A Thesis Submitted in Partial Fulfilment of the Requirements for the Degree of Doctor
of Philosophy in Life Sciences of the Nelson Mandela African Institution of Science and
Tobacco (Nicotiana tabacum L.) is globally known to be a primary source of nicotine
worldwide. This study was conducted to determine the effects of nicotine on the rhizosphere
and subsequent maize crop yield. Pot experiments were carried at the Nelson Mandela
African Institution of Science and Technology (NM-AIST), using levels of root and leaf
extracts drenching to maize seedlings. In the first year, the field experiment comprised of six
treatments (a) fertilized tobacco, (b) fertilized maize, (c) fertilized tobacco incorporated with
tobacco stalks, (d) unfertilized maize, (e) unfertilized tobacco and (f) fallow with a plot size
of 6 m x 6 m, 1.2 m from the ridge to ridge and 0.50 m from plant to plant. In the second
year, all plots with exception to fallow plots were planted with maize to observe the effects of
tobacco on soil nutrients, bacteria diversity and maize yield. Soil samples were taken to
measure nutrients, nicotine and study bacteria diversities. Results showed that fertilized
tobacco, released higher nicotine into the soil (10.27 mg ha
-1
) than unfertilized tobacco (3.07
mg ha
-1
). High levels of nicotine released in soils 7.59 mg kg
-1
were found at a depth of 30 -
50 cm and lowest level 5.50 mg kg
-1
at a depth of 0 - 10 cm. Maximum adsorbed and
desorbed nicotine were found to be 4.61 and 2.21 mg kg
-1
, respectively. Maize absorbed
nicotine but at a very low concentration (0.001%) in maize grain. Maize planted not after
tobacco had the highest grain yields (3.86 t ha
-1
), but maize planted as subsequent crop after
tobacco had the lowest grain yields (3.53 t ha
-1
). The low yields were due to the low
absorption of P and K nutrients following extreme uptake of these nutrients by the tobacco
plant. In tobacco rhizosphere, bacteria under Proteobacteria, influence solubilization of P, K,
S, Cu
2+
, Fe
2+
, Zn
2+
, Mn
2+
hence increased uptake of macronutrients and reduced their levels
in soils; and less uptake of micronutrients and increased their levels in soils. This study
recommends further studies to re-calibrate new recommendations for P and K on maize crop
planted after tobacco.