Response of chickpea (Cicer arietinum) to zinc fertilization and its critical level in soils of semi-arid tropics

In a greenhouse experiment the response of chickpea (Cicer arietinum) to zinc fertilization was examined using 27 soils from the semi-arid tropics. The critical level of DTPA extractable soil Zn was evaluated. Zinc additions to the soil increased the dry matter yield of six weeks old plant shoot, grain and straw significantly at the 5 mg kg^–1 level, but tended to decrease it at the 10 mg kg^–1 level.The DTPA extractable Zn of the soils ranged from 0.28 to 1.75 ppm and was negatively correlated at 1 per cent level with pH (r = – 0.81) and positively with organic carbon (r = 0.79) and Olsen's P (r = 0.63). The per cent yield increase or decrease over zero zinc ranged from 67 to – 16 in respect of grain yield and was positively correlated with available Zn (r = 0.86^**). Zinc concentration in plants was greatly increased with the application of Zn and accumulation of Zn was higher in grain than straw. The critical level of available zinc in soil below which plant response to Zn fertilization may be expected was 0.48 mg Zn kg^–1 soil. Soils between 0.48 to 0.70 mg kg^–1 of DTPA extractable Zn appear boarderline and a negative response to applied Zn was observed in soils of high Zn category. The results show the suitability of DTPA soil test for demarcating soils on the basis of plant response to zinc fertilization.     

Direct and residual effect of fertilizer zinc application on the yield and chemical composition of rice-wheat crops in an alkali soil

A field experiment was conducted on an alkali soil to evaluate the direct and the residual effect of six levels of zinc i.e. 0, 2.25, 4.5, 9.0, 18.0 and 27.0 kg Zn ha^–1 added either once to the first crop only or continuously to each crop on the growth, yield and chemical composition of plants grown in a rice-wheat cropping sequence. The soils were amended with gypsum applied at the uniform rate of 14 t ha^–1. Zinc was supplied as zinc sulphate. Application of zinc at the rate of 2.25 kg ha^–1 to both rice and wheat crops or an annual application of 4.5 kg Zn ha^–1 only to rice was found optimum for rice-wheat sequence. Higher zinc applications increased the availability of zinc in the soil and its content in the plants but did not increase crop yield. DTPA extractable zinc build up was more for zinc applied at the rate of 2.25 kg ha^–1 to each crop compared to a single zinc application of equivalent amount. Results of these studies have shown that continuous Zn application up to 27 kg Zn ha^–1 to each crop did not induce nutrient imbalances and had no adverse effect on crop yield.     

Effect of Continuous use of Sodic Irrigation Water with and without Gypsum, Farmyard manure, Pressmud and Fertilizer on Soil Properties and Yields of Rice and Wheat in a Long term Experiment

A long term field experiment was conducted for 8 years during 1994–2001 to evaluate the effect of N, P, K and Zn fertilizer use alone and in combination with gypsum, farmyard manure (FYM) and pressmud on changes in soil properties and yields of rice and wheat under continuous use of sodic irrigation water (residual sodium carbonate (RSC) 8.5 meq l⁻¹, and sodium adsorption ratio (SAR) 8.8 (m mol/l)^1/2 at Bhaini Majra experimental farm of Central Soil Salinity Research Institute, Karnal, India. Continuous use of fertilizer N alone (120 kg ha⁻¹) or in combination with P and K significantly improved rice and wheat yields over control (no fertilizer). Phosphorus applied at the rate of 26 kg P ha⁻¹ each to rice and wheat significantly improved the yields and led to a considerable build up in available soil P. When N alone was applied, available soil P and K declined from the initial level of 14.8 and 275 kg ha⁻¹ to 8.5 and 250 kg ha⁻¹ respectively. Potassium applied at a rate of 42 kg K ha⁻¹ to both crops had no effect on yields. Response of rice to Zinc application occurred since 1997 when DTPA extractable Zn declined to 1.48 kg ha⁻¹ from the initial level of 1.99 kg ha⁻¹. Farmyard manure 10 Mg ha⁻¹, gypsum 5 Mg ha⁻¹ and pressmud 10 Mg ha⁻¹ along with NPK fertilizer use significantly enhanced yields over NPK treatment alone. Continuous cropping with sodic water and inorganic fertilizer use for 8 years slightly decreased the soil pH_e and SAR from the initial value of 8.6 and 29.0 to 8.50 and 18.7 respectively. However, treatments involving the use of gypsum, FYM and pressmud significantly decreased the soil pH and SAR over inorganic fertilizer treatments and control. Nitrogen, phosphorus and zinc uptake were far less than additions made by fertilizer. The actual soil N balance was much lower than the expected balance thereby indicating large losses of N from the soil. There was a negative potassium balance due to greater removal by the crops when compared to K additions. The results suggest that either gypsum or FYM/pressmud along with recommended dose of fertilizers must be used to sustain the productivity of rice – wheat system in areas having sodic ground water for irrigation.     

