sativus L.) + oat (Avena sativa L.) (FR+O), forage radish + Austrian winter peas (Pisum sativum L.) (FR+P), forage radish + rye (Secale cereal L.) (FR + R), winter peas + rye (P+R), and winter peas + oat (P+O) were planted in the field with a grain drill planter on August 25 and 28 in 2013 and 2014, respectively.
In addition, a no cover crop plot was considered to compare cover crop treatments with no cover (NCC). Cover crop tissue samples were taken from cover crop stands at different growth stages before frost (Late November). Also, soil samples were taken three times during the growth form three different soil depths (20, 40, and 60 cm) and before soil frost. Among cover crops, forage radish, oat, and winter peas were winter killed while rye can survive the winter and continue its growth in the spring. Nitrate collector capsules were sampled at equal intervals during the growing season and before the cover crop termination. Resins were then extracted from the capsules and rinsed with distilled water. Afterwards, the resins were put into Erlenmeyer flasks mixed with 3 M KCl and shaken on a reciprocal shaker for one hour. The samples were then diluted and analyzed colorimetrically with the vanadium chloride method. After the first phase of the experiment in fall and winter 2013 and 2014, experimental plots were disked and prepared for planting potatoes. Rye was ploughed under and disked along with the winter peas, oat, and forage radish, which were already winterkilled.
A week prior to planting, potato tubers were cut in half or third, each piece having at least two eyes and stored in room temperature for 5 days for the cut surface to be healed to prevent fungal/virus infection. Potatoes were then planted late April in both years using a small potato planter. The distance between rows was considered 36 inches while the distance between plants on the rows was considered 9 inches. Four levels of nitrogen fertilizer including no application of nitrogen (N0), 50 kg N/ha (N1), 100 kg N/ha (N2), and 150 kg N/ha (N3) supplied as urea (46% nitrogen). Since half of the applied nitrogen fertilizer could be lost through leaching, de-nitrification, and volatilization, we banded urea in the plots.
Potatoes were harvested early to mid August in both years and after weighing tubers in each plot, sub-samples were taken to determine nutrient density of tubers. Tubers were washed and rinsed with distilled water, cut and dried in the oven for 72 hours. Afterwards, dried samples were weighed again and ground using a Willey Mill and then passed through a 1-mm mesh screen. To turn ground samples into ash, 0.500 g of ground samples were weighed and put in crucibles and heated in a furnace at 500o C over night. The ashed samples were then acidified with 25 mL of 10% HCl, filtered and stored in plastic bottles to measure the concentration of nutrients in each treatment. Another set of ground samples were weighed (0.250 g) and acidified with 3.5 mL of sulfuric acid, heated for 1 hour (according to the Kjeldahl procedure), diluted with 46 mL of distilled water, and stored in culture tubes to measure the total nitrogen (TKN) using a Lachat Quickchem machine.
Weed biomass was collected using 0.1 m2 quadrates from center rows of each plot with a hand mower and dried in a forced air oven at 55?C for 72 hrs and weighed. Before dying in the oven, weeds were separated and biomass was weighed based on the most dominant species in field, which were crabgrass (Digitaria sanguinalis L. Scop), common lambsquarters (Chenopodium album L.) and yellow nutsedge (Cyperus esculentus L.) in descending order and the other weed species were separated , weighed, and classified as other species. Analysis of variance was conducted by PROC MIXED procedure of SAS (SAS, v. 9.2, Cary, NC). Nitrogen fertilizer levels and cover crops were considered as fixed effects, while year and block were considered random. Data presentaed in the tables and figures are averaged over two years. Effects were considered significant at P ≤ 0.05 by the F test, and when the F test was significant, Least Significant Difference Test was used for mean separations. Trends due to N application were assessed by regression analysis.
The results of ANOVA showed that potato tuber yield was affected significantly by N fertilization, cover crops, and N by cover crop interaction effect (Figure 2).
