Nitrogen dynamics following grain legumes and subsequent catch crops and the effects on succeeding cereal crops

The effects of faba bean, lupin, pea and oat crops, with and without an undersown grass-clover mixture as a nitrogen (N) catch crop, on subsequent spring wheat followed by winter triticale crops were determined by aboveground dry matter (DM) harvests, nitrate (NO₃) leaching measurements and soil N balances. A 2½-year lysimeter experiment was carried out on a temperate sandy loam soil. Crops were not fertilized in the experimental period and the natural ¹⁵N abundance technique was used to determine grain legume N₂ fixation. Faba bean total aboveground DM production was significantly higher (1,300 g m^?2) compared to lupin (950 g m^?2), pea (850 g m^?2) and oat (1,100 g m^?2) independent of the catch crop strategy. Faba bean derived more than 90% of its N from N₂ fixation, which was unusually high as compared to lupin (70–75%) and pea (50–60%). No effect of preceding crop was observed on the subsequent spring wheat or winter triticale DM production. Nitrate leaching following grain legumes was significantly reduced with catch crops compared to without catch crops during autumn and winter before sowing subsequent spring wheat. Soil N balances were calculated from monitored N leaching from the lysimeters, and measured N-accumulation from the leguminous species, as N-fixation minus N removed in grains including total N accumulation belowground according to Mayer et al. (2003a). Negative soil N balances for pea, lupin and oat indicated soil N depletion, but a positive faba bean soil N balance (11 g N m^?2) after harvest indicated that more soil mineral N may have been available for subsequent cereals. However, the plant available N may have been taken up by the grass dominated grass-clover catch crop which together with microbial N immobilization and N losses could leave limited amounts of available N for uptake by the subsequent two cereal crops.     

Nitrogen supply from fertilizer and legume cover crop in the transition to no-tillage for irrigated row crops

In spite of potential benefits and positive assessments of reducing primary tillage operations, only a small part of irrigated row crops is currently managed using reduced tillage, for reasons that include concerns about its agronomic suitability for certain crop rotations. Three years of a tomato/corn rotation under standard and no-tillage management were used to understand the fate of a fertilizer and cover crop nitrogen (N) application. Uptake of both inputs was reduced under no-tillage during the year of application, in this case a tomato crop. As a result, more input N was retained in the soil in this system. The initial challenge of reduced tomato yields diminished as no-tillage management remained in place and the soil N reservoir developed. Corn production was not affected by tillage treatment. Inclusion of a legume cover crop increased the amount of fertilizer N retained in the soil over time, more so under no-tillage than under standard tillage, emphasizing the benefit of cover crops in reducing the amount of fertilizer required to maintain productivity. While acceptance of reduced tillage ultimately depends on economic performance, the results of this study support its agronomic viability for irrigated row crops.     

Long-term effects of crop rotations with and without perennial leys on soil carbon stocks and grain yields of winter wheat

A change from cultivated land to grassland generally increases soil organic matter (SOM) content and is a potential option to mitigate greenhouse gas emissions. We investigated the effects of two-year perennial grass and mixed grass/legume leys in a six-year crop rotation on topsoil (0–0.25 m depth) carbon content and on grain yields of winter wheat over a period of 31 years. Different nitrogen fertilisation regimes were included and no manure was added to the experimental plots. We used data from long-term crop rotation experiments at three sites in southern Sweden: Säby (59°49′ N/17°42′ E), Lanna (58°20′ N/13°07′ E) and Stenstugu (57°36′ N/18°26′ E). At Säby, the reduction in topsoil carbon content was smaller in the ley crop rotations than in the crop rotation with only annual crops. There were no statistically significant effects of crop rotation on topsoil carbon at the other two sites. At Lanna, the grain yield increase in winter wheat over time was higher in the mixed legume/grass ley crop rotation than in the other two rotations. Together, these effects of ley on topsoil carbon and winter wheat yield suggest that replacing annual crops with leys in the crop rotation could reduce losses of soil carbon without decreasing total yield of annual crops on a regional scale. We also applied the Introductory Carbon Balance Model (ICBM) to simulate topsoil carbon content at the three sites. Based on the results, measures to improve the model predictability are proposed.     

