GREEN MANURE AS A MEANS TO SUSTAIN SUGARCANE PRODUCTION

Over 50% of the sugarcane in Thailand is produced in the Western belt. More than 50% of the soil in this area has a silty clay loam texture (Kamphangsean series : hydromorphic, noncalcic brown soil). As silt is the dominant particle (55%), Prammanee et al (1993) reported that when this soil is planted with cane for a long period of time it loses its structure, becoming compact. A sub-surface hard pan occurs making growth and development of cane roots difficult. The water- holding capacity of the soil is also decreased, causing increased runoff and erosion risk. Surface compaction of the soil makes it difficult for cane to emerge from the ground. In addition cane burning, the introduction of heavy harvesters and fully loaded cane trucks running over the land make soil conditions even worse. The end results are a reduced cane yield and increased land preparation costs.

To reduce further degradation of the soils in the western belt, applications of organic matter (OM) such as animal manure, compost and green manure is an option. For green manuring legume species are the best option. However, green manuring is not commonly practiced in Thailand, particularly in the western region. As there is a lack of information on the effects of green manuring on cane production, a series of experiments were carried out at Suphan Buri from 1991-94.

The objectives were as follows:

to introduce green manuring into the cane cropping system and hence make the Thai sugar industry more sustainable,
to evaluate the effects of a green manure on soil properties and cane yield improvement in the western area.
to determine the quantity on N released from decomposed green manure and to examine the efficiency of applying N fertilizers in combination with green manure.

Preliminary field study

. A preliminary field experiment was conducted in 1991. Two factors were combined in a randomized complete block design with 4 replications, i.e. with and without green manure in the presence and absence of chemical fertilizer, Sesbania (Sesbania rostrata) and sunnhemp (Crotalaria juncea) were sown together in green manure treatments. Green manure incorporation tends to increase soil OM leading to improved soil physical conditions as reflected by bulk density (Table 1). The improved soil physical properties resulted in increased cane yields although they were not statistically significant (Table 2)

Yield response to N application rates under green manure management

The results confirmed the findings obtained from the preliminary trial. Incorporation of sesbania or sunnhemp can increase cane yields by up to 20 t/ha (Table 3). Green manure increased cane yield by increasing the number of millable stalks. When green manure was incorporated cane produced more tillers due to better soil physical properties. Chemical fertilizer was necessary for cane even when green manuring was practiced. Incorporation of sunnhemp before cane planting increased cane yields by only ca. 8 t/ha over the control (without green manure). A yield increase of 30 t/ha was achieved when 114-76-76 kg/ha of N-P2O5-K2O were applied together with the sunnhemp.

Studies on the cane uptake of 15Nitrogen tracer originating from decomposed green manure

Where mung beans had been incorporated into the soil, the total N they contributed was 28 kg/ha as compared to 72 kg/ha when green manure (sesbania and sunnhemp) was used. The 15N analysis indicated that cane plants recovered ca. 15 kg N/ha from green manure compared to 7 kg N/ha from mung beans residues, with these quantities constitu- ting 17 and 8% respectively of the total N in cane. The supply of 72 kg N/ha by green manure as found in this study may enable a reduction in the application rate of fertilizer N normally recommended to farmers.

Microplot experiment

Recovery of N fertilizer one day after application was higher in the absence of green manure. Almost 21% of applied N had been transformed into nitrate and nitrite. The lower recovery of fertilizer N when ammonium sulfate was applied following incorporation of sunnhemp and sesbania may be due to temporary unavailability due to immobilization favored by the green manure crops. Another reason for the lower recovery of N upon green manure incorporation is probably due to the leaching of nitrate + nitrite below the 10 cm layer. At day 15 recovery of applied N was 56-59% ; there was no significant difference between treatments. After 45 days the recoveries of applied N were almost similar to those noted at day 15, indicating either that there was no N loss during this period or the loss was being compensated for by N released from decom- posed green manure. At day 60 the recovery of applied N was 50% and was again similar in the three treatments.

Use of green manure in the farmer's field

The results confirmed the previous findings that incorporation of green manure may increase cane and sugar yields. Incorporation of sword bean before cane planting showed the best results increasing cane yield by 12 t/ha over the control while sunnhemp or sorghum did not show any yield advantages over the control. In the plots where sunnhemp and sorghum were incorporated, there was poor drainage resulting in long periods of waterlogged conditions during heavy rainfall in August. These conditions limited cane growth and development.

Conclusions

By using green manure the soil physical conditions and the organic matter level were improved. Increases in cane yield were found when green manuring was practiced in combination with chemical fertili zation. 15N studies indicated that the green manure contributed ca. 15% of the N taken up by the sugarcane.

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