Increasing spinach yield and water productivity with cow manure in deficit irrigation: investigating soil-plant health links

This study evaluated the combined effects of manure application and deficit irrigation on soil health, spinach plant health, and water productivity. A two-year field experiment compared cow manure (FM) and mineral (M) fertilization at three irrigation levels (W1, full irrigation; W2, 85% of W1; and W3, 70% of W1), with three replicates. Cow manure was applied to raise topsoil organic matter to 3%. Manure application improved soil aggregation and water retention and enhanced the macro- and micronutrient status of both the soil and the plants. Deficit irrigation, however, reduced spinach growth and yield by impairing plant nutrition and physiological responses. Principal component analysis of all soil properties and yield indicated that improved soil aggregation, higher nitrate concentrations, and greater water retention were positively associated with spinach yield, whereas high bulk density, calcium carbonate content, and pH were negatively associated with yield. Principal component analysis of all plant properties showed that yield was closely related to leaf water content and stomatal conductance, but only weakly correlated with macronutrient and micronutrient contents, except for calcium. The membrane damage index was the strongest indicator of plant water stress and the associated yield reduction. Combining deficit irrigation with manure application improved physiological responses and increased both spinach yield and water productivity. Cow manure had a particularly pronounced effect, increasing yield by 33.6% under moderate deficit irrigation and by 103.6% under severe deficit irrigation. The FM-W1 treatment achieved the highest water productivity, at 72.8 kg m⁻³. Relative to the mineral-fertilizer treatments, water productivity increased by 27.6% under FM-W1, 35.3% under FM-W2, and 106.3% under FM-W3. In conclusion, full irrigation is preferable when cow manure replaces mineral fertilizer because it maximizes both water and crop productivity. In regions with limited water resources, the FM-W3 treatment is preferable because it maximizes water productivity relative to mineral fertilization.