Wireless underground sensor networks (WUSNs) are becoming ubiquitous in many areas and designing robust systems requires extensive understanding of the underground (UG) channel characteristics. In this paper, UG channel impulse response is modeled and validated via extensive experiments in indoor and field testbed settings. Three distinct types of soils are selected with sand and clay contents ranging from 13% to 86% and 3% to 32%, respectively. Impacts of changes in soil texture and soil moisture are investigated with more than 1,200 measurements in a novel UG testbed that allows flexibility in soil moisture control. Time domain characteristics of channel such as RMS delay spread, coherence bandwidth, and multipath power gain are analyzed. The analysis of the power delay profile validates the three main components of the UG channel: direct, reflected, and lateral waves. It is shown that RMS delay spread follows a log-normal distribution. The coherence bandwidth ranges between 650 kHz and 1.15MHz for soil paths of up to 1m and decreases to 418 kHz for distances above 10m. Soil moisture is shown to affect RMS delay spread non-linearly, which provides opportunities for soil moisture-based dynamic adaptation techniques. The model and analysis paves the way for tailored solutions for data harvesting, UG sub-carrier communication, and UG beamforming.