Fertile Clay Soil Research Articles

Passive regeneration of subtropical grassland vegetation in a chronosequence of ex‐cultivated fields in Australia

AbstractAimIn Australia, natural grasslands largely occur on fertile clay soils and have consequently been extensively cultivated since European settlement. Other studies have demonstrated that passive restoration of subtropical grassland is possible in environments where there is substantial native remnant vegetation in the surrounding landscape. This study examined a chronosequence of abandoned fields to investigate the recovery of fragmented grassland in a matrix of cultivated land with significant exotic species that may impede succession.LocationDarling Downs, Queensland, Australia.MethodsIn this study we surveyed a chronosequence of ex‐cultivated subtropical grasslands in Queensland's Darling Downs, where only 1% of the original grasslands remain. Floristic surveys were carried out in 80 fallow sites up to 51 years old, and data were compared with 16 previously surveyed remnant grasslands on the same land type.ResultsRichness of native species increased with time since cultivation and approached, but did not reach, that of remnant grasslands after 51 years, whereas the richness of exotics declined with time since cultivation. Rainfall and grazing positively and negatively affected exotic plant abundance respectively and these factors had no significant direct or indirect effect on the native species richness. In terms of composition, fallow grasslands supported 54% of the native plant species from remnant grasslands after 51 years. The native perennial grasses Dichanthium sericeum and Aristida leptopoda were the only dominant species in the remnants that were present in more than 60% of the fallow grasslands. Of the species from remnants that were “missing” from fallow sites, most were native perennial forbs and grasses. Compositional dissimilarity of fallow grassland to remnant grasslands remained high after 51 years (78%). Grassland species that are uncommon in nearby hill woodlands were preferentially absent in fallow grassland and this partly explains the slow compositional recovery.ConclusionIn a highly fragmented landscape, passive restoration is only partially effective. The restoration of grassland is probably enhanced by repositories of native seeds from hill woodlands that are too rocky to cultivate. However, some species may have to be actively reintroduced to achieve complete recovery. In this landscape, where grazing is a dominant land use, exotic species do not seem to impede succession.

Effect of limiting vertical root growth on maize yield and nitrate migration in clay and sandy soils in Northeast China

Increasing rooting depth via subsoiling is suggested to increase nitrogen (N) uptake and reduce nitrate leaching in maize. In a two-year field experiment, this hypothesis was evaluated with two levels of N supply (120 and 240 kg N ha−1) in two soils with different leaching potential (infertile sandy soil and fertile clay soil). Maize rooting depth was limited to either the 0–20 cm (D20) or 20–40 cm (D40) soil layer using a nylon net (30 μm), or was not limited (control). There was no water stress during the growing seasons for both soils. Compared to the control, root length with the D20 and D40 treatments was longer in the 0–20 and 20–40 cm soil layers, respectively. However, total root length per plant differed less among the treatments. In clay soil, the D20 treatment did not affect N uptake, shoot biomass, or maize yield, but increased nitrate leaching. In sandy soil, the D20 treatment greatly reduced plant N accumulation, shoot biomass, and grain yield, while nitrate leaching was similar across the three treatments. It is concluded that deeper roots in maize contribute to increase N uptake and grain yield in sandy soil, but only help to reduce nitrate leaching in clay soil.

Allometry of the pyrophytic Aristida in fire-maintained longleaf pine-wiregrass ecosystems.

