Continuous Potato Research Articles

Altering soil carbon and nitrogen stocks in intensively tilled two-year rotations

Information is needed on the ability of different crop management factors to maintain or increase soil C and N pools, especially in intensively tilled short crop rotations. Soil samples from field experiments in Maine were used to assess the effect of cover crop, green manure (GM) crop, and intermittent or annual amendment on soil C and N pools. These field experiments, of 6–13 years’ duration, were all characterized by a 2-year rotation with either sweet corn ( Zea mays L.) or potato ( Solanum tuberosum L.), and primary tillage each year. Total, particulate organic matter (POM), and soil microbial biomass (SMB)-C and -N pools were assessed for each experiment. Total C and N stocks were not affected by red clover ( Trifolium pratense L.) cover crop or legume GM, but were increased by 25–53% via a single application of papermill sludge or an annual manure and/or compost amendment. With the exception of continuous potato production which dramatically reduced the SMB-C and SMB-N concentration, SMB-C and -N were minimally affected by changes in cropping sequence, but were quite sensitive to amendments, even those that were primarily C. POM-C and -N, associated with the coarse mineral fraction (53–2,000 µm), were more responsive to management factors compared to total C and N in soil. The change in soil C fractions was a linear function of increasing C supply, across all experiments and treatments. Within these intensively tilled, 2-year crop rotations, substantial C and N inputs from amendments are needed to significantly alter soil C and N pools, although cropping sequence changes can influence more labile pools responsible for nutrient cycling.

Characterization of soil microbial communities under different potato cropping systems by microbial population dynamics, substrate utilization, and fatty acid profiles

The effects of 11 different 2- and 3-yr potato crop rotations on soil microbial communities were characterized over three field seasons using several techniques. Assessments included microbial populations determined by soil dilution plate counts on various general and selective culture media, microbial activity by fluorescein diacetate (FDA) hydrolysis, single carbon source substrate utilization (SU) profiles, and fatty acid methyl ester (FAME) profiles. Potato rotation crops evaluated in research plots at Newport, ME, included barley/clover, canola, green bean, millet, soybean, sweet corn, and a continuous potato control. Soil populations of culturable bacteria and overall microbial activity tended to be highest following barley, canola, and sweet corn rotations, and lowest with continuous potato. Differences among rotations were less apparent during the potato phase of the rotations. Populations of actinomycetes and fluorescent pseudomonads tended to be greater in barley rotations than in most other rotations. SU profiles derived from BIOLOG GN2 plates indicated that certain rotations, including barley, canola, and sweet corn tended to have higher overall microbial activity, and barley and sweet corn rotations averaged higher substrate richness and diversity. Soybean and potato rotations tended to have lower substrate richness and diversity. Principal component analyses of SU data revealed differences among rotation soil communities in their utilization of individual carbon sources and substrate guilds, including carbohydrates, carboxylic acids, amines/amides, and amino acids. Analyses of soil FAME profiles demonstrated distinct differences among all the rotation soils in their relative composition of fatty acids, indicating differences in their microbial community structure. Fatty acids most responsible for differentiation among rotation soils included 16:1 ω5c, 16:1 ω7c, 18:2 ω6c, 18:1 ω9c, 12:0, and 13:0 anteiso, with 16:1 ω5c being the single greatest determinant. Overall, monounsaturated fatty acids, particularly 16:1 ω5c, were most prevalent in sweet corn rotations and polyunsaturates were highest in barley and millet rotations. Straight chain saturated fatty acids comprised the greatest proportion of fatty acids in soils under continuous potato. FAME biomarkers for microorganism groups indicated barley and millet rotations had the highest ratio of fungi to bacteria, and soybean and continuous potato had the lowest ratio. This research has demonstrated that different crop rotations have distinctive effects on soil microbial communities that are detectable using a variety of techniques. Further studies will identify more specific changes associated with particular rotations and relate these changes to potential effects on disease management, crop health, and crop productivity.

