Effects of limestone and gypsum application to a Malaysian ultisol on soil solution composition and yields of maize and groundnut

A field experiment was conducted on an Ultisol in Malaysia to assess changes in soil solution composition and their effects on maize and groundnut yields, resulting from limestone and gypsum application. The results showed that soil solution Ca in the lime treatment remained mainly in the zone of incorporation, but in the gypsum treatment some Ca moved into 15–30 cm zone. Al³⁺ and AlSO₄ ⁺ were dominant Al species in the soil solution of nil treatment. Liming decreased Al³⁺ and AlSO₄ ⁺, but increased hydroxy-Al monomer activities. However, gypsum application resulted in an increase of AlSO₄ ⁺ activity and in a decrease of Al³⁺ activity. Relative maize and groundnut yields were negatively correlated with Al³⁺, Al(OH)²⁺ and Al_sum activities. Likewise, relative yields were negatively correlated with Al concentration and the Al concentration ratio and positively correlated with soil solution Mg concentration and Ca/Al ratio.     

Net nitrogen mineralization and net nitrification along a tropical forest-to-pasture chronosequence

Soil inorganic nitrogen pools, net mineralization and net nitrification rates were compared during the dry season along a chronosequence of upland (terra firme) forest, 3-, 9- and 20-year-old pastures in the western Brazilian Amazon Basin state of Rondônia to investigate the influence of forest conversion to pasture on soil nitrogen cycles. Surface soil (0 to 10 cm) from forest had larger extractable inorganic nitrogen pools than pasture soils. In the forest, NO ₃ ^– pools equaled or exceeded NH ₄ ⁺ pools, while pasture inorganic N pools consisted almost exclusively of NH ₄ ⁺ . Rates of net N mineralization and net nitrification in seven -day laboratory incubations were higher in the seven - day forest than in the pastures. Net N mineralization rates did not differ significantly among different-aged pastures, but net nitrification rates were significantly lower in the 20-year-old pasture. Higher net N mineralization and net nitrification rates were measured in laboratory and in situ incubations of sieved soil, compared with in situ incubations of intact soil cores. Rates calculated in seven-day incubations were higher than determined by longer incubations. Sieving may increase N mineralization and/or decrease N immobilization compared with intact cores. We concluded that 7-day laboratory incubation of sieved soil was the most useful index for comparing N availability across the chronosequence of forest and pasture sites. High net nitrification rates in forest soils suggest a potential for NO ₃ ^– losses either through leaching or gaseous emissions.     

Vesicular-arbuscular mycorrhizal effectiveness in an acid soil amended with fresh organic matter

A greenhouse investigation was undertaken to determine the influence of fresh organic matter on the formation and functioning of vesicular-arbuscular mycorrhizal symbiosis in Leucaena leucocephala grown in an acid aluminum-rich ultisol. In soil not amended with fresh organic matter or lime, plants failed to grow. Mycorrhizal infection level, mycorrhizal effectiveness measured in terms of pinnule P content of L. leucocephala leaves and dry matter yield of the legume increased with increase in fresh organic matter. Although VAM colonization level and dry matter yield of L. leucocephala were significantly higher if the test soil was limed (7.2 cmole OH^–) than if amended with fresh organic matter, the latter was as effective as lime in off-setting the detrimental effect of aluminum on mycorrhizal effectiveness. The lower mycorrhizal colonization level and the lower dry matter yield noted in the soil treated with fresh organic matter appears to be related to the inadequacy of Ca in the soil amended with fresh organic matter. These observations are supported by the low calcium status of soil and plant tissues in the absence of lime. It is concluded that while fresh organic matter, in appropriate amounts, could protect sensitive plants and VAM symbiosis against Al toxicity in acid soils, maximum mycorrhizal inoculation effects are not likely to be attained unless the soils are also amended with Ca.Contribution from Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No 3740.     

Residual effects of natural bush,Cajanus cajan and Tephrosia candida on the productivity of an acid soil in southeastern Nigeria

An experiment was established in 1986 to examine the contribution of Tephrosia candida and Cajanus cajan shrubs to improving the productivity of an acid soil. The main treatments were N levels (0 and 60 kg ha^-1) with subplots of maize/natural bush, maize/Tephrosia candida, maize/Cajanus cajan, maize + cassava/natural bush, maize + cassava/Tephrosia candida, and maize + cassava/Cajanus cajan. In 1988, all plots were cleared and maize uniformly planted to study the residual effects of the treatments. No residual effects of N application were observed. Tephrosia candiada and Cajanus cajan increased surface soil organic carbon and total N levels over the natural bush. However, only Tephrosia candida plots produced improved maize grain and stover yield. Highly significant correlations were found between maize grain yield and earleaf N (r=0.73^**), grain N (r=0.51^**), and stover N (r=0.54^**) contents. These results suggest that Tephrosia candida increased N availability in the soil. Therefore, the shrub has potential for improving the productivity of acid soils under traditional systems, where N is limiting due to the absence of N₂-fixing legumes in the natural bush fallow.     

