Balanced fertigation and improved sustainability of June bearing strawberry cultivated three years in open polytunnel

3.1.1Fruit yield

There was no interaction concerning total fruit yield or marketable fruit yield, neither for fertilization vs year nor for fertilization vs cultivar, and therefore is fruit yield presented as average of cultivars (Table 1). Considering the two-year mean, the level of fertilizer had effect on total fruit yield. The highest fertilizer level (III) improved yield compared with level II and control, but fruit yield at level II was not significantly different to control. However, there was no significant differences between marketable yields in average of years. Fruit weight increased by fertilization. The percentage of small fruits was highest in control in average of the two years. Percentage of misshaped fruits and rest was not significantly different between fertilization levels.

Results varied between years. In 2014 (first crop year) there was no significant effect of fertilization neither for total nor marketable yield. This is comparable to results achieved in the first harvesting year for ‘Korona’ and ‘Bounty’ grown in open field at the same location and on the same soil type as here, fertilized with a nutrient solution of Calcinit (Yara, Norway) [14]. There were no misshapen fruits that year, and fruit weight equal to or smaller than 25 mm diameter (Sf) and rest was unaffected by fertigation. There was no significant effect on fruit size by fertilizer.

In the second crop year strongest fertilization increased total fruit yield compared to control. However, marketable fruit increased but not significantly; the main reason was the strong infestation of mildew on ‘Korona’ and to some extent on ’Florence’, at strongest fertilization, which dominated the rest portion. Fertilization had no significant effect on fruit weight, but fruit weight was higher than in 2014; in spite of higher yields than the year before. The differences in yield between the years could partly be explained by the much warmer September and October in 2014 (T_average 9.1°C) than in 2013 (T_average 5.0°C), that provided better conditions for flower bud development in the autumn of 2014 and favored high fruit yield and fruit size in 2015. The percentage of small and misshapen fruits was lower for fertilized plants than for control in 2015.

3.1.2Soil ion concentration

The concentration of ions in soil water changed throughout the three seasons, as a reflection of fertilization that increased during the establishing season parallel to increasing plant size, and in the two crop seasons because of vegetative development and fruit ripening (Fig. 2, Tables 2–4). Since strawberry is sensitive to salinity [15, 16], we suggest that the species should be in saline class weak of Wolf [15]. That imply EC-levels in soil (1soil:2water) from 0.51 to 1.50 mS cm^–1, where the lower part of the scale may indicate too little N and K for rapid growth [16], which also are in agreement with practice in present Spanish strawberry growing. The ion values are from soil water sampled the same day as fertigation took place. We fertilized at weekly intervals on the same week-day and at the same time of the day. The level of ions would drop between one fertigation and the next, depending on the level of watering between the days of fertigation (not tabulated), and uptake by plants.

Fig.1

Program showing weekly fertigation in minutes for two fertilization levels (FL II and III = 1 and 3 g of N m^–2, respectively, in 2013; and 3 and 6 g N m^–2,respectively in 2014 and 2015. The third level Control (I) = 0 fertilizer is not shown.

Fig.2

Effect of fertilization on NO_3 ÷ concentration in soil water at two depths. In 2013 the sampling was taken at one date in July and one in September. In 2014 and 2015 the average of all samples in the the period before flowering (Grow), during flowering (Flow), during harvest (Harv) and after harvest (Aut) is shown. In 2015 fertilization stopped after harvest.

July 29th 2013 was the first soil water sampled. Results show that concentration of macro nutrient cations and EC recorded was higher at fertigation level III (3 g N m^–2) than at control and level II (1 g N m^–2). The difference between control and fertilizer level II was not significant for any parameter except EC, which was lower for level II than for control at 15 cm depth and higher at 25 cm depth (Table 2). Also, NO₃^– concentration was higher at fertigation level III compared with control and fertilization level II; but not too high since the EC value was relatively low (Fig. 2, Table 2). The samples from 11 September 2013, showed Ca–, Mg–and K–ion values with not much difference for control and fertigation level II (Table 2); the concentrations of nitrate were relatively low (Fig. 2). However, at strongest fertigation the content of cations increased; and the concentrations of nitrate increased at 25 cm depth compared to 15 cm depth. EC level at highest fertigation was low and do not indicate to high use of fertilizer. However, the results indicate movement of excess nitrate downwards in the soil profile in summer and autumn, and similar for calcium in summer; probably because of too long fertigation pulses.