Organic matter and aggregation in a degraded potato soil as affected by raw and composted pulp residue

Soils under intensive potato (Solanumtuberosum L.) production are often low in organic matter content and microbial activity. Pulp fibre residue addition may restore the quality of these soils. An experiment was initiated in which raw or composted pulp fibre waste was added to a Fredericton sandy loam (Orthic Humo-Ferric Podzol) in New Brunswick (Canada). Each material was applied at 45 Mg (dry weight) ha–1 and 90 Mg (dry weight) ha–1 before planting. Soil was under continuous potato (cv. Russet Burbank) for 3 years. The raw and the composted pulp greatly increased soil organic matter (SOM) content, the C/N ratio, macroaggregation, C mineralization, microbial biomass C (MBC) and enzyme activities for the whole duration of the experiment but had no effect on its total N. Most of the SOM increase was found in the macroorganic matter fraction. The SOM, C/N ratio, macroaggregation, C mineralization and MBC decreased from the time of application. No major difference was found between materials except for CO2-C release in the incubation which was higher for the composted than for the raw pulp. This study indicated that pulp fibre, either applied raw or composted, increased SOM and macroaggregation and promoted microbial growth and activity in this potato soil which was low in C content. The beneficial effects on soil physical and biochemical properties were still present after 3 years of continuous cropping.

Soil organic matter quality and aggregate stability under eight potato cropping sequences in a fine sandy loam of Prince Edward Island

Soil degradation and soil organic matter (SOM) reduction have long been suspected to occur under intensive potato (Solanum tuberosum L.) production in Prince Edward Island. Thus, this study was conducted to evaluate the effect of eight potato cropping sequences involving barley (Hordeum vulgare L.), ryegrass (Lolium multiflorum L.), red clover (Trifolium pratense L.) and barley under-seeded with red clover on aggregate stability and SOM properties of a Charlottetown fine sandy loam (Orthic Humo-Ferric Podzol). Total C and N, light-fraction (density < 1.7 g cm−3) C (LF-C) and light-fraction N (LF-N), microbial biomass C (MBC), carbohydrate content, alkaline phosphatase activity (APA) and aggregate stability were determined in the 0- to 15-cm soil layer in the 6th and 10th years of the trial. Average C and N contents were 20 to 27% greater in sequences that included 3 or 4 yr of potatoes and a high frequency of red clover than in a 9-yr potato sequence; and were attributed to the less frequent tillage and higher organic matter inputs associated with the forage component. The response for MBC, LF-C, LF-N and APA was greater than those for total C and N suggesting that the former parameters may better reveal SOM changes caused by variations in potato management than the latter. Light-fraction N was particularly sensitive to the inclusion of perennial legumes in the cropping sequence. Wet-aggregate stability was 33% higher after 6 yr in the sequence with 50% frequency of red clover than in the continuous potato system, but no effect was noted after 10 yr. The results of this study indicate that sequences which are limited to 30 to 40% of potatoes and have a high frequency of perennial forage such as red clover, lead to greater SOM content and quality of a Charlottetown fine sandy loam compared to continuous potatoes. Trends in aggregate stability generally followed those of SOM fractions but the effects were relatively smaller and varied with time. Key words: Potato, Solanum tuberosum L., soil aggregation, organic matter, carbon, nitrogen, light fraction, microbial biomass

Potato response to crop sequence and nitrogen fertilization following sod breakup in a Gleyed Humo-Ferric Podzol

