A farm-level evaluation of nitrogen and phosphorus fertilizer use and planting density for pearl millet production in Niger

Mineral fertilizer use is increasing in West Africa though little information is available on yield response in farmers' fields. Farmers in this region plant at low density (average 5,000 pockets ha^–1, 3 plants pocket^–1), which can affect fertilizer use efficiency. A study was conducted with 20 farmers in Niger to assess the response of pearl millet [Pennisetum glaucum (L.) R. Br.] to phosphorus and nitrogen fertilizers under farm conditions. In each field, treatments included control, single superphosphate (SSP) only, SSP plus N (point placed near plant), and either SSP or partially acidulated phosphate rock (PAPR) plus N broadcast. N and P were applied at 30 kg N ha^–1 and 30 kg P₂O₅ ha^–1. Farmers were allowed to plant, weed, etc., as they wished and they planted at densities ranging from 2,000 to 12,000 pockets ha^–1. In the absence of fertilizer, increasing density from 2,000 to 7,000 pockets ha^–1 increased yield by 400%. A strong interaction was found between fertilizer use and density. Farmers planting at densities less than 3,500 pockets ha^–1 had average yields of 317 kg grain ha^–1 while those planting at densities higher than 6,500 pockets ha^–1 showed average yields of 977 grain ha^–1. Though phosphate alone increased yields significantly at all densities, little response to fertilizer N was found at densities below 6,000 pockets ha^–1. Significant residual responses in 1987 and 1988 were found to P applied in high-density plots in 1986. Depending on fertilizer and grain prices, analysis showed that fertilizer use must be be combined with high plant density (10,000 pockets ha^–1) or no economic benefit from fertilizer use will be realized.     

Phosphorus response of grain sorghum in the Guinea savanna of Nigeria as influenced by rates,placement and plant spacing

Improved grain sorghum (Sorghum bicolor L.) Moench responded to applied P up to 22 kg P ha^–1 on an Alfisol situated in the Nigeria savanna. Hill placement of phosphorus fertilizer gave higher grain yield of sorghum, one thousand grain weight, P content of leaves and residual phosphate than furrow P application. Grain yields were observed to be higher at population of 55 555 plants ha^–1 than at 31 750 and 24 700.     

Mineral nutrition and fertilizer response of grain sorghum in India — A review over the last 25 years

Researches on the mineral nutrition and fertilizer response of grain sorghum (Sorghum bicolor (L) Moench) carried out during the last 25 years in India are reviewed here. In general, N,P,K, Fe and Mn concentrations in vegetative plant parts decreased with crop age, while the concentrations of Ca, Mg and Cu increased. The concentration of N and P increased in panicle or grains of sorghum with advance in crop age. The seasonal change for other nutrients has not, however, been studied.Accumulation and uptake of N,P, and K by grain sorghum were characterized. Usually N and P accumulated slowly compared with the rapid accumulation of K in early crop growth stage and vice-versa in later stages of growth. As against the sizable mass of N and P into panicle, K was partitioned into stalk.Fertilizer responses to N and P were observed throughout India. Improved varieties and hybrids of sorghum responded to N rates ranging from 60 to 150 kg N ha^–1, whereas a response to P application was observed up to 40 kg P ha^–1. Although responses to K application had been inconsistent, an increase in grain yield of sorghum was observed due to 33 kg K ha^–1. A balanced fertilizer schedule consisting of 120 kg N ha^–1, 26 kg P ha^–1, 33 kg K ha^–1 and 15–25 kg Zn50₄ ha^–1 is recommended for improved productivity of grain sorghum.It is concluded that systematic research efforts should be directed so as to identify problem soils showing deficiencies and toxicities of different nutrients. Characterization of the seasonal changes in the concentration and uptake of different nutrients and determination of critical concentration and hidden hunger of different nutrients in plant tissues would lead to the recommendation of balanced fertilization for different sorghum-growing regions in India.A part of the paper presented in the Silver Jubliee Conference of Indian Society of Agronomy held at H.A.U., Hissar (India) in March, 1981     

Response of sorghum to fertilizer phosphorus and its residual value in a Vertisol

