Inherent Soil Fertility Research Articles

Yields and Nutrient Use-Efficiency in Groundnut-Sunflower Cropping System in Punjab, India

ABSTRACT Long-term continuous cereal cropping systems without returning crop residues to soil has led to depletion of inherent soil fertility in semiarid subtropical soils of South Asia. The inclusion of legumes and their residues assumes great significance to restore sustainability. Groundnut (Arachis hypogaea L.) and sunflower (Helianthus annus L.) are important oilseed crops of the region and have the feasibility to be grown in annual double crop rotation with the expansion of irrigation facilities. A 4-year field experiment with irrigated groundnut-sunflower rotation was conducted on a loamy sand soil (Typic Haplustept) to evaluate the effects of different rates of fertilizer N (0, 30 and 60 kg N ha−1) and P (0, 20, 30 and 40 kg P2O5 ha−1 corresponding to 0, 8.7, 13 and 17.5 kg P ha−1, respectively), applied either to one crop (direct for current crop and residual for succeeding crop) or to both crops (cumulative) each year, with or without 4 t ha−1 of groundnut crop residues (CR) on crop yields, nutrient uptake, fertilizer use efficiency and soil productivity parameters (pH, organic carbon, available N and P). The residual effects of P and CR were assessed on the succeeding groundnut crop. Winter-grown sunflower responded significantly and consistently to applied fertilizer N and P. Seed yield of sunflower increased significantly up to 60 kg N and 30 kg P2O5 ha−1 and there was no further increase with directly applied 40 kg P2O5 ha−1. Cumulative application of P produced sunflower yields comparable with those from direct applications. Incorporation of groundnut residue in conjunction with 30 kg N ha−1 significantly increased the sunflower yields that were statistically at par with those obtained with the lone application of 60 kg N ha−1. All through 4 years, the combined application of CR with 60 kg N ha−1 and 30 kg P2O5 ha−1 further improved the yield potential of sunflower by 10–11% and of succeeding groundnut by 10–24%, illustrating the benefit that fertilizer N and P could not achieve. Residual fertilizer P (30 kg P2O5 ha−1) produced optimum yields of groundnut, which were comparable with that from directly applied 20 kg P2O5 ha−1. After 4 years of study, soil organic carbon swelled by 22% and Olsen-P increased from initial 12 kg P ha−1 to 24–43 kg P ha−1 in fertilizer and CR-treated plots. These results illustrated that in groundnut-sunflower rotation, (1) 60 kg N and 30 kg P2O5 ha−1 are adequate for the sustaining optimal sunflower production; (2) whereas the application of fertilizer P to sunflower crop is required, P requirement of groundnut can be met from soil-derived and residual fertilizer P; and (3) incorporation of groundnut crop residues in conjunction with the adequate rates of N and P has complementary effects in maximizing the yields, uptake of N and P, and nutrient use-efficiency in sunflower as well as its residual effects on succeeding groundnut crop, and in sustaining long-term soil productivity.

Millet nutrient use efficiency as affected by natural soil fertility, mineral fertilizer use and rainfall in the West African Sahel