To control leaching we monitored water content in soil at three depths. This helped to adjust the fertilization program since water was the carrier of fertilizer. In the year of establishment the relation of the fertigation solution to the stock solution was 1 : 200 and water peaks at fertigation went high above FC (Field Capacity) (Figs. 1 and 3); we considered that a similar practice in 2014 would result in even higher water peaks since we increased length of watering pulses [17]. Therefore, we changed the relation to 1 : 100 in the two cropping years, and the peaks above FC the day of fertigation became acceptable in the growth and flowering stages, but still too high during harvest (Figs. 4 and 5). To avoid this in practice, we suggest fertigation twice a week with a shorter pulse, instead of once a week, to reduce the peaks. On a lighter soil than in our experiments adding fertilizer at every occasion of watering would be an option, but that would need a secure control of EC, and amount of water and fertilizer given as suggested by others [18]. In the establishing year, it seems like fertilizer equivalent to 1 g N m^–2 s in balance with plant growth in a fertile soil. The experimental field was in open high polytunnel to avoid rainfall. In open field and especially on light soils, rainfall could wash out nutrient. Under such conditions, it is essential that the nutrient concentration in soil is monitored to make it possible to adjust the fertilization program.

Fig.3

Percentage plant available water at three soil depths (Red = 10 cm, Green = 20 cm, Blue = 30 cm) in the establishing year (2013). Easily available water (70–100%) is within the turkis zone. 100% available water = FC.

Fig.4

Percentage plant available water at three soil depths (Red = 10 cm, Green = 20 cm, Blue = 30 cm) in the first harvest year (2014). Easily available water (70–100%) is within the turkis zone. 100% available water = FC.

Fig.5

Percentage plant available water at three soil depths (Red = 10 cm, Green = 20 cm, Blue = 30 cm) in the second harvest year (2015). Easily available water (70–100%) is within the turkis zone. 100% available water = FC.

In the first cropping year (2014) fertilization gradually increased from spring and throughout the harvest season and gradually decreased after harvest (Fig. 1). Using 3 g N m^–2 (level II) during growth and flowering, did not alter nitrate and cation concentrations or EC in soil water much from the situation in July the previous year using 1 g N m^–2 (Fig. 2, Tables 2 and 3). That could be because the plants were larger and had the capacity to use more nutrients, and that the temperature in 2014 was warmer than in 2013 enhancing respiration and uptake [17]. However, fertilization level III (6 g N m^–2) was on the high side concerning concentration of nitrate at the flower stage. The good thing was that the concentration reduced from 15 to 25 cm depth, which also was the case for cations and EC (Fig. 2, Table 3). Plants can store large amounts of nitrate in their vacuoles and have in addition at least four different transport systems operating. Additionally many plant species can modify their root architecture to improve uptake and lateral roots will proliferate in nutrient rich soil patches [19]. We demonstrated in our experiments making a vertical cross cutting of one of the beds (using a spade), that roots concentrated just below the drip pipes seeking for water and nutrients (Fig. 6). There is not much information indicating what the nutrient level in soil should be in a strawberry field, but optimal mineral content in plant tissues is known [20], and it was shown that using a nutrient recipe for soilless culture (like we did) was not appropriate for fertilization in soil [8]. The situation in the field trial changed strongly moving into the harvest stage with a maximum EC of 0.25 for strongest fertilization. Control had relatively high levels of minerals compared to earlier development strages, but nitrate was close to zero probably because of a strong uptake. The reason for this could be high temperatures in July that year (average 19.7°C), with a potential to increase plant physiological processes and thereby uptake of nutrients, compared with a normal July with an average of 13.2°C [21]. It is obvious that the plants utilized the nutrients in the soil to provide for the generative development. At fertigation level III there is a positive gradient from 15 to 25 cm depth for Ca and Mg ions, but K ions decreased. The nitrate values were low for all fertilization levels indicating that the amount of fertilizer in the harvest period should be increased (Table 3, Fig. 2). The fertilization in autumn at fertilization level II (3 g N m^–2) seems appropriate, while level III may be a little too high; although there is a negative gradient for all ions and EC from 15 to 25 cm depth. However, the total amount of fertilizer at strongest fertigation given for the whole season seem appropriate. It is generally a question of moving some of the fertilizer given during flowering (and early growth of unripe fruit) and in autumn into the harvest season.

