Calibration accuracy of requirement factor and sorption studies for fertilizer recommendation

Abstract

The inconsistent and incoherent approaches by fertilizer recommendations to index crop response has prompted the search for alternative approaches. Some of the problems stem from the overlooking of fundamental soil properties that govern the soil solution, which is where plant roots absorb nutrients for growth. A comparison was made between two contrasting equilibration techniques to evaluate their precision in estimating crop response. Sorption isotherms for phosphorus (P) and potassium (K) were compared to requirement factors. Phosphorus sorption isotherms were determined following the batch equilibration technique. Potassium was developed following equilibration with graded K levels. The requirement factors of both P and K were determined following a 6‐week incubation with four different levels of fertilization. Cowpea (Vigna unguiculata), mustard (Brassica juncea), and maize (Zea mays) were used as test crops. The growth parameters measured included biomass (g), height (cm), and leaf area index. At harvest, yield (g pot−1) and uptake (mg pot−1) were also recorded. Linear correlation studies were carried out to evaluate the association between treatments and the growth parameters of the tested crops. Results showed no significant difference (p < 0.05) in maize growth parameters between the equilibration methods, despite the sorption isotherms estimating higher levels of P and K. The sorption isotherms for P and K were 1.7 and 9.8 times higher than their respective requirement factors. The crop response, although relatively similar in both methods, was weakly correlated with the sorption‐estimated nutrient levels, indicating an overestimation of nutrients. Therefore, the requirement factors were deemed to be a more precise equilibration technique for estimating nutrient levels.