The response of crops to added P in Vertisols is generally less predictable than in other soil types under similar agroclimatic conditions. Very few studies have considered the residual effects of P while studying responses to fresh P applications. Field experiments were conducted for three years to study the response of sorghum to fertilizer P applied at 0, 10, 20 and 40 kg P ha^–1, and its residual value in a Vertisol, very low in extractable P (0.4 mg P kg^–1 soil), at the ICRISAT Center, Patancheru (near Hyderbad), India. In order to compare the response to fresh and residual P directly in each season, a split-plot design was adopted. One crop of sorghum (cv CSH6) was grown each year during the rainy season (June-September).The phenology of the sorghum crop and its harvest index were greatly affected by P application. The days to 50% flowering and physiological maturity were significantly reduced by P application as well as by the residues of fertilizer P applied in the previous season. In the first year of the experiment, sorghum grain yield increased from 0.14 t (no P added) to 3.48 t ha^–1 with P added at the rate of 40 kg P ha^–1. Phosphorus applied in the previous year was 58% as effective as fresh P but P applied two years earlier was only 18% as effective as fresh P.     

Growth and yield responses of dryland grain sorghum to nitrogen and phosphorus fertilization in a ferruginous tropical soil (Haplustalf)

Field trials were conducted during the 1980–82 seasons to study the response dryland sorghum to nitrogen and phosphorus fertilization in a ferruginous tropical soil. Treatments tested were factorial combinations of three rates of nitrogen (0, 60 and 120 kg N ha^–1) and four rates of phosphorus (0, 11, 22 and 33 kg P ha^–1). Grain and straw yields and yield components were enhanced by nitrogen fertilization in two out of three years. The optimum N rate for grain yield was 60 kg N ha^–1 while straw yield responded up to 120 kg N ha^–1. The optimum P rate for dryland sorghum was 11 kg P ha^–1. Both N and P enhanced grain weight per head, grain number, test weight and tillering significantly but it was only N which enhanced 1000-grain weight and flag leaf area. Dry matter productin was increased by N fertilization but not by P. There were no significant N × P interactions for any of the parameters studied. Dryland sorghum response to N and P fertilization was influenced by season, time of planting and rainfall distribution.     

Relative efficiency of ammonium polyphosphate and orthophosphates for wheat and their residual effects on succeeding cowpea fodder

The relative efficiency of ortho and polyphosphates as P sources for wheat were studied in a field experiment with five sources—TSP, DAP, NP, APP (solid) and APP (Liquid) at three levels—13, 26 and 39 kg P ha^–1 and the residual effect was studied on a succeeding cowpea (fodder). Both the crops were grown each year at same location. Wheat responded upto 39 kg P ha^–1 in the first year but only upto 26 kg P ha^–1 in the second year. The growth parameters—plant height and dry matter production and yield attributes-ears m^–1, grains ear^–1 and test weight were favourably influenced by P application. The rate of P uptake initially exceeded the dry matter production. Wheat when fertilized with 26 or 39 kg P ha^–1 left behind a significant residual effect to raise the soil P availability and dry matter yield of cowpea. The grain yield of wheat was higher with APP's than with NP or DAP but the residual effect was the highest with NP. The magnitude of build up of soil available P with NP and APP's were similar and higher than DAP or TSP.     

Effects of different phosphate fertilizers on yield of barley and rape seed on reddish brown soils of the Ethiopian highlands

The effectiveness of six phosphorus sources at 4 rates were tested for two seasons on reddish brown soil at Holetta, Ethiopia, using barley (Hordeum vulgare L.) and rape (Brassica napus L.) as test crops. The fertilizer sources include: basic slag (BS), bone meal (BM), Ethiopian rock phosphate (ERP), Gafsa rock phosphate (GRP), triple superphosphate (TSP) and mixture of TSP and GRP in the ratio 1:4 (MIX). Yield, P uptake by both crops as well as available soil P showed a marked response to the application of the various P sources. On continuously cropped field, grain yield increase over the unfertilized plot ranged from 2.5 to 16.4 dt ha^–1 for barley and rape respectively. On newly cleared field no significant effects of the different P sources on barley were observed. On the other hand for rape, a grain yield increase over the unfertilized plot ranging from 10.6 to 17.8 dt ha^–1 was recorded. The highest agronomic effectiveness relative to TSP (RAE) for both crops was obtained with BS. Rape was found to utilize P not only from the reactive rock phosphate (RP) but also from the unreactive one, which had a total P content of only 3% and 0.4% ammonium citrate soluble P. Barley, on the contrary, could not utilize P from this magmatic rock phosphate and failed to grow.     