Aboveground biomass (AGB) of herbaceous vegetation is a primary source of fuel in frequent surface fires that maintain grasslands, savannas, and woodlands. Methods for nondestructively estimating AGB are required to understand the mechanisms by which fuels affect fire behavior and the effects of time since the last burn. We developed allometric equations to estimate AGB in wiregrass (Aristida beyrichiana/A. stricta), a dominant bunchgrass in Pinus palustris ecosystems and a key species for ecological restoration. We collected wiregrass from North Carolina to Florida, across a range of time-since-last burn and site types. We tested 32 mixed effect models to see which predictors were best at predicting live, dead, and total AGB. We also examined how time since burn (TSB) affected the live-to-dead ratio (LDR) using regression. Wiregrass AGB was found to increase with increasing latitude (relative to tussock volume), possibly due to an increase in precipitation, and was greater on more fertile clay soils and flatwoods than on sandy soils. The LDR decreased as a power function with TSB, resulting in rapid accumulation of dead, highly flammable, biomass in the fire-free period. Greater biomass will support fires of higher intensity. Our models can be useful in the parameterization of future physics-based models to predict fire behavior. Understanding the environmental variables that influence the allometry of wiregrass should help increase the precision of AGB estimates and the subsequent effects on fire behavior and effects on neighboring vegetation.

The heavy plow and the agricultural revolution in Medieval Europe

This research sheds new light on the much-debated link between agricultural productivity and development. We do so by estimating the causal impact of a large shock to agricultural productivity—the introduction of the heavy plow in the Middle Ages—on long run development. We build on the work of Lynn White, Jr. (1962), who argued that it was impossible to take proper advantage of the fertile clay soils of Northern Europe prior to the invention and widespread adoption of the heavy plow. We implement the test in a difference-in-difference set-up by exploiting regional variation in the presence of fertile clay soils. Using a high quality dataset for Denmark, we find that historical counties with relatively more fertile clay soil experienced higher urbanization after the heavy plow had its breakthrough, which was around AD 1000. We obtain a similar result, when we extend the test to European regions. Our findings substantiate that agricultural productivity can be an important driver of long-run development.

Characterization of the plant traits contributed to high grain yield and high grain nitrogen concentration in maize

During the past several decades, maize (Zea mays L.) grain yield (GY) has increased dramatically, while grain nitrogen concentration (GNC) has declined in modern hybrids. Genetic improvement to increase both high GY and GNC is necessary to improve maize nutritional quality. In the present study, we characterized the plant traits in three maize cultivars (YD13, ZD958, XY335) with contrasting GY and GNC in a two-year field experiment in two soils (Fu-jia-jie with infertile sandy soil and Quan-yan-gou with fertile clay soil). The hybrid YD13 as a control had low yield and high GNC. In comparison to YD13, ZD958 had higher GY but lower GNC, whereas XY335 had higher GY and similar GNC. Both ZD958 and XY335 had higher total and post-silking dry matter (DM) accumulation and N uptake than YD13, and were also characterized by delayed leaf senescence and a sustained net photosynthetic rate after silking. In addition, XY335 also had higher N remobilization efficiency (NRE) and higher photosynthetic nitrogen use efficiency (PNUE) than ZD958. It is supposed that higher NRE together with higher N and DM accumulation are the target traits to improve the modern stay-green cultivars to increase GNC without penalty in GY in the area of northeast China. Increasing PNUE of the leaves may be a possible way to maintain whole-plant photosynthesis and DM accumulation under efficient N remobilization.

The Heavy Plough and the Agricultural Revolution in Medieval Europe

This research tests the long-standing hypothesis put forth by Lynn White, Jr. (1962) that the adoption of the heavy plough in Northern Europe led to increased population density and urbanization. White argued that it was impossible to take proper advantage of the fertile clay soils of Northern Europe before the invention and widespread adoption of the heavy plough. We implement the test in a difference-in-difference set-up by exploiting regional variation in the presence of fertile clay soils across European regions as well as across Danish historical counties. Consistent with the hypothesis, we find that regions with relatively more fertile clay soil experienced higher urbanization and population growth after the heavy plough had its breakthrough, which was approximately around the closing of the first millennium AD. Our findings suggest that the heavy plough accounts for around 10% of the increase in urbanization and population density during the High Middle Ages.

