Effect of potassium, boron and zinc fertilizers on growth and nutrient uptake in watermelon grown in gravel-mulched fields

【Objective】The gravel-mulched land has a porous layer of gravel and sand about 6-15 cmthick that lies on the soil surface and can reduce the risk of crop failure. It is a unique and traditional wa-ter-saving technique that has been used for centuries in the loess regions of northwestern China. The lowrainfall of about 120 mm during the growth period of watermelon and the sand layer on the soil surface inthis regions increase difficulty in fertilizer uptake. Besides, the traditional fertilization in this region in-volves chiefly application of nitrogen and phosphate fertilizers while potassium and trace elements are ne-glected. Therefore, drought stress, low fertilizer efficiency and imbalanced fertilization are the limiting fac-tors in gravel-mulched fields. This study examined the effects of potassium, boron and zinc fertilizers onphotosynthetic characteristics, yield, quality and nutrient uptake in watermelon grown in a gravel-mulched field under the same nitrogen and phosphorus fertilization and microjet irrigation.【Methods】Thestudy was carried out in Gaolan, Gansu Province, China, where watermelon cultivation in gravel-mulchedfields has a long history. The mean annual precipitation in the region was only 260 mm, and the availabili-ty of potassium, born and zinc in the soil were 94.56 mg·kg-1, 0.52 mg·kg-1and 0.78 mg·kg-1, respective-ly. A late mature watermelon cultivar‘Longkang 9'was used as material and given 5 fertilization treat-ments(NP, NPK, NPKB, NPKZn, and NPKBZn). The treatments were arranged in a randomized mannerwith three replicates, and the area of test plots was 32 m2. The amounts of nitrogen, phosphorus and potas-sium applied were 200 kg·hm-2, 170 kg·hm-2and 200 kg·hm-2, respectively. The volume of irrigation was105 m3·hm-2and conducted at vine stage and fruiting stage. At vine stage, irrigation volume was 45 m3·hm-2plus 50% and 40% of N and K, respectively. At fruiting stage, irrigation amount was 60 m3·hm-2with20% of N and 60% of K, and B and Zn were applied twice at a concentration of 0.2%. The photosyntheticcharacteristics, yield, quality and NPK uptake in watermelon were determined. 【Results】The diurnalchange in photosynthetic rate in the leaves followed a unimodal pattern, peaking at 10 am. Addition of Band/or Zn increased photosynthetic rate and reduced transpiration rate compared with the other treatmentswithout B and Zn application. Therefore, addition of B and Zn could improve water use efficiency. The wa-ter use efficiency in NPKB, NPKZn and NPKBZn was 20.47%, 30.89% and 39.40% higher than NP treat-ment, respectively. The watermelon yield in the treatments with potassium fertilization was increased by12.34% to 23.87% compared with those with no potassium application, and the yield in the treatment ofNPKBZn reached 48632.70 kg·hm-2, which was the highest. Meanwhile, the watermelon quality of treat-ments with potassium added were superior to that in treatments with no potassium. For example, the con-tents of sugar at both the center and edge of the fruit, vitamin C and proteins increased by 9.0%-13.0%,12.3%-22.2%, 12.4%-33.7% and 26.7%-43.6% respectively, suggesting that potassium plays an impor-tant role in quality formation in watermelon. Zinc was effective in increasing sugar at the edge of fruit, andit reduced sugar gradient and increased sugar-acid ratio, as NPKZn treatment increased the sugar contentat fruit edge and sugar-acid ratio by 6.52% and 5.74% respectively, and reduced sugar gradient by 30.0%compared to NPK treatment. Boron was beneficial to increasing vitamin C content in fruit, as Vc contentin NPKB treatment was increased by 9.94% compared to NPK treatment. The amount of nutrition ab-sorbed in watermelon was in the order of K>N>P, and the N:P:K ratio was 1:0.18:1.65 for whole growingstages. The contents of nitrogen and phosphorus in the leaves were the highest, while fruit was the lowest,but potassium content in the stem was the highest, and the nutrition accumulation in fruit was higher thanleaves and stem for influenced by biomass. The total accumulation amounts of nitrogen, phosphorus andpotassium in watermelon were increased significantly by 24.80%, 32.57% and 43.01%, respectively, byapplying potassium fertilizer compared with no potassium treatment. Born seemed favorable for nitrogenabsorption and accumulation, as the nitrogen accumulation in NPKB treatment was 16.87% higher com-pared to NPK treatment. Zinc could promoted absorption of potassium in fruit, as NPKZn treatment in-creased potassium accumulation by 12.96% compared to NPK treatment. The total amounts of nitrogen,phosphorus and potassium in fruit were the highest in NPKBZn treatment, where they were 24.41%,22.59% and 13.70% higher compared to NPK treatment respectively, and potassium utilization efficiencyincreased by 45.53% compared with the treatment with only potassium fertilizer applied.【Conclusion】From the above analysis, potassium, boron and zinc have positive effects on yield, quality and nutrition ab-sorption in watermelon grown in gravel- mulched fields, and they have different roles in watermelongrowth. The combination of NPK fertilizers significantly increased photosynthetic rate, yield, quality andnutrient accumulation in watermelon compared with NP treatment. Boron was beneficial to vitamin C accu-mulation and nitrogen absorption; zinc is helpful for sugar accumulation and potassium absorption. Thecombination of potassium, boron and zinc fertilizers could further promote fruit growth yield, quality andnutrient absorption in watermelon.