2017 ASHS Annual Conference

Growth of several crops in south Florida has benefited from the use of sun hemp (Crotalaria juncea L.) as a cover crop, which is grown for about 3 months (generally to a height of 2.5-3 meters) and then mowed and incorporated into the soil prior to crop planting. When sunn hemp reaches that height, stems are heavily lignified and decomposition in the soil is slow. Thus, it may be beneficial to mow and incorporate sunn hemp into the soil at an earlier growth stage. An experiment was conducted to investigate physiology and growth of ‘Red Lady’ papaya (Carica papaya L.) with ‘Tropic Sun’ sunn hemp incorporated into the soil prior to planting papaya. Sunn hemp was planted in the field in Krome very gravelly loam soil at one time and mowed and incorporated into the soil at 3 different times, resulting in 1-, 2- or 3-month-old sunn hemp treatments. A fourth treatment had no sunn hemp. Papaya were planted in 56.7-L plastic pots containing soil from one of the four sunn hemp treatments and placed in the field. Each sunn hemp treatment was subdivided into two nitrogen treatments: standard nitrogen (typical grower application rate starting at 3.1 g plant^-1) and low nitrogen (starting at 1.6 g plant^-1) applied to the soil at 2-week intervals. Thus, there were 4 sunn hemp treatments and 2 nitrogen treatments for a total of 8 treatments with 5 replications per treatment. The amount of inorganic nitrogen added to the soil of each nitrogen treatment was increased over time to account for plant growth. Soil nitrogen and organic matter contents, and papaya leaf gas exchange (net photosynthesis, stomatal conductance, transpiration), plant height and stem diameter were assessed at six-week intervals for plants in each treatment. There was no significant interaction (P>0.05) between sunn hemp and nitrogen treatments for any other the variables measured. There was no significant effect of sunn hemp treatment on net photosynthesis, but stomatal conductance and transpiration were higher for plants in the 3-month-old treatment than the other sunn hemp treatments on one measurement date. There were no significant differences (P>0.05) in plant height or stem diameter among sunn treatments. Data are still being collected from this study, but the results to date indicate that incorporation of 1- 2- or 3-month-old sunn hemp into a gravelly loam soil in southern Florida does not increase leaf gas exchange or growth of papaya.