Potato production in the tropics: Crop requirements

Considerable efforts have been made to alleviate constraints affecting potato production in the tropics. In spite of substantial progress, particularly in the area of plant breeding, overall yield increase due to this improved technology has been marginal at best. The major factor restricting production appears to be the limited adaptation of potato to tropical conditions. Also, the lack of a clear understanding of the requirements of the crop hampers a more efficient use of the available technology. As a partial answer to these concerns, this paper reviews current knowledge on the environmental, soil physical and soil chemical requirements of potato in tropical regions.     

Crop production through conservation-effective tillage in the tropics

Agriculture can be intensified and food production increased in the tropics through conservation-effective tillage, with other benefits being soil and water conservation, energy saving and improved timeliness of planting.

The agronomic and economic performance of conservation-effective tillage is extremely location-specific. Problems important in semi-arid regions may not be significant in humid tropical areas. This paper attempts to avoid broad generalizations and to indicate ways of developing the best combination of practices for each local situation. The widespread use of on-farm research and demonstration plots to obtain site-specific information would aid the rapid adaptation and adoption of conservation farming practices in developing countries within different regions. Acceptance of conservation tillage by small farmers in developing countries would be an evolutionary process, and their adoption of improved simple farm equipment and appropriate practices to control weeds should be intesively promoted.

A concerted effort to understand and respond to the needs of farmers, crops and soils will be essential in achieving the goal of increasing agricultural productivity while maintaining oil through conservation-effective tillage in the tropics.     

Crop production in the tropics: A review

Intensive land use and concomitant land degradation are strongly undermining the sustainability of current crop production systems in the tropics. Thus, finding alternative cropping strategies to overcome soil and environmental stresses is an integral part of the ongoing process of maintaining and improving crop productivity. In this paper, major features of crops and crop production systems were reviewed. Production systems such as pastures, perennial crops, and agroforestry were found to be highly effective in promoting high and stable crop productivity, restoring and/or enhancing soil fertility, and minimizing erosion risk.     

Crop management and phenology trends in the U.S. Corn Belt: Impacts on yields, evapotranspiration and energy balance

Crop yields are affected by many factors, related to breeding, management and climate. Understanding these factors, and their relative contributions to historical yield increases, is important to help ensure that these yield increases can continue in the future. Two important factors that can affect yields are planting dates and the crop's growing degree day (GDD) requirements. We analyzed 25 years of data collected by the USDA in order to document trends in planting dates, lengths of the vegetative and reproductive growth periods, and the length of time between maturity and harvest for corn and soybeans across the United States. We then drove the Agro-IBIS agroecosystem model with these observations to investigate the effects of changing planting dates and crop GDD requirements on crop yields and fluxes of water and energy. Averaged across the U.S., corn planting dates advanced about 10 days from 1981 to 2005, and soybean planting dates about 12 days. For both crops, but especially for corn, this was accompanied by a lengthening of the growth period. The period from corn planting to maturity was about 12 days longer around 2005 than it was around 1981. A large driver of this change was a 14% increase in the number of GDD needed for corn to progress through the reproductive period, probably reflecting an adoption of longer season cultivars. If these changes in cultivars had not occurred, yields around 2005 would have been 12.6 bu ac⁻¹ lower across the U.S. Corn Belt, erasing 26% of the yield increase from 1981 to 2005. These changes in crop phenology, together with a shortening of the time from maturity to harvest, have also modified the surface water and energy balance. Earlier planting has led to an increase in the latent heat flux and a decrease in the sensible heat flux in June, while a shorter time from maturity to harvest has meant an increase in net radiation in October.     

Constraints for potato production in the tropics

Potato tuber yields in the tropics are extremely low compared to those of temperate zones. The low yields merely reflect the inability of the current technology to overcome soil and environmental constraints. Given the various constraints affecting potato production in the tropics, few viable soil and crop management options remain. This paper reviews key constraints for potato production, assesses the impact of indigenous and/or adapted technology in managing them, and examines basic relationships describing productivity changes as management takes place. The overall intent of this paper is to outline the scientific basis for developing management strategies to sustain and improve potato production in the tropics.     