Meadow soils could supply significant amounts of N for several years and reduce N requirements for the potato (Solanum tuberosum L.). We examined a combination of three crop sequences, three cultivars and six N rates in relation to tuber yield and quality. Superior, Kennebec and Snowden were grown as follows on a Tilly silty loam between 1993 and 1995: (1) 3-yr continuous potato cropping of Superior, (2) 3-yr continuous potato cropping starting with Snowden and followed by 2 yr of Kennebec, and (3) 3-yr sequence of Kennebec, oat (Avena sativa L. 'Marion') and Superior. There were six N treatments (0, 70, 105, 140, 175 and 210 kg N ha−1) and a split application of 140 kg N ha−1 (half at seeding and half before hilling). The seasons were either favorable to growth (1993), relatively wet (1994) or relatively dry (1995). In 1993 and 1994, total yield varied between 37 and 50 t ha−1. In 1995, continuous cropping of Kennebec and Superior produced 28 t ha−1 compared with 37 t ha−1 under crop rotation. The rotation increased total yield of Superior by 34% and graded yield by 42% in 1995. The N requirement was closely related to yield, rather than cultivar. Compared with continuous cropping of Superior, tuber-quality index was highest with the rotation for rhizoctonia (Rhizoctonia solani) (67 vs. 57) and common scab (Streptomyces scabies) (90 vs. 82). The incidence of both diseases decreased when at least 70 kg N ha−1 was applied. For common scab, skin coverages was higher at rates of 0, 175 and 210 kg N ha−1 than at intermediate rates of 70–140 kg N ha−1. Mineral N accumulated in the 0–60-cm layer in the interrow and increased rapidly when N rates exceeded 105 kg N ha−1. After sod breakup an N rate in the range of 70–100 kg N ha−1 and crop rotation with cereals sustained the productivity of the potato during 3 yr while minimizing the environmental impact of mineral N. Key words: Crop rotation, common scab, rhizoctonia, reduced N rates, Solanum tuberosum L.

Crop Rotation Effects on Sweet Potato Yield and Quality

As the suite of fungicides, pesticides, and fumigants available to vegetable producers continues to shrink, growers may begin to reexamine crop rotations as an alternative method of disease, weed, and pest control. Although many studies have examined field crop rotations, relatively few have examined a vegetable crop as a part of that rotation. In 1985 a study was initiated to evaluate the effect of various crop rotations on sweet potato (Ipomoea batatas L.) yield and quality. Specific rotations were: (i) continuous sweet potato, (ii) sweet corn (Zea mays L. var. rugosa) and sweet potato, (iii) soybean [Glycine max (L.) Merr.] and sweet potato, (iv) 2 yr sweet corn and one yr sweet potato, (v) 2 yr bahiagrass (Paspalum notatum Flugge.) and 1 yr sweet potato, and, (vi) soybean, sweet corn and sweet potato. Two complete cycles (1988-1990 and 1991-1993) of the rotations were analyzed. The crop rotation producing the largest annual yield (4.70 tons/acre) and one of the largest cumulative (11.35 tons/acre) sweet potato yields was 2 yr of bahiagrass followed by sweet potato. Continuous sweet potatoes produced the lowest annual (2.12 tons/acre) and one of the highest total marketable cumulative yields (11.39 tons/acre) during the 6 yr. In general, a rotation containing a grass (bahiagrass, sweet corn) produced higher annual sweet potato yields than those without.

Quantifying Total, N, and Non-N Related Crop Rotation Effects Without 15N

ABSTRACT The N benefits attributed to legumes in crop rotations are often overestimated because the technique for quantifying this benefit without 15N includes rotation effects not directly related to N. This study was conducted to test an approach proposed in the literature for delineating such effects without requiring the use of 15N. Potato (Solanum tuberosum L. cv. Norwis) was grown continuously and in two-year rotations with alfalfa (Medicago sativa L. cv. Nitro), hairy vetch (Vicia villosa Roth), or white lupin (Lupinus albus L. cv. Ultra). Potato was fertilized with 0, 90, or 180 kg N ha−1 as (NH4)2S04. The total, N, and non-N effects of crop rotation were calculated based on tuber dry weight and plant N uptake for the rotation systems and the tuber dry weight vs. plant N uptake relationship for continuous potato. The total effect of crop rotation generally declined upon increasing N fertilizer from 0 to 90 kg N ha−1, apparently due to decreasing importance of the N effect with added fertilizer. Non-N related effects mostly remained static across fertilization regimes. Those effects attributed to N generally declined upon adding 90 kg N ha−1 in a relatively dry year; and upon adding 90 and 180 kg N ha−1 in a year with more favorable precipitation. Calculated N effects with no N fertilizer reflected the quantity and quality of legume crop residues. This method does allow assessment of total, N, and non-N related rotation effects. However, the approach appears sensitive to climatic conditions, and data should only be subjected to analysis where a yield response to N exists.