Resilience of a high‐conservation‐value, semi‐arid grassland on fertile clay soils to burning, mowing and ploughing

AbstractIn grassland reserves, managed disturbance is often necessary to maintain plant species diversity. We carried out experiments to determine the impact of fire, kangaroo grazing, mowing and disc ploughing on grassland species richness and composition in a nature reserve in semi‐arid eastern Australia. Vegetation response was influenced by winter–spring drought after establishment of the experiments, but moderate rainfall followed in late summer–autumn. Species composition varied greatly between sampling times, and the variability due to rainfall differences between seasons and years was greater than the effects of fire, kangaroo grazing, mowing or disc ploughing. In the fire experiment, species richness and composition recovered more rapidly after spring than autumn burning. Species richness and composition were similar to control sites within 12 months of burning and mowing, suggesting that removal of the dominant grass canopy is unnecessary to enhance plant diversity. Two fires (separated by 3 years) and post‐fire kangaroo grazing had only minor influence on species richness and composition. Even disc ploughing caused only a small reduction in native richness. The minor impact of ploughing was explained by the small areas that were ploughed, the once‐off nature of the treatment, and the high degree of natural movement and cracking in these shrink‐swell soils. Recovery of the composition and richness of these grasslands was rapid because of the high proportion of perennial species that resprout vegetatively after fire and mowing. There appears to be little conservation benefit from fire, mowing or ploughing ungrazed areas, as we could identify no native plant species dependent on frequent disturbance for persistence in this grassland community. However, the ability of the Astrebla‐ and Dichanthium‐dominated grasslands to recover quickly after disturbance, given favourable seasonal conditions, suggests that they are well adapted to natural disturbances (e.g. droughts, fire, flooding and native grazing).

Root-lesion nematodes (Pratylenchus thorneiandP. neglectus): a review of recent progress in managing a significant pest of grain crops in northern Australia

Two species of root-lesion nematode (predominantly Pratylenchus thornei but also P. neglectus) are widespread pathogens of wheat and other crops in Australia's northern grain belt, a subtropical region with deep, fertile clay soils and a summer-dominant rainfall pattern. Losses in grain yield from P. thornei can be as high as 70% for intolerant wheat cultivars. This review focuses on research which has led to the development of effective integrated management programs for these nematodes. It highlights the importance of correct identification in managing Pratylenchus species, reviews the plant breeding work done in developing tolerant and resistant cultivars, outlines the methods used to screen for tolerance and resistance, and discusses how planned crop sequencing with tolerant and partially resistant wheat cultivars, together with crops such as sorghum, sunflower, millets and canaryseed, can be used to reduce nematode populations and limit crop damage. The declining levels of soil organic matter in cropped soils are also discussed with reference to their effect on soil health and biological suppression of root-lesion nematodes.

Variable grain legume yields, responses to phosphorus and rotational effects on maize across soil fertility gradients on African smallholder farms

Promiscuous soyabean varieties have potential to contribute significantly to income generation, food security and soil N budgets on smallholder farms. One of the major factors limiting this potential is farmers’ preference to allocate nutrient resources to food security cereal crops on the most fertile fields, leaving grain legumes to grow on residual fertility on infertile fields. Two experiments were conducted to: (i) compare the current farmer practice with targeting manure and single super phosphate (SSP) to soyabean in a three-year rotation cycle on two fields with different soil fertility: an infertile sandy soil and a more fertile clay soil; and (ii) assess the effects of variability of soil fertility within and across farms on productivity of soyabean and groundnut. In the first experiment, soyabean (<0.2 t ha−1) and maize yields (<0.7 t ha−1) without fertilizer were poor on a degraded sandy soil. Both crops responded poorly to SSP due to deficiency of other nutrients. Manure application significantly increased soyabean and maize yields, led to yield stabilization over three seasons and also significantly increased the proportion of N2 fixed by soyabean (measured using 15N natural abundance) from 60% to 83%. On the sandy soil, P was used more efficiently and gross margins were greater when SSP and manure were applied to maize in a maize–soyabean rotation. Soyabean and maize yields without fertilizer inputs were larger on clay soil with moderate fertility (0.4–0.7 t ha−1 and 2.0–2.3 t ha−1 respectively) and were significantly increased by application of SSP and manure. Within rotations, P recovery was higher when manure and SSP were applied to maize (43 and 25%) than when applied to soyabean (20 and 19%). However, application of manure to soyabean on the clay was more profitable than application to maize for individual crops and within rotations. In the second experiment, soyabean and groundnut yields were largest (∼1 and ∼0.8 t ha−1 respectively) on plots closest to homesteads on wealthy farms, which were more fertile due to good past management. Yields were poor (< 0.5 t ha−1) on other fields which previously had received little nutrient inputs. Soyabean and groundnut yields correlated well with available P (R 2 = 0.5–0.7) and soil organic C (SOC) contents (R 2 = 0.4–0.6). For smallholder farmers to maximise benefits from legume production they need to focus attention on the more fertile plots, although production should be optimized in relation to maize. Targeting nutrients to maize as currently practiced by farmers was more efficient and economic under poor soil fertility conditions, whilst potential exists to increase income by targeting manure to soyabean on the more fertile soils.