The search for carbon sinks in the tropics

The study and modeling of the global C cycle have been dominated by the assumption that the atmosphere and the biota were in C steady state prior to the industrial revolution. This view led to the perception that most of the terrestrial biota was neutral with regards to the C concentration of the atmosphere. Recent evidence suggests that neither the atmosphere nor the biota were in C steady state prior to, or since, the industrial revolution. Therefore, it is now necessary to re-visit the role of natural processes in the global C cycle, study the C cycle in its totality, and focus attention on the magnitude of potential C sinks in ecosystems previously thought to be neutral with respect to atmospheric C. “An improved understanding of the CO₂ cycle is essential to predict the future rate of any atmospheric CO₂ increase and to plan eventually for an international CO₂ management strategy” Tans et al. (1990).     

Tillage systems and soils in the semi-arid tropics

Even though conservation tillage may be ideal for the semi-arid tropics (SAT) in view of results from studies and tillage practices in the U.S.A. and Australia, studies conducted in semi-arid regions of Africa appear to support the use of conventional tillage systems. Some of the reasons for this apparent discrepancy are because of the physical properties of the soils in semi-arid Africa, particularly the Sahelian zones where the soils are sandy, have high bulk densities and therefore low total porosities and form crusts upon wetting and drying. Consequently, no-till or reduced tillage systems that do not have the soil surface covered by residue in irder to prevent formation of crust as a result of raindrop impact, tend to lose water through runoff in a region where water economy is essential. Also, because these soils have inherently high bulk densities, conventional tillage systems appear to be suitable since they increase the macropores, reduce both bulk density and strength and thus ensure prolific root distribution and the resultant exploration of water and nutrients at greater soil depths. Notwithstanding, it seems that since most of the SAT soils are structurally unstable, further conventional tillage even though it has ephemeral advantages, may in the long term be exacerbating the problems of structural instability and their deleterious effect on water and soil conservation and therefore on crop production. We suggest that at this stage soil tillage research in the semi-arid regions of Africa and Asia should re-examine some of the concepts of conservation tillage in relation to soil physical properties and processes in order to obtain a tillage system that ensures high crop yields without destruction of the soil resource.     

Rainfall simulator for tillage research in the tropics

A new concept is proposed that enables rainfall simulation to be carried out in the tropics, where technical facilities and know-how are usually limited. A rainfall simulator developed on the basis of this concept produced uniform rainfall intensities (uniformity coefficient, C_u, 0.8–1.0) of 4–5, 5–6, 6–7 and 7–8 mm/h on areas of 131, 88, 25 and 8.6 m², respectively. Possibilities of using the rainfall simulator for erosion research are discussed.     

Denitrification losses from puddled rice soils in the tropics

Summary Although denitrification has long been considered a major loss mechanism for N fertilizer applied to lowland rice (Oryza sativa L.) soils, direct field measurements of denitrification losses from puddled rice soils in the tropics have only been made recently. This paper summarizes the results of direct measurement and indirect estimation of denitrification losses from puddled rice fields and reviews the status of research methodology for measurement of denitrification in rice fields. The direct recovery of (N₂+N₂O)-¹⁵N from ¹⁵N-enriched urea has recently been measured at sites in the Philippines, Thailand, and Indonesia. In all 12 studies, recoveries of (N₂+N₂O)-¹⁵N ranged from less than 0.1 to 2.2% of the applied N. Total gaseous N losses, estimated by the ¹⁵N-balance technique, were much greater, ranging from 10 to 56% of the applied urea-N. Denitrification was limited by the nitrate supply rather than by available C, as indicated by the values for water-soluble soil organic C, floodwater (nitrate+nitrite)-N, and evolved (N₂+N₂O)-¹⁵N from added nitrate. In the absence of runoff and leaching losses, the amount of (N₂+N₂O)-¹⁵N evolved from ¹⁵N-labeled nitrate was consistently less than the unrecovered ¹⁵N in ¹⁵N balances with labeled nitrate, which presumably represented total denitrification losses. This finding indicates that the measured recoveries of (N₂+N₂O)-¹⁵N had underestimated the denitrification losses from urea. Even with a probable two-or threefold underestimation, direct measurements of (N₂+N₂O)-¹⁵N failed to confirm the appreciable denitrification losses often estimated by the indirect difference method. This method, which determines denitrification losses by the difference between total ¹⁵N loss and determined ammonia loss, is prone to high variability. Measurements of nitrate disappearance and ¹⁵N-balance studies suggest that nitrification-denitrification occurs under alternate soil drying and wetting conditions both during the rice cropping period and between rice crops. Research is needed to determine the magnitude of denitrification losses when soils are flooded and puddled for production of rice.     

