J. N. O'Sullivan

Leaf litter decomposition of Piper aduncum, Gliricidia sepium and Imperata cylindrica in the humid lowlands of Papua New Guinea.

No information is available on the decomposition and nutrient release pattern of Piper aduncum and Imperata cylindrica despite their importance in shifting cultivation systems of Papua New Guinea and other tropical regions. We conducted a litter bag study (24 weeks) on a Typic Eutropepts in the humid lowlands to assess the rate of decomposition of Piper aduncum, Imperata cylindrica and Gliricidia sepium leaves under sweet potato (Ipomoea batatas). Decomposition rates of piper leaf litter were fastest followed closely by gliricidia, and both lost 50% of the leaf biomass within 10 weeks. Imperata leaf litter decomposed much slower and half-life values exceeded the period of observation. The decomposition patterns were best explained by the lignin plus polyphenol over N ratio which was lowest for piper (4.3) and highest for imperata (24.7). Gliricidia leaf litter released 79 kg N ha−1, whereas 18 kg N ha−1 was immobilised in the imperata litter. The mineralization of P was similar for the three species, but piper litter released large amounts of K. The decomposition and nutrient release patterns had significant effects on the soil. The soil contained significantly more water in the previous imperata plots at 13 weeks due to the relative slow decomposition of the leaves. Soil N levels were significantly reduced in the previous imperata plots due to immobilisation of N. Levels of exchangeable K were significantly increased in the previous piper plots due to the large addition of K. It can be concluded that piper leaf litter is a significant and easily decomposable source of K which is an important nutrient for sweet potato. Gliricidia leaf litter contained much N, whereas imperata leaf litter releases relatively little nutrients and keeps the soil more moist. Gliricidia fallow is more attractive than an imperata fallow for it improves the soil fertility and produces fuelwood as additional saleable products.

Nitrogen use efficiency of taro and sweet potato in the humid lowlands of Papua New Guinea

Root crops are an important staple food in the Pacific region, Yields are generally low and inorganic fertilizers are deemed an option to increase root crop production. The effects of inorganic N fertilizers on upland tare (Colocasia esculenta (L.) Schott) and sweet potato (Ipomoca batatas (L.) Lam.) were quantified with the aim to investigate relationships between inherent soil fertility, N uptake, N application raters and crop yield. The research took place on a sandy, Typic Tropofluvents in the humid lowlands of Papua New Guinea. Five levels of fertilizer N (0,100, 200, 300 and 400 kg ha(-1)) were given in split applications. The yield of marketable tare corms was not affected by N fertilizer but non-marketable corm yield doubled at high N fertilizer rates. High N applications yielded 8-11 Mg ha(-1) more taro tops. Marketable and non-marketable sweet potato yield was negatively affected by N fertilizers, High N applications yielded 26 Mg ha(-1) more vines than the control treatment. Nitrogen fertilizer significantly reduced the harvest index in both crops. When no fertilizer was applied, the total N uptake of tare was 32.0 kg ha(-1) of which 9.7 kg was taken up in the marketable corms. At 400 kg N ha(-1) the total N uptake was 67.5 kg ha(-1) of which 23% was taken up by the marketable corms. Uptake of N in the marketable sweet potato tubers was less than 11 kg ha(-1) and for most treatments more N was taken up in the non-marketable tubers than in the marketable yield. Up to 156 kg N ha(-1) was taken up with the sweet potato vines. Despite the negative effect of N on sweet potato yield, sweet potato had a higher N use efficiency than rare due to a higher above-ground biomass production. The N fertilizer recovery was 25% for the sweet potato but only 9% for thr: tare indicating considerable N losses, likely caused by leaching

Yield decline of sweet potato in the humid lowlands of Papua New Guinea

Sweet potato (Ipomoca batatas (L,) Lam) is the major staple crop in Papua New Guinea and experiments were conducted investigating factors affecting yield decline. Yields; of unfertilized plots were related to rainfall and measured changes in soil properties, nematode (Meloidogyne sp., Rotylenchulus reniformis) and sweet potato weevil (Cylas formicarius) populations. The research took place at two locations (Hobu and Unitech) on Eutropepts and Fluvents, respectively. Yields at Hobu decreased from 18 Mg ha(-1) in the first season to around 7 Mg ha(-1) in the third season, but no significant yield trend was observed at Unitech. Vine biomass was not affected by the number of cropping seasons at Hobu brit it decreased at Unitech with time. Marketable tuber yield at both sites was significantly correlated to rainfall, i.e, the more rain the lower the yield. Significant changes in soil chemical properties included a decrease in pH and base saturation (Hobu) and a decrease in CEC and exchangeable K (Unitech). No significant changes in soil bulk density were found and no obvious pattern was found in the nutrient concentrations of leaf samples with time. Nematode populations were high and tripled between the first and third season at Hobu. Half of the vines at Hobu and all of the vines at Unitech were damaged by sweet potato weevils, but tuber damage was higher in Hobu although the damage was only superficial. Despite the considerable variation in yield and yield determining factors, the study showed that the decline in sweet potato yield may be attributed to the high nematode infestation, accompanied by an increase in vine damage by weevils and a declining soil fertility