Mussoorrie rock phosphate-pyrite mixture as phosphate fertilizer

Mussoorrie rock phosphate (MRP), MRP + pyrite (25% by weight), diammonium phosphate (DAP), ammonium polyphosphate (APP) and nitrophosphate (NP) were compared in a field experiment as fertilizers for wheat. At 20 kg P ha^–1, MRP was only 6 per cent as effective as DAP. However, when it was mixed with pyrite, the efficiency of MRP increased to 64 per cent at 20 kg P ha^–1 compared with 97 per cent at 40 kg P ha^–1. The P requirement for a targeted yield for 4.5 t ha^–1 decreased from 39.4 kg P ha^–1 as MRP to 23.7 kg P ha^–1 as MRP + pyrite. Of the other P fertilizers studied, NP was as effective as DAP, whereas APP was 9 to 37 per cent more effective than DAP. However, the P requirement as DAP, NP and APP for a targeted yield of 4.5 t ha^–1 was similar (11 ± 0.5 kg P ha^–1).     

Effect of rate and method of phosphate placement on productivity of durum wheat in mediterranean environments

Cereals grown on calcareous soils in a dry Mediterranean climate have generally responded to P fertilizer. Broadcasting is the common practice used by most farmers.The effects of rate and method of phosphate placement on dry matter production (TDM), grain yield and P uptake by durum wheat (Triticum vulgare L.) were investigated. Factorial experiments with four levels of P (0, 17.5, 35, and 52.5 kg P ha^–1) and 2 methods of application (banding by drilling with the seeds, and broadcasting before surface tillage and sowing) were conducted for three consecutive seasons at three experimental sites: Breda (Typic Calciorthid, 281 mm); Tel Hadya (Chromoxeretic Rhodoxeralf, 328 mm) and Jindiress (Palexerollic Chromoxerert, 471 mm). Three levels of P were used in the third season only. Treatments were replicated in three blocks. The average values of NaHCO₃-extractable P in the plow layer were 2.8, 3.7 and 2.1 mg P kg^–1 at Breda, Tel Hadya, and Jindiress, respectively.Above-ground total dry matter (TDM) was determined at tillering and anthesis. Grain and straw yields were estimated separately at harvest. The P concentration in plant material was determined at both stages. At tillering, TDM and total P uptake (TPU) increased linearly with the rate of P applied. It was found that (i) 17.5 kg P ha^–1 banded was as effective in increasing TDM and TPU as 52.5 kg P ha^–1 broadcast, and (ii) the average relative percentage increase in TDM at tillering due to P banded as compared to P broadcast was 54%, with significantly less response to banding, by maturity and harvest. The response to P placement was found to be site related. At Breda, banding was found superior to broadcasting in all 3 seasons with a relative increase in grain yields equivalent to 34, 63 and 23% in the 3 successive seasons, when optimum amounts of P were applied. However, in two out of three seasons only banding was found superior to broadcasting at both Tel Hadya and Jindiress.As a result, banding P is recommended for durum wheat grown on the calcareous soils of the Mediterranean regions because it stimulates growth and P uptake early in the season and increases the crop potential for good grain and straw yields especially in dry areas. Given equal rates of P application, banding was found superior to broadcasting in most cases.     

