Field experiments were conducted in Wisconsin for 3 years on irrigated sandy and silt loam soils in an attempt to better calibrate potato phosphorus (P) needs and to determine which of several P fertilizer placements/timings were best for crop yield and quality or could improve P fertilizer use efficiency. In 2 of 3 years at both locations, positive yield and tuber quality responses were seen with phosphate rates ranging from 45 to 200 kg P2O5 ha−1 on the sand and 124 kg > 224 kg P2O5 ha−1 on the silt loam. Row placement improved P fertilizer use efficiency on the sand but not on the silt loam. In-season P applications improved plant P concentrations, but did not always result in tuber yield or quality responses, although this method may be effective as a rescue treatment. Early season (~35 days after emergence, dae) petiole P analysis was useful for predicting P sufficiency with a critical level of 0.35% P. Later sampling (> 50 dae) did not adequately separate responsive from non-responsive treatments. Based on this research, row-placed P at modest rates (about 100 kg P2O5 ha−1) is recommended on sandy soils, whereas on the medium-textured soil all placement/timings performed similarly except the in-season treatment. A higher rate was needed (~160 kg P2O5 ha−1) on this soil to optimize yield and quality.

In an attempt to quantify the nitrogen (N) contribution from forage legumes to succeeding potatoes, trials were conducted on 28 Wisconsin alfalfa or red clover fields. Potato total yield and crop value were determined following in-season fertilizer N applications of 0 to 280 kg ha−1. Only 2 of the 12 alfalfa sites and 6 of the 16 red clover sites responded to applied fertilizer N when total yield was considered. However, using crop value, responses were seen at 5 alfalfa and 7 red clover sites. Apparent fertilizer replacement value ranged from 0 to 224 kg N ha−1 for both alfalfa and red clover, and averaged 129 or 102 kg N ha−1 for the respective forages. Due to the wide variation, consideration of criteria in addition to preceding crop is necessary to accurately estimate potato N needs following legume forages. Use of the petiole nitrate-N test did not improve the predictive capability.

Research has shown that while fumigation and use of ammonium N can both reduce the severity of verticillium wilt (Verticillium dahliae Kleb.) of potatoes (Solanum tuberosum L.), the use of the two practices together raises concerns over feeding the crop only ammonium N under reduced nitrification conditions. To assess the validity of this concern, we conducted two 3-year field split-plot experiments with both using metam sodium fumigant (none, fall or spring applied) as the main plot. For the first experiment, N source (134 kg N ha−1 as ammonium sulfate, urea, or ammonium nitrate) was the split, whereas for the second trial in-season N rate (0, 67, 134, or 202 kg N ha−1 all as ammonium sulfate) was the split. For both trials, in 2 of the 3 years, fumigation significantly increased tuber yield by an average of 9.9 Mg ha−1 and decreased late-season verticillium severity ratings from 77 to 45%. In some years, fumigation also increased the proportion of U.S. No. 1 tubers and tubers >170 g. No differences in crop yield or quality were observed between the various N sources applied. This was true even on spring-fumigated areas with the highest rate of ammonium N applied. These experiments confirm that the choice between in-season potato N fertilizer should be based on factors such as potential for benefits or N losses, cost, and convenience of use rather than concern over an interaction between fumigation and ammonical N. While both fumigation and N rate reduced verticillium severity ratings in some years, the lack of interaction suggests these factors are functioning independently.

The availability and use of crop protectants including azoxystrobin in combination with fumigation has extended the Central Wisconsin effective growing season by 2 to 4 weeks. This study, evaluating the influence of these crop protection practices on the optimum nitrogen rate and time of application for Russet Burbank potato (Solanum tuberosum L.), was established as two 3-year field trials designed as split-split plot experiments. Both experiments used metam sodium as the main plots and fungicide treatment (chlorothalonil Zn alone or chlorothalonil Zn alternated with azoxystrobin for the first six sprays) as the first split. In-season fertilizer N rate (179, 224, 269, or 313 kg N ha−1) or in-season N timing (N split into two, three, or four applications at 269 kg N ha−1) was the second split. Not fumigating resulted in significantly higher verticillium ratings and severely repressed crop yield and tuber quality responses to both fungicide treatment and N rate. On average, fumigation increased total yield 13.6 Mg ha−1yr−1, U.S. No. 1 tubers by 9 % and U.S. No. 1 tubers >170 g by 5 % over where fumigation was not used. In 2 of the 3 years when azoxystrobin was included in the fungicide program early blight severity was reduced by about 50 %, and on fumigated areas yields were increased 4.8 Mg ha−1, whereas no yield increase was seen from this fungicide treatment on the non-fumigated plots. In these same years, fumigation increased optimum N rate by about 50 kg ha−1; however, there was no apparent interaction with fungicide treatment. Although fumigation, fungicide treatment, and time of N application each influenced tuber yield or tuber quality in some years, in the two more responsive years some interactions between these factors were statistically significant, with benefits generally only seen where plots were fumigated.