Spatial and temporal analysis of vegetation change in agricultural landscapes: A case study of two brigalow (Acacia harpophylla) landscapes in Queensland, Australia

The majority of landscapes around the world have been modified or transformed by human activities to meet the needs of human societies. The loss of native vegetation for agricultural development affects the sustainability of growing proportion of the world's ecosystems. Factors such as land tenure, roads and agricultural intensification, together with biophysical properties, have been cited as drivers of deforestation. This paper combined analysis of the historical drivers of change with analysis of the trends of deforestation since 1945 in two brigalow landscapes (100,000 ha) in sub-tropical Australia. A selection of these drivers were then applied at a property-level (1000 ha) to test their influence on native vegetation retention. Regression trees were used to identify significant human drivers and biophysical properties, and then a generalised linear modelling approach was used to quantify the effect of these factors on the proportion of remnant native vegetation. Results showed that until the mid-20th century, government policies to intensify settlement did not result in increased agricultural production, but since this time, landscape change has been rapid, and has particularly affected ecosystems on fertile clay soils. Although socio-economic factors were critical in driving deforestation, after 60 years of agricultural intensification by far the most significant explanatory variable determining the proportion of native vegetation retained at a property scale was the suitability of the soil for agriculture. Property size was an important secondary influence. The results were not, by and large, consistent with other studies of landscape change and suggest that generalised principles explaining deforestation may be elusive. Solutions to the problem of over-clearance of native vegetation, therefore, need to be tailored to the specific regional situations encountered.

Preliminary assessment of gidgee (Acacia cambagei) woodland thickening in the Longreach district, Queensland

Gidgee (Acacia cambagei) forms woodlands and forests on moderately fertile clay soils throughout semi-arid areas of north-eastern Australia. Analysis of soil data from Queensland indicates that gidgee’s environmental domain has a broad overlap with treeless Mitchell grassland although a preference for slightly coarser soils with higher phosphorus and lower potassium content. Mapping of woody cover change for an area of central Queensland suggests that gidgee thickening and spread has occurred between 1951 and 1994. However, this increase has been more than offset by loss of gidgee cover by clearing in other areas. Gidgee thickening and encroachment onto grassland is a problem for graziers because the trees suppress grass and are unpalatable except when very young. Landholder surveys and other information indicate that gidgee has recruited sporadically and that thickening occurred with a massive seedling recruitment in the 1950s with a smaller recruitment wave in the 1970s. These waves coincide with relatively wet times and the extreme rainfall abundance in the 1950s has a multi-century expected recurrence. There are historical records describing gidgee death during the extreme droughts in the first part of the 20th century. Young gidgee plants are fire sensitive but well-established gidgee stands are fire retardant, and the importance of fire for control of gidgee thickening is unknown. One scenario explaining gidgee dynamics emphasises the role of environment (mostly climate and soil), while another emphasises management (mostly fire). Research priorities are presented and may contribute to appropriate management of gidgee woodlands.