Growth, composition, biological N2 fixation and nutrient uptake of a leguminous cover crop mixture and the effect of their removal on field nitrogen balances and nitrate leaching risk

Cover crops (CC) are an important source of nitrogen (N) in organic farming systems. Only few data are available about the effect of management activities (liquid slurry amendments, crop residue management) on growth, nutrient uptake and biological N₂ fixation (BNF) of a CC mixture. Furthermore, little information is available about the effect of CC harvesting on nutrient flows, nitrate leaching risk and soil mineral N supply of the succeeding main crop. The objectives of the presented field trials were (1) to measure the impact of organic manuring (straw residues and liquid slurry applications) on growth, composition, and BNF of a CC mixture with legumes and oil radish as components; (2) to determine the effect of CC species composition on nutrient content and uptake (N, P, K, Mg); and (3) to evaluate the effect of CC removal on field N balances and nitrate leaching risk. A CC mixture with legumes and non-legumes was able to compensate for many environmental and cultivation effects by influencing the competitive ability of the partners. For example, an increase of soil N supply due to additions of slurry or removal of cereal straw promoted growth of non-legumes at the expense of the legumes, resulting in N shortage at the end of the growing period, as shown by lower N contents and a wider C/N ratio of the non-legume partner. Low N availability at the beginning of the CC growth enhanced legume growth and/or reduced non-legume growth, resulting in a higher N supply in later periods of CC growth. A high legume percent composition within a CC mixture increases overall N content in the aboveground biomass and the N content of non-legumes within the mixture, and decreases the C/N ratio. Large amounts of nutrients were removed from the field by the harvesting of the CC aboveground biomass, significantly reducing the nitrate leaching risk. However, a reduction of the nitrate leaching risk was found only on fields where the green manure was incorporated in autumn.     

Integrated soil management for the savanna zone of W. Africa: legume rotation and fertilizer N

Integrated soil management with leguminous cover crops was studied at two sites in the northern Guinea savanna zone of northern Nigeria, Kaduna (190 day growing season) and Bauchi (150 days). One-year planted fallows of mucuna, lablab, and crotalaria were compared with natural grass fallow and cowpea controls. All treatments were followed by a maize test crop in the second year with 0, 30, or 60 kg N ha^–1 as urea. Above ground legume residues were not incorporated into the soil and most residues were burned early in the dry season at the Kaduna site. Legume rotation increased soil total N, maize growth in greenhouse pots, and dry matter and N accumulation of maize. Response of maize grain yield to 30 kg N ha^–1 as urea was highly significant at both sites and much greater than the response to legume rotation. The mean N fertilizer replacement value from legume rotation was 14 kg N ha^–1 at Kaduna and 6 kg N ha^–1 at Bauchi. W ith no N applied to the maize test crop, maize grain yield following legume fallow was 365 kg ha^–1 higher than natural fallow at Bauchi and 235 kg ha^–1 higher at Kaduna. The benefit of specific legume fallows to subsequent maize was mostly related to above ground N of the previous legume at Bauchi, where residues were protected from fire and grazing. At Kaduna, where fallow vegetation was burned, maize yield was related to estimated below ground N. The results show that legume rotation alone results in small maize yield increases in the dry savanna zone.     