Yield and growth response to potassium of grain sorghum as influenced by variety in a savanna soil of Nigeria

Field trials were conducted at Samaru over a three-year period (1980–82) to study the yield, growth and nutrient concentration of three grain sorghum varieties (L. 187, SK5912 and FFBL) in relation to potassium fertilization in a savanna soil. Potassium application rates were 0, 25, 50 and 75 kg K ha^–1. Year × potassium interactions were not significant although there were significant variety × K interactions. The highest grain yields for var. L.187, SK5912 and FFBL occurred from the application of 25, 50 and 75 kg K ha^–1 respectively. Straw yield was generally increased by K application, which also promoted tillering and hastened flowering in grain sorghum. Although grain weight per head, head number per m², grain number and 1000-grain weight were unaffected by this nutrient, weight per head was reduced by 22.8 per cent. K application enhanced N concentration of sorghum plants but caused decline in P concentration. The highest K rate gave the highest K concentration in each of the three varieties at 7 weeks after planting. Optimum K requirement of grain sorghum would seem to be between 25 and 50 kg K ha^–1.     

Fate of phosphorus applied in slurry and mineral fertilizer: accumulation in soil and release into surface runoff water

Phosphorus (P) accumulation on the soil surface and its effect on the concentration of dissolved orthophosphate P (PO₄-P) in surface runoff water were studied after three years of surface application of slurry and mineral fertilizer to grass ley on a sandy soil, poor in P. The total amount of P applied was 107–143 kg ha^–1>, of which 72–119 kg ha^–1> was applied on the soil surface during two or three years without incorporation or mixing. The addition of slurry and mineral fertilizer resulted in an increase in inorganic P in the 0–5 cm but not the 5–25 cm soil layer, but organic P was not affected. The measured changes in inorganic P deviated only by 4–6 kg ha^–1> from the values derived from inputs and outputs of P (crop uptake + losses in surface runoff and drainage water). The increase in inorganic-P was accompanied by increases in the degree of P saturation (DPS) and in P extracted with acid am monium acetate (P_Ac ), sodium bicarbonate (P_Olsen) and anion-exchange resin (P_Resin). In surface runoff, 10–18 months after the last surface application of P, the mean flow-weighted concentration of PO₄-P was linearly increased with the values of DPS, P_Ac, P_Olson and P_Resin in the 0–5 cm soil layer. PO₄-P was lowest (0.033 mg l^–1> ) in the control plots and highest (0.62 mg l^–1>) in the plot where 143 kg ha^–1> P had been applied in slurry and fertilizer. On that plot, the corresponding values of DPS, P_Ac, P_Olson and P_Resin were 16%, 13 mg kg^–1>, 85 mg kg^–1> and 71 mg kg^–1 , even within a few years, and multiply the P loading to surface runoff from the site. A very shallow soil sampling (< 5 cm) is needed to assess P loading potential in a soil where P has been surface-applied.     

Residual effects of growing mungbean and uridbean on the yield and nitrogen uptake of a succeeding wheat crop

A two year field experiment was carried out at the Indian Agricutural Research Institute, New Delhi - 110012, India to assess the effect of mungbean (Vigna radiata L.) and uridbean (Vigna mungo L.) residues on the yield and N uptake of a succeeding wheat crop as compared to sorghum fodder. Sorghum produced 3.5–7.5 times more dry matter and removed 2–3 times more nitrogen than mungbean or uridbean during same duration (80 ± 10 days) of their growth. Without N application the grain yield of wheat following mungbean and uridbean (without residue incorporation) was 0.45 and 0.48 t ha^–1 more than the yield of wheat following sorghum fodder. These yields were equivalent to that predicted when 36 and 38 kg urea-N ha^–1, respectively, was directly applied to wheat. The residual effects of these grain legumes were higher when succeeding wheat was fertilized with 60 kg urea-N ha^–1; at this level mungbean and uridbean spared 52 and 43 kg urea-N ha^–1, respectively, in succeeding wheat. The residual effect of mungbean and uridbean further increased when their residue was incorporated in soil; with this practice they spared 94 and 115 kg urea-N ha^–1, respectively, without N application to wheat and 74 and 82 kg urea-N ha^–1, respectively, with an application of 60 kg urea-N ha^–1 to wheat.Mungbean and uridbean, without residue incorporation, increased aboveground plant-N uptake of succeeding wheat by 11.5–34.9 and 10.8–34.0 kg N ha^–1, respectively; whereas with residue incorporation, they increased aboveground plant-N content of succeeding wheat by 26.1–45.8 and 32.7–47.7 kg N ha^–1, respectively.The results of the present study indicate that there is both an indirect sparing effect and a direct residual effect of mungbean and uridbean on the nitrogen needs of succeeding wheat, more so when their residues are incorporated in soil.     

