Reintroduction of a native Glomus to a tropical Ultisol promoted grain yield in maize after fallow and restored the density of arbuscular mycorrhizal fungal spores

Maize (Zea mays L.) is an important crop in central Thailand where fallow is widely practiced and farmers are interested in crop rotation and beneficial soil biota. A pot experiment using a Typic Paleustult (topsoil + subsoil) from the National Corn and Sorghum Research Centre, Nakhonratchasima Province, Thailand was undertaken over three successive crops to evaluate effects of agronomic practices on populations of arbuscular mycorrhizal (AM) fungi and to determine whether reintroduction of a local Glomus was beneficial to maintain maize yield. The three crops and their treatments were: (1) preceding crop: maize grown in all pots; (2) subexperiment 1: agronomic practices [maize, fallow ± soil disturbance, fallow with solarization, non–AM host (cabbage)]; and (3) subexperiment 2: maize ± Glomus sp. 3 at three rates of P fertilization (0, 33, 92 kg P ha^–1). The AM‐fungal community was established under the preceding crop. In subexperiment 1, the three fallow treatments decreased (30%–40%) the total AM spore number in the topsoil whereas there was no change under maize or cabbage. Glomus, the dominant genus, showed sensitivity to fallow. In subexperiment 2, inoculation with Glomus sp. 3 enhanced total AM spore number and root colonization when applied following the three fallow treatments. Furthermore, inoculation promoted grain yield; at nil P following fallow ± soil disturbance, at 33 kg P ha^–1 following fallow without soil disturbance, and following solarization. Two treatments, maize following maize and maize following cabbage, did not respond to inoculation with Glomus sp. 3. Overall, the results suggest that reintroduction of Glomus sp. 3, a local AM fungus in this soil, may overcome negative effects of fallow and promote effectiveness of P fertilizer. Further work is needed to evaluate the benefits of other indigenous AM species that persist under modern fertilization practices.     

A framework for determining the efficient combination of organic materials and mineral fertilizer applied in maize cropping

Application of organic manures and composts in crop production has been strongly encouraged in many places but often without due consideration to their quality and price. Since organic amendments can vary greatly in composition and mineralization rate, a framework is needed to make rational choices on their use as replacements of inorganic fertilizer, especially when considering poor quality organic materials. A field experiment was carried out with maize grown annually for 5 years on a Rhodic Kandiustox in Thailand to test response to mineral fertilizer (at 0-0 to 125-55 kg N-P ha⁻¹ yr⁻¹), compost (0.59% N, 0.31% P and 0.55% K at 0-7500 kg ha⁻¹ yr⁻¹) and stubble removal. The DSSAT model was calibrated to predict yields using the first year's trial data and then used to predict treatment yields for the following 4 years. The Seasonal Analysis module of DSSAT using Dominance Analysis showed that mineral fertilizer (125-55 kg N-P ha⁻¹ yr⁻¹) with stubble return gave the highest net profit whereas the highest rate of compost without mineral fertilizer gave the biggest loss. The yield response was attributed primarily to N supply rather than P. Effects of compost, mineral fertilizer, stubble management, and their interactions on yield and profit were not related to bulk density or soil available water capacity even though soil organic matter (SOM) levels increased. With stubble return, the highest rate of mineral fertilizer increased SOM whereas with compost application or stubble removal it did not. The DSSAT simulation of yield indicated that the low quality compost would only be as profitable as mineral fertilizer if the N concentrations are 3-4 times higher than the present compost (1.8-2.4% N) or if the compost price is greatly reduced. The DSSAT yield simulation and Seasonal Analysis provided a framework whereby the suitability of compost as a N fertilizer replacement for maize could be determined based on its composition, rate of application and price. Further validation of this approach is needed where the organic amendments have significant effects on soil physical properties and where other nutrients besides N are a significant factor in the crop yield response.