Fig.6

Cross section of a bed showing how strawberry roots distribute in the profile in August 2014 at fertilization level II. There were earthworm burrows, going deep into the plow layer.

In the second cropping year (2015) the sum of fertilizer given was equal to 2014, but experiences from 2014 made us change the fertigation program moving more of the fertilizer from the growth and flowering stage into the harvest season, compared with 2014. The fertilization stopped after the last harvest date 2 September (Fig. 1). EC was in some situations higher than recommended (0.51–1.50) for a not saline tolerant species like strawberry [15, 16]. In spite of the reduced use of fertilizer in the vegetative- and flowering-phase compared with 2014, the ion concentrations and EC were surprisingly high (Table 4, Fig. 2) in these stages. In the growth stage, concentration of nitrate at fertigation level III was close to 1400 mg l^–1 at 15 cm depth. However, the roots were effective in taking up ions down to 25 cm lowering the concentration to 1000 mg l^–1 (still high compared to a maximum of 200–350 ppm of nitrate, considered optimal for strawberry in a silt clay soil). For the other ions as well, there was a strong reduction in concentration at 25 cm soil depth compared to 15 cm. The high concentrations could be an influence of the large differences in temperature between the two years. The temperature averages in May, June and July 2014 and 2015 was 10.2, 13.0 and 19.6 and 8.4, 10.3 and 13.4, respectively [21], which would reduce the uptake of nutrients in 2015 because of low respiration, compared with the relative warm conditions in 2014 [22]. Additionally, evapotranspiration was higher in 2014 than in 2015, which would increase transport of ions into the plant lowering the levels in soil. However, we observed no visible injury on the above ground parts of the plants in our research, but EC close to 2 mS cm^–1 at strongest fertilization during growth and flower development could give injury in saline sensitive plants like strawberry [14].

At fruit harvest, the ion concentration decreased for all fertigation levels except control compared with growth and flower stage, and it was generally higher than in 2014 as expected, probably because of a combination of changes in the fertilization scheme and the lower temperature (Fig. 1). The nitrate concentration differed little between fertilization levels II and III at 15 cm depth, but was lower at 25 cm depth for fertilization level II than for level III (Fig. 2). However, concentrations of cations and EC were higher at both depths for fertilization level III than for level II, but not considered too high and there was a decrease in concentrations down to 25 cm depth. What we learn from this is that we should have moved more of the nutrients given at growth and flower stage into the harvest stage. We therefore find that balanced fertigation equivalent to 6 g N m^–2, giving 4 g N m^–2 from two weeks before and during harvest is recommendable for a fruit yield of 3 kg m^–2, as for ‘Sonata’ in 2015.

When watering in between the single days of weekly fertigation, the level of soil-ions decreased. Since we avoided high water peaks above field capacity, this practice should reduce leaching of nutrients. Therefore, it looks like the strawberry roots absorbed nearly all fertilizer given during harvest. As shown, roots are effective in taking up nutrients reducing the amount of ions in the soil profile between 15 and 25 cm. The roots developed well in the bed profile down to 35 cm and concentrated just below the drip pipes. A few roots moved even deeper into the hard clay layer (Fig. 6). This indicate that roots were active taking up nutrients down to at least 35 cm. However, in spite of roots going relatively deep the flux of water and fertilizer can be higher than the crop ability to absorb it, resulting in percolation and nutrient leaching.

3.1.3Nitrogen balance index (NBI) of strawberry leaves

NBI level was equal for all three fertilization levels May 2014 (Fig. 7). However, during harvest in July NBI of control (0) dropped strongly, while levels of fertigation level II and III were relatively unchanged, with a small reduction for level II and a small increase for level III. The next recordings were in September 2015 sampled after harvest, and it show that NBI for control was strongly reduced compared with the May and July recordings of the previous year; while the levels of fertilized plots had a similar appearance as the year before. The results correlate to the ion levels caused by differences in fertilization. It also indicate that optimum NBI level (average of adaxial and abaxial side) for strawberry is between 9.5 and 11.5, lower than for Blackcurrant and sourcherry [11].

Fig.7

NBI index of strawberry leaves in two years influenced by three fertilization levels (Control = 0, II = 3 g N m^–2, III = 6 g N m^–2) as average of three cultivars.