The residual effectiveness of previously applied copper fertiliser for grain yield of wheat grown on soils of south-west Australia

The residual effectiveness of copper (Cu) applied 18 to 21 years previously was estimated for grain yield of wheat. In one field experiment, current levels of Cu fertiliser were applied and its effectiveness was compared to that of the same level of Cu applied previously. The effects of nitrogen (N) fertiliser on the Cu concentration in the youngest emerged blade and in the grain, as well as the effects of N levels on the grain yield of wheat, were also studied.Where the recommended level of Cu fertiliser had been applied previously, its residual effectiveness depended on the soil type. On the grey sands over clay and gravelly sands over clay, the residual Cu would last approximately 20 years where wheat is grown in rotation with a legume crop (Lupinus augustifolius L.) and where N fertiliser is applied at high levels (92 kg N ha^–1). On the yellow brown sandy earths of the Newdegate district, the residual value was in excess of 30 years.When Cu levels in the soil are marginal, high levels of N applied to wheat crops grown on stubbles of legume crops (high soil N) could suffer from induce Cu deficiency which could reduce grain production.Critical concentrations of Cu in the youngest emerged blade of less than 1.2 mg Cu kg^–1 at Gs50–59 would indicate Cu deficiency. Cu concentrations of less than 1.1–1.2 mg Cu kg^–1 in the grain suggest that the wheat crop is marginally supplied with Cu. In both situations, Cu fertiliser needs to be applied before the next crop.     

Recovery of 15N-labelled fertilizer applied to winter wheat and perennial ryegrass crops and residual 15N recovery by succeeding wheat crops under different crop residue management practices

The recovery of ¹⁵N-labelled fertilizer applied to a winter wheat (120 kg N ha^–1) and also a perennial ryegrass (60 kg N ha^–1) crop grown for seed for 1 year in the Canterbury region of New Zealand in the 1993/94 season was studied in the field. After harvests, ryegrass and wheat residues were subjected to four different residue management practices (i.e. ploughed, rotary hoed, mulched and burned) and three subsequent wheat crops were grown, the first succeeding wheat crop sown in 1994/95 to examine the effects of different crop residue management practices on the residual ¹⁵N recovery by succeeding wheat crops. Total ¹⁵N recoveries by the winter wheat and ryegrass (seed, roots and tops) were 52% and 41%, respectively. Corresponding losses of ¹⁵N from the crop-soil systems represented by un-recovered ¹⁵N in crop and soil were 12% and 35%, respectively. These losses were attributed to leaching and denitrification. The proportions of ¹⁵N retained in the soil (0-400 mm depth) at the time of harvest of winter wheat and ryegrass were 36% and 24%, respectively. Although the soil functioned as a substantial sink for fertilizer N, the recovery of this residual fertilizer by subsequent three winter wheat crops was low (1-5%) and this was not affected by different crop residue management practices.     

Nitrogen fixation of red clover interseeded with winter cereals across a management-induced fertility gradient

The incorporation of legume cover crops into annual grain rotations remains limited, despite extensive evidence that they can reduce negative environmental impacts of agroecosystems while maintaining crop yields. Diversified grain rotations that include a winter cereal have a unique niche for interseeding cover crops. To understand how management-driven soil fertility differences and inter-seeding with grains influenced red clover (Trifolium pratense) N₂ fixation, we estimated biological N₂ fixation (BNF) in 2006 and 2007, using the ¹⁵N natural abundance method across 15 farm fields characterized based on the reliance on BNF derived N inputs as a fraction of total N inputs. Plant treatments included winter grain with and without interseeded red clover, monoculture clover, monoculture orchardgrass (Dactylis glomerata), and clover-orchardgrass mixtures. Fields with a history of legume-based management had larger labile soil nitrogen pools and lower soil P levels. Orchardgrass biomass was positively correlated with the management-induced N fertility gradient, but we did not detect any relationship between soil N availability and clover N₂ fixation. Interseeding clover with a winter cereal did not alter winter grain yield, however, clover production was lower during the establishment year when interseeded with taller winter grain varieties, most likely due to competition for light. Interseeding clover increased the % N from fixation relative to the monoculture clover (72% vs. 63%, respectively) and the average total N₂ fixed at the end of the first growing season (57 vs. 47 kg N ha⁻¹, respectively). Similar principles could be applied to develop more cash crop-cover crop complementary pairings that provide both an annual grain harvest and legume cover crop benefits.     

