Effects of lime and chicken manure on nutrient cycling, soil properties and plant response in acid South African ultisols

Abstract

In South Africa's Eastern Cape Province, little information is available from which the influence of soil acidity and liming on organic matter decomposition, nutrients cycling and plant growth responses can be predicted. Accordingly, the laboratory studies described herein were conducted to obtain information on the effects of soil liming on amounts of carbon (C) and nitrogen (N) released during the decomposition of chicken manure applied to soils. Additionally, liming, organic and/or inorganic N fertilizers were compared for their effects on maize growth, N uptake, added N recovery and selected soil properties. The two soils used in these studies were collected from two sites (Magusheni and Nikwe) in the Eastern Cape Province. Both soils are classified as Humic Hapludults. A comparison of chemical properties of virgin topsoil (0-15 cm) indicated that the soils differ in fertility and potential productivity, with the experimental soil from Magusheni site being the poorer of the two. The Magusheni soil had a pH(KCI) of 4.78, exchangeable (Al + H) acidity was 1.40 cmolc kg-1 and acid saturation was 24.7%. The percentage Ca, Mg and K saturation levels were 30.1, 41.4 and 3.9 %, respectively. In comparison, the Nikwe soil had a pH(KCI) of 5.06, exchangeable (Al + H) acidity was 0.38 cmolc kg-1 and acid saturation was 4.3%. The percentage Ca, Mg and K saturation levels were 46.6, 34.0 and 15.0 %, respectively. Results of the 56-day soil laboratory incubation experiment (chapter 1) indicated that the effects of liming on microbial activity and N mineralization varied between the soils. In the low acid saturated (Nikwe) soil, total CO2 evolution and N mineralization increased as the rate of chicken manure application increased. However, liming and the accompanying increase in soil pH (5.0 to 5.9) had no apparent effect on total CO2 evolution and N mineralization rates. The results indicated that microbial activity and N mineralization were limited by C and N supply and not by soil acidity in the Nikwe soil. The lack of a liming effect on microbial activity and N mineralization in the Nikwe soil can be attributed to the relatively high initial soil base status (95.7 %), high % Ca2+ saturation (47 %) and the low percent acid saturation (4.3 %). In the Nikwe soil, there would be no appreciable benefit on microbial activity nor N mineralization by liming soil when the base saturation is high and the soil acid saturation percentage is low. In the high acid saturated (Magusheni) soil, total CO2 evolution increased as both lime and chicken manure application rates increased. However, among all chicken manure rates, the total amount of CO2 evolved was lower in the unlimed than in the lime amended treatments. The results indicate that N mineralization decreased as lime application rate and soil microbial activity increased. The decrease in the accumulation of inorganic N in lime amended treatments as soil respiration rates increases, suggest a rapid immobilization of mineralized N by an active microbial population. Therefore, liming is essential to supply Ca2+ and correct the acidity in the Magusheni soil. However, addition of large amounts of lime, especially when combined with broiler chicken manure, are expected to stimulate greater microbial activity and are likely to cause an initial N immobilization or a longer lag period in N mineralization. The long-term effect under field conditions, however, is expected to be a net increase in N availability. The results of this study do show that the short-term effects of lime on microbial activity and N mineralization varies among soils and highlight the influence of chemical soil properties other than soil pH on microbial activity and N mineralization in organic waste amended soils. The findings presented in chapter 1 agreed with the results obtained in the greenhouse pot experiment reported in chapter 2. Results of another greenhouse pot experiment (chapter 3) also showed that the effect of lime x N fertilizer interaction on maize plant height, biomass production, total N uptake and N concentration varied between the Magusheni and Nikwe soils. In the Magusheni soil, the lime x N fertilizer interaction was positive in that lime enhanced plant growth responses to N application. Based on soil analysis results, responses in plant height, aboveground biomass production, and total N uptake to N application were lower in the unlimed than in limed soil due to the higher levels of soil acidity in the former soil. In the unlimed soil, the percent soil acid saturation was 47%, which is more than double the critical 20% permissible acid saturation level for a maize crop. The percent base saturation and Ca2+ saturation were also much lower than in limed soil. In the limed soil, the percent base saturation, Ca2+ saturation, and soil acid saturation were 91%, 50%, and 9.3%, respectively. These greatly improved soil conditions may have contributed to the positive plant responses to N application in the limed soil. Therefore, soil acidification must been corrected in order to optimize maize response to applied N fertilizer in the Magusheni soil. In the Nikwe soil, the plant height, aboveground belowground biomass production and total N uptake was either similar or lower in limed than in the limed soils. Soil analysis results, indicated that chemical characteristics of the unlimed and limed soils were similar. In both the unlimed and the limed soils, the percent base saturation, Ca2+ saturation, and soil acid saturation were 95%, 50%, and 5.0%, respectively. Therefore, lime had little or no effect on soil chemical properties in the Nikwe soil. The results presented, suggest that as long as the soil base status remains high and acid saturation levels low, there would be little or no benefit to total N uptake, maize growth and development due to lime application in the Nikwe soil. However, the fact that large responses to applied N were obtained in the Nikwe and Magusheni soils confirms that N was the limiting nutrient in the control treatments.