Field experiments were designed to investigate the effectiveness of integrated soil fertility management (ISFM), comparing fertilizer use efficiency and its impact on millet, cultivated close to the homestead (“infields”) and away from the homestead (“outfields”). Millet yields and response to N (0, 30, and 60 kg ha−1) and P (0, 15, and 30 kg ha−1) were determined on nine infields and nine outfields over a period of 3 years (from 1999 to 2001) in the southern Sahel of Niger. Rainfall was 650, 470, and 370 mm during the three successive years, interaction between decreasing rainfall and millet yield performance was also analyzed. While soil organic carbon (1.5 g kg−1 on outfields and 1.6 g kg−1 on infields) and pH-H2O (4.8 on outfields and 5.1 on infields) were comparable, total-N, plant available P (measured as P-Olsen and P-Bray), and exchangeable Ca, K, and Mg levels were higher on infields as compared to outfields. Without fertilizer, average grain yield (GY) and stover yield obtained on infields were three times as high as on outfields. GY across years and fertilizer treatments was higher on infields as compared to outfields (P < 0.001). Average yield was 800 kg ha−1 on outfields and 1,360 kg ha−1 on infields (P < 0.001). On outfields, average GY was stagnant over the 3-year experimental period. Despite declining rainfall, millet GY across all treatments gradually increased over time on infields (P < 0.001). P fertilization alone resulted on both field types to steadily and substantial yield increases while yield response to N fertilization was only obvious when fertilizer P was applied. With no fertilizer applied, N uptake on infields (19 kg N ha−1) was more than twice as high as on outfields (7 kg ha−1), and P uptake was four times higher on infields (3 kg ha−1) than on outfields (0.8 kg ha−1). Indigenous soil N supply was on average 24 kg N ha−1 on outfields and 46 kg N ha−1 on infields. Average value for indigenous soil P supply was 4 kg P ha−1 on infields and 2 kg ha−1 on outfields. Apparent recovery of fertilizer N applied varied considerably among treatments and ranged from 17 to 23% on outfields and 34 to 37% on infields (P < 0.001). Average apparent recovery of fertilizer P applied was significantly higher (P < 0.001) on infields (31%) than on outfields (18%) over the 3-year growing period, illustrating ISFM-induced positive effect on millet nutrient N and P use. Results indicate higher inherent soil fertility, underline ISFM-induced drought tolerance of soils on infields as compared to outfields, and highlight the crucial role of fertilizer P (especially on outfields) for millet production. These call for site-specific nutrient management and support, even under low rainfall conditions, the potential value of fertile infields for efficient and productive external input use and sustainable millet production in West African Sahel.

Mixtures of papermill biosolids and pig slurry improve soil quality and growth of hybrid poplar

AbstractHybrid poplar plantations in Quebec, Canada, are generally established on marginal agricultural lands characterized by low pH and low inherent soil fertility. Here, we tested the hypothesis that two potential organic fertilizer (OF) sources, papermill biosolids (PBs) and liquid pig slurry (LPS), would improve soil quality and the growth performance of hybrid poplars (Populus trichocarpa × Populus deltoides), especially if applied in mixtures rather than separately. The fertilizer treatments included an unfertilized control, inorganic fertilizer (IF) (calcium ammonium nitrate and triple superphosphate) and OFs (PBs alone, LPS alone and two combinations of PBs and LPS) applied at two rates. Fertilizers were broadcast within 1 m of tree trunks and unincorporated, to prevent damage to tree roots. Hybrid poplar growth was the greatest in plots fertilized with a combination of PBs and LPS, suggesting that the two OFs complemented themselves and/or interacted to improve soil nutritional quality. PBs were the most efficient at raising soil pH, providing plant‐available Ca and increasing nitrification rates over the long term, whereas LPS provided more readily available NO3‐N, P and K. Applied together, PBs and LPS interacted to provide more extractable P and mineralizable NH4‐N than when applied separately. OFs increased soil biological activity, notably basal respiration, microbial biomass, metabolic quotient and mineral N production rates. Community‐level catabolic profiles of the extractable soil microflora in plots with OFs differed significantly from the control and IF treatments. This implies that surface‐applied OFs may induce fundamental changes to the diversity and composition of microbial communities in the underlying rooting zone. Although this study has shown beneficial effects of OF mixtures on soil quality and hybrid poplar growth, further research should focus on their possible environmental impacts.

