Assessment of potentially available organic nitrogen in some Ghanaian soils and its effect on the growth of maize

Nitrogen mineralized from soil organic matter can be an important source of N for crop uptake. The difficulty in predicting the contribution of mineralized-N continues to be a serious obstacle to implementing best practices for fertilizer N application. In Ghana the potential of soil N or organic manure sources to supply N to growing crops is routinely assessed by the total Kjeldahl N and total C by the Walkley and Black method. However, total N and C do not reflect the actual potential of a soil or manure to supply N to growing crops in the season because total N and C usually include recalcitrant N and C containing compounds resistant to mineralization to release N. This study was aimed at testing the effectiveness of a chemical extraction procedure, phosphate-borate buffer at pH 11.2 rapid steam distillation (RSD) for estimating potentially mineralizable (potentially available organic) N and available inorganic N against some conventional biological mineralization procedures such as aerobic incubation (AT), anaerobic incubation (AnI) and initial inorganic N by K2SO4 extraction. These methods were assessed using maize growth and N uptake at increasing levels of poultry manure-N in pot experiments in a greenhouse. Three benchmark soils - Kumasi series (Plinthi-Ferric Acrisol) from the forest zone, Nyankpala series (Ferric Luvisol) and Tingoli series (Plinthic Lixisol) both from the savanna zone were used for the study. The soils were sampled from a depth of 0-20 cm and analyzed for aerobic and anaerobic incubation N, RSD, K2S04 extraction and total Kjeldahl N. A comparison of the various methods for extracting available N, RSD, Aerobic incubation (Al), Anaerobic Incubation (AnI), K2S04 extraction of zero incubation samples and N uptake from 0 kg N/ha were made. The correlation analyses were used to compare how the extraction procedures relate. The methods were positively and highly significantly correlated with correlation coefficient values ranging from 0.9958 to 0.8485 at P<0.01. The ANOVA indicated that all the methods extracted significantly different amounts of N except RSD-N and AnI-N which were not significant in the Kumasi series, but were significant in the Tingoli series. The aerobic incubation extracted the highest available N and K2S04 the least. A linear regression model of RSD-N using various soils was significant (R2 of 0.7601 and P<0.05). Poultry manure rates of 0, 45, 90 and 135 kg N/ha (calculated based on the RSD extracted N) were applied to the soils and planted with maize to monitor the crop response to N levels in a greenhouse. The performance of maize dry matter accumulations at 20, 40 and 60 DAP was assessed. Quadratic regression models ‘of maize plant growth rate, girth diameter and leaf area in response to levels of poultry manure N were highly significant (with R2 values between 0.9992 and 0.8551 at P< 0.001). A graphical relation of maize plant dry matter weight, maize girth diameter maize leaf area and maize height with time (in weeks) showed a steady increase in growth at the different increasing poultry manure N levels. Cost analyses calculated on the basis of chemicals used, technician time and overhead cost gave total costing which decreased in the order of AI> AnI > TK > RSD. It was concluded that the different methods extracted different amounts of available N but were correlated significantly. RSD-N had a linear regression with total Kjeldahl N on a variety of selected soils. Linear regression equations, like y = 0.044x +15.552 with an R2 = 0.76 (in which, y = RSD-N and x = TK-N) could be used to quickly estimate total Kjeldahl in soil analytical service laboratories in Ghana. Maize growth increased with increasing levels of RSD extracted N. Maize on the Kumasi series was found to reach optimum growth at poultry manure N level of around 90 kg/ha, whereas maize growth on the Nyankpala and Tingoli series reached their optimum at around 135 kg N/ha. Results of this study show that RSD can be used to assess the ability of soils or organic manure to supply available N to growing crops and this may measure the mineralizable potential of a soil. However, further experiments will be needed to assess RSD in field conditions such as in different soil types, soil temperature and moisture levels. Thus, after further experimentation, RSD could be used as one of the factors for working out N indices.

Source: KNUST