The efficient use of fertilizer nitrogen (N) is critical for potato production in regions with sandy soils as concerns for groundwater contamination have become more apparent. The interactive effects of different hill shapes and distribution of in-season N fertilizer applications at various timings were evaluated in a 3-year potato (Solanum tuberosum L. cv. Russet Burbank) field experiment on a sandy soil in central Wisconsin. A split-plot design was used with hill shape (standard, shaped-plateau, or pointed) as the main plots and 202 kg N ha−1 divided into two, three, or four applications as the split plots. Broader, flatter hills provided tuber yield increases of 7 to 10 %, tuber size and grade improvements of 8 to 25 %, and increased tuber N uptake an average of 22 % in some years; however, post-emergence hilling operations negatively affected yield and tuber size and grade out in 1 of 2 years. Splitting the N into three in-season applications (emergence, early tuberization, and tuberization + 20 days) increased tuber yield by about 4 % or tuber size by 19 % in years where rain increased leaching potential on this sandy soil, but further splitting increased the proportion of small tubers that passed a 5.1-cm screen. This study confirmed that more blocky-shaped hills with only one hilling operation at emergence can significantly benefit potato yield and quality, and fertilizer N use efficiency on these sandy soils.

Early-season nitrogen (N) is necessary for optimal potato vegetative growth and creating an optimal growing condition for high yields; however, on sandy soils it also increases the risk of losing fertilizer N through leaching. This 3-year field experiment evaluated whether a smaller amount of N placed near the plant roots could provide the benefits associated with higher rates of early N applications that were less well placed. Two rates of N applied at emergence (40 or 80 kg N ha−1) were spot-placed (5 to 7 cm around each plant), banded along the row, or broadcast applied, and compared to no N or where all of the in-season N was applied at tuberization. All plots except the zero N controls received a total of 170 kg ha−1 of in-season N. Where emergence N was spot-applied in some years, tuber numbers were reduced compared to where the N was broadcast, and in these situations, resulted in increased tuber size and higher yields of prime-sized tubers (U.S. No. 1, 170 to 370 g). Where differences existed, results from banded treatments were intermediate between those from the spot and broadcast treatments. However, in spite of apparent N placement effects likely associated with having a higher concentration of N near the plant roots early in the season, no differences were evident between the two rates of emergence N within a given placement. In this experiment, total yields were not affected by rate or placement of emergence N. Overall, this experiment provides support for the concept of placing early-season N near the plant roots, and band applications along the row may be a grower-manageable alternative for achieving this goal.

As hill shape significantly influences water infiltration into potato hills, modification of hill shape may be an opportunity for improving fertilizer nitrogen use efficiency on sandy soils. The interactive effect of different hill shapes and rate of nitrogen (N) fertilizer application on N use efficiency was assessed in a 3-year potato (Solanum tuberosum L. cv, Russet Burbank) field experiment on Plainfield loamy sand soil at Hancock, Wisconsin, USA. A split-plot design was used with hill shape (shaped-plateau, pointed, or standard) as the main plots and in-season N rates (0, 135, 202, 269 kg N ha−1) as the split plots with four replications in randomized complete blocks. In 1 of 3 years, potato yield and quality were increased and less N was needed to optimize yield and quality where the hills were shaped. In the other 2 years, the more blocky hills (shaped-plateau and standard) showed consistent tendencies (p = 0.02 to 0.19) toward better crop performance; however, time of hill formation was influencing these results with root pruning likely the influencing factor. Results of this study show more blocky hills with only one hilling operation at emergence can significantly improve potato yield and quality and N use efficiency on these sandy soils.

