Availability of Potassium, Calcium, Magnesium, and Sodium in “Bulk” and “Rhizosphere” Soil of Field‐Grown Corn Determined by Electro‐ultrafiltration

Abstract

The available levels of K+, Ca2+, Mg2+, and Na+ in bulk and rhizosphere soil of corn (Zea mays var. Forban) were evaluated by the electro‐ultrafiltration (EUF) technique. The nutrients extracted at 20°C and 200 V (EUF‐I = easily available fraction), at 80°C and 200 V (EUF‐II = available reserve), and at 80°C and 400 V (EUF‐III = retained reserve) were determined. The field experiment was carried out with four treatments, N + P + K (F1), N + P (F2), N + K (F3), and N (F4) in a completely randomized experimental design. Results revealed that the pH of rhizosphere soil was slightly lower than that of bulk soil with significant differences at 40 days after sowing (DAS). Organic carbon levels were higher in rhizosphere soil with differences that tend to increase with crop age. The EUF‐I‐K+ was significantly (P < 0.001) higher in rhizosphere soil at 40 DAS. Differences were significant (P < 0.05) for all the EUF fractions also at 60 and 120 DAS. The EUF‐II‐K+ and the EUF‐III‐K+ pools were high and just slightly lower than the EUF‐I fraction (128–153 vs. 143–201 mg kg−1), respectively. The EUF‐Tot‐K+ (EUF‐I + EUF‐II + EUF‐III) of rhizosphere soil surpassed the optimum threshold of 150 mg kg−1. Starting from 60 DAS all the EUF‐Ca2+ fractions were significantly (P < 0.001) lower in rhizosphere soil. The EUF‐II‐Ca2+ was about three‐fold lower than the EUF‐I‐Ca2+, 140 mg vs. 380 kg−1, whereas the EUF‐III‐Ca2+ was almost the same order of magnitude as EUF‐I‐Ca2+ (300 vs. 350 mg kg−1). The EUF‐Tot‐Mg2+ was quite similar in bulk and rhizosphere soil and unlike what was found for K+ and Ca2+. Likewise Ca2+ and beginning from 60 DAS even the levels of EUF‐Mg2+ of rhizosphere soil were lower than those of bulk soil. The EUF‐Tot‐Na+ varied between 120–300 and 130–350 mg kg−1 in bulk and rhizosphere soil, respectively. The EUF‐II‐Na+ and EUF‐III‐Na+ were lower than the EUF‐I‐Na+. The EUF‐I‐Na+ of rhizosphere soil was always lower than that of bulk soil with a peak of −28.3% at 60 DAS and during F1 treatment (N). No significant differences (P < 0.001) were found among EUF‐Na+ fractions with respect to the composition of the fertilizing supply.