Available technologies to replenish soil fertility in East Africa

Low inherent soil fertility in the highly weathered and leached soils largely accounts for low and unsustained crop yields in most African countries. But in particular, the major nutrients, nitrogen (N) and phosphorus (P), are commonly deficient in these soils. This scenario of nutrient depletion is reflected in food deficits and hence the food aid received continuously, specifically in sub-Saharan Africa. Undoubtedly, substantial efforts have been made in the continent to replenish the fertility of degraded soils in attempts to raise crop yields, towards self-sufficiency and export. Such efforts consist of applications of both organic and inorganic resources to improve the nutrient status of soils and enhanced nutrient uptake by crops, provided that soil moisture is adequate. Overall, positive crop responses to these materials have been obtained. Thus in the East African region, maize (staple) yields have been raised in one growing season from below 0.5 t/ha without nutrient inputs, to 3–5 t/ha from various nutrient amendments at the smallhold farm level. However, in spite of the positive crop responses to nutrient inputs, farmers are generally slow to adopt the soil fertility management technologies. In this paper we review the impact of some technologies, focussing the use of nutrient resources of different characteristics (qualities) in relation to improved crop yields, with an overall goal to enhance technology adoption. Thus, inorganic resources or fertilizers often give immediate crop responses, but their use or adoption is rather restricted to large-scale farmers who can afford to buy these materials. Organic resources, which include crop residues, water hyacinth and agroforestry shrubs and trees, are widely distributed, but they are generally of low quality, reflecting the need to apply large quantities to meet crop nutrient demands. Moreover, most organics will add N mainly to soils. On the other hand, phosphate rocks of varying reactivity are found widely in Africa and are refined elsewhere to supply soluble P sources. The recently developed soil fertility management options in East Africa have targeted the efficient use of N and P by crops and the integrated nutrient management approach. Some people have also felt that the repackaging of inputs in small, affordable quantities, such as the PREP-PAC described in this paper, may be an avenue to attract smallhold farmers to use nutrient inputs. Nonetheless, crop responses to nutrient inputs vary widely within and across agroecozones (AEZs), suggesting specificity in recommendations. We highlight this observation in a case study whereby eight soil fertility management options, developed independently, are being tested side- by-side at on-farm level. Farmers will be empowered to identify technologies from their own choices that are agronomically effective and economically friendly. This approach of technology testing and subsequent adoption is recommended for technology development in future.

Soil fertility in three cropping systems after conversion from conventional to organic farming

Temporal changes in the scores of selected soil fertility indices were studied over six years in three different cases of organic crop rotation located in southern, eastern and central Norway. The cropping history and the initial scores of fertility indices prior to conversion to organic cropping differed between the sites. Crop yields, regarded as an overall, integrating fertility indicator, were in all rotations highly variable with few consistent temporal trends following the first year after conversion. On the site in eastern Norway, where conversion followed several years of all-arable crop rotations, earthworm number and biomass and soil physical properties improved, whereas the system was apparently degrading with regard to P and K trade balances and contents in soil. On the other two sites, the picture was less clear. On the southern site, which had a relatively fertile soil before conversion, the contents of soil organic matter and K decreased during the six-year period, but the scores of other fertility indices showed no trends. On the site in central Norway, there were positive trends for earthworm-related indices such as worm biomass and tubular biopores, and negative trends for soil porosity. The results, especially those from the eastern site, illustrate the general difficulty in drawing conclusions about overall fertility or sustainability when partial indicators show divergent trends. Consequently, the study gave no unambiguous support to the initial working hypothesis that organic farming increases inherent overall soil fertility, but rather showed that the effect varied among indicators and depended on status of the cases at conversion. It is concluded that indicators are probably better used as tools to learn about and improve system components than as absolute measures of sustainability.

Factors affecting rice yield and fertilizer response in rainfed lowlands of northeast Thailand

Rice-based ( Oryza sativa L.) rainfed lowlands are the major cropping system in northeast Thailand. Earlier research on nutrient management of rainfed lowland rice produced conflicting results with respect to inherent soil fertility, fertilizer response, and the importance of organic fertilizers, most probably because of highly variable soil quality and water resources. The objectives of this paper were to advance the understanding of soil fertility and fertilizer response in northeast Thailand and thereby provide a basic framework for improved nutrient management of rainfed lowland rice. For this purpose, we analyzed an existing database on fertilizer trials conducted between 1995 and 1997 at eight different sites in northeast Thailand, which were previously described by Wade et al. [Wade, L.J., Amarante, S.T., Olea, A., Harnpichitvitaya, D., Naklang, K., Wihardjaka, A., Sengar, S.S., Mazid, M.A., Singh, G., McLaren, C.G., 1999a. Nutrient requirements in rainfed lowland rice. Field Crops Res., 64, 91–107]. Average annual rainfall across sites and seasons was 1300 mm, but half of all rainfed trials (12 of 23) experienced substantial water stress during the growing season. Average grain yield in N-omission plots was low (1.6 t ha −1), even when compared with that of rainfed lowlands in neighboring Lao PDR. Nitrogen was clearly the most limiting element, whereas PK treatments increased yields significantly in only 6 out of 78 observations. Average agronomic efficiency of applied N was good (16 kg grain kg −1 N), but highly variable among sites. Two groups of soils (i.e., sites) were separated because of their distinct differences in reaction to inorganic and organic fertilizer. Better nutrient availability improved crop performance at all field water stress levels occurring at the trial sites. However, yield reductions caused by water stress seemed to interact with the level of nutrient supply, that is, absolute yield differences between different fertilizer treatments decreased with increasing water stress. We concluded that efficient fertilizer use in rainfed rice of northeast Thailand can be achieved, but that existing uniform recommendations do not provide farmers with much useful advice. Therefore, we proposed a set of basic guidelines for improved nutrient management, which, after further efforts of all stakeholders involved, could contribute to increased system productivity.