This study was established to determine whether the timing of liquid dairy manure application prior to planting potatoes influenced potato crop performance and the occurrence and severity of common scab (Streptomyces spp.) and verticillium wilt (Verticillium dahliae). Four potato (Solanum tuberosum L.) varieties with varying levels of resistance to common scab and verticillium wilt were planted on a site treated with manure applied at 280,500 L ha−1 to separate plot areas 18, 6, or 1 month before potato planting in each of 2 years. Total tuber yield for 2001 showed a significant interaction between time of manure application and variety, with more scab-susceptible varieties (Russet Norkotah and W-1151R) showing lower yields compared to the fertilizer control when the manure application was made 1 month before planting. In 2002, the proportion of U.S. No. 1 tubers was significantly lower when manure was applied 1 month before planting, especially for W-1151R. Tuber dry matter was significantly reduced by manure application in both years, but was not consistently related to time of manure application. Total yield was significantly decreased as common scab levels increased, and this relationship was primarily driven by the low yield/high scab levels associated with W-1151R and Russet Norkotah planted 6 or 1 month after manure application in 2001. Late-season verticillium evaluations showed highest levels in 2001 when manure was applied 1 month before planting to susceptible varieties Superior and Russet Norkotah. This experiment showed that when manure was applied immediately ahead of potato planting yields, and quality of some varieties decreased in 1 of 2 years. Therefore, it may be safer to apply manure at least 18 months prior to planting potatoes.

Over a 5-year period, field evaluations of several rates of ACA, an acronym for Agricultural Crop Additive, were conducted on irrigated sandy Wisconsin soils using potato (Solanum tuberosum L.) as the test crop. In some years, statistically significant tuber total yield increases of about 4 Mg ha−1 were seen for one rate, but not others, with the response rate varying between years. In 1 year, similarly sized yield increases were seen for all rates. In most years, the use of ACA (at one or more rates) increased the size of harvested tubers but did not affect other measured growth parameters including stem or tuber number, early-season vegetative or root growth, or midseason leaf nutrient concentrations. In the year when ACA was evaluated across three potato varieties, the responses appeared to be variety specific. No clear mode of action could be identified where the positive responses were seen other than the apparent influence of the additive on harvested tuber size.

Recent evaluations of soil water use by potato (Solanum tuberosum L.) plants have confirmed that a dry zone develops mid to late in the growing season when potatoes were grown in a ridge and furrow system under sprinkler irrigation oil some sandy soils. Although trickle irrigation has been shown to reduce the dry zone in potato hills, a more cost- and labor-effective solution Could be the use of a surfactant to change soil water surface tension and thereby promote more uniform water distribution into hills. Water content in surfactant-treated potato hills was compared with no-surfactant-treated hills using data from time domain reflectometry probes collected at 15-minute intervals, In addition, nitrate-nitrogen (NO3-N) concentration in soil water collected 1 m below the crop row with porous cup samplers was evaluated to assess NO3-N leaching below the root zone of potatoes. Data from surfactant applications at 9.35 L ha(-1) at Planting with the seed piece in 1998, 1999, and 2003 through 2005 generally resulted in significantly increased movement of water into the dry portion of potato hills and in many cases decreased soil water NO3- concentrations at a depth of I In beneath potato hills. Comparison of several surfactants suggested that water movement into the dry zone can be accomplished with a number of different products. Some trends of increased potato yield with surfactant applications as compared with no surfactant were noted in a few cases; but in all cases (P = 0.16-0.24), these were not statistically significant at the 95% level.

The effective use of manure nutrients requires an accurate assessment of their availability to the specific crops being grown. A fertilizer equivalence approach was used to evaluate the availability of nitrogen (N) and phosphorus (P) from liquid dairy manure to potatoes in field experiments conducted in northeast Wisconsin in 2000, 2001, and 2002. Crop responses to moderate and high rates of liquid dairy manure (93,500 and 187,000 L ha−1) were compared to results obtained from N or P fertilizer applied at five rates (0 to 269 kg ha−1). Availability estimates were made using the fertilizer equivalence method based on tuber yield, harvested tuber N or P concentration and uptake, petiole nitrate or total P concentration, and soil nitrate or extractable P phosphate levels. Results showed an apparent availability of manurial N from 10% to 40%, with an overall average across all 3 years and all parameters of 28.6% for the lower rate and 24.6% for the high rate. These values are slightly less than average availability measured where corn has been used as the test crop. Although all parameters did not show responses to P fertilizer additions in these trials, where estimates were possible, P availability ranged from 20% to 90% with an overall average across all 3 years of 55.4% for the low and 50.5% for the high manure rates. Despite potato being more shallow-rooted and somewhat less efficient in N and P use compared to corn, it appears that only minor adjustments to manurial nutrient availability estimates are warranted when potatoes are grown.