Center for Agronomic Research, University of Costa Rica, San Pedro, Costa Rica and Agronomy Department, University of Wisconsin-Madison, 1575 Linden Drive, Madison, WI 53706, USA

Background and objectives
Mulch-based agriculture has been in existence for centuries in tropical areas, especially where high rainfall makes the burning of vegetation as practiced in slash-and-burn difficult. Although in the past the system has been used for many crops, from bananas to rice, a mulch-based system (MBS) called slash-mulch is an option still chosen by farmers in the neotropics for the production of dry beans, corn and sorghum. The key component of this method is a mulch of second-growth vegetation grown in a fallow of 9 months to 4 years.

The objective of this 12-year study is to compare agro-ecological parameters (production, nodulation, mycorrhizae, diseases) of this traditional, low-input MBS with several other systems of bean (Phaseolus vulgaris) production: (i) fertilized MBS, 10N-30P-10K; (ii) alley-cropped MBS (four species of N-fixing trees, singly and in combination); and (iii) a newly-introduced, high-input soil-based system (SBS) without mulch.

Results and discussion
The decomposing mulch of the MBS provides nutrients for the beans, and where fertilizers are not applied it yields significantly more than the SBS. Of N, P and K, the MBS is P-limited [1]. Where fertilizers were applied, 14 kg elemental P/ha applied to the MBS was more nutrient efficient than 28 and 43 kg applied to the MBS, or than any level of fertilizer applied to the SBS in long-term experiments. Over 6 years, yields of the MBS at 28 kg P/ha were approximately double that of the SBS at 43 kg [2].

Mulch affected bean symbioses: nodulation was greater under the MBS in dry years and although percentage mycorrhizal fungus colonization was not affected by the system, colonization decreased more in the fertilized MBS than SBS. The mulch also prevented the sprouting of weed seeds. In alley-cropped plots, specific trees changed both the biomass and composition of weed species.

Under humid tropical conditions, beans are severely affected by the foliar diseases Thanatephorus cucumeris (sexual stage of Rhizoctonia solani) and Colletotrichum lindemuthianum, and farmers have developed methods of prevention: mulch and planting on east-facing slopes. In wet years, less foliar disease was found in the MBS than SBS. Root-knot nematode (Meloidogyne sp.) infection was significantly less in the MBS than SBS, and longer fallow reduced the nematode gall count.

Major advantages of the mulch in addition to disease prevention are: nutrients from mulch decomposition, efficiency of applied fertilizers, weed prevention and enhanced symbioses under some conditions, as well as increased soil organic matter, better soil aggregation and erosion control.

1. Rosemeyer ME, Gliessman SR, 1992. Agriculture, Ecosystems and Environment 40, 61-70.
2. Rosemeyer ME, 1995. CIAT International Workshop on Low Phosphorus in the Common Bean, San Jose, Costa Rica, pp. 157-163.