Seasonality in the Trans-Atlantic Slave Trade

Rainfall, crop type and agricultural calendars

Rainfall, temperature, sunlight, humidity and soil type determines crop choice and regulates agricultural calendars. Maximizing plants’ nutritional content requires precise growing cycles under optimal ecological conditions. For most subsistence and cash crops, farmers plant just before or during rainy months and harvest during dry, sunny months. Crops needing long periods of sunshine to maximize yield include grains, cereals, and cane starches. Intense ultraviolet light damages coffee and other berries. Yams and other tubers are long-growing tropical foods, requiring 8-10 months underground. Except for rice, most crops do not grow well in rainforests, because downpours leach soils of nutrients and roots cannot tolerate waterlogging. Cold winters in the continental climates of the New World kill sugar, coffee, cacao, and cotton plants. Rainfall loosens soils to facilitate digging and sowing, and all seeds and cuttings require water to propagate; as sunlight and warm, dry weather ripen plants, caloric content increases. In ecosystems that support short-growing plants and have two rainy seasons, farmers can produce two crops per year.

Millet, sorghum, rice, maize, yams, and cassava, principal African crops, grow in ecosystems that dictate agricultural calendars. Millet and sorghum are often the only food plants grown in the semi-arid and arid 10-15° N belt, three hundred miles inland from the African Atlantic coastline. Farmers plant these cereals during the first rains in June, which soften the rock-hard soils, and in early November, at the end of the rainy season when floodwaters begin to recede. The short-season crops flower in 90-180 days; harvests occur in September-December and February-May, depending on rainfall. The two cereals also thrive in the long dry winter seasons of the Congo savannah, and may have grown further west before being displaced by manioc. Rice is the staple from the Lower Gambia south to Sierra Leone and along the Windward Coast, rainy coastlines that allow rice to grow in its requisite water depth of 4-6 inches. It grows from June (rainy season) to November (onset of dry season). Maize, a New World crop imported in the 1600s, requires sufficiently long, dry, sunny periods, and thrived mainly in the central Gold Coast. South of 10° N one finds ideal conditions, as in much of Nigeria, for yam cultivation: 85° F temperatures, rainfall totalling 60 inches, a 2-3 month dry season, sufficient sunlight, and free-draining soils.

Sugar, tobacco, coffee, and rice were the major New World cash crops. In the tropical Americas, sugar (with its by-products rum and molasses) was the principal plantation commodity. Planting occurred during rainy months, June-October in most of the West Indies, and the cane grows over a 14-18 month period. Saccharine matter reaches its greatest content during the ripening period when stalks dry. In the West Indies, dry seasons usually occur from January to May, though there are microclimates in the larger and mountainous islands, such as Haiti (before 1804, French St. Domingue), Dominica, and Jamaica. The best ecosystems for tobacco were located in the Chesapeake Lowcountry and Bahia, where high summer humidity keeps growing leaves moist and drier fall air allows them to dry and be cut. In the late 1700s, coffee groves became important in well-draining, shaded mountain ecosystems, the six-month fruit cycles ending during dry-season berry picking. Wet rice proved profitable in humid, low-lying areas prone to flooding, as in the coastal Carolinas, Georgia, Surinam, and northeast Brazil. On South American rice fields, slaves cleared land during the August-November dry season, planted in winter rains, and harvested between March and May. The Carolina rice and indigo cycles began in February and ended in November. Though a crop associated strongly with plantation slavery, cotton did not dominate many areas until the 1800s, and comparatively few African-born slaves worked on cotton plantations.