The optimum combination of controlled-release potassium chloride and irrigation for efficient greenhouse tomato production

Objectives Fertilizer and water are the two important inputs for efficient crop production. Excessive fertilization is the main cause of soil salinization under facility cultivation. Here, we studied the potential of reducing potassium input using controlled-release potassium chloride fertilizer (CR-KCl) and a matching irrigation level.

Methods A pot experiment was conducted in a greenhouse, using tomato cultivar ‘Luo la’ as test material. The two controls were no K application and conventional KCl (K₂O 0.95 g/kg) application under 90%–100% field capacity irrigation (CK, CF). The nine fertilizer and water combination treatments comprised three CR-KCl dosages 100% (K₁₀₀), 80% (K₈₀), and 60% (K₆₀) of CF) and three irrigation levels 90%–100% (W₁₀₀), 72%–80% (W₈₀), and 54%–60% (W₆₀) of the field capacity. Leaf photosynthetic efficiency, soil pH, and soil EC were measured during the main growth stages of tomato. Also, the tomato yield and quality were investigated.

Results The average fruit yield and water efficiency of tomato were K₈₀>K₁₀₀>K₆₀. Compared with K₁₀₀ and K₆₀, K₈₀ (P<0.05) increased tomato K uptake (plant and fruit) by 3.4%–7.8%, Vc by 3.4%–3.9%, soluble sugar content by 2.2%–6.5%, soluble solid by 3.5%–7.4%, enhanced the net photosynthetic rate by 4.3%–10.9%, stomatal conductance by 6.5%–11.9%, and transpiration rate by 3.6%–9.5%. Compared with W₁₀₀ and W₆₀, W₈₀ (P<0.05) increased tomato yield by 9.8%–16.0%, K utilization efficiency by 6.3–7.1 percentage points, Vc by 6.8%–24.0%, soluble sugar by 5.6%–8.8%, soluble solid by 6.6%–9.2%, net photosynthetic rate by 4.1%–10.3%, transpiration rate by 8.4%–16.6%, SPAD value at flowering, fruit enlargement, and ripening stage by 4.8%–12.0%, 1.7%–9.4% and 4.6%–14.5%, soil pH value by 0.9%–2.1%, and EC value decreased by 4.9%–8.2%. Among the 9 CR-KCl and irrigation combinations, except for K₆₀W₁₀₀ and K₆₀W₆₀, tomato fruit yield and water use efficiency were (P<0.05) higher in other treatments than CK and CF, K₈₀W₈₀ achieved the highest yield and quality, and K and water use efficiencies were also high.

Conclusions CR-KCl and irrigation affected tomato yield, quality, and photosynthetic efficiency. The optimum combination of CR-KCl and irrigation was 20% less K₂O dosage in the one-time basal application and irrigation level of 72%–80% field water capacity (W₈₀). This combination had the highest fruit yield and quality, improved soil pH, reduced EC value, and had high fertilizer and water use efficiency.