Nitrification suppression and provitamin A content of spinach

Abstract

Bloomsdale Longstanding' spinach was grown in the field and in growth chambers to determine the effect of soil NH4 and soil NO3 on spinach yield, green color, leaf NO3, and provitamin A (carotene) content. Nitrapyrin (Np) was adsorbed onto (NH4)2SO4 crystals and applied as a band in the field and blended with a soil-perlite mix in the growth chambers. Field treatments received (NH4)2SO4 fertilizer at 90 kg N/ha and differed only by the addition of 140 g/ha (0.0625 ppm soil basis or 0.156% based on N) Np to one treatment. Significant increases in yield, green color, and carotene content were noted for the Np treatment in the field and attributed to increased soil N retention. A green color difference between treatments with and without Np in the field, as measured by a reflectance meter, of 0.05 relative units correspond with a 15% increase in carotene content for the Np treatment. Carotene yield (kg carotene/ha) of the Np treatment was 43% greater than from the same treatment without Np. Growth chamber treatments varied soil NH4 -N and NO3 -N concentration while maintaining equal fertilizer N applications to expose spinach to a range of NH4/NO3 ratios in a nonleaching system. Fertilizers for the four growth chamber treatments supplied equal amounts of N (400 mg N/2 kg soil mix) for each and were (NH4)2SO4, protected with Np and unprotected, Ca(NO3)2, and a mix of protected (NH4)2SO4) and Ca(NO3)2 to provide a 0.33 NH4 -N/NO3 -N ratio. Np rate was 3 mg/2 kg soil mix or 0.75% based on N. Ammonium toxicity symptoms and greenest co lor were noted for spinach grown with essentially all NH4 -N. Spinach receiving (NH4)2SO4 as sole N source with or without Np, produced statistically equal as well as the greatest amount of carotene. A 48% increase in carotene yield of spinach resulted when NH4 was the available source of N compared to NO3. Large differences in leaf carotene correspond to small differences in leaf color in spinach. Proper manipulation of the soil NH4/NO3 ratio by means of nitrification suppression can increase spinach yield and provitamin A content as well as improve green color with lower leaf nitrate thus bettering consumer acceptability.