CHANGE IN SPECIES COMPOSITION WITH REPEATED SHIFTING CULTIVATION: LIMITED ROLE OF SOIL NUTRIENTS

The effect of repeated, long-fallow shifting cultivation on tree species composition in secondary forest fallows was examined in a village established over 200 years ago in West Kalimantan, Indonesia. Seven 9–12-yr-old fallows that had previously been cultivated 1–10 times were compared with three primary forest stands. Species composition in secondary forests was strongly related to the number of prior cultivation cycles experienced at a site. A systematic shift resulted from significant increases or decreases in the abundance of 27 of the 59 most common species with repeated cultivation. Seventeen additional species were limited to “early” (1–4 cycles) or “late” (6–10 cycles) in the history of cultivation. Nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca), and carbon (C) stocks in the top 15 cm of soil were higher in sites that had experienced more cultivation–fallow cycles. For Ca, Mg, and K, these differences were due only to inherent soil fertility, highest in sites with the longest cultivation history. In contrast, P, N, and C stocks increased significantly with the number of prior cultivation cycles after accounting for inherent soil fertility. Soil nutrients were significantly related to the basal area of more than half of 21 species tested. For 13 species, the response to nutrients (positive, negative, or neutral) was consistent with their response to repeated cultivation. Soil nutrient effects only partially explained the systematic shift in species composition that occurred during 200 years of long-fallow shifting cultivation. We propose that, for some species, dispersal ability compensated for a negative response to repeated cultivation, and dispersal failure doomed other species that initially responded positively.

Soil water-retention characteristics and fertility of afforested arable land

From the standpoint of evaluating site quality for growing forest trees, the soil chemical and physical properties of cultivated arable land are poorly known. Therefore, soil matrix, water-retention characteristics and fertility of afforested arable land were studied. Subsequently, the implications of these soil characteristics for productivity of tree stands were assessed. The results indicated that the soil properties of afforested arable land differ markedly from those of forest soils. The changes in the composition of the soil matrix due to former agricultural land use manifested as increased organic matter content in mineral soils and, in contrast, as increased mineral matter content in peat soils. In general, afforested arable lands were characterized by soils having high contents of nutrients, high content of organic matter, high pH, and low air-filled porosity at field capacity, which resulted from the predominance of small pores. In all investigated soils, the air-filled porosity was less than 20% in the topmost soil layer and decreased significantly with increasing soil depth. The results suggested that soils of afforested arable land commonly have critically low aeration for tree growth. It appears that due to management history, afforested arable lands have a higher level of soil fertility compared to their inherent soil fertility. The high soil fertility of former arable land was, however, also attributed to the inherent properties of fine-grained soils. Consequently, based on of densities of Ca, P, silt and pH in the 0-10 cm soil depth, derived discriminant functions classified all sites from afforested arable land into forest site types of high productivity. Among these soils, those with high clay and silt content were the most fertile. The changes in the physical and chemical properties of soils due to former agricultural land use seem to be significant and long lasting to such a degree that afforested arable land is very unlikely to regain its inherent soil fertility even over an extended period of time.

BIOMASS ACCUMULATION AFTER 10–200 YEARS OF SHIFTING CULTIVATION IN BORNEAN RAIN FOREST

I examined the effect of repeated long-fallow shifting cultivation on biomass accumulation in a rain-forest landscape occupied for over 200 years in West Kalimantan, Indonesia. The diameters of stems >5 cm dbh were measured in nine secondary forests using a stratified random design of nested plots (3000 m2 per site). The stands were 9–12 years old and had experienced from 1 to 10 or more cycles of shifting cultivation. Accounting for differences in soil fertility, analysis of covariance (ANCOVA) demonstrated that aboveground live biomass increment (ABI, in megagrams per hectare per year) was significantly greater after four cycles than after six or more cycles. ABI was lowest after two cycles, highest after one or four cycles and intermediate after 6–10 cycles. Differences were driven by the density and biomass of trees >10 cm dbh, which followed a similar pattern. Although the diameter increment and contribution to total biomass of trees >10 cm dbh increased significantly with inherent soil fertility, va...