Relative yield and zinc uptake by rice from zinc sulphate and zinc oxide coatings onto urea

Zinc (Zn) deficiency is prevalent worldwide and is a barrier to achieving yield goals in crops. It is also now recognized as a leading risk factor for disease in humans in developing countries. In general, soil application of 5–17 kg of Zn ha⁻¹ year⁻¹ as zinc sulphate (ZnSO₄) or more is recommended. However, in developing rice growing countries of Asia, ZnSO₄ of desired quality is not readily available and is also quite expensive, so the farmers generally fail to apply Zn, resulting in rice crop yield loss. Availability of Zn-coated urea guarantees not only the availability of quality Zn but also ensures its application. Field experiments were therefore conducted during the rice seasons of 2005 and 2006 at the Indian Agricultural Research Institute, New Delhi, to evaluate the relative efficiency of 0.5, 1.0, 1.5 and 2.0% Zn as ZnSO₄- or zinc oxide (ZnO)-coated ureas for rice. Soil application of ZnSO₄ was also compared in 2006. Rice grain and straw yields, Zn concentrations in grain and straw, and Zn uptake by rice increased with the level of Zn coating onto urea. Crop response was the highest with 2.0% ZnSO₄-coated urea, and higher than with the same rate of ZnO-coated urea, possibly related to the higher water solubility of Zn in ZnSO₄. Crop response with ZnSO₄-coated urea was also higher than with the same rate of ZnSO₄ and urea applied separately to the soil. However, apparent recovery data suggest that 1.0% coating with ZnSO₄ may be a better choice from the point of view of the utilization of applied Zn. Increased Zn concentrations in rice grain due to application of Zn-coated urea is important from the point of view of Zn nutrition of humans, since rice is the staple food in developing countries of Asia. Also, increased Zn concentrations in rice straw is of importance as regards cattle nutrition since in developing countries of Asia rice straw is the major feed for farm cattle.     

Effect of zinc-enriched urea on productivity,zinc uptake and efficiency of an aromatic rice–wheat cropping system

Zinc deficiency is prevalent worldwide and is a barrier in achieving yield targets in crops. It is also now recognized as a leading risk factor for disease in humans in developing countries. Generally, soil application of 5–17 kg Zn ha⁻¹ y⁻¹ (25–85 kg zinc sulphate heptahydrate ha⁻¹ y⁻¹) or more is recommended for rice. However, in the developing rice-growing countries of Asia, zinc sulphate of desired quality is not readily available and is also quite expensive, and the farmers generally fail to apply Zn, resulting in crop yield loss in rice. Availability of zinc-enriched urea (ZEU) makes possible not only the availability of quality zinc, but also assures its application. Therefore, field experiments were conducted for two consecutive years at the research farm of Indian Agricultural Research Institute, New Delhi, India, during rainy (rice) and winter (wheat) seasons of 2004–2006 on a sandy clay-loam soil to study the effect of various concentrations of zinc enrichment of urea on productivity, zinc concentrations, its uptake and use indices of aromatic rice–wheat cropping system. Eight treatments comprising prilled urea (PU) and 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5% zinc-enriched urea, replicated three times, were compared in a randomized block design. The enrichment of PU was done through zinc oxide containing 80% zinc. The results of this study revealed that the zinc-enriched urea (ZEU) had a significant effect on growth, yield attributes and yields of aromatic rice. Highest values for all these attributes and yields were recorded at the highest enrichment (3.5%) of the PU with zinc. The highest zinc concentration and uptake in rice grain and straw were also significantly higher with the highest level (3.5%) of zinc enrichment. The highest total zinc uptake recorded was 1,168 and 1,353 g ha⁻¹, during 2004 and 2005, respectively, with 3.5% ZEU. However, a major increase in grain yield of rice was recorded up to 1.0% zinc enrichment. The residual effect of zinc-enriched urea on succeeding wheat yield and zinc uptake was significant only at a higher level of zinc-enriched urea and only in the second year of study. Overall, 1.0% zinc-enriched urea recorded significantly higher productivity and zinc uptake over PU in the rice–wheat cropping system and is recommended for Delhi and adjoining areas. The recommendation is also made keeping in view the fact that with increased levels of zinc enrichment of urea, the partial factor productivity, agronomic efficiency, apparent recovery and physiological efficiency of applied zinc in a rice–wheat system decreased significantly. Considering all the economic parameters (benefit, benefit:cost ratio, IR gained IR⁻¹ invested in zinc), 1.0% ZEU proved the most economic source for aromatic rice–wheat cropping system and therefore is recommended for rice–wheat cropping system in Delhi and adjoining areas of north India.     