Flow injection chemiluminescence determination of sudan I in hot chilli sauce

A chemiluminescence method based on the luminol-H2O2 system with flow injection technology was proposed for the determination of sudan I in hot chilli sauce. It was found that sudan I could enhance chemiluminescence intensity generated from the luminol-H2O2 system. The increment of chemiluminescence intensity was proportional to the concentration of sudan I, giving a calibration graph linear over the concentration from 10 pg mL-1 to 7 ng mL-1 (R 2 = 0.9980) with the detection limit of 3 pg mL-1 (3sigma) and the quantification limit of 7.5 pg mL-1. At a flow rate of 2.0 mL min-1, one analysis cycle, including sampling and washing, could be accomplished in 60 s with a relative standard deviation of <5.0%. The method has been applied successfully to the determination of sudan I in Pixian douban, Golden Mark guilin chilli sauce, and Golden Mark satay sauce, and the recovery was 90.6-110.0%.     

Classifying soils at the ultimate stage of weathering in the tropics

Oxisols cover ≈ 23% of the land surface in the tropics and are utilized extensively for agricultural purposes in the tropical countries. Under the variable input types of agricultural systems practiced locally, some of these soils still appear to have problems in terms of proper soil classification and subsequently hinder attempts to implement sustainable agro‐management protocols. The definition for Oxisols in Soil Survey Staff (1999) indicates that additional input is still required to refine the definition in order to resolve some of the outstanding classification problems. Therefore, the objective of this study is to examine the properties of some Oxisols and closely related soils in order to evaluate the classification of these soils. Soils from Brazil, several countries in Africa, and Malaysia were used in this study. Field observations provided the first indication that some of the presently classified kandi‐Alfisols and kandi‐Ultisols were closer to Oxisols in terms of their properties. Water‐retention differences and apparent CEC of the subsurface horizons also supported this idea. The types of extractable Fe oxides and external specific surface areas of the clay fractions showed that many kandic horizons have surface properties that are similar to the oxic horizons. Micromorphology indicated that the genetic transition from the argillic to the oxic involves a diminishing expression of the argillic. Properties, such as CEC, become dominant. The kandic horizon is therefore inferred as a transition to the oxic horizon. It is proposed that the Oxisols be keyed out based only on the presence of an oxic horizon and an iso–soil temperature regime. The presence of a kandic horizon will be reflected at lower levels in Oxisols. The Oxisols will now be exclusive to the intertropical belt with an iso–soil temperature regime. The geographic extend of the Oxisols would increase and that of kandi‐Alfisols and Ultisols would decrease. A few kandi‐Alfisols and Ultisols in the intertropical area will have low CEC which would fail the weatherable mineral contents. The kandic subgroups of some Alfisols and Ultisols will be transitional between the low (< 16 cmol_c [kg clay]^–1)‐ and high (> 24 cmol_c [kg clay]^–1)‐activity clay soils. The proposed changes to classification will contribute to a better differentiation of the landscape units in the field. Testing of the proposed classification on some Malaysian soils showed that the new definition for Oxisols provides a better basis for the classification of the local soils and the development of meaningful soil‐management groups for plantations.     

The retention of nitrate in acid soils from the tropics

Abstract. In most soils of temperate regions nitrate is not held on soil surfaces and moves freely in solution. But when soils carry positive charges, nitrate is held as an exchangeable anion. As a result, leaching of nitrate is delayed relative to the movement of water. The delay can be predicted provided the anion exchange capacity (AEC) can be measured and the concentration of counter-anions is known. For soils with variable charge, the AEC varies with both pH and ionic strength, and the effective AEC should be determined under conditions similar to those in soil solution. A simple leaching method is described which satisfies this requirement. Delays in the leaching of nitrate measured in columns of repacked soil were strongly related to the AEC.     

Amelioration of subsoil acidity in an Oxisol of the humid tropics

Summary This work investigated the effectsof amendments of fertilizer N and lime on subsoil acidity and maize rooting depth in an acid soil of the central Amazon basin. A split-plot designed field experiment was conducted on a clayey Oxisol (Typic Acrudox) during a 16-month period. Main plots received 0 or 4 Mt ha^-1 of lime. Subplots were four crop sequences: (1) Maize-green manure (Canavalia ensiformes); (2) maize-green manure (Mucuna aterrima); (3) maize-bare fallow, with the maize receiving 300 kg ha^-1 of urea-N; and (4) bare fallow, with an application of 300 kg ha^-1 of urea-N at the same time as sequence 3. Plots were periodically sampled to 1.2 m. The experimental site received 4265 mm of precipitation during 16 months; approximately 60%–90% of this rain percolated through the profile. Substantial amounts of Ca were leached from the 0–30 cm horizon during the experimental period, but only limited amounts accumulated in the subsoil. Base saturation below 45 cm was less than 50% at the end of the experiment regardless of lime treatment. Roots of maize were concentrated in the 0–30 cm layers in limed plots and the 0–20 cm layers in unlimed plots. In all treatments less than 5% of the roots was found below 50 cm. An acidity balance indicated that considerable acidity was leached below the plow layer and out of the profile.     