Combining ability for seed yield traits of Chenopodium quinoa breeding lines

For a better understanding of the inheritance of seed yield traits in Quinoa (Chenopodium quinoa) Willd., a half-diallelic crossing experiment with six diverse but uniform breeding lines was conducted. True hybrid plants were detected by means of differences in panicle colour in 14 out of the 15 crosses performed. The agronomic performance of 14 F2 populations and six parental lines was evaluated in field trials on fertile clay soil. General and specific combining ability of the lines (GCA and SCA) were estimated. Highly significant differences in GCA effects were found between the lines for plant height at maturity, early flowering, early maturity, seed yield and thousand seed weight. Significant SCA effects were only found for plant height at maturity. However, SCA effects were rather small and accounted for 7% of the total variance. The finding of large GCA effects and low SCA effects suggests that best selection results could be expected from crosses between the agronomic best performing genotypes.

Effect of culture conditions on the antimicrobial activities of UV-mutants of Streptomyces corchorusii and S. spiroverticillatus against bean and banana wilt pathogens

The potentiality of 36 strains of actinomycetes species isolated from fertile clay soils in Egypt for the production of antimicrobial substances against a variety of test organisms was investigated. Most of the isolated organisms inhibited gram positive, gram negative and acid fast bacteria as well as yeasts and filamentous fungi. Nine isolates showed antagonistic activities against the French bean-wilt fungus Fusarium oxysporum f.sp. phaseoli, and 19 isolates antagonized the bacterial-wilt pathogen of banana Pseudomonas solanacearum in vitro. The most active isolates against the preceding pathogens were identified as Streptomyces corchorusii and S. spiroverticillatus. These were subjected to UV-irradiation treatment in order to induce mutants with increased antagonistic activities. Eleven mutants were obtained from S. corchorusii and mutant M155 was the most active against the Fusarium -wilt pathogen on agar medium (diameter of inhibition zone was 15 mm after 10 days compared with 9 mm for the wild-type). All five mutants obtained from S. spiroverticillatus were inhibitory to P. solanacearum ; mutant M281 was the most potent in liquid medium (diameter of inhibition zone was 24 mm after 2 days compared with 12 mm for the wild-type by the paper disc method). Different concentrations of the ingredients of starch-nitrate medium stimulated by nearly 2-fold the inhibitory effects of M155 and M 281 as compared with their wild types. Under such conditions, S. corchorusii M155 lysed completely the mycelium of F. oxysporum f. sp. phaseoli after 10 days.

Influence of edaphic specialization on pteridophyte distribution in neotropical rain forests

The abundance of terrestrial pteridophytes was studied in eight sites in Western Amazonia, all representing lowland tropical rain forest in non‐inundated terrain (tierra firme). In each site, a sample plot ranging from 0.25 ha to 1.00 ha in area was inventoried and all terrestrial pteridophytes were recorded. In total, 27,000 individuals representing sixty‐four species were encountered, with each plot having between fourteen and thirty‐three species. The plots represented a gradient from nutrient‐poor sandy soils to relatively fertile clay soils, and it was found that their floristic compositions clearly correlated with the differences in the soils. Two‐thirds of the species were restricted either to poor, intermediate or rich soils, while less than a tenth were found growing on all soil types. Analyses of species lists from another twenty neotropical localities supported the result that some pteridophyte species associate, whereas other species are never found growing together. The floristic similarities among sites showed no relationship to the geographical distances between them, but rather reflected similarities in soils. It is concluded that pteridophytes are good indicators of such ecological conditions, and that when interpreting biogeographical patterns in the neotropical rain forests, it is important to take into account the distribution of edaphically different habitats.