Nutrient indexing of benchmark sites of Erode district in Tamil Nadu

In order to forecast the emerging micro- and macro-nutrient deficiencies or toxicities in crops and soils resulting from intensive cropping, 15 benchmark sites have been identified in Gobichettipalayam taluk of Erode district in Tamil Nadu, based on the soil physico-chemical properties. The experimental soils were neutral in pH, free from salinity, low in organic carbon, medium in available P &amp; K. The available micronutrient status has shown that the soil available Zn &amp; Cu and Fe &amp; Mn is in the range of deficiency and sufficiency levels, respectively. The soils belong to Irugur (red non-calcarcous), Chikkarasampalayam (black calcareous) and Sathyamangalam (black calcareous) series. Two intensive cropping systems such as Rice - Rice and Sugarcane - Sugarcane - Turmeric were chosen for the study. The soil data indicated that both intensive cropping systems have depleted the soil nutrient status heavily. However, as both cropping sequences have been supplied with sufficient quantities of fertilizer inputs carrying macronutrients, soils in the benchmark sites have maintained the inherent soil fertility status. On the other hand, non-addition of micronutrients has caused deficiencies of Zn, Cu and B as indicated by the available nutrient status in both cropping sequences. Plant samples showed sufficiency levels of both macro and micronutrients. The overall data indicated that there is an utmost necessity for application of micronutrients especially Zn, B and Cu to enable to maintain the crop yields in the intensive cropping systems in Erode district. Further, soil nutrient balance has to be maintained by proper substitution of organic and inorganic sources of macronutrients.

Research Priorities in Natural Systems Agriculture

SUMMARY Over the course of millennia and most dramatically in recent years, agriculture has eroded natural capital as it has supplied human demands. These tendencies can be offset by infusions of fossil fuels for fertility, pest control and traction, but only partially and not over the long term. This could be called the “problem of agriculture,” in contrast to problems in agriculture. Natural ecosystems are unmatched for efficient nutrient recycling, solar energy use, and biodiversity preservation, but they cannot feed dense human populations. An ecological agriculture that is a synthesis of natural and agricultural systems can save soils from erosion and reduce chemical and water use. Reducing chemical pollution of air, water and soils will conserve wildlife habitat, improve water quantity and quality and protect human health. By practicing an agriculture that has conservation as a direct result, farmers can become better stewards of the land. Lower input costs and inherent sustained soil fertility will mean more profit for farmers and communities. Principles of natural systems agriculture (NSA) are applicable to any food- or fiber-production system worldwide. In the central Great Plains of the United States, agriculture should mimic the native prairie, which is a polyculture of herbaceous perennial plants. The Land Institute has spent 25 years studying the structure and function of prairies. Now we and other research organizations are breeding perennial grain crops, including cool- and warm-season grasses, legumes, and composites. We are following two parallel strategies: selecting wild perennials for greater seed production and other traits of domestication and hybridizing wild perennials with annual crops to combine desired traits. Although the breeding process will take many years, the effort will be repaid many times over if we achieve our goal: a lasting solution to the 10,000-year-old problem of agriculture.

Diversification of rice with pigeonpea in a rice–wheat cropping system on a Typic Ustochrept: effect on soil fertility, yield and nutrient use efficiency

