Solanum tuberosum L. cv. Casablanca (85G040-080 × Picasso; Suttons Seeds, Woodview Road, Paignton, Devon, UK, TQ4 7NG), cv. Rooster (OP 2532 64 × Pentland Ivory) and cv. Shelford (FL1625 × Hermes) were used in this study. Casablanca was used in the greenhouse trials. Rooster and Shelford crops were tested in field trials on commercial farms.
Chitted Casablanca tubers of approximately 7–8 cm in length were planted in May 2017, in 13.5-L−1 buckets perforated at the base, containing J. Arthur Bowers John Innes No. 2 compost (Westland Horticulture Ltd., Co., Tyrone, UK), at a rate of one per container, placed 4–6 cm below the soil surface. The pH of this compost is 5.5–6.0, and it provides appropriate macro- and micronutrients to plants in all treatments for the first 3 to 4 weeks, including boron and calcium. Plants were allowed to grow to an average stem height of 10–12 cm prior to experimental treatments (June), in a heated and ventilated glasshouse under natural light (PPFD 200–1000 μmol m2 s−1), in Preston, northern England, UK. Nighttime temperature was 12–16 °C and daytime temperature was 16–32 °C. Plants were watered by hand to soil capacity as required, taking care not to over-water. Each of the three treatments (see below) comprised of 12 replicate potato plants (seven for the first harvest, five for the final harvest) which were randomised within an area of 8.0 × 1.5 m2. Two harvests were carried out: one in August and one in September 2017.
Plants were treated with foliar sprays containing nitrogen (N) fertiliser in three formulations: an industry-standard N–P–K control (IS), stabilised amine nitrogen (SAN) in a formulation called ‘Lono’ (supplied by Levity Crop Science Ltd., Preston, UK) and standard unstabilised urea (SU). SAN was supplied at 0.1 mmol m−3, equivalent to a rate of 5.0 L ha−1 in 200 L water. IS was supplied at 1.4 mmol m−3. SU was supplied at 0.03 mmol m−3. SAN contains 15% N (by weight), and the IS and SU treatments were designed to provide the same amount of N to the plants (given that IS contains 24% N and SU contains 46% N by weight). Commercial IS contains a mixture of ureic and ammonium nitrate N. All three treatments were supplemented with industry-standard N–P–K (IS) at one-quarter recommended strength every 10–12 days, approximately mid-way between main treatment dates, ensuring access to sufficient micronutrients and P–K. Plants were treated with N fertiliser over the course of the following weeks at the times detailed in Table 1, at a rate of 20 cm3 per square metre of planting area.
Over the course of the experiments (24 days or 46 days), plants were treated with the appropriate N compound every 10–14 days (as specified in Table 1) and leaf relative chlorophyll content and shoot length, number and extension rates were measured on several occasions (Table 1). One group of plants (seven replicates) was harvested shortly after tuber initiation, and divided into tissue types (shoots, seed potato, roots and new tubers (1–2-cm stage)) and weighed immediately. A second group of plants (five replicates) was harvested 3 weeks later at mid-bulking. Shoots and new tubers (4–6 cm) were weighed and counted, and a version of ‘marketable’ tuber weight was designated as 5–25 g FW−1.
Relative chlorophyll content was measured in leaves as an index, with a FieldScout CM 1000 Chlorophyll Meter (Spectrum Technologies Inc., IL, USA). ‘Point-and-shoot’ technology instantly measures the reflectance of ambient and reflected 700-nm and 840-nm light in a conical viewing area on the adaxial leaf surface 30–180 cm from the light receptor. Laser guides outline the edges of the sampling area, allowing replication of the position of this between plants (we chose a 0.5-cm-diameter area mid-way between the midrib and the leaf edge of the most recently matured leaves). The light receptor comprises four photodiodes: two for ambient light and two for reflected light from the leaf. Measurement units are calculated as an index of relative chlorophyll content: 0–999 ± 5%.
The length of each shoot per plant was hand-measured with a ruler from the soil surface, and was also combined to give an overall length comprising all shoots per plant.
Means and standard errors of each measurement type per treatment are displayed as bar charts, and correlations between data sets are depicted on scatter graphs. The significance of the differences between treatments in bar chart form was calculated using a one-tailed t test for two independent means, and where treatments are significantly different from each other (p < 0.1), this is denoted by ‘a’, ‘b’ or ‘c’, above the appropriate column on the graphic representations of the data. Best-fit trend lines on scatter graphs were calculated (linear or exponential) on Microsoft Excel, and r2 values are provided. The significance of the r value—the strength of the relationship—was calculated using the Pearson product-moment correlation and is represented as one asterisk (p < 0.1), two asterisks (p < 0.05) or three asterisks (p < 0.01).
Summer–Autumn 2016 Field Trials
Hampshire, England, UK: Shelford
Conditions on farm and dates of crop treatments, from sowing to harvest, are described in Table 2. Five replicate plots for each of the three experimental treatments were laid out in a randomised block design: (1) soil-applied commercial urea controls, (2) foliar-stabilised amine nitrogen (SAN) applied three times, (3) foliar SAN applied four times. Foliar SAN applications were carried out at a rate of 5.0 L ha−1 in 200 L water (0.1 mmol m−3): the first application occurring at tuber initiation and the last at late bulking. Where treatments were applied only three times, either the tuber initiation treatment was withheld or the late bulking treatment was withheld.
Yield data is presented as follows: tuber count per plot, tuber yield per plot (kg), tuber count per hectare, yield per hectare (metric tonnes: t ha−1). Tubers were graded into categories by tuber size (< 40 mm, 40–60 mm, 60–80 mm, > 80 mm (split tuber number was assessed visually)), and marketable yield was 40–80 mm.
Means and standard errors of yield data for the three treatments (control, SAN 3 applications, SAN 4 applications) are displayed as bar charts. The significance of the differences between treatments in bar chart form was calculated using a one-tailed t test for two independent means, and where treatments are significantly different from each other (p < 0.1), this is denoted by ‘a’, ‘b’ or ‘c’, above the appropriate column on the graphic representations of the data.
County Meath, Ireland: Rooster
Conditions on farm and dates of crop treatments, from sowing to harvest, are described in Table 3. Three different nitrogen treatments were applied to four replicated plots each, set out in a randomised block design: (1) soil-applied commercial N fertiliser controls (55 kg ha−1), (2) SAN applied foliarly three times, (3) SAN applied foliarly four times. Foliar SAN was applied at a rate of 5.0 L ha−1 in 200 L water (0.1 mmol m−3), with the first application occurring at tuber initiation and the last at late bulking. Where treatments were applied only three times, the tuber initiation treatment was withheld.
Green leaf area (percentage of all leaves) was assessed visually on 13 September, in 3 plants per plot. Yield data is presented (t ha−1) for small tubers (< 45 mm), marketable tubers (45–65 mm), oversized tubers (> 65 mm) and gross weight (minus large tubers).
Means, standard errors and significance levels of green leaf area and yield data were calculated as described for the Shelford trial above, and presented as bar charts.