Evaluation of lime in ameliorating soil acidity for improved yields of intercropped sugarcane and soybean in Western Kenya

A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY OF SOKOINE UNIVERSITY OF AGRICULTURE. MOROGORO, TANZANIA. 2016

Farmers in Western Kenya contribute significantly to the Kenyan economy through food and cash crop production such as sugarcane. However, the acidic nature of the soil reduces the yields due to soil fertility problems. Acidity is further accelerated by the long term sugarcane monoculture and long term use of acidifying fertilizers. Cropping systems such as intercropped sugarcane referred to crop diversification is currently advocated and practiced by smallholder farmers. The productivity of this system is under threat due to the soil fertility problems associated with acidity. Liming offers the opportunity to ameliorate soil acidity, increase nutrient availability, nutrients content and yields of intercropped sugarcane. However, liming costs are prohibitive due to the large quantities required, coupled with the lime broadcast method of application. Various lime placement methods, including lime rates and nitrogen rates under sugarcane monoculture and soybean intercropped were investigated. The overall objective was to determine the best lime placement method for ameliorating soil acidity, increase soil nutrient availability, nutrients content by sugarcane and yields. The findings are presented in three chapters. Chapter two findings were on the effects of cropping systems (intercropped sugarcane with soybean; monoculture sugarcane), lime placement methods [broadcasted, shallow banded (0 – 15 cm) and deep banded (15 – 30 cm)] and lime rates (0, 1 and 2 t ha-1) on soil pH, soil nutrient and sugarcane nutrient content. Lime rate at 2 t ha-1 significantly (P ≤ 0.05) increased soil pH from 5.5 to 6.4 in water. Increased lime rate led to decreased levels of manganese from 203 to 172 mg kg-1 and iron from 147 to 129 mg kg-1. Lime deep banded to 15 – 30 cm soil depth increased soil pH to the highest level of 6.20 and highest available phosphorus to 23.91 mg kg-1. Intercropping system (IC) led to a lower soil pH level (5.94) but higher soil organic carbon (1.41 %) as compared to monoculture system which led to a higher soil pH (6.20) and lower soil organic carbon (1.21 %). Intercropped sugarcane (IC) led to higher content of sugarcane leaf calcium (0.44 %) and Mn (76.96 mg kg-1) compared to sugarcane monoculture which recorded 0.38 % sugarcane leaf Ca and 64.93 mg kg-1 sugarcane leaf Mn. Broadcasted lime led to high content of sugarcane leaf nitrogen (0.47 %) and phosphorus (0.08 %). Lime shallow banded led to highest content of sugarcane leaf Ca (0.44 %) and Zinc (17.93 mg kg-1). To reduce soil acidity for acidic Cambisols of Kibos in Kisumu, lime rate at 2 t ha-1 is recommended. Also lime broadcasting (L-BC) is preferred to ameliorate acidity at 0 – 15 cm depth while banded lime shallow and banded lime deep are preferred to reduce sub soil acidity. It was found that that, use of lime placement methods depends on the soil acidity stratification with depth which therefore needs further investigation. Chapter three presents results on the effects of cropping systems, lime placement methods and lime rates on sugarcane yields and quality. Intercropped sugarcane led to higher sugarcane yields (136 TCH) than the monoculture sugarcane (133 TCH). No significant effect was observed for ratoon crop harvest. Shallow banded lime gave the highest sugarcane quality of 15.09 pol % cane and 13.83 commercial cane sugar (CCS) while lime broadcasted gave the least at 14.59 pol % cane and 13.29 CCS. There was significant (P ≤ 0.05) reduction of yield and quality (pol % and brix % juice) from plant crop to ratoon crop cycle both under sugarcane monoculture and also intercropped sugarcane. Under monoculture, sugarcane yield was reduced from 133 TCH in plant crop to 116.6 TCH in ratoon crop. Pol % juice was reduced from 19.47 % in plant crop harvest to 17.57 % in ratoon one harvest. It was found that, liming plays a limited role on the direct effect on sugarcane yield, but a significant and direct role on amelioration of acidity and nutrient transformations. Liming should therefore be integrated with other cropping and nutrient management strategies for increased yields. Results in chapter four were on the effects of lime placement methods, lime rates and nitrogen rates (0, 50 and 100 kg N ha-1) on soil pH, soil nutrient availability, leaf nutrient content, yield and quality of sugarcane. Lime rates significantly affected soil pH at both 0 – 15 cm and 15 – 30 cm depths. Lime rate at 2 t ha-1 led to the highest soil pH of 6.34 and 5.23 for 0 – 15 cm soil depth and 15 – 30 cm soil depth respectively. Lime placement methods (LPM) and nitrogen rates (NR) did not affect soil acidity. Some soil chemical properties, specifically, soil available Ca, Mn and Zn and also soil organic carbon were significantly (P ≤ 0.05) affected by the lime rates but not the lime placement methods. Lime rate at 2 t ha-1 led to highest soil available Ca at 23.74 cmol (+) kg-1 but least soil extractable Mn (178 mg kg-1), Zn (1.44 mg kg-1) and soil OC (1.30 %). Lime placement methods affected content of sugarcane leaf K, Ca, Mn and Zn. Lime shallow banded led to the highest sugarcane leaf K (0.57 %), leaf Ca (0.68 %), leaf Mn (60 mg kg-1) and leaf Zn (18.4 mg kg-1). Lime rate at 2 t ha-1 using the broadcasting placement method or lime shallow banding (0 – 15 cm depth) is recommended for Cambisols of Kibos, Kisumu County, Kenya. It is further recommended that routine soil and plant analysis be carried out for judicious soil and crop management.