Zinc and phosphorus interaction in a wheat-maize cropping system

To study the interaction effect of Zn and P in a wheat-maize cropping system, a field experiment was conducted at the H.P. Agricultural University Research Station, Palampur (India). Zinc was applied as ZnSO₄·7H₂O at the rate of 0, 20 and 40 kg per ha and P as superphosphate at the rate of 0, 60 and 120 kg per ha. The direct Zn-P interaction effect was investigated on wheat (S—308) and its residual effect on maize (early composite). Added Zn did not increase the grain and straw yield of wheat when P was not applied, but when P was applied, 20 kg per ha added Zn responded significantly. Contrary to this, in maize, only 20 kg per ha added Zn responded significantly when P was not applied, but when P was applied, even 40 kg per ha Zn increased the grain and straw yield of maize. The grain and straw yield of wheat and maize were higher under limed than under unlimed conditions.The concentration of Zn increased with the application of Zn and decreased with the application of P. The concentration of Zn was comparatively higher in grain than in straw of wheat and maize. The P concentration in wheat and maize plants decreased with the increasing levels of applied Zn. The concentrations of Zn were lower under limed than under unlimed condition, whereas the reverse was true for P concentrations.The respective absorption of Zn and P in wheat was 9.7 and 7.3 per cent upto tillering, 47.9 and 49.4 per cent between tillering and flowering, and 42.3 and 43.3 per cent between flowering and maturity. The corresponding absorption of Zn and P in maize was 11.7 and 9.4, 59.9 and 52.1, and 29.3 and 38.5 per cent before booting stage, between booting and tasseling stage and between tasseling and maturity stage, respectively. At maturity, about 53.1 and 13.0 per cent of the Zn and P taken up were retained by wheat straw and 46.9 and 87.0 per cent by wheat grain. The respective values for Zn and P in maize straw and grain were 66.8 and 30.3 and 33.2 and 69.7 per cent. When more Zn was applied, less Zn was translocated to grains; when more P was added, more Zn was translocated to grains. The effects of P and Zn on P distribution at maturity were opposite to that of Zn distribution.     

Response of pearl-millet to zinc fertilization in relation to DTPA extractable zinc

The response of pearl-millet (Pennisetum americanum) grown on forty eight diverse soils to applied zinc fertilization was examined in a screenhouse experiment. The DTPA-extractable soil zinc ranged from 0.34 to 1.42 mg kg^–1. In many of the soils yield was increased by the addition of zinc and there were large differences in the size of the response. The critical level of zinc in soil and plant — below which response to applied zinc may be expected — was determined by a graphical method. The values found were 0.65 and 18 mg kg^–1, respectively. Bray's percent yield was positively and significantly related with both soil Zn (r = 0.88) and plant Zn (r = 0.72).     

Determination of critical level of zinc in non-calcareous soils for predicting response of wheat to applied zinc

Studies were conducted in ten non-calcareous arid brown soils (India) to determine the critical level of soil Zn for predicting response of wheat to zinc fertilization. The per cent mean response at 5 mg kg^–1 added Zn varied from 1.3 to 51.4 with a mean value of 17.5 per cent over control in terms of grain yield (g pot^–1). Further, Zn application resulted in significant increase in Zn concentration in various plant parts in all the soils irrespective of the initial Zn status. The critical level of Zn in soil and plant below which response to applied Zn may be expected was found to be 1.75 mg kg^–1 for 0.1 N HC1 extractable soil Zn and 1.7 mg kg^–1 for plant tissue Zn.     

Zinc equilibria in submerged sodic soils as influenced by application of amendments

In a pot culture experiment, the effect of gypsum (50% of soil requirement), pyrites (equivalent to gypsum), farmyard manure (0.5 g per 100g soil) and Zn (10mg Zn kg^–1 soil) on Zn equilibria in Ghabdan and Langrian series of sodic soils was studied. The equilibrium soil solutions collected anaerobically after 1, 14, 28, 42, 56, 70 and 84 days of submergence were analysed for pH, EC, HCO₃ and Zn. Submergence markedly decreased soil solution pH and