ROOT DISTRIBUTION OF YAM (DIOSCOREA ALATA) DETERMINED BY STRONTIUM TRACER

Strontium was used as a tracer to detect root activity of yam (Dioscorea alata) at distances from 0.5 to 5.5 m from the plants crown, and at depths of 7, 15, 25 and 40 cm, in field plantings separated by plastic root barriers. Tracer bands were installed at planting, and leaves were sampled at intervals throughout the growing season. Background Sr concentrations in leaves were found to be closely related to leaf Ca concentrations, allowing the expected background Sr to be calculated from the Ca concentration in each sample. The ratio of observed to expected Sr did not alter significantly with time of sampling or leaf age for control plants providing a robust parameter for comparison across harvests. Plants at all horizontal distances from the Sr placement up to 5.5 m showed significant elevation of leaf Sr by 13 weeks after planting (WAP). The depth placement at 15 cm showed earliest Sr elevation, followed by the 7 cm placement. However, both 25 cm and 40 cm placements resulted in significant leaf Sr elevation by 13 WAP, and their final Sr concentrations at 27 WAP did not differ significantly from those of shallower placements. Three fertilizer placement options were compared with respect to Sr uptake by the treated plants and by those in adjacent rows. Placement in a ring around the crown of the seed sett resulted in high leaf Sr concentration at 5 WAP, declining considerably at subsequent samplings. Placement underneath the seed sett produced a similar pattern but at lower concentrations. Banding midway between the plant rows, 0.5 m from plants, yielded a small increase at 5 WAP increasing to 13 WAP, and ultimately attaining similar concentrations to the other two placements. Plants in adjacent rows significantly accessed all three placements by 13 WAP. The results indicate greater reach of yam roots, both horizontally and vertically, than has been previously reported, and raise concerns about the adequacy of treatment separation in previously published fertilizer experiments with yams.

CROP DEVELOPMENT AND ROOT DISTRIBUTION IN LESSER YAM ( DIOSCOREA ESCULENTA ): IMPLICATIONS FOR FERTILIZATION

A growth analysis study involving monthly excavation of Dioscorea esculenta plants revealed that the root system developed fully in the period before tuber initiation, and extended radially for a distance between 2.3 and 4.3 m. Primary roots initially remained in the top 10 cm of the soil profile, but descended to approximately 30 cm near their full extension. Tuber number increased from initiation around 21 weeks after planting (WAP) until maximum vine growth was reached around 33 WAP, but tuber dry weight continued to increase throughout the senescence period of the vine, to 45 WAP. Tubers lost dry matter but not fresh weight during dormancy in the soil, to 55 WAP. The results indicate that a distance of at least 4.5 m is required to separate experimental fertilizer treatments, and that post-establishment burial of fertilizers around the mound or ridge risks damage to roots near their base, while inter-row application is accessible to roots and may be preferable.

Effects of Sodium on Potassium Nutrition in Three Tropical Root Crop Species

ABSTRACT The potassium (K) nutrition and high K requirement of tropical root crops may be affected by their sodium (Na) status, as has been observed in a number of plant species. Solution culture was used to study the effects of K and Na supplies in tannia [Xanthosoma sagittifolium (L.) Schott.], sweet potato [Ipomoea batatas (L.) Lam.] and taro [Colocasia esculenta (L.) Schott]. At low K supply, Na ameliorated symptoms of K deficiency and increased growth in tannia, and to a lesser extent in sweet potato, but not in taro. None of the species responded to Na at adequate K supply. Differences in response to Na were attributed to differences in Na translocation to plant tops. At maximum Na supply, the Na concentration in index leaves averaged 1.82% in tannia, 0.205% in sweet potato, and 0.0067% in taro. An increase in the supply of Na resulted in a shift in the critical K concentration for deficiency (i.e., 90% of maximum yield) in index leaves from 2.9% to 1.2% in tannia, and from 4.8% to 2.5% in sweet potato. The critical K concentration in taro was 3.3%, irrespective of Na supply. To overcome the problem in tannia and sweet potato of determining the critical concentration relevant to a leaf sample of unknown K status, a relationship was established for each species relating the critical K concentration to the concentration of Na in the index leaves.

Mineral nutrient disorders of root crops of the Pacific: preliminary observations on sweet potato (Ipomoea batatas)

Solution culture was used to characterise deficiencies or toxicities of several essential elements in Ipomoea batatas cv. Wanmun, and to define the critical concentrations of these elements in young mature leaves during vegetative growth. Tentative critical concentrations for deficiency, expressed on the basis of dry weight of leaf blade, were: nitrogen 3.8%, phosphorus 0.17%, potassium 2.4%, magnesium 0.12%, manganese 20μg/g and zinc 10μg/g. For manganese and zinc toxicities it was possible only to designate the range within which the critical concentration occurred. Visible symptoms are briefly described.

The Burden of Durable Asset Acquisition in Growing Populations

While most analyses of economic impacts of population growth have been equivocal, this article describes a new perspective from which the effects are strongly negative. The economies and diseconomies of population size are largely circumstantial and empirically inconsistent, but those of growth rate are intrinsic and consistent. These impacts are not apparent on income and per capita GDP, but on costs. The article estimates these costs using the logic of calculus rather than marginal accounting. Specifically, the cost of maintaining per capita capacity of durable assets, including infrastructure, equipment and skilled personnel, is increased by population growth by a factor proportional to the working lifespan of the asset class.

Resource use peak dates distract from real planetary limits

(2015) reported “The rates at whichhumans consume multiple resources such as food and woodpeaked at roughly the same time, around 2006. This means thatresources could be simultaneously depleted, so achievingsustainability might be more challenging than was thought.” However, does the paper tell us anything useful about scarcity orsustainability? I suggest that it does not. Although some of the media coverage may have extrapolatedbeyond the paper’s intent, the