The effect of plant residues on plant uptake and leaching of soil and fertilizer nitrogen in a tropical red earth soil

Two field experiments, in which differing amounts and types of plant residues were incorporated into a red earth soil, were conducted at Katherine, N.T., Australia. The aim of the work was to evaluate the effect of the residues on uptake of soil and fertilizer N by a subsequent sorghum crop, on the accumulation and leaching of nitrate, and on losses of N.Stubble of grain sorghum applied at an exceptionally high rate (~ 18 000 kg ha^–1) reduced uptake of N by sorghum by 13% and depressed the accumulation of nitrate under a crop and particularly under a fallow.Loss of fertilizer N, movement of nitrate down the profile, and uptake by the crop was studied in another experiment after application of N as¹⁵NH₄ ¹⁵NO₃ to field microplots. By four weeks after fertilizer application 14% had been lost from the soil-plant system and by crop maturity 36 per cent had been lost. The pattern of¹⁵N distribution in the profile suggested that losses below 150 cm had occurred during crop growth. The recovery of¹⁵N by the crop alone ranged from 16 to 32 per cent. There was an apparent loss of N from the crop between anthesis and maturity. Residue levels common to sorghum crops in the region (~ 2000 kg ha^–1) did not significantly affect uptake by a subsequent sorghum crop, N losses, or distribution of nitrate in the profile.     

Phosphorus transformations as affected by sampling date, fertilizer rate and phosphorus uptake in a soil under pasture

A phosphorus (P) fertilization study was conducted in the southeast of the Buenos Aires province in Argentina, to determine the effect of P fertilizer rate and sampling date on microbial biomass P, and of organic and inorganic P extractable with 0.5M NaHCO₃, in a soil under pasture. In addition, Bray-P, dry-matter production and P uptake were measured. Soil was sampled at different times over one year, and two to three years after application of 0, 50, 100 and 200 kg P ha ^–1. The addition of P fertilizer significantly increased total soil P and the labile fraction of P extractable with NaHCO₃, with the greatest change in the labile inorganic P form, but had no effect on microbial biomass P. Fractions of P showed different patterns of seasonal variation. Microbial biomass P had a peak in winter and a lowest value in summer, the opposite occurring with NaHCO₃-extractable organic P, while NaHCO₃-extractable inorganic P remained relatively constant throughout the year. The cumulative dry-matter yield after three years was 31% higher in the fertilized than in the unfertilized treatments; the highest being 27660 kg ha^–1 for 200 kg P ha^–1. Concentration of Bray-P increased by 0.18 mg P kg ^–1 for each additional kg P ha^–1 added, but remained relatively constant over the year. A significant correlation was found between available Bray-P and microbial biomass P (r = 0.53), and NaHCO₃-extractable organic P (r = 0.47), suggesting that these organic fractions may contribute to plant nutrition.     

Uptake of NPK and yield of rainfed groundnut (Arachis hypogaea L.)

Experiments were conducted on sandy loam soils of Tirupati campus of Andhra Pradesh Agricultural University for two rainy seaons of 1980 and 1981 to study the effect of split application of NPK fertilizers on Spanish bunch groundnut. The fertilizer doses were 40 N, 20 P and 40 K kg ha^–1 in 1980 and 30 N, 10 P and 25 K kg ha^–1 in 1981.In 1980, uptake of N (48 kg ha^–1), P (7 kg ha^–1) and K (37 kg ha^–1) was maximum with the application of 10 N, 5 P and entire 40 K kg ha^–1 as basal and 30 N and 15 P kg ha^–1 at 30 days after sowing, leading to highest pod yield (0.76 t ha^–1). In 1981, application of 20 N, 10 P and 25 K kg ha^–1 as basal dose and 20 N kg ha^–1 at 30 days after seeding resulted in highest uptake of N (114 kg ha^–1), P (17 kg ha^–1) and K (58 kg ha^–1) and hence the pod yield (2.36 t ha^–1).Differences in the uptake of NPK and pod yield in 1980 and 1981 was due to variation in total rainfall and its distribution during the crop period. Rainfall was equally distributed throughout the crop period in 1981, whereas there were two prolonged dry spells of more than 40 days in 1980.     

