Control of nitrogen and water in full-grain barn for a sustainable transition to increased taste and storage quality

Main goal in the study was to achieve a higher fruit quality in strawberry trough sustainable irrigation and nitrogen (N) fertilization. The project focused on the relation between irrigation and N fertilization with fruit firmness and taste quality. Taste quality was quantified by the ratio between sugars and acids.

Irrigation trials were set up between 2014 and 2017 at Proefcentrum Hoogstraten (PCH) and Proefcentrum Fruitteelt (PCFruit). In these trials an optimal irrigation treatment was compared with a deficit irrigation treatment. In the optimal irrigation treatment plants were irrigated according to the water demand of the plant. In the dry irrigation treatment irrigation water supply was lowered to 70% of the optimal irrigation treatment. In the two irrigation treatments plant and soil water status was monitored. Irrigation demand was highest in 2015 and 2017, which resulted in a different moisture status between the two irrigation treatments. Yield results of the trials in 2015 indicated the water stress sensitivity of strawberry. In the trials at PCH as well at the trials at PCFruit strawberry yield was 15% lower in the dry treatment, were soil water potential decreased to -100 kPa. In 2015 as well as in 2017 a dryer irrigation regime could be linked to smaller fruit size with a higher concentration of total soluble solids (°Brix). The irrigation regime has however a lower influence on fruit quality compared to time of fruit picking, strawberry variety and the duration of sunshine before harvest. Optimal soil water potential, as observed from the irrigation trials for the variety Elsanta grown in open air, was ‑30 kPa to ‑50 kPa. When Elsanta variety is grown under plastic cover, and for the strawberry variety Portola in general, optimal soil water potential was -40 to -70 kPa.

Since the start of the project three N-fertilization trials were included in the experiment. A ‘low’ N‑fertilization treatment, where no mineral N-fertilization was applied was compared to a ‘mid’ N‑fertilization treatment and a ‘high’ N‑fertilization treatment. In the ‘high’ fertilization treatment N‑fertilization dose was double of the ‘mid’ fertilization treatment. Only in 2015 the influence of N‑fertilization on strawberry yield was significant on a trial at PCH. In this trial strawberry plants were planted and fertilized in august 2014, stored in the fridge during winter, and planted out in the open field in the spring of 2015. In all other experiments there was no significant influence of N‑fertilization on strawberry yield. The limited N-fertilization response of strawberry was confirmed by the crop analysis, where N‑content in the plant was measured. In total 100 kg N was exported by the strawberry variety Portola in 2017 on a total strawberry yield of 5.3 kg/m². In June bearing Elsanta in 2015 in total 90 kg N/ha was exported by strawberry. The soils of the trial fields at PCH and PCFruit were, just as many other strawberry fields, highly fertile. N incubation experiments in the laboratory showed an N mineralization rate of 1.3 kg/ha N per day. In the entire growing season up to 100 kg N/ha is set available to the plant by the soil trough mineralization of soil organic matter. The high mineralization rate is caused by the plastic cover which increases the soil temperature, the high carbon concentration in the fertile strawberry soils and the presence of irrigation, which all favour mineralization of soil organic matter. Excessive N fertilization was linked to lower fruit firmness for both the Elsanta variety as for the Portola variety.