The regression analysis of potato tuber yield showed that increasing nitrogen rates resulted in increased tuber yield in a quadratic trend (Figure 1).
Fertilized plants at 50 or 100 kg N ha-1 produced 22 and 47% more yield than unfertilized plots (Figure 1). Nitrogen is one of the most important key nutrients for plant growth and development, and an adequate early season nitrogen supply is important to support vegetative growth of potato tubers and high tuber production. Several studies have reported a potato yield increase as a result of nitrogen fertilization. However, increasing nitrogen rate from 100 to 150 kg N ha-1 did not increase potato tuber yield significantly (Figure 1). Probably, higher nitrogen rates in addition to the released nitrogen from cover crop residues delayed tuber maturation by increasing the vegetative growth, resulting in equal or even lower yields. According to Rosen and Bierman (2008), excessive soil nitrogen can delay maturity of tubers and suppress tuber initiation, limit yields, and decrease the specific gravity in some cultivars. In addition, Sweetlove and Hill (2000) claimed that there is an apparent tradeoff between allocation of nitrogen to maintain photosynthesis activity of existing leaves and storage organ weight gain at high nitrogen rates. In contrast, Sinick et al. (2008) reported a linear increase in potato tuber yield with increasing nitrogen supply up to 225 kg ha-1.
Cover crop mixtures showed a significant difference compared with no cover crop plots in terms of potato tuber yield (Figure 2). No cover crop plots produced lower tuber yield than potatoes planted after cover crop mixtures (Figure 2). While the highest level of nitrogen fertilizer increased tuber yield in no cover crop plots, mixtures produced higher tuber yield at lower nitrogen rates (Figure 2). The highest tuber yield was obtained from O+FR (24.9 Mg ha-1) and O+P (25.8 Mg ha-1) mixtures, fertilized at 100 kg N ha-1 (Figure 2). Application of 150 kg N ha-1 decreased tuber yield in O+FR and P+FR plots compared to 100 kg N ha-1. O+FR mixture showed to be more efficient in providing nutrients for the following potatoes when no nitrogen fertilizer was applied (Figure 2). Our results confirmed those reported earlier by Vos and Van der Putten (2004) who found increased potato tuber yield when potatoes were planted after cover crops. The quantity of cover crop N available to a subsequent crop is species–dependent and usually associated with greater availability of N from legumes than from nonlegume cover crops (Dekker et al., 1994; Vyn et al., 1999).
Presumably, higher tuber yield in potatoes planted after forage radish or winter peas could be explained by more synchrony with potato nitrogen demands and nitrogen release from the cover crop residues than the other cover crop mixtures. Several studies have reported that potato yield after legumes and non-grass cover crop mixtures were higher than following cereal cover crops (Vyn et al., 1999; Kremen and Weil, 2006, Delgado et al., 2007). The overall results of this study indicates that potatoes planted after cover crops were less dependent on synthetic fertilizer and significant savings in fertilizer purchase and lower risk of environmental pollution enhance sustainability of potatoes cultivation in Northeast region .