Evaluation of partially acidulated phosphate fertilisers and reactive phosphate rock for hill pastures

The initial and residual effectiveness of two partially acidulated fertilisers, a single superphosphate (SSP): reactive phosphate rock (RPR) physical mix (SSP:RPR) and a partially acidulated phosphate rock (PAPR), and a RPR, North Carolina, were compared with SSP at two phosphate (P)-responsive sites in hill country. One site had received small annual inputs of SSP (125 kg ha^–1 y^–1) fertiliser for 10 years (LF) and the other site no SSP in the past 5 years (NF). The SSP, PAPR and RPR were applied at 3 rates (20, 40 and 60 kg P ha^–1) and SSP:RPR at one rate (40 kg P ha^–1) once only in the first year. Fertiliser treatments were applied with or without Grasslands Huia white clover (Trifolium repens L.) seed. Initial and residual pasture and legume responses were measured over two years.In the first year, pasture and legume response to applied P was much greater at the LF than NF site. A deterioration in legume content and vigour brought about by withholding fertiliser, rather than a difference in soil-fertiliser reactions, appears to be the main reason for the different response at the two sites in the first year. At the LF site the fully (SSP) and partially (SSP:RPR and PAPR) acidulated fertilisers were far more effective in stimulating legume growth than the RPR, while at the NF site no differences in pasture or legume production were found between fertilisers in the first year. Where fertiliser has been withheld for a number of years the use of SSP appears to be a wasteful and inefficient use of a processed fertiliser.Residual effects of RPR were greater than those of SSP, as shown by the greater yield of legume at both sites in the second year. The residual effectiveness of both the partially acidulated materials was much less than that of the RPR. Mixing and sowing white clover with the fertilisers had some beneficial effects on legume content at the NF site in both years and improved legume production at this site in the second year.     

Response of lowland rice and common bean grown in rotation to soil fertility levels on a Varzea soil

Brazil has approximately 30 million hectares of lowland areas, known locally as Varzea, but very little is known about their fertility and crop production potential. A field experiment was conducted for three consecutive years to evaluate response of lowland rice (Oryza sativa L.) grown in rotation with common bean (Phaseolus vulgaris L.) on a Varzea (low, Humic Gley) soil. Rice was grown at low (no fertilizer), medium (100 kg N ha^–1, 44 kg P ha^–1, 50 kg K ha^–1, 40 kg FTE-BR 12 ha^–1), and high (200 kg N ha^–1, 88 kg P ha^–1, 100 kg K ha^–1, 80 kg FTE-BR 12 ha^–1 fritted trace element-Brazil 12 as a source of micronutrients) soil fertility levels. Green manure with medium fertility was also included as an additional treatment. Average dry matter and grain yields of rice and common bean were significantly (P < 0.01) increased with increasing fertilization. Across the three years, rice yield was 4327 kg ha^–1 at low fertility, 5523 kg ha^–1 at medium fertility, 5465 kg ha^–1 at high fertility, and 6332 kg ha^–1 at medium fertility with green manure treatment. Similarly, average common bean yield was 294 kg ha^–1 at low soil fertility, 663 kg ha^–1 at medium soil fertility, 851 kg ha^–1 at high fertility, and 823 kg ha^–1 at medium fertility with green manure treatment. Significant differences in nutrient uptake in bean were observed for fertility, year, and their interactions; however, these factors were invariably nonsignificant in rice.     

