Effects of irrigation, Nitrogen Fertilization, Plant Population and Variety on the Physico-Chemical Properties of Maryland Tobacco: I. Agronomic Effects

Irrigation experiments on Maryland tobacco have been conducted since 1955 on Monmouth fine sandy loam, a typical tobacco soil of Southern Maryland. An overhead sprinkler system was employed. Moisture levels in the soil were studied, using Bouyoucous gypsum blocks and tensiometers. The following conclusions were made from this study:

1. In a year of severe drought, 1966, ample irrigation increased yields, unit value and total value. Irrigated yields were 175 per cent of non-irrigated; unit value was 141 per cent, and total value of irrigated tobacco was 247 per cent of non-irrigated.

2. Nitrogen fertilizer rates of 100.9, 134.5 and 168.1 kg ha were used. Under irrigation, gains in yield and total value, and some loss in unit value were obtained from the first added increment of fertilizer nitrogen above the base amount. Addition of the second increment produced smaller gains in yield, a greater loss in unit value, and losses in total value.

3. In the absence of irrigation, the supply of soil water could limit nitrogen assimilation by the plants. The higher rates of nitrogen fertilization were effective in increasing yield and total value over the lower increments. Even unit value was not adversely affected.

4. The Wilson broadleaf variety of Maryland tobacco consistently outyielded the Catterton medium broadleaf variety, but due to lower unit value, probably caused by darker colours, it had a lower total value.

5. Plant populations of about 18000 per hectare produced increased yields and gross return over a population of about 12000. Populations of 24000 plants per hectare were excessive in most cases, and produced losses in yield and unit value.

6. Comparative response to irrigation depends on severity of drought, but late depletion of soil moisture levels may prevent the full potential development of the crop, and result in lower yields and total value.