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Is pigeon pea a good option for cover cropping in a humid climate?

Answer: Pigeon pea (Cajanus cajan) is an upright trifoliate herb that, if allowed to, can become a small tree in some varieties. It is a weak perennial that is most often used as an annual but can live three to five years with the most productive seed bearing occurring in the first two years. There are several wild relatives of pigeon pea on the Indian Sub-Continent, supporting the idea of it being domesticated in that region ~3,500 years ago. Pigeon peas are a well-known food crop eaten throughout the tropical world and are being researched for their antimalarial properties. They have been traditionally used for that purpose in Ghana (Merel et al., 2004; Duker-Eshun et al., 2004).

Pigeon peas are drought-tolerant and highly adaptable. They even perform well in nutrient-depleted soils. This crop has been known to produce a harvest even after others have wilted and withered away. Pigeon peas account for about 5% of total world legume production (Odeny, 2007). As a cover crop, they have been shown to increase corn yields by almost 33% in Togolese production systems, which average around 90° high temperatures throughout the year (Sogbedji et al., 2006). Pigeon peas incorporate phosphorous from the soil very efficiently from orthophosphates and can even pull phosphorous from soils with aluminum. Its upright habit lends itself to being used as windbreaks in adjacent plots. For instance, in a 1-acre field, the center third of the field can mowed and incorporated into the soil, while leaving the edges intact. This will allow the outside to act as a windbreak or a sort of nurse crop for the center. Later, the sides can be mowed, incorporated and then replanted with either a cash crop or more cover crops (Valenzuela, 2011).

Pigeon peas are good nitrogen fixers that supply quite a bit of that nitrate to the subsequent crops through their residue. Most legumes fix nitrogen, but some impart it to other crops better than others. Tree legumes like Leucaena, Calliandra, and Acacia can, in some circumstances, only convert less than 5% of their nitrates into mineralized forms that other plants can use. Pigeon peas, on the other hand, make about 20% of their total nitrogen available to the next crop. These numbers seem to be environmentally dependent, however, because in other circumstances, Leucaena contributed 12% to 28% of its nitrates to subsequent crops (Palm, 1995).

There are other cover crops suitable for humid climates, and you can learn about them in the ATTRA publication Cover Crops for Hot and Humid Areas. It discusses the characteristics of cover crops that are better suited for areas with hot, humid summers, like the southern portions of Texas and Florida and along the Gulf Coast, the Caribbean, Hawaii, and points beyond with similar climatic conditions. It includes a table that will allow you to make the best decision for your situation about which cover crops may suit your individual needs. It also includes a general inoculant guide for legume crops.

The National Center for Appropriate Technology, ATTRA’s parent organization, is conducting the Subtropical Soil Health Initiative, a Conservation Innovation Grant project in the Lower Rio Grande. The project is a collaboration with a University of Texas Rio Grande Valley team that is testing different cover crops in the field to determine the best species for farmers in the hot, humid Lower Rio Grande Valley. Sunn Hemp appears to be the best option to date.

Be sure to check out these related ATTRA resources:

Cover Crops podcast

Using Cover Crops in an Organic Walnut Orchard podcast

Overview of Cover Crops and Green Manures publication

Sunn Hemp (Crotalaria juncea) video

References:

Duker-Eshun, G., J.W. Jaroszewski, W.A. Asomaning, F. Oppong-Boachie, and S.B. Christensen. 2004. Antiplasmodial constituents of Cajanus cajan. Phytotherapy Research. February. p. 128–130.

Merel, H., A. Akoègninou, and J. van der Maesen. 2004. Medicinal plants used to treat malaria in Southern Benin. Economic Botany. December. p. S239-S252.

Odeny, D.A. 2007. Th e potential of pigeonpea (Cajanus cajan (L.) Millsp.) in Africa. Natural Resources Forum. Vol. 31. p. 297–305.

Palm, C.A. 1995. Contributions of agroforestry trees to nutrient requirements of intercropped plants. Agroforestry Systems. Vol. 30. p. 105-124.

Sogbedji, J., H.M. van Es, and K.L. Agbeko. 2006. Cover cropping and nutrient management strategies for maize production in Western Africa. Agronomy Journal. Vol. 98, No. 4. June. p. 883-889.

Valenzuela, H., and J. Smith. 2002. Lablab. Sustainable Agriculture Green Manure Crops. SA GM-7. CTAHR, Manoa, Hawaii.