作物次级群体的研究进展

用于作物数量性状基因座(QTL)定位研究的次级群体有近等基因系、导入系、染色体片段代换系等。次级群体是用于QTL鉴定、精细定位、互作分析、图位克隆、品种改良等的良好材料。相对于初级群体而言,次级群体是在相似的遗传背景下进行QTL分析,消除了大部分遗传背景的干扰和QTL之间的互作,提高了QTL分析的灵敏度和准确性。概述了作物次级群体库的构建及利用的研究进展,讨论了基于次级群体理论发展起来的染色体单片段代换系在作物遗传育种中的利用价值。     

干旱缺水地区森林植被蒸散耗水研究

 系统总结近年来在宁夏固原六盘山、北京延庆等干旱缺水地区进行的森林植被蒸散耗水研究结果。乔、灌、草作为土壤水分限制型生态系统的坡面植被,蒸散都是水分平衡的最大分项,其中植被蒸腾又是蒸散的最大分项。植被蒸散量一般表现为高大乔木林>亚乔木林>灌木林>自然草地,但人工草地>自然草地。可依坡面产水功能将不同植被分类,自然草地和灌丛为水源生产型,亚乔木林为水源平衡型,高大乔木林和人工草地为水源消耗型。植株密度不是坡面植被蒸散大小的决定性因子,其作用更多的是调控蒸腾量及其占蒸散比例。虽然降低植株密度一般会减小蒸散,但并不是相同比例地线性下降,不同植被类型反应也不一样,表现出降低密度减少蒸散的作用从乔木、亚乔木到灌木而变弱的趋势,降低密度减少蒸散的作用是有限的。对降低密度减少蒸散的作用大小,作为调控措施的有效性,有效的密度调控范围等,还需严格的对比实验和理论研究。估计和评价植被蒸散耗水时,用叶面积指数或叶生长量指标可能比密度更符合生物学逻辑。从在干旱缺水地区建立节水、稳定、高效、多功能的坡面植被的角度而言,草地和灌丛的蒸散低于乔木林,建立稀树草原或稀树灌丛式的植被可能更利于流域产水和植被稳定。从小流域管理的角度而言,还需考虑在土壤水分承载力空间差异的基础上,探讨能兼顾产水功能、水土保持、水源涵养、植被稳定的植被空间优化配置的理论和技术。     

Tillage systems for root and tuber crops in the tropics

Although limited information is available on tillage practices for root crops, published results show that tillage methods vary widely depending upon the specific root crop, the soil type, the previous vegetation, as well as the socio-economic conditions of farmers. These aspects are discussed in this review.

In general terms, it has been found that root and tuber crops are sensitive to soil compaction, inadequate aeration or poor drainage and therefore respond favorably to intensive tillage, followed by ridging or mounding. However, large differences exist between crops, with potato, sweet potato and yam requiring more intensive cultivation than cassava and taro. On light-textured soils, cassava can be grown without or with minimum tillage as long as weeds are controlled; in heavy or compacted soil cassava responds favorably to tillage and yields tend to increase when grown on ridges. In order to reduce erosion as well as production costs, cassava should be grown with as little tillage as possible as long as high yield can be maintained. Contour ridging and mulching are other practices that not only tend to increase yields but also reduce erosion losses.

Organized data for use in a classification of the tillage requirements of different soils for the various root crops is lacking. We suggest that research efforts should be directed towards the characterization of the physico-chemical and biological factors which determine the tillage requirements of a given soil for a given root crop.     

引入平流影响的蒸散估算

本文用波文比--能量平衡观测资料计算的麦田潜热，验证了引入平流影响的平流--干燥蒸散估算模型，结果表明，平流--干燥模型能较好地模拟麦田蒸散潜热日变化过程。该模型只需一个高度的气象资料，有推广应用前景。