Cover crop mixtures differed significantly in terms of nitrogen uptake and dry matter accumulation. All of the mixtures were well established in both years with dry matter production levels generally within the expected range for the Northeast region (SARE, 2007). O+FR produced the highest dry matter yield (4.13 kg ha-1) followed by P+FR and R+FR (Figure 3). Forage radish and oats have a rapid growth and establishment in the fall and can accumulate substantial amount of biomass if planted early (Williams and Weil, 2004). In contrast, P+FR produced the highest nitrogen yield per hectare (195 kg N ha-1). Overall, nitrogen accumulation in cover crop mixtures were in the order of: P+FR > O+P > R+P>O+FR>R+FR (Figure 3). Austrian winter peas produces abundant vining forage and nitrogen fixing plants are considered as top nitrogen producers that can yield from 50 to 190 kg N ha-1, and at times up to 300 kg N ha-1 (SARE, 2007). Therefore, mixtures containing winter peas accumulated more nitrogen than those without winter peas (Figure 3). With respect to the nitrogen contribution of cover crop mixtures, P+FR and O+FR showed to have a higher potential to provide nitrogen with the succeeding crops than with the other ratios. Regardless of winter peas, which is expected to be a high nitrogen yielding plant, relatively high nitrogen content of forage radish could be attributed to the roots ability in scavenging residual nitrogen from top soil and lower levels of soil (Kermen, 2006). Forage radish produces a large fleshy taproot, typically 3 to 6 cm in diameter and 15 to 30 cm in length, in addition, the long singular taproot of forage radish may penetrate as deep as 5 feet in the soil, making it able to scavenge the leached nitrate from lower levels of soil. While major part of nitrogen in peas, as a legume, was derived from N2 fixation, nitrogen in forage radish, rye, or oats came from mineralized soil organic N or residual mineral N following the summer crop (Schomberg et al., 2006). Our results showed that mixtures with rye, as a component, may not provide the succeeding crop with sufficient amount of nitrogen (Figure 3). This was partly due to the fact that rye was killed early in the spring so experimental plots were prepared for planting potatoes. This resulted in unsatisfactory biomass accumulation which could explain less tuber yield in rye plots compared with mixtures with forage radish, winter peas, or oat.
Cover crops have been introduced as an effective way of decreasing nitrogen leaching into the soil and decreasing nitrogen fertilization. The results of our study showed that cover crop mixtures significantly decreased nitrate leaching compared with the no cover crop plots, which showed the highest nitrate leachate (135 mg kg resin-1) (Figure 4). Amongst cover crop mixtures, R+FR and R+P showed the least nitrate leachate (36 and 52 mg kg resin-1). Several researches show that cereal and grass cover crops are very effective in scavenging residual nitrogen in soil and nitrate leaching prevention because of their specific root structure. However, winter peas has a shallower root system compared with cereals and may not be as effective in scavenging residual nitrogen and nitrate leaching prevention (Figure 4).
Cover crop mixtures were different than no cover crop plots in terms of weed population. The dominant weed species in potato plots were crabgrass, common lambsquarters, and yellow nutsedge in descending order (Table 1). Cover crops were significantly different in terms of weed biomass production (Table 1). The highest crabgrass biomass (1248 kg ha-1) was in no-cover crop plots and among cover crop mixtures, crabgrass biomass was the highest in P+FR mixture (1108 kg ha-1) followed by O+P and R+P (825 and 688 kg ha-1, respectively). Overall, cover crop mixtures were less affected with weed infestation than in no-cover crops plots (Table 1). Also, mixtures with a forage radish or rye component suppressed more weeds than mixtures with winter peas. Higher weed biomass in no cover crop plots and mixtures with peas suggested that winter peas is not a suitable cover crop for controlling weeds, especially infested with crabgrass. Also, relatively high weed pressure in this study could be due to the soil disturbance before planting, an action that may encourage weed emergence from the soil seed bank. Campiglia et al. (2009) reported that rapeseed (Brassica napus L.) and ryegrass (Lolium perenne L.) were the most efficient weed suppressors and had the least proportion of weed biomass (<1%) of the total produced by the cover, and they also suppressed weed emergence in the following potato crops. Chemical compounds released from cover crop residue have potential to stimulate or inhibit weed germination and growth (Liebman and Mohler 2001).
There was a linear increase in weed biomass in response to nitrogen fertilizer application in our study (Table 1). Weed biomass was consistently higher in fertilized plots than in unfertilized plots for all three weed species, as well as other families (Table 1). The lowest (902 kg ha-1) and the highest (1226 kg ha-1) weed biomass were collected from 0 and 150 kg N ha-1 treatments, respectively. Weeds are strong competitors with agronomic crops for the growth resources, and, therefore, higher nitrogen rates resulted in increased weed biomass in our study.