Towards a Revised Coefficient for Estimating N2O Emissions from Legumes

The Intergovernmental Panel on Climate Change (IPCC) standard methodology to conduct national inventories of soil N₂O emissions is based on default (or Tier I) emission factors for various sources. The objective of our study was to summarize recent N₂O flux data from agricultural legume crops to assess the emission factor associated with rhizobial nitrogen fixation. Average N₂O emissions from legumes are 1.0 kg N ha⁻¹ for annual crops, 1.8 kg N ha⁻¹ for pure forage crops and 0.4 kg N ha⁻¹ for grass legume mixes. These values are only slightly greater than background emissions from agricultural crops and are much lower that those predicted using 1996 IPCC methodology. These field flux measurements and other process-level studies offer little support for the use of an emission factor for biological N fixation (BNF) by legume crops equal to that for fertiliser N. We conclude that much of the increase in soil N₂O emissions in legume crops may be attributable to the N release from root exudates during the growing season and from decomposition of crop residues after harvest, rather than from BNF per se. Consequently, we propose that the biological fixation process itself be removed from the IPCC N₂O inventory methodology, and that N₂O emissions induced by the growth of legume crops be estimated solely as a function of crop residue decomposition using an estimate of above- and below-ground residue inputs, modified as necessary to reflect recent findings on N allocation.     

Effect of N-rates on dryland wheat in relation to mulching previous crop or fallow

In many rainfed areas a rainy season crop is followed by a post rainy season (rabi) crop and the yield of the latter is directly related to the profile-stored water at the time of its sowing. Mulching is known to help increase soil moisture storage. The yields of dryland crops are also limited by availability of nutrients, particularly nitrogen. We studied the effect of organic mulching in rainy season maize or fallow on the yield of maize and the yield and water use efficiency (WUE) of the following wheat fertilized with different rates of N.Mulching increased yield of maize by 9–12% and that of the following wheat by 25–28%. The increase in wheat yield with mulching in the previous crop of maize is attributable to greater residual moisture after maize particularly in the seed-zone and enrichment of soil with nutrients. In wheat following mulched maize, the yields with 0 and 40 kg N ha^–1 were comparable to those with 40 and 80 kg N ha^–1 in the absence of mulch. For a given N rate the yield of wheat was a function of available water supply. The yield increased with water supply up to a certain level beyond which it decreased with additional water supply. The threshold water supply and the corresponding yield increased with increase in N rate. Favourable effects of mulching in maize on the yield of wheat decreased with increase in water supply. The results suggest that to achieve optimum yield of wheat in double cropping the previous crop should be mulched and the N rate for wheat should be chosen in relation to available water supplies.     

Changes in soil available NPK in multiple cropping systems based on short duration sugarcane crops relative to a conventional sugarcane cropping system

Changes in soil available NPK were studied in four intensive crop rotations based on short duration (8 months) sugarcane crops (1. short duration plant cane/1st ratoon/2nd ratoon; 2. short duration plant cane/1st ratoon/finger millet/cotton; 3. finger millet/short duration plant cane/1st ratoon/wheat; and 4. finger millet/maize/short duration plant cane/1st ratoon). These rotations were compared with the conventional duration (12 months) sugarcane crop sequence (one plant + one ratoon) in a cycle of 24 months.Soil available nitrogen (SAN) declined when 100 or 150 kg N ha^–1 was applied in the short duration sugarcane based systems, but was either maintained or improved at a higher N application rates (200 or 250 kg ha^–1). The conventional system showed a sharp decline in SAN of about 14% from its original status at the end of the sequence. Close row spacing (60 cm) of sugarcane improved the soil N level over that in the conventionally spaced rows (90 cm) probably through greater rhizosphere biomass additions.Available P declined sharply from its original level in the soil in sequence 2, the decline being marked after cotton. In all the other short duration based sequences it was maintained. The conventional system also showed reduced soil available P at the end of the sequence. Soil available K declined in all crop sequences.Nitrogen uptake was far less than additions made by fertilizer. The actual soil N balance was much lower than the expected balance thus indicating large losses of N from the soil. Phosphorus removal was also less than the additions made and thus there were improvements in the soil available P status at the end of the crop cycle. In all the sequences, there was a negative potassium balance due to greater removal by the various crops when compared to K additions. However, in the system as a whole there were net gains of K as larger amounts were recovered than had been added.     