Crop rotation and N fertilization effects on growth, yield, and disease incidence in potato

Crop rotation can be an effective mechanism for reducing disease incidence and contributing nitrogen (N) to succeeding crops. Interactions of plant pathogen suppression and soil nutrient availability may also exist, adding to the cropping system complexity. This study examined the impact of crop rotation, N fertilization, and their interaction on growth, yield, andRhizoctonia solani incidence in potato (Solanum tuberosum L. Norwis). Potato was grown continuously and in two-year rotations with annual alfalfa (Medicago sativa L. Nitro), hairy vetch (Vicia villosa Roth), white lupin (Lupinus albus L. Ultra), and oat (Avena sativa Astro). Fertilizer was banded at potato planting with 0, 45, 90, 135, 180, or 225 kg N ha−1 as (NH4)2SO4. Approximately 58% of continuous potato possessed stem lesions caused byR. solani, but only 12 to 22% of potato stems from other rotations possessed lesions. Tuber dry weight was affected by crop rotation in 1989, a dry year, but not in 1990. Apparent N fertilizer replacement values for hairy vetch, Nitro alfalfa, white lupin, and oat were 65, 43, 26, and 11 kg N ha−1, respectively. All crop rotations studied appeared to enhance potato production by reducing stem infection byR. solani. Vetch and alfalfa provide additional benefits through their N contributions.

Influence of crop rotation on selected chemical and physical soil properties in potato cropping systems

Crop yields are often increased through crop rotation. This study examined selected soil chemical and physical properties that may constitute the N and non-N related effects of crop rotation in potato cropping systems. Potato (Solanum tuberosum L. Norwis) was grown continuously and in two-year rotations with annual alfalfa (Medicago sativa L. Nitro), hairy vetch (Vicia villosa Roth), white lupin (Lupinus albus L. Ultra), and oat (Avena sativa Astro). Hairy vetch contributed more residue N than any other crop rotation, ranging from 110 to 119 kg N ha−1. Inorganic N concentrations in potato soils were related to the previous crop’s residue N contents, and were highest following vetch and alfalfa and lowest following oat and potato. The highest mineralizable N concentration was found following vetch (46.6 mg N kg−1). Saturated soil hydraulic conductivity in potato following all rotations ranged from 9.88 to 11.28 cm h−1 compared to 5.71 cm h−1 for continuous potato. Higher soil water contents were maintained in the 30 to 45 cm depth for all rotations compared to continuous potato. Thus several parameters indicate substantial N effects associated with particular crop rotations. Soil hydraulic conductivity and soil water status may also represent significant components of the rotation effect not directly related to N for these cropping systems.

Short-term effects of crop rotations and wood-residue amendments on potato yields and soil properties of a sandy loam soil

A 3-yr study (1988–1990) was conducted on a Beaurivage sandy loam soil located in St-Lambert, Québec, Canada, to evaluate the short-term effects of crop rotations and organic amendments on soil properties and potato yields (Solanum tuberosum, L.). Treatments consisted of yearly fall rye (Secale cereale, L.) as a green-manure cover crop, barley (Hordeum vulgare, L.) grown every 3-yr in potato crop rotation, partially humified bark residues and fresh tree clippings applied once at 100 m3 ha−1 in the beginning of the experiment, and continuous potato in monoculture used as check. Fertilizer rates were applied at 150, 100, 160 and 40 kg ha−1 rates for N, P2O5, K2O5 and Mg, respectively. Results indicated that more rapid changes of soil C content were observed with ligneous material incorporation than with fall rye and barley residue additions. Compared with fresh tree cuttings, partially humified bark induced rapid changes in soil organic C and cationic exchange capacity (CEC). A temporary soil structural stability improvement was observed in 1989 under fall rye cover crop. However soil bulk density increased significantly (P < 0.05) in these plots in 1990, and this was also related to low potato yields. Barley residues and ligneous amendments significantly improved soil water content during the critical flowering stage, and this increased potato yields and specific gravity (P < 0.05). In general, soil amendment would improve potato yields and quality through improved soil water content on a short-term. Key words: Barley, rye, rotation, cover crop, ligneous amendments, potato yields, soil physical and chemical properties