In general, potatoes planted after cover crop mixtures accumulated more nutrients than those grown in no cover crop plots table 2. Nitrogen concentration in potato tubers planted after FR+P was the highest (1.96 % DW) followed by O+P and R+P (Table 2). The lowest nitrogen content was recorded form plots where potato was planted after R+FR or NCC. However, NCC showed significantly lower nitrogen content compared with the cover crops. Cover crop mixtures with winter peas component were dominant in terms of nitrogen concentration, as expected with a high N yielding legume such has winter peas. Potatoes planted after cover crop mixtures were similar in terms of phosphorus (P) concentration and showed 26% increase in P content on average compared with NCC (Table 5). Rye in general require and uptake more K from soil than legumes. Among cover crop mixtures and NCC plots, potato tubers tended to accumulate more K in R+FR and R+P plots whereas the difference between O+P and P+FR mixtures were not significant (Table 2). According to Eckert (1991), rye can increase the concentration of exchangeable potassium (K) near the soil surface, by bringing it up from lower in the soil profile, an action that may be the reason for higher concentration of K in potatoes planted following rye in this study.
Magnesium (Mg) and Ca concentrations in potato tubers followed the same trend in no-cover crop plots and cover crop mixtures with NCC treatment having the lowest concentration (Table 2). While concentration of Sulfur (S) was not affected in most of the cover crop mixtures, potatoes planted after FR+R accumulated a significant higher percentage of S in their tubers (Table 2). Also, potato tubers accumulated more B in FR+R compared to the other treatments whereas other cover crop mixtures and no cover crop plots were similar with this regard (Table 2). Forage radishes can take up large amounts of nutrients, especially N, P, S, Ca, and B because of its specific root characteristics (SARE, 2007, and 2013), results that may explain higher concentrations of some macro or micronutrients in potatoes planted after cover crop mixtures with forage radish as a component. The reason FR+R accumulated more nutrients than FR+P and FR+O may be explained by higher dominance of forage radish in competing with rye compared with oats or winter peas. Potatoes also accumulated more Zn or Mn in cover crop mixtures compared with no cover crop plots; however, the Zn and Mn changes in tubers were not significantly different among cover crop mixtures (Table 2).
Nitrogen concentration in potato tubers followed a quadratic trend in response to N fertilization (Table 3), and increasing N rate from 0 to 50 and to 100 kg ha-1 resulted in 96 and 240% increase in N concentration, respectively. However, application of 150 kg N ha-1 did not further increase N content of tubers (Table 3). Increased nitrogen concentration as a result of N fertilization has been reported in other studies (Alva, et al., 2002; Haase et al., 2007). While P and K concentrations were not influenced by N application, Mg concentration increased linearly in respond to the N application (Table 3). Unfertilized potatoes along with plants fertilized at 50 kg N ha-1 did not show a significant difference in terms of Mg concentration; however, 100 or 150 kg N ha-1 increased Mg concentration to 1.72 and 2.13%, respectively. Calcium concentration also followed the same trend and ranged from 0.76% in un-fertilized plants to 1.84% where 150 kg N ha-1 was applied (Table 3). Our results were in accordance to those reported by Chase and Henry (1997) who found a slight increase in Mg and Ca concentration in response to N application and in contrast to Barunawati (2013) who found no difference in concentration of the nutrients affected by N fertilization. Also Ciampitti et al. (2013) reported that P, K, and S levels were not influenced by N application. In contrast, Heidari et al. (2012) reported a positive relationship between P and K concentration and N treatments.
Education & Outreach Activities and Participation Summary
Information that we have learned from this experiment is presented as several outreach and extension type activities including field days, fact sheets, and regional and national scientific meetings (Please see attached). Also, two manuscripts are under preparation to be submitted to peer-reviewed journals.
