Improving phosphorus availability in two upland soils of Vietnam using Tithonia diversifolia H.

Abstract

Phosphorus was added to two acidic upland soils (a Cambisol and a Ferralsol) at two rates (9 mg P kg −1 and 145 mg P kg −1 ) either in an inorganic P form (KH 2 PO 4 ) or as a green manure (Tithonia diversifolia H. at 2.5 g kg −1 and 40 g kg −1 ). The effect of P source on the chemical availability of P was assessed in an incubation experiment by measuring resin extractable P, soluble molybdate reactive (DMR-P) and unreactive P (DMU-P). Soil pH and extractable Al were monitored during the incubation period of 49 days. Green manure addition caused an immediate and sustained increase in soil pH and an immediate and sustained decrease in extractable Al. Labile P (resin P + DMR-P + DMU-P) was increased more by P added as a green manure than when added in inorganic form in one soil (Ferralsol), while it decreased or did not differ in the other one (Cambisol). In both soils, the concentrations of soluble DMU-P were frequently higher where Tithonia had been added. The effects of green manure amendment on physical factors governing the phosphorus supply through diffusive transport were also investigated. Aggregate size distribution was substantially changed by green manure amendment due to a shift in the percentage of microaggregates (<250 µm in diameter) to larger sizes. Changes in soil aggregation as a consequence of green manure amendment led to a reduction in specific surface area (SSA) of the whole soil. Coupled with the large increase in effective cation exchange capacity caused by green manure amendment in both soils, and the decrease in SSA, there was an increase in the net negative surface charge density in both soils. In summary, at a large addition rate -and in addition to the well-known effect derived from the extra supply in P, green manure amendment may improve the chemical availability and diffusive supply of P through the following mechanisms: (i) an increase in soil pH increasing the solubility of phosphate sources; (ii) a decrease in extractable Al reducing the fixation of added P; (iii) increased macro-aggregation and reduced specific surface area and porosity leading to fewer sorption sites for P and hence enhanced diffusion rates; and (iv) increased negative charges and reduced positive charges at the soil surface resulting in a net increase in repulsive force for P. The induced changes in most measured soil properties were smaller in the Ferralsol than in the Cambisol.

Effects of Organic and Inorganic Materials on Soil Acidity and Phosphorus Availability in a Soil Incubation Study

“…The soil pH, however, declined by 16 WAI with tithonia-treated soils showing the highest pH reductions. The increase in soil pH due to application of OMs at 4 WAI in this study is consistent with results reported by several other workers (e.g., [15,16]). The principal mechanisms involved in increasing soil pH by various types of OMs differ considerably and according to [17], and a broad distinction can be made between the mechanisms of undecomposed plant materials such as tithonia and humified materials such as FYM and composts.…”

“…Another mechanism that has been proposed to explain the increase in soil pH by such materials as FYM is the specific adsorption of humic material and/or organic acids (the products of decomposition of OMs) onto hydrous surfaces of Al and Fe oxides by ligand exchange with corresponding release of OH − as suggested by [21]. On the other hand, [15] attributed the soil pH changes observed with fresh materials, for example, tithonia, in an incubation study, mainly to …”

“…In this incubation study, soils were amended with the OMs in a closed system without growing plants. Therefore, the effects of plant uptake, root exudates, and leaching are not relevant and the processes responsible for the pH changes are limited to the decomposition and nutrients held in tithonia and N transformations [15]. Under anaerobic conditions, NH 4 + produced by the ammonification process would accumulate due to inhibition of nitrification, and the pH would increase.…”

Integrated soil fertility management in sub-Saharan Africa: unravelling local adaptation

“…In order to improve the productivity of acid soils, exchangeable and soluble Al contents need to be reduced. While acid soils may be managed in several ways, including the use of crop species that are tolerant to high levels of exchangeable Al or concentrating relatively high levels of organic resources near the planting hole (Cong and Merckx, 2005), liming is the most established means for correcting Al toxicity (The et al, 2006; Crawford et al, 2008). However, management of Al toxicity has received little attention in recent years in SSA, mainly because (i) Al toxicity is believed to be localized to only a particular few areas of central Africa where highly weathered and leached soils occur (Crawford et al, 2008) and (ii) the use of lime has been constrained by limited infrastructure for mining lime deposits and transporting the final product.…”

Comparative study of earthworm communities, microbial biomass, and plant nutrient availability under 1-year Cajanus cajan (L.) Millsp and Lablab purpureus (L.) Sweet cultivations versus natural regrowths in a guinea savanna zone

“…There are some reports on the potential of some legumes to improve plant available P by dissolving immobilized soil P through exudation of organic acids (Kamh et al 1999;Lyasse et al 2002). Other studies reported that supplying organic residues to low-P soils leads to improved plant P availability through decreasing the precipitation of P as insoluble compounds, desorbing the fixed P in soils and providing extra supply of P during their decomposition (Cong and Merckx 2005;Ikerra et al 2006).…”