The physical and chemical responses of a degraded sandy clay loam soil to cover crops in southern Nigeria

The physical and chemical responses of a degraded sandy clay loam Ultisol to two leguminous and four grass cover crops in southern Nigeria were studied after five years to assess the rejuvenative effects of the covers. There were relative increases of 26% and 112% in soil organic carbon and phosphorus levels and also appreciable improvements in the CEC and Ca levels under vegetative covers compared with the initial conditions. The improvements were more pronounced with legume covers than with grass covers. Furthermore, the vegetative covers improved mean organic carbon level by 28% and appreciably improved mean CEC, Ca, and Mg levels over the values for the bare soils. The percentage of water-stable aggregates >1.0 mm was significantly reduced under bare fallow ( = 27.7%) compared with soils under vegetative cover = 79.3%). The correlation between water-stable aggregates > 1.0 mm and exchangeable aluminum was negative and significant (r = −0.705°) at p = 0.05. There were highly significant treatment effects (P = 0.01) for penetrometer resistance, pore size distribution, water infiltration, water retention and saturated hydraulic conductivity. Grass and legume fallows which protect the soil and guarantee regular additions of organic materials are ecologically sound and socially acceptable components of sustainable agricultural production. Indications, however, are that this degraded tropical Ultisol would require a period exceeding five years under vegetative covers for restoration of its fertility to acceptable productive status. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effects of pH and calcium on the growth ofLeucaena leucocephala in an oxisol and ultisol soil

Leucaena was grown for eleven weeks in a pH and calcium amended oxisol and ultisol to determine whether poor growth in acid soils is related to calcium deficiency or low pH induced effects, such as aluminium toxicity. Soil pH was ameliorated by the addition of either CaCO₃, or SrCO₃, while CaCO₃ or Ca(NO₃)₂ were used to modify calcium supply. In both soils, CaCO₃, and SrCO₃ application resulted in an increase in pH and a reduction in exchangeable aluminium although the response of leucaena to the applied amendment varied between the two soils. In the oxisol there was little effect of increased pH on growth, but a marked response to Ca application. In the ultisol, growth was improved by increasing pH but there was no response to increased calcium. The effects of liming on these soils are discussed in relation to alternative strategies available for utilising leucaena and other tree legumes.     

Studies on potassium-magnesium interaction in coconut (Cocos nucifera)

The widespread deficiency of magnesium in coconut acts as a limiting factor to increased production. Two ongoing experiments on potassium-magnesium (K-Mg) interaction in coconut conducted in lateritic gravels (ultisols), in the wet and intermediate agroclimatic zones of Sri Lanka showed significant yield responses (P=0.01) to differential K treatments, in the wet zone, but not in the intermediate zone. Differential Mg treatments, however, did not give rise to yield responses. Leaf and nut water analysis showed significant changes in the concentrations of Na, K and Cl (P=0.001), with a distinct inverse relationship between Na and K when K was applied. Differential Mg applications showed a significant effect only for leaf Mg (P=0.001), in the fourth year of the experiment, in the wet zone. Results indicate the usefulness of nut water analysis as an additional diagnostic tool, for Na, K and Cl.     

Soil phosphorus fractions,aluminum, and water retention as affected by microbial activity in an Ultisol

Increased organic matter input into weathered and infertile soils through agricultural techniques such as minimum tillage or agroforestry can improve P availability to crops. Organic matter is an energy source for microbes, and their activity may be responsible in part for increased levels of labile P. The objective of the work reported here was to examine, in a highly weathered Ultisol, the influence of microbial activity in mobilizing P, maintaining it in a plant-available state, and preventing its fixation, and the effect of N and biocides on these processes. Exchangeable aluminum and soil moisture were also determined, since they interact with microbes and soil P.Results showed that increased microbial activity reduced sorption of dissolved and organic P by soil, maintained inorganic P in soluble and labile pools, increased microbial P, decreased mineral P, increased exchangeable Al, and increased water retention. Additions of N and biocides had variable effects, probably due to complex interactions between N, degrading biocides, and microbial populations.