Presentations in ASA/CSSA/SSSA annual meetings in 2014 and 2015:
Jahanzad, E., A.V. Barker, M. Hashemi, A. Sadeghpour. 2015. Effect of Nitrogen Fertilization and Cover Crop Mixtures on Potato Tuber Yield, Nutrient Density, and Nitrate Leaching. ASA, CSSA, SSSA, Minneapolis, MN, Nov 15-18. (Session No.427- SSSA Division- Soil fertility and plant nutrition- 2:30PM).
Jahanzad, E., A.V. Barker, M. Hashemi, A. Sadeghpour. 2014. Cover Crop Mixtures May Reduce Nitrate Leaching and Fertilizer Application in Potato Production. ASA, CSSA, SSSA, Long Beach, CA, Nov 2-6. (Session No. 166- SSSA Divvision – Soil fertlity and plant nutrition- 4:00 PM).
Presentations in field days in 2014 and 2015:
Jahanzad, E., A. V. Barker, M. Hashemi, T. Eaton, A. Sadeghpour. 2014. Cover Crop and Nitrogen Management for Sustainable Potato Production. Crops, Dairy, Live stock & Equine News letter. 17(2):8.
Jahanzad, E., A.V. Barker, M. Hashemi, T. Eaton, A. Sadeghpour. 2015. Using cover crop mixtures to reduce nitrate leaching and fertilization in potato prodcution. UMass Agricultural Field Day. Crops and Animal Resarch Education Center, June 24th, p.14-15.
Aber, J. D., Goodale, C. L., Ollinger, S. V., Smith, M. L., Magill, A. H., Martin, M. E., Stoddard, J. L. 2003. Is nitrogen deposition altering the nitrogen status of northeastern forests? BioScience. 53(4): 375-389.
Aghighi Shahverdi Kandi, M., Tobeh, M., Gholipoor, A., Jahanbakhsh, S., Hassanpanah , D., Sofalian, O. 2011. Effects of Different N Fertilizer Rate on Starch Percentage, Soluble Sugar, Dry Matter, Yield and Yield Components of Potato Cultivars. Australian Journal of Basic and Applied Sciences. 5(9): 1846-1851.
Barunawati, N., Giehl, R. F. H., Bauer, B., Von Wirén, N. 2013. The influence of inorganic nitrogen fertilizer forms on micronutrient retranslocation and accumulation in grains of winter wheat. Frontiers in Plant Science, 4:320.
Berry, J.K., Delgado, J.A., Pierce, F.J., Khosla, R. 2005. Applying spatial analysis for precision conservation across the landscape. J. Soil Water Conservation. 60:363–370.
Campiglia, E., Paolini, R., Colla, G., Mancinelli, R. 2009. The effects of cover cropping on yield and weed control of potato in a transitional system. Field Crops Research. 112(1): 16-23.
Cao, W., Tibbitts, T. W. 1991. Physiological Responses in Potato Plants under Continuous Irradiation. J. American Society of Horticultural Science. 116(3):525-527.
Chase, C. R., Henry, G. M., Gallaher, R. N. 1994. Yield and nutrient concentration and content in response to nitrogen fertilization in forage and grain sorghum. Southern Conservation Tillage Conference for Sustainable Agriculture.
Ciampitti, I. A., Camberato, J. J., Murrell, S. T., Vyn, T. J. 2013. Maize nutrient accumulation and partitioning in response to plant density and nitrogen rate: I. Macronutrients. Agronomy Journal. 105(3): 783-795.
Collins, H. P., Delgado, J. A., Alva, A. K., Follett, R. F. 2007. Use of nitrogen-15 isotopic techniques to estimate nitrogen cycling from a mustard cover crop to potatoes. Agronomy Journal. 99:27–35.
Decker, A.M., Clark, A.J., Meisinger, J.J., Mulford, F.R., McIntosh, M.S. 1994. Legume cover crop contributions to no-tillage corn production. Agronomy Journal. 86:126–135.
Delgado, J.A., Follett, R.F. 2002. Carbon and nutrient cycles. Journal of Soil Water Conservation. 57:455–464.