Continuous adoption of rice–wheat cropping system (RWCS) has led to depletion of inherent soil fertility resulting in a serious threat to its sustainability in the Indo-Gangetic plain region (IGPR) of India. The inclusion of legumes in RWCS assumes a great significance to restore soil fertility. But farmers in the IGPR rarely grow legumes in the system. We, therefore, carried out farmers’ participatory diagnostic survey in the Upper Gangetic plain zone (UGP) to understand farmers’ fertilizer management practices for wheat ( Trititicum aestivum L . Emend Fiori & Paol) following rice ( Oryza sativa L.) or pigeonpea ( Cajanus cajan(L.) Millsp). The survey indicated that most of the farmers in UGP grew pigeonpea in place of rice under RWCS as only a break crop at a 2–3 year interval. The farmers applied, on average, 11 kg N ha −1 and 24 kg P ha −1 to wheat sown after rice, and 12 kg N ha −1 and 19 kg P ha −1 to wheat sown after pigeonpea. Wheat yields, however, were lower (3.3 t ha −1) when sown after pigeonpea than after rice (3.7 t ha −1). The survey was followed by a field experiment at Modipuram (29° 4′ N), Meerut, India that continued during the three consecutive years (1998–1999 to 2000–2001) to examine the effect of inclusion of pigeonpea in place of rice on soil fertility, N and P use efficiency and yields of wheat. In 1998–1999, wheat yields after pigeonpea were lower than after rice, but improved significantly ( p < 0.05) by 11.4–15.1% in pigeonpea plots compared with those in rice plots during 1999–2000 and 2000–2001, respectively. The use efficiency of applied N and P fertilizers in wheat, measured as agronomic efficiency and apparent recovery, was increased with combined use of fertilizer N and P at recommended rate, and also with inclusion of pigeonpea in place of rice. The post-wheat harvest NO 3–N in soil profile beyond 45 cm depth was significantly greater under rice–wheat system than under pigeonpea–wheat system, suggesting that inclusion of pigeonpea may help in minimizing NO 3–N leaching to deeper profile layers beyond root zone. Similarly, in the treatments receiving both 120 kg N and 26 kg P ha −1, NO 3–N beyond 45 cm soil depth was lower compared to those receiving N or P alone. Inclusion of pigeonpea in place of rice enhanced carbon accumulation in the soil profile. The available P content was, however, invariably low under pigeonpea plots as compared to that under rice. With continuous rice–wheat cropping, the bulk density (BD) of soil was increased, especially in the 30–45 cm soil profile. Inclusion of pigeonpea in the system not only helped maintaining soil BD at initial level in the surface (0–15 cm) soil layers, but also in decreasing ( p < 0.05) BD in sub-surface layers (15–30 cm and 30–45 cm). Compared to rice, a statistically significant ( p < 0.05) positive effect of pigeonpea on root volume (58%) and root weight (99.5%) of succeeding wheat was also recorded. The net economic returns under pigeonpea–wheat system were greater compared with rice–wheat system.

Chlorophyll Meter– and Leaf Color Chart–Based Nitrogen Management for Rice and Wheat in Northwestern India

Overapplication of N in cereal crops leads to low N recovery efficiency and risk of NO3 pollution of ground water. The chlorophyll meter, also known as SPAD meter, is a simple, portable diagnostic tool for identifying crop N status. We used it to test need‐based N management approaches for rice (Oryza sativa L.) and wheat (Triticum aestivum L.) on a loamy sand in northwestern India. Applying 30 kg N ha−1 each time the SPAD value fell below the critical value of 37.5 resulted in application of 90 kg N ha−1, which produced rice yields equivalent to those with 120 kg N ha−1 applied in three splits. Using a SPAD value of 35 was inadequate for the two rice cultivars because it resulted in application of only 60 kg N ha−1 and, thus, low yields. With high inherent soil fertility resulting in rice yield of &gt;3 Mg ha−1 in zero‐N plots, applying N basally or a week after rice transplanting did not further increase yield. Limited experimentation with leaf color chart (LCC) indicated that N management based on LCC shade 4 helped avoid overapplication of N to rice. Wheat responded to N application at maximum tillering (MT) when SPAD value fell below 44. Wheat yield increased by 20% when 30 kg N ha−1 was applied at SPAD value of 42 at MT. Results show that plant need–based N management through chlorophyll meter reduces N requirement of rice from 12.5 to 25%, with no loss in yield.