Endophytic bacterial systems governing red clover growth and development

SUMMARYEndophytic competent bacteria capable of promoting both beneficial and detrimental growth responses in red clover (Trifolium pratense L.) were recovered from three adjacent areas of farmland each having a different cropping history — continuous red clover, continuous potatoes (Solanum tuberosum L.) or a 2 yr rotation of red clover and potatoes. The population composition of these rhizobacteria was altered by the various crop sequences. The greatest instance of significant growth responses (beneficial or detrimental) occurred with those bacteria derived from the clover‐potato soil, suggesting increased interactive ‘competition’ among bacterial populations at the ‘interface’ between different crop rotations. Whether bacterial strains promoted or inhibited growth appeared to depend on the cropping history and prior exposure of pre‐bacterised clover plants to the natural microflora in the peat‐based growing media. The interaction between bacterial colonists influenced plant trait expression to the degree that some characteristics were completely masked. Improvements in plant growth were interpreted as an allelopathic side‐effect of the competition between endophytes for the same ecological niche, from which the plant inadvertently benefits.

Fertility and management practices to control verticillium wilt of the russet burbank potato

Management practices for the suppression of Verticillium wilt of Russet Burbank potato include sanitation, use of optimum sprinkler-irrigation practices, soil solarization, and an adequate soil fertility program. Among all cultural factors considered, nitrogen (N) deficiency in potato tissue was most commonly associated with the severity of Verticiilium wilt in Russet Burbank potato. Field studies have shown that increased N availability suppresses Verticiilium incidence on cv Russet Burbank while having no effect on cv Norgold Russet. Studies from both greenhouse and field show that the colonization ofV dahliae in potato stem tissue is suppressed in cv Russet Burbank when the availability of Phosphorous (P) is increased to the optimum. Following one season of cropping with Russet Burbank potato, the treatment providing the optimal N availability also suppressed the increases ofV dahliae populations in the soil during the following year of consecutive cropping. Similarly, after two seasons of continuous potato cropping, treatments with optimal P had lower soil populations ofV. dahliae in soil. Results show the suppression of Verticiilium wilt with optimal fertility. Verticiilium wilt [caused by eitherVerticiilium dahliae Kleb, (microsclerotial form) orVerticiilium albo- atrum Reinke and Berthold (dark mycelial form)] is one of the most severe diseases of potato in the United States. Potato yields, tuber size, and specific gravity may be substantially reduced by this disease, depending on severity, time of occurrence, and growing season. In Idaho and other arid growing regions of the United States, Verticiilium wilt is caused byVerticillium dahliae Kleb. Idaho field studies involving cropping practices, soil fumigation treatments, solarization, and Verticillium-resistant potato clones all support the importance ofV dahliae upon potato yield. Data from these studies show that yield losses due toV. dahliae commonly approximate 5 to 12 metric T/ha (5, 6). Table 1 illustrates the effect of several cropping practices upon relativeV dahliae populations in soil with the effects upon both disease severity and potato yield. With continuous cropping of Russet Burbank,V dahliae populations in soil increased, and yields were reduced by 18 to 19 metric Tlha when compared to locations that had been allowed to remain either weed-free and fallow or where corn had been previously cropped. It was estimated thatV dahliae was influencing yield by about 10 to 12 metric Tlha based on a regression analysis of yield as a function ofV dahliae populations in soil. Similarly, whenV dahliae was suppressed by fumigation treatments involving mixtures of dichloropropene and picfume, yields more significantly increased by 6.5 to 12 metric T/ha (5). Although many factors may influence yields, clones with higher Verticillium resistance than Russet Burbank generally out-yield Russet Burbank. Figure 1 demonstrates the effect of solarization (a technique involving the laying of clear plastic on soil for the purpose of elevating soil temperatures to killV dahliae). When Verticillium was controlled in this manner (lo), there was no significant yield difference between the Russet Burbank potato and the highly resistant A68113-4 clone. The A68113-4 clone grown in non-solarized soil out-yielded the Russet Burbank in non-solarized soil by 31 percent while the solarization treatment significantly increased yield for the Russet Burbank and A68113-4 clones by 46 and 18 percent, respectively. These yield responses were observed in a field where inoculum levels ofV dahliae were relatively low (2.10 cfulgrn of soil). With these losses, the need to develop control procedures is great and to achieve this control the need also exists to accurately evaluate the effect of treatments uponV dahliae. Our Idaho studies have routinely utilized such a procedure (9) in combination with comparisons of both disease severity and yield.