Fertilizer inputs,nutrient balance and soil nutrient supplying power in intensive,irrigated rice system. III. Phosphorus

Data from long-term experiments at 11 sites in Asia with a wide range of nutrient input treatments and yield levels were used to quantify crop P requirements of rice (Oryza sativa L.) and the P balance in intensive, irrigated rice systems. Uptake of 1.8–4.2 kg P was required to produce one ton of grain yield. Physiological P use efficiency varied between 220 to 900 kg grain kg P^-1. Without added P, there was a net loss of 7 to 8 kg P ha^-1 per crop; with added P there was a net gain of 4 to 5 kg P ha^-1 per crop. Phosphorus adsorption kinetics on mixed-bed ion-exchange resin capsules provided an integrative measure of soil P status, P diffusion, and acid-induced P solubilization. The resin capsule was a sensitive tool to characterize buildup or depletion of soil P as a result of different P balances. Both Olsen-P and the resin capsule were suitable methods to predict P uptake of tropical lowland rice. It is hypothesized that both methods measure a similar soil P pool which is soluble under alkaline, aerobic conditions but transformed into acid-soluble P froms as a result of submergence and reduction. Present recommendations for P fertilizer use on rice of 20–25 kg P ha^-1 are adequate to maintain yields of 5–6 t ha^-1, but sustaining higher yields of 7–8 t ha^-1 will require farm-specific management strategies based on knowledge of the long-term P balance and soil P-supplying capacity.     

Phosphorus budget and phosphorus availability in soils under organic and conventional farming

The aim of this work was to assess to which extent organic farming practices would affect the accumulation of total and available phosphorus (P) in a cropped soil in comparison to conventional practices. In order to achieve this, soil samples were taken from a long-term field trial comparing a non-fertilised control (NON), two conventionally cultivated treatments (MIN, CON), and two organically cultivated treatments (ORG, DYN). Soil samples were taken from each treatment at two depths (0-20 and 30-50 cm) before starting the field trial (1977) and at the end of every three crop rotations (1984, 1991 and 1998). They were then analysed for total P (P_t), total inorganic P (P_i), total organic P (P_o) and isotopically exchangeable P_i. After 21 years, the average P input-output budget reached -20.9 kg P ha^–1 a^–1 for NON, -7.8 for DYN, -5.7 for ORG, -5.0 for MIN and +3.8 for CON. Total P, P_i as well as the amount of P_i isotopically exchangeable within 1 minute (E₁) were positively correlated to the P budget. Comparison between P budget and P_t in the top- and subsoils of the fertilised treatments suggested a net transfer of P from the 0–20 to the 30–50 cm layers between 13 and 26 kg P ha^–1 a^–1during the first rotation and between 3 and 12 kg P ha^–1 a^–1during the second rotation. During the third rotation a net upward movement of P from the subsurface to the topsoil ranging between 3.7 and 10.5 kg P ha^–1 a^–1was estimated. In the topsoil, E₁decreased from an initial value of 12 mg P kg^–1 to 11 in CON, 8 in MIN, 6 in ORG, 5 in DYN and 2 in NON after 21 years. In the subsoil, E₁ increased from an initial value of 2 mg P kg^–1 to 4 in MIN, ORG, DYN and NON and to 6 in CON. These results show that, with the exception of NON, all treatments had still an adequate level of available P after 21 years of trial and that, in this low to moderately P sorbing soil, an equilibrated input-output budget allows to maintain P availability at a constant level. In the organic systems, yields have so far partly been attained at the expense of soil reserves or residual P from earlier fertiliser applications.     

Response of three sorghum varieties to N supply and plant density in a tropical environment

Field trials were conducted at Samaru, Nigeria to investigate the growth, yield and grain quality response of three grain sorghum (Sorghum bicolor L. Moench) varieties (L. 187, SK 5912 and FFBL) to N fertilization under varying plant densities (33300, 50000 or 66600 plants ha^–1). Year × N interactions were significant for yield components and so were variety × N and variety × plant density interactions. Grain yield increased 41, 42, and 126% with application of 60 kg N ha^–1, the optimum N rate, a response which was associated with variations in grain weight per panicle, panicle weight and grain number. Varieties SK 5912 and FFBL produced more straw in response to added N than did var. L. 187 while yield components in var. SK 5912 and L.187 responded better than those in var. FFBL. Yield components declined in var. SK 5912 and L. 187 as plant density was increased to either 50000 or 66600 plants ha^–1. Grain crude protein (CP) content and protein yield were increased 8 and 52% respectively by 60 kg N ha^–1 but CP content declined as plant density was increased. Grain tannin content was virtually unaffected by increasing N supply. Optimum plant density for grain sorghum production in this environment is in the range of 50000 plants ha^–1.