How winter cover crops and tillage intensities affect nitrogen availability in eggplant

This study evaluates the fate of nitrogen (N) content in winter cover crops under different tillage intensities. Field trials were conducted over a 2-year period in a Mediterranean environment adopting a cover crop–eggplant sequence. The treatments were: three cover crops (hairy vetch, oat and oilseed rape); three tillage intensities (residue left on soil surface, shallow green manure and deep green manure). The measurements included: cover crop and eggplant characteristics, N mineralization from cover crops, soil inorganic N and soil CO₂ emission. At cover crop termination, N accumulated in the cover crops was 207, 77 and 77 kg N ha^?1 in hairy vetch, oat and oilseed rape, respectively. Tillage intensity affected biomass decomposition and N mineralization from cover crop residues which were slower when residues were left on soil surface (54 and 71%, respectively) than when incorporated into the soil (66 and 79%, respectively). Hairy vetch showed a greater ability to supply N to eggplant (151 kg N ha^?1), due to the fast decay of its residues, consequently, the N balance index was always high after hairy vetch throughout eggplant cultivation. N mineralized by cover crops was positively correlated with total soil CO₂ emission and soil inorganic N. Placing cover crop residues on soil surface enhances synchronization between N mineralized and eggplant N demand in hairy vetch, while in oat it appears to mitigate the shortage of soil inorganic N for the following vegetable. These findings may also be extended to other summer vegetables which have similar requirements to the eggplant.     

Effect of the timing and rate of nitrogen fertilization on the growth and recovery of fertilizer nitrogen within the potato (Solanum tuberosum L.) crop

The effect of the timing of N fertilizer application on the uptake and partitioning of N within the crop and the yield of tubers has been studied in two experiments. In 1985 either none, 8 or 12 g N m^–2 was applied and in 1986 none, 12 or 18 g N m^–2. Fertilizer N was applied either at planting, around the time of tuber initiation or half at planting and the remainder in four foliar sprays of urea during tuber bulking.¹⁵N-labelled fertilizer was applied to measure the recovery of fertilizer N in the crops.There was an apparent pre-emergence loss of nitrate from the soil when N was applied at planting in 1986, thereby reducing the efficiency of fertilizer use. Applying the N at tuber initiation delayed and reduced the accumulation of N in the canopy compared with crops receiving all their fertilizer at planting. Foliar sprays of urea slightly increased both tuber yields and tuber N contents when compared to a single application at planting. The proportion of the fertilizer N recovered in the crop was little affected by the rate of N application, but a greater proportion of foliar-applied N was recovered than N broadcast at planting, due partly to pre-emergence losses of nitrate in 1986. It is suggested that late applications of N was foliar sprays can be of benefit to crops with a long growing season and reduce environmental losses of N.     

Effects of a catch crop and reduced nitrogen fertilization on nitrogen leaching in greenhouse vegetable production systems

Greenhouse vegetable cultivation has greatly increased productivity but has also led to a rapid accumulation of nitrate in soils and probably in plants. Significant losses of nitrate–nitrogen (NO₃-N) could occur after heavy N fertilization under open-field conditions combined with high precipitation in the summer. It is urgently needed to improve N management under the wide spread greenhouse vegetable production system. The objective of this study was to evaluate the effects of a summer catch crop and reduced N application rates on N leaching and vegetable crop yields. During a 2-year period, sweet corn as an N catch crop was planted between vegetable crops in the summer season under 5 N fertilizer treatments (0, 348, 522, 696, and 870 kg ha⁻¹) in greenhouse vegetable production systems in Tai Lake region, southern China. A water collection system was installed at a depth of 0.5 m in the soil to collect leachates during the vegetable growing season. The sweet corn as a catch crop reduced the total N concentration from 94 to 59 mg l⁻¹ in leached water and reduced the average soil nitrate N from 306 to 195 mg kg⁻¹ in the top 0.1-m soil during the fallow period of local farmers’ N application rate (870 kg ha⁻¹). Reducing the amount of N fertilizer and using catch crop during summer fallow season reduced total N leaching loss by 50 and 73%, respectively, without any negative effect on vegetable yields.     

