Summary Slash‐and‐burn farmers in northern Laos consider weeds, insufficient rainfall and rodent damage as the most important constraints to upland rice (Oryza sativa L.) production. Labour inputs of 140‐190 days ha‐1 for weed control result in low labour productivity. Average weed cover observed in rice fields was 5.6, 4.1, 2.1, 1.7, 0.7 and 0.7 cm m‐1 (transect) for Chromolaena odorata (L.) King and Robinson, Ageratum conyzoides L., Commelina spp., Lygodium flexuosum (L.) Sw., Panicum trichoides Sw. and Corchorus spp, respectively. Chromoiaena odorata, which was introduced in the 1930s, has become the main fallow species and is considered a desirable fallow plant by most farmers. Average fallow periods reported for the 1950s, 1970s and 1992 were 38, 20 and 5 years respectively. Reduced fallow periods in the last decades have re‐suited in a marked increase in weeding requirements. Above‐ground biomass for rice stem, herbaceous plants and trees after rice harvest was 168, 67 and 60gm‐2 in 1991 and 115, 43 and 24 gm‐2 in 1992. Weeding at 14‐day intervals did not increase rice grain yield.

Decline in soil fertility accelerated by shorter fallow periods was expected to be a major constraint in slash-andburn rice production systems in northern Laos. In this paper we describe relationships between fallow period, soil fertility parameters, weeds and rice yield. Soil infertility is not perceived a major yield constraint by the farmers. Of the various soil parameters observed only soil organic matter showed consistent association with rice yield (r =0.42, p<0.01). Fallow period and rice yield showed no association and the relationship between fallow and organic matter was very weak (r=-0.16, p<0.01). Rice yield was negatively related to densities of Ageratum conyzoides and Lygodium flexuosum. Soil loss during the cropping period ranged from 300-29.300 kg ha -l . For the same period organic matter, total N, available P and available K content in the top 0-3 cm decreased by 11,12,17, and 17%, respectively, and loss of total N for the soil depth of 0-25 cm was estimated at 400 kg ha-I. Soil physical properties, moisture stress and available N are the most likely detriments to rice yields. Further attempts to relate soil properties to rice yield should include repeated measurements during the cropping season and observations on soil physical properties.

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Crop rotation of soybean [Glycine max (L.) Merr.] with grain sorghum [Sorghum bicolor (L.) Moench], and application of N fertilizer or manure generally increases grain sorghum yield. Little is known about rotation and fertilization effects on soybean yield in the Great Plains. Grain yields were measured from 1981 to 1987 in a cropping experiment started in 1980 on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic Typic Argiudoll). The cropping treatments included continuous soybean, continuous grain sorghum, and grain sorghum‐soybean rotation. Fertilizer treatments consisted of control, manure (15.8 Mg dry matter ha−1 yr−11) and N (45 kg ha−1 for soybean and 90 kg N ha−1 for sorghum). Volumetric soil water content was determined with a neutron probe in 1985, 1986, and 1987. Soil water content was unaffected by fertilizer treatment. Water content in the upper 30 cm was generally greatest with continuous grain sorghum and least with continuous soybean. Soil water depletion to 120 cm in September was 10 to 36 mm greater with soybean than with grain sorghum. Crop rotation increased soybean yield, but N application did not. Manure application reduced soybean yield in 1986, but had no effect in the other years. Rotation and fertilization increased sorghum grain yield. The soybean yield advantage from crop rotation decreased as 1 April to 31 May rainfall increased. Cropping‐system induced differences in soil water content early in the growing season may be partly responsible for higher soybean yield with crop rotation.

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Vesicular-arbuscular mycorrhizal fungi (VAMF) can reduce plant stress resulting from nutrient deficiencies, drought, and other factors. The objective of this work was to measure the effect of soybean \Glycine max (L.) Merr.] and grain sorghum [Sorghum bicolor (L.) Moench] rotation and fertilization on plant response and VAMF root colonization and diversity, and relate effects to soil environment. Fertilizer treatments consisted of no fertilizer, N, and manure. Rooting densities correlated with previous crop, VAMF colonization, and soil NO,. Root colonization by VAMF was affected by previous crop, rooting density, N fertilization, soil P, and water-filled pore space. Root colonization by VAMF ranged from 93% at 15 cm to 15% at the 120-cm soil depth. Root density and VAMF colonization were least when soybean was grown the previous year and manure was applied. Root colonization by VAMF for control, N, and manure treatments were 54, 53, and 30%, respectively, for continuous soybean and 61, 55, and 44%, respectively, for soybean from rotation plots. Root colonization by VAMF for control, N, and manure treatments were 69, 59, and 54%, respectively, for continuous grain sorghum and 56, 48, and 31%, respectively, for grain sorghum from rotation plots. These agricultural soils contained a diverse mixture of 26 VAMF species, which is probably a major factor in the region's soil productivity. Plants stressed due to cropping system or fertilizer practice have greater VAMF colonization and VAMF activity. A diverse VAMF population could increase the ability of VAMF to respond to different stresses. C OLONIZATION OF PLANT ROOTS by VAMF often reduces adverse effects of plant stresses (water, nutrient, and environmental factors) and has beneficial effects in maintaining crop yields. Grain sorghum and soybean are colonized by VAMF. Rotating crops and application of N fertilizer may affect VAMF colonization and final crop yield. Clegg (1982) and Roder et al. (1989a) showed that rotating these crops improved crop yield, compared with continuous cropping. They were unable, however, to explain the rotation effect based solely on N availability. Since VAMF colonize the root system and serve as a link between plant roots and soil, changes in cropping systems that affect the soil environment should affect both roots and VAMF. The rate and patterns of root growth vary with soil water, soil strength, bulk density, soil aeration, soil temperature, chemical and biological soil properties,