Chlorophyll Meter– and Leaf Color Chart–Based Nitrogen Management for Rice and Wheat in Northwestern India

Overapplication of N in cereal crops leads to low N recovery efficiency and risk of NO3 pollution of ground water. The chlorophyll meter, also known as SPAD meter, is a simple, portable diagnostic tool for identifying crop N status. We used it to test need-based N management approaches for rice (Oryza sativa L.) and wheat (Triticum aestivum L.) on a loamy sand in northwestern India. Applying 30 kg N ha−1 each time the SPAD value fell below the critical value of 37.5 resulted in application of 90 kg N ha−1, which produced rice yields equivalent to those with 120 kg N ha−1 applied in three splits. Using a SPAD value of 35 was inadequate for the two rice cultivars because it resulted in application of only 60 kg N ha−1 and, thus, low yields. With high inherent soil fertility resulting in rice yield of >3 Mg ha−1 in zero-N plots, applying N basally or a week after rice transplanting did not further increase yield. Limited experimentation with leaf color chart (LCC) indicated that N management based on LCC shade 4 helped avoid overapplication of N to rice. Wheat responded to N application at maximum tillering (MT) when SPAD value fell below 44. Wheat yield increased by 20% when 30 kg N ha−1 was applied at SPAD value of 42 at MT. Results show that plant need–based N management through chlorophyll meter reduces N requirement of rice from 12.5 to 25%, with no loss in yield.

Response of lowland rice (Oryza sativa) to green-manuring, levels and sources of phosphatic fertilizer in coastal Orissa

An experiment was conducted during the rainy seasons of 1987 and 1988 to study the effect of green-manuring with and without chemical nitrogen and with different sources and levels of phosphatic fedlizer on lowland rice (Oryur s d v a L.). Green-manuring with dhahcha or prickly -ban [Sapbank c-btta (Retz.) Pers.; syn. S. aculePra Pers.] alone gave yield of 3.48 tonnes, whereas green-manuring + 20 kg Nha topdressed at panicle initiation gave 3.81 tonnerha of unhusked rice which was on a par with 60 kg N/ha through prilled urea No significant diierence in grain yield due to Mussoorie rockphosphate (100%) and Mussoorie rockphosphate + single superphosphate (50% each) was evident. But both the treatments were superior to single superphosphate (100%). Lowland rice did not respond to higher levels of phosphatic fertilizer and 40 kg P~ha gave as much yield as that of 60 kg Pha. Similar trend was observed for N and P uptake. Inherent soil fertility and productivity was sustained. Soil-available P and K status were found either equal to or higher than initial values at the end of 2 seasons showing sustained fenility.

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 taro ( Colocasia esculenta (L.) Schott) and sweet potato ( Ipomoea batatas (L.) Lam.) were quantified with the aim to investigate relationships between inherent soil fertility, N uptake, N application rates 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 taro 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 taro 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 taro due to a higher above-ground biomass production. The N fertilizer recovery was 25% for the sweet potato but only 9% for the taro indicating considerable N losses, likely caused by leaching.

Soil microbial biomass and mineralization of carbon and nitrogen in ecological, integrated and conventional forage and arable cropping systems

In a cropping systems experiment in southeastern Norway, ecological (ECO), integrated (INT) and conventional (CON) forage (FORAGE) and arable (ARABLE) model farms were compared. After 5 experimental years, topsoil was sampled in spring from spring grain plots and incubated for 449 days at controlled temperature (15 °C) and moisture content (50% water-holding capacity). There were no detectable differences between model farms in terms of total soil C or N. For INT and CON, however, values of microbial biomass C and N, microbial quotient (Cmic/Corg), and C and N mineralization were, or tended to be, higher for FORAGE than for ARABLE. For the ECO treatment, values were similar for FORAGE and ARABLE and did not differ significantly from that of CON-FORAGE. For INT and CON, the metabolic quotient (qCO2) was lower for FORAGE than for ARABLE. Again, for the ECO treatment, values were similar for FORAGE and ARABLE and did not differ significantly from that of CON-FORAGE. We estimated the sizes of conceptual soil organic matter pools by fitting a decomposition model to biomass and mineralization data. This resulted in a 48% larger estimate for CON-FORAGE than for CON-ARABLE of physically protected biomass C. For physically protected organic C the difference was 42%. Moreover, the stability of soil aggregates against artificial rainfall was substantially greater for CON-FORAGE than for CON-ARABLE. On this basis, we hypothesized that the lower qCO2 values in the FORAGE soils were mainly caused by a smaller proportion of active biomass due to enclosure of microorganisms within aggregates. Altogether, our results indicated a poorer inherent soil fertility in ARABLE than in FORAGE rotations, but the difference was small or absent in the ECO system, probably owing to the use of animal and green manures and reduced tillage intensity in the ECO-ARABLE rotation.