Growth, yield and composition of four winter cereals. II. Nitrogen and carbohydrate economy

A field experiment with 3 cultivars each of wheat, rye, triticale and barley, grown at a density of about 320 plants/m, was conducted in 1986 on a fertile clay soil at East Flevoland, Netherlands. N at 120 kg/ha for wheat and triticale and 60 kg/ha for rye and barley was split-dressed in 2 applications. N yield was highest in wheat (196 kg/ha) and lowest in rye (123 kg/ha). The amounts taken up were influenced by the N rate. The triticale cv. Lasko and the barley cv. Marinka had a higher N-uptake than the other triticale and barley cultivars. N harvest index (i.e. the ratio of N in grains and N in above-ground DM at final harvest) was lowest in rye and highest in barley. N concentration in plant organs (grains, chaff, leaves, stems and roots) was higher in wheat and triticale than in rye and barley. This was probably caused by the difference in the level of N application. N use efficiency, expressed as grain DM production/kg N taken up, was 53 in wheat, 68 in rye, 50 in triticale and 61 in barley. In all species, the largest reserves of water-soluble carbohydrates (WSC) were found in the stems. Rye allocated more dry matter to stem growth before flowering than wheat, triticale and barley. Averaged over these cereals, 26% of WSC, produced before flowering, was used for redistribution and respiration during grain production.

Growth, yield and composition of four winter cereals. I. Biomass, grain yield and yield formation

A field experiment with 3 cultivars of each of 4 winter cereals (wheat, rye, triticale and barley), sown at about 320 plants/msuperscript 2, was conducted on a fertile clay soil in the central Netherlands. The N fertilizer was split-dressed: 120 kg/ha in total for wheat and triticale, and 60 kg/ha for rye and barley. The fewest shoots/msuperscript 2 were found in triticale (828/msuperscript 2), and the most in barley (1477/msuperscript 2). The average decrease in number of shoots during shoot/ear development was 51% in wheat, 54% in rye, 49% in triticale and 67% in barley. The rate of crop development was largest in rye and barley; they flowered and matured earlier than wheat and triticale. Rye was the first to attain a closed canopy (LAI &gt;3), and had the lowest maximum LAI and shortest leaf area duration. Wheat and triticale stayed green longest. Average specific leaf weight was 4.3 mg/cmsuperscript 2 in wheat and triticale, and 3.7 mg/cmsuperscript 2 in rye and barley. The growth rate of the grains at the linear stage was fastest in barley (1.89 mg grain-1 d-1) and slowest in rye (0.89 mg grain-1 d-1). Total aboveground DM production was 18 790 kg/ha in wheat, 15 230 kg/ha in rye, 18 300 kg/ha in triticale and 12 460 kg/ha in barley. Grain yields (t/ha) were 8.74 (wheat), 6.64 (rye), 8.20 (triticale) and 6.62 (barley). Cultivar differences in grain yield and in yield components were mostly smaller in wheat and rye than in triticale and barley. The harvest index was highest in barley (53.3%) and lowest in rye (43.7%); in some plant species there were marked differences between cultivars, but in others there were not.

Leaf nutrient analysis in Salix viminalis (L.) energy forest stands growing on agricultural land

AbstractThe seasonal pattern of morphological leaf characteristics, leaf nutrient and carbohydrate contents in two energy forest stands growing on fertile clay soil and consisting of Salix viminalis (L.) was investigated. One of the stands was irrigated and liquid fertilized (IL) daily from May to August while the other stand was untreated (C). The study was carried out on shoots growing for their second year on five‐year‐old roots.A pronounced seasonal variation in length, weight, and area of leaves of the same developmental stage was observed, while differences between the stands were small. The leaf content of carbohydrates and starch was low. Nitrogen was the only nutrient that was significantly higher in the IL stand compared with the C stand throughout the growing season. The small differences between stands were most probably an effect of the initially high soil fertility. Different bases for expressing plant nutrient status during the growing season, i.e. nutrient amount per leaf dry weight with and without carbohydrates, nutrient amounts per leaf or leaf area, nutrient proportions in leaves, and nutrient amounts in the total canopy, were compared and discussed. It was concluded that the common expression, nutrient weight per dry weight of leaves, without correction for non‐structural carbohydrates, was adequate to describe the nutrient situation in well‐growing basket willow plantations. The most appropriate time for leaf sampling was proposed to be the phase of most intensive growth.

Bacterial composition and N2‐fixation of some Egyptian soils cultivated with wheat (Triticum aestivum L.)