Urban Nature Conservation in Britain

This study was conducted to identify chemicals in root exudates and their effect on Rhizoctonia solani in potato cropping systems. Root exudates were collected from the fields with 5 years of continuous potato cropping in comparison with rotational cropping of potato and other crops, using in-house made root boxes at the seedling and squaring stages. Chemicals in the root exudates were identified using the GC-MS method. The results showed that glucide concentration was the highest in the root exudates, followed by organic acids. Compared with the rotational cropping, the continuous cropping significantly decreased the glucide content and increased the content of organic acids in the root exudates. The contents of almitic acid in root exudates under continuous cropping was 0.94% at seedling stage and 1.4% at squaring stage, the dibutyl phthalate was 0.15%, whereas under rotational cropping, those values were decreased to 0.15%, 0.2%, and being negligible, respectively. The root exudates promoted the growth of R. solani, especially under continuous potato cropping. The simulation test showed that the palmitic acid and dibutyl phthalate in root exudates could promote the growth of R. solani.

The effect of crop rotation and granular nematicides on the incidence ofRhizoctonia solani in potato

The incidence ofRhizoctonia solani in potato was studied in two crop rotation experiments from 1981 to 1985 inclusive. The greater the frequency of potato cropping, the more severe the attack on stems and stolons byR. solani. Severity of black scurf on progeny tubers and cropping frequency were also correlated, but less significantly. In fields with antagonists toR. solani, continuous potato cropping caused less black scurf on tubers than did 1∶2 rotations.

Quantity and quality of potato yield as influenced by unbalanced and excessive fertilization

Potato (var. Rekord) was cultivated for three years in a sandy field. It was given various levels of three fertilizers the N, P and K composition of which in percentages was 7-11-12, 18-3-12 and 27.5-0-0. The N levels were 0, 80/60, 140/120 and 300/240 kg/ha (1983/1984-85), but the P and K levels varied according to the composition of the fertilizers. In the first year the yields were high regardless of fertilization. The following years the yields were clearly highest in the plots where the fertilizer 7-11-12 was used. This was due to insufficient P and K supply in the other plots. With increasing N fertilization the starch content and mealiness of the tubers decreased and the flavor was degraded. The two higher N levels yielded a considerably higher nitrate content than was measured in tubers from unfertilized plots and those receiving the lowest levels of fertilizers. The present results point out the significance of appropriate N fertilization for potato. A certain N amount was necessary to produce a quantitatively satisfactory yield but excessive amounts had an adverse effect on the quality. P and K were of minor importance for the quality in this experiment, but abundant P and K fertilization was needed for optimum yield formation. Maintenance of soil K level proved difficult in continuous potato cultivation.

SOIL REDISTRIBUTION ON THREE CULTIVATED NEW BRUNSWICK HILLSLOPES CALCULATED FROM 137Cs MEASUREMENTS, SOLUM DATA AND THE USLE

137Cs, solum data and the USLE were used to estimate soil erosion on three medium-textured soils in New Brunswick that had been in nearly continuous potato monoculture over the past 15 yr. Sampling transects were located on slopes ranging from 425 to 1040 m long, and from 4.8 to 6.0% slope. Ten equally spaced sites were sampled on each transect. The 137Cs data were used to divide the sampling sites into four groups: deposition, little erosion or deposition, moderately eroded and severely eroded areas. The four groups generally agreed well with soil profile properties. Solum, A horizon thickness and organic carbon content of the A horizon all decreased going from depositional sites to sites with severe erosion. Variations in A horizon thickness and organic carbon content explained about 65% of the variation in 137Cs. Soil losses predicted by the USLE were better correlated with soil losses calculated from 137Cs when the 137Cs data were used to delineate slope segments that were subject to erosion than when 137Cs and USLE soil losses were calculated on a field basis. The USLE overestimated soil losses at high erosion rates. Key words: Erosion, deposition, 137Cs, USLE, soil profile