Delgado, J.A. 2002. Quantifying the loss mechanisms of nitrogen. Journal of Soil Water Conservation. 57:389–398.
Eckert, D.J. 1991.Chemical attributes of soils subjected to no-till cropping with rye cover crops. Soil Science Society of America J. 55:405-409.
Essah, S.Y.C., Delgado, J.A., Dillon, M., Sparks, R. 2012. Cover crops can improve potato tuber yield and quality. HortTecnology. 22 : 185-190.
Haase, T., Schüler, C., Hess, J. 2007. The effect of different N and K sources on tuber nutrient uptake, total and graded yield of potatoes (Solanum tuberosum L.) for processing. European Journal of Agronomy. 26(3): 187-197.
Heidari, M., Mohammad, M. M. 2012. Effect of rate and time of nitrogen application on fruit yield and accumulation of nutrient elements in Momordica charantia. Journal of the Saudi Society of Agricultural Sciences. 11(2): 129-133.
International Potato Center, http://www.cipotato.org/pressroom/facts figures/let them eat potatoes.asp, 2009.
Khajehpour, M. 2004. Industrial crops. Isfahan Jahade Daneshghahi Press, pp: 580.
Kremen, A.E. 2006, Nitrogen mineralization from brassica cover crops. M. S. Thesis, University of Maryland.
Legg, J.O., Meisinger, J.J. 1982. Soil nitrogen budgets. p. 503–566. In F.J. Stevenson et al. (ed.) Nitrogen in agricultural soils. Agron. Monogr. 22. ASA, Madison, WI.
Liebman, M. Mohler, C.L. 2001. Weeds and the soil environment. p. 210-268. In: M. Liebman, C.L. Mohler, and C.P. Staver (eds.), Ecological Management of Agricultural Weeds, New York: Cambridge University Press.
Rosen, C. J., Bierman, P.M. 2008. Best management practices for nitrogen use: irrigated potatoes. Minnesota Extension, University of Minnesota.
SARE. 2007. Managing cover crops profitably, 3rd edition. Sustainable Agriculture Network, Beltsville, MD.
Schomberg, H.H., Endale, D., Calegari, A., Peixoto, R., Miyazawa, M. 2006. Influence of cover crops on potential nitrogen availability to succeeding crops in a Southern Piedmont soil. Biology and Fertility of Soils. 42: 299–307.
Shoji, S., Delgado, J.A., Mosier, A., Miura, Y. 2001. Use of controlled release fertilizers and nitrification inhibitors to increase nitrogen use efficiency and to conserve air and water quality. Communications in Soil Science Plant Analysis. 32:1051–1070.
Sincik, M., Turan, Z.M., A. Göksoy, A.T. 2008. Responses of potato (Solanum tuberosum L.) to green manure cover crops and nitrogen fertilization rates. American Journal of Potato Research. 85:150–158.
Stark, J.C., Porter, G.A. 2005. Potato nutrient management in sustainable cropping systems. American Journal of Potato Research. 82:329– 338.
Sweetlove, L.J., Hill, S.A. 2000. Source metabolism dominates the control of source to sink carbon flux in tuberizing potato plants throughout the diurnal cycle and under a range of environmental conditions. Plant Cell Environment. 23: 523-529.
Vos, J. ,Van Der Putten, P.E.L. 2004. Nutrient cycling in a cropping system with potato, spring wheat, sugar beet, oats, and nitrogen catch crops: II. Effect of catch crops on nitrate leaching in autumn and winter. Nutrient Cycling in Agro ecosystems. 70:21–31.
Vyn, T.J., Janovicek, K.J., Miller, M.H., Beauchamp, E.G.1999. Spring soil nitrate accumulation and corn yield response to preceding small grain N fertilization and cover crops. Agronomy Journal. 91:17–24.
Williams, S. Weil, R. 2004. Brassica cover crop root channels may alleviate soil compaction effects on subsequent soybean crop. Soil Science Society of America. J. 68: 1403-1409.