Effectiveness of applying urea solution by furrow irrigation to wheat grown on raised beds of an impervious red-brown earth

The distribution and recovery of urea N (25 kg ha^–1) applied in solution by low-flow furrow irrigation to wheat, direct-drilled in rows in 1.5m wide permanent beds of a red-brown earth, was determined using¹⁵N labelled fertilizer. This method of fertilizer application resulted in an uneven distribution of applied N across the soil bed. Fertilizer N was recovered mainly in the upper 0.15m horizon of the soil. Forty seven percent of the applied N was recovered in the soil and plants within 0.20m of the point of application. Recovery rapidly decreased with increasing distance from the furrow and less than 4% of the urea N was recovered by the plants in the fourth row, 0.67m from the middle of the furrow. The recovery of fertilizer N in the crop was 23, 28, 47 and 40% at 13, 32, 59 and 86 days, respectively; the corresponding total recovery in the crop plus soil was 77, 91, 87 and 75%; the mean being 83 ± 8%.The results suggest that with this method of fertilizer application and these soil properties the furrows should be less than 0.75m apart in order to get uniform distribution of the fertilizer.     

Evaluation of iron pyrites as sulphur fertilizer

A field experiment was conducted for three consecutive winter crop seasons commencing in 1979–80 on the Typic Ustochrept of Pura to evaluate iron pyrites as S fertilizer. Four crops viz, wheat, chickpea, mustard and Egyptian clover were tested for their responsiveness to added pyrites. All the crops responded significantly to added pyrites. Mustard proved most sensitive to S deficiency in soil and wheat the least. Between the two legumes, Egyptian clover was more sensitive to S stress than chickpea. Average biomass production by Egyptian clover was highest followed by wheat, mustard and chickpea. Mustard and Egyptian clover required more S to achieve maximum biomass production compared with wheat and chickpea but they also recovered from the soil a large proportion of added S than wheat and chickpea. Addition of pyrites increased availability of S in soil. Pyrites enhanced mobilization of soil P and its utilization by the crops.     

Pea–barley intercropping and short-term subsequent crop effects across European organic cropping conditions

Grain legumes are known to increase the soil mineral nitrogen (N) content, reduce the infection pressure of soil borne pathogens, and hence enhance subsequent cereals yields. Replicated field experiments were performed throughout W. Europe (Denmark, United Kingdom, France, Germany and Italy) to asses the effect of intercropping pea and barley on the N supply to subsequent wheat in organic cropping systems. Pea and barley were grown either as sole crops at the recommended plant density (P100 and B100, respectively) or in replacement (P50B50) or additive (P100B50) intercropping designs. In the replacement design the total relative plant density is kept constant, while the additive design uses the optimal sole crop density for pea supplementing with ‘extra’ barley plants. The pea and barley crops were followed by winter wheat with and without N application. Additional experiments in Denmark and the United Kingdom included subsequent spring wheat with grass-clover as catch crops. The experiment was repeated over the three cropping seasons of 2003, 2004 and 2005. Irrespective of sites and intercrop design pea–barley intercropping improved the plant resource utilization (water, light, nutrients) to grain N yield with 25–30% using the Land Equivalent ratio. In terms of absolute quantities, sole cropped pea accumulated more N in the grains as compared to the additive design followed by the replacement design and then sole cropped barley. The post harvest soil mineral N content was unaffected by the preceding crops. Under the following winter wheat, the lowest mineral N content was generally found in early spring. Variation in soil mineral N content under the winter wheat between sites and seasons indicated a greater influence of regional climatic conditions and long-term cropping history than annual preceding crop and residue quality. Just as with the soil mineral N, the subsequent crop response to preceding crop was negligible. Soil N balances showed general negative values in the 2-year period, indicating depletion of N independent of preceding crop and cropping strategy. It is recommended to develop more rotational approaches to determine subsequent crop effects in organic cropping systems, since preceding crop effects, especially when including legumes, can occur over several years of cropping.     