Productivity and residual effects of legumes in rice-based cropping systems in a warm-temperate environment: II. Residual effects on rice

Field experiments were conducted over the period 1994–1996 to investigate the residual effect on rice of a wide range of temperate legume species, grown during the preceding winter season in a warm-temperate environment in Nepal. The incorporation of large quantities of above- and below-ground legume biomass (roots and foliage) resulted in substantial residual effects on the subsequent upland rice crop. The effects on rice yield in terms of percent increase over the control were more pronounced at a site of comparatively lower inherent soil fertility where grain yields of rice grown after high yielding legume crops such as bitter lupin ( Lupinus mutabilis) and Persian clover ( Trifolium resupinatum) were almost twice as high (up to 7.6 t DM ha −1) than those of the control treatment (rice after wheat). In contrast, the residual effects of below-ground biomass (roots, nodules) on the following rice crop were not very pronounced and resulted in similar residual effects as rice, after fallow. Although up to 480 kg ha −1 of legume foliage N had been applied to rice, only a small proportion of the legume N (up to 70 kg N ha −1) was recovered by the following rice crop. Total rice dry matter yields were highly correlated with the amount of legume N applied, yet other factors such as residue quality and residue management practices appeared to also affect the magnitude of the residual effects. Future work should aim at investigating the effect of residue quality as affected by legume species and residue management practice on decomposition and N mineralisation processes in rice soils.

Influence of Site and Fertiliser Addition on Nutrient Cycling in Eucalyptus globulus Plantations in Gippsland, South-eastern Australia. I. Foliage and Litter Quality

The productivity of Eucalyptus plantations on many sites in south-eastern Australia is limited by nitrogen and phosphorus supply. Therefore, after canopy closure, nutrient return and decomposition are key processes maintaining productivity. To gain a better understanding of the effects of site and fertilisers on these processes, foliage and litter quality in E. globulus (Labill.) plantations in Gippsland, south-eastern Australia were characterised on three sites covering a range of soil types, inherent soil fertility and fertiliser treatments. Foliage and litter quality were estimated by sequential extraction of labile forms of N, P and C with cold, then hot, trichloroacetic acid (TCA). Selected treatments were sampled in N × P factorial fertiliser trials of 6-year-old trees where nutrients were added up to 2 years of age. Foliage and litter were categorised as recent or old depending on sampling position. Site significantly influenced concentrations of total and labile N and P (P &lt; 0.0001) in foliage and litter. Phosphorus fertiliser increased total P concentrations in old foliage at two sites, with the greatest absolute and relative increases at the least fertile site (Glencoe). Inorganic P extracted by cold (4°C) TCA accounted for 30-55% of total leaf and litter P and was the fraction most responsive to P fertiliser addition. Total N concentration and N fractions in foliage and litter were not influenced by N fertiliser addition. Inorganic N extracted by cold and hot (90°C) TCA accounted for less than 2% of total N and was not significantly different among fertiliser treatments. Both sugar and phenol concentrations in foliage and litter varied significantly between sites, with the least fertile site showing significantly higher concentrations of phenols in recent litter. Sugars and phenols extracted in cold TCA decreased from old foliage to litter at all sites and were not influenced by N and P fertiliser addition. The results show that additions of 200 kg ha-1 of P cause perturbations in P cycle that are bigger in magnitude and are sustained for longer periods of time compared to changes in N cycle with 400 kg ha-1 of N additions.

Pasture renovation practices of farmers in the neotropical savannahs

Degradation of sown pastures in the neotropics is a well known but poorly documented phenomenon. The paper reports the results of a survey conducted in the eastern savannahs of Colombia to quantify farmers' reactions to perceived pasture degradation. The survey was conducted in an area of 23 644 km2. The sample consisted of 46 farms, which had 19 012 ha of Brachiaria decumbens pastures distributed in 374 paddocks. Of these paddocks, 196 had been sown with vegetative seed, and 281 had received modest rates of P fertilization at planting. Pastures sown on flat lands had younger modal ages (three years) and were renovated more frequently than those sown on hilly or dissected savannahs (10 years). These differences were probably a result of variations in infrastructure and relative stage of development of the two topographies, but slight differences in inherent soil fertility may also have played a role. We found no relationship between grazing management and frequency of pasture renovation. Implications for future research on pasture reclamation are discussed. © 1998 John Wiley & Sons, Ltd.