AbstractThe composition of the microflora, N2‐fixing bacteria particularly, in different soils cultivated with wheat in Egypt was investigated in some samples collected from the fields after applying the agricultural practices recommended for wheat cultivation and just before sowing. The influence of carbon sources, mineral nitrogen and water regimes on potential dinitrogen fixation (acetylene reduction assay) in soils was investigated. The bacterial population densities including‐N2‐fixing organisms were related to a number of environmental factors such as organic matter content. Among diazotrophs, Azotobacter spp. and Azospirillum spp. were encountered in higher densities in comparison with clostridia. Unamended soils showed a lower acetylene‐reducing activity (0.5–61.5 nmoles C2H4 g−1 h−1). Addition of glucose (1% w/w) greatly enhanced such activity being the highest (86.9–2846.5 nmoles C2H4 g−1 h−1) in the clay soil with the highest organic carbon content (1.42%). Glucose amendment had no significant influence on acetylene reduction in the saline soil. N2‐fixation in barley straw‐amended (1%) soils was not much higher than in unamended soils. Concentrations of up to 70 ppm ammonium‐nitrogen depressed N2‐fixation in soils that received barley straw. Acetylene reduction in submerged soil increased after addition of cellulose. Non‐flooded conditions favoured N2‐fixation in the fertile clay soil amended with sucrose.

Frankiasymbiosis as a source of nitrogen in forestry: a case study of symbiotic nitrogen-fixation in a mixed Alnus-Picea plantation in Scotland

SynopsisAlnus rubragrowing in the field in Scotland is nodulated effectively by local strains ofFrankia. Strains which have been isolated fromAlnus rubraat Lennox Forest show different growth medium requirements and colony morphology compared with isolates fromAlnus glutinosaand several different strains have been isolated from the same locality. Preliminary evidence suggests that some spore negative, northwest AmericanFrankiastrains may be more effective than local spore positive crushed nodule inoculum for nitrogen-fixation inAlnus rubra.On the moderately fertile clay soil of Lennox Forest, a mixed plantation of alternatePicea silchensisandAlnus rubrashowed no improvement in growth of spruce compared with pure spruce plots. However, the presence of alder increased upper soil nitrogen status by 585 kg ha ' which approximates the standard 150 kg N ha 'of fertiliser nitrogen applied in practice to nitrogen deficient stands at about 5-year intervals. Although the alder had penetrated the subsoil, there was no apparent effect on spruce rooting depth in the mixed stands in this high clay soil. In addition to nitrogen content, the total phosphorus of the upper soil horizons was improved in the mixed plots by an estimated 3–6 kg ha-1y1and it is suggested that this phosphorus may be brought from the subsoil by the deeper rooting alder and deposited on the surface in its litter.Although improved growth of spruce in mixture withAlnus rubrais only likely where pure spruce stands are stressed for nutrients, the potential benefits of symbiotic nitrogen-fixation in silvicultural practice make it desirable to investigate other species and provenances ofAlnussuited to British conditions, to achieve maximum symbiotic fixation of nitrogen by selection and inoculation with superior strains ofFrankiaand to include such plants in trials of mixtures on sites where nitrogen-availability may be critical.

Witchweed (Striga hermonthica) on rain‐grown pearl millet in nitrogen‐deficient sandy soil of the central Sudan

SUMMARYSpraying with 1 lb./acre of 80% sodium 2,4‐D 2 weeks after sowing reduced incidence of the phanerogamic root parasite Striga hermonthica (Scrophulariaceae, ‘witchweed’) attacking rain‐grown pearl millet (Pennisetum typhoides) in sandy soil of the Central Sudan, but this control resulted in increased yields only on land very deficient in available nitrogen and not when ammonium sulphate fertilizer was added. Nitrogen deficiency may be more important than Striga attack in limiting yields of millet in these sandy soils, although in inherently fertile clay soils elsewhere in the Central Sudan Striga has been reported to cause heavy crop losses to sorghum (Sorghum vulgare) before signs of soil exhaustion appear.