Contribution of relay intercropping with legume cover crops on nitrogen dynamics in organic grain systems

Nitrogen (N) management is a key issue in livestock-free organic grain systems. Relay intercropping with a legume cover crop can be a useful technique for improving N availability when two cash crops are grown successively. We evaluated the benefits of four relay intercropped legumes (Medicago lupulina, Medicago sativa, Trifolium pratense and Trifolium repens) on N dynamics and their contribution to the associated and subsequent cash crops in six fields of organic farms located in South-East France. None of the relay intercropped legumes affected the N uptake of the associated winter wheat but all significantly increased the N uptake of the succeeding spring crop, either maize or spring wheat. The improvement of the N nutrition of the subsequent maize crop induced a 30 % increase in grain yield. All relay intercropped legumes enriched the soil–plant system in N through symbiotic fixation. From 71 to 96 % of the N contained in the shoots of the legumes in late autumn was derived from the atmosphere (Ndfa) and varied between 38 and 67 kg Ndfa ha^?1. Even if the cover crop is expected to limit N leaching during wintertime, the presence of relay intercropped legumes had no significant effect on N leaching during winter compared to the control.     

Effect of Phenolic Acids in Soil under and Between Rows of a Prior Sorghum (Sorghum bicolor) Crop on Germination, Emergence, and Seedling Growth of Peanut (Arachis hypogea)

Sorghum bicolor is an allelopathic crop that reduces the yield of succeeding crops. We have assessed its effect on the germination, emergence, and seedling growth of Arachis hypogea sown in soil that had had a prior sorghum cropping. A. hypogea was sown on rows and interrows of a previous sorghum crop in 1997 and 1998 in Senegal. Seedling establishment (germination rate and seedling weight) was better between rows than on rows of the previous crop. The highest concentrations of phenolic compounds occurred in the rows in 1998, while contents of row and interrow soils were similar in 1997. Vanillic acid was the main component of the six chemicals found in 1997 soils, whereas the 1998 soil samples contained mainly p-hydroxybenzoic acid, p-hydroxybenzaldehyde, vanillic, and p-coumaric acids (10 phenolics identified). The germination of peanut seeds in water (control), soil water extracts, and mixtures of pure phenolics (equivalent to those in 1997 and 1998 soil samples) was tested. All extracts inhibited germination compared to controls, but there was no significant difference among treatments, i.e., the inhibition was the same for seeds in soil solutions and those in the respective phenolic mixtures. Similarly, there were no significant differences among the germination rates in soil water extracts of rows and interrows or in the pure phenolic mixtures of rows and interrows. We propose a geometrical sowing pattern for peanuts between the rows of the previous sorghum crop to escape the latter's "allelopathic heritage."     

Sub-surface injection of liquid waste

The sub-surface injection of sewage sludge in grassland conditions has been investigated showing that injection is a viable alternative technique to surface applications. With the large amounts of existing liquid wastes, either municipal or agricultural, the land required for injection has risen over the available grassland. This study has considered the sub-surface injection of sewage sludge in a growing crop of spring wheat.The agronomic response of spring wheat has been compared at three different levels of crop development. Five different techniques were used to inject the sludge, and compared with two control situations. Using a three leg injector, with the outside tines bent, and placing gaps between rows for the tines and wheels, the crop showed a better agronomic response at the final levels of development.Analysing row by row the dry matter production in the injected plots, it was clear that the lower global production per square metre was due to the damage caused in the rows by the legs and traffic.