The role of phosphorus in growing tomatoes in near water-saturated soil

Abstract

High-yielding, waterlogged cultivations with considerable nitrogen input are widely practiced in Kyushu Island, Japan. Our objective was to determine the role of available phosphorus in relation to nitrogen and soil microorganisms on tomato growth in such systems. Tomato seedlings were grown in a pasteurized soil with ample KNO3 in addition to different amounts of sodium or potassium phosphates to assess effects of phosphorus on growth enhancement and water-use efficiency in the seedlings grown with copious soil water (>－0.7 kPa). Both monobasic and dibasic phosphates applied at 40 mM to near-saturated soil markedly promoted seedling growth when fertilized with 120 mM KNO3 or NH4Cl, but not in the soil without nitrogen. The final concentration of NO3－ near the soil surface was maximized in the nitrated soil with no added PO43－ but declined significantly when fertilized with any phosphates except Na2HPO4. No significant accumulation of NO3－ was detected in the water-saturated bottom soils regardless of soil fertilization with any plant minerals. Increased seedling growth was accompanied by lower dry root/shoot ratios and marked increases in evapotranspiration efficiency. Such positive effects of PO43－ were greater with NO3－ than NH4＋ but diminished with increased concentrations of Na＋ up to 160 mM. Nonetheless, growth promotions by ample NO3－ with PO43－ were nearly negated in the non-pasteurized soil. Apparently, the activities of indigenous soil microbes were more significant than soil nitrogen and phosphorus in limiting the growth potential of tomato plants in near-saturated soils.