Nitrate and chloride distribution and balance under continuous potato cropping

A study was conducted between 1977 and 1982 to determine N losses from sandy loam soil under continuous potato cropping in Southern Ontario, Canada. A monthly climatic water budget was used to evaluate the timing and amount of soil drainage in relation to changes in the distribution of NO 3 −-N and Cl − in the soil profile. Significant temporal variations in NO 3 −-N and Cl − concentration were associated with percolation. The presence of concentration peaks below 183 cm, adjacent to a shallow water-table aquifer underlying the field, indicated definite leaching patterns. Ground water in the aquifer and in springs located down-gradient from the field had NO 3 −-N concentrations > 10 mg l −1. Little leaching occurred during the summer, but considerable amounts of NO 3 −-N and Cl − were lost during major episodes of soil drainage in autumn before freeze-up and in the early spring thaw period. The similar pattern of NO 3 −-N and Cl − concentration in the 0–3-m soil profile suggested that denitrification was not an important N-loss mechanism. This conclusion was supported by laboratory incubations of soil samples which showed that 8–20% of the initial NO 3 −-N was denitrified over a 14-day period in samples from 0–15-cm depth, whereas soils from greater depths exhibited very low denitrification potentials. Mass balance data for 1979–1982 indicated that annual N losses from the 0–183-cm soil profile ranged between 78 and 220 kg ha −1 under potato crops which received annual fertilizer applications of 160–210 kg N ha −1. Above-normal precipitation in late summer and autumn may have increased the rates of NO 3 −-N leaching into the shallow aquifer during the study period.

The profitability of continuous potatoes versus rotations including potatoes and other cash crops: I

In a continuing experiment, at two locations in Ontario, continuous Norchip and Kennebec potatoes are being compared for profitability with rotations including potatoes followed by malting barley or grain corn or soybeans. At the end of the first seven-year cycle Kennebec may have started to decline in yield in continuous production, but continuous potatoes remains the most profitable cropping sequence.

EFFECTS OF ROTATION AND NITROGEN ON YIELD AND QUALITY OF POTATOES

The effects of eight rates of N, ranging from 0 to 245 kg/ha, on yield, specific gravity and petiole nitrate content of potatoes (Solanum tuberosum L.) grown continuously in the same soil and in a 2-yr rotation with corn were evaluated over a 4-yr period. Yields under continuous potato culture were lower than yields in the corn-potato rotation. Soil infestation with Verticillium wilt (Verticillium albo-atrum) was a factor contributing to this yield reduction. The response pattern to added N was identical for both planting systems each year and was best described by a quadratic curve. The optimum rate of N for maximum yield under the irrigated, coarse-textured soil conditions of this experiment was about 175 kg/ha. This is 40 kg higher than the current N recommendation for potatoes in Ontario. Specific gravity under continuous potato culture was significantly reduced each year, but rates of N sufficient for maximum yield did not reduce specific gravity. Tissue nitrate-N reflected the rate of N applied and was significantly higher in petioles from continuous potato plots on the second sampling date.

EFFECTS OF CROP ROTATION ON SOME SOIL-BORNE PATHOGENS OF POTATO

Crop rotations had a marked effect on the incidence and severity of wilt disease on potato crops, and consequently on crop yields. The primary cause of the disease appeared to be the "dauermycelium" form of Verticillium albo-atrum. Colletotrichum coccodes was also isolated and a few pink Fusaria were noted. The highest level of wilt occurred in the continuous potatoes and in the second potato crop of a 6-year rotation, and the lowest level occurred in a 3-year rotation. The first potato crop of a 6-year rotation had an intermediate degree of wilt. Black dot (C. coccodes) occurred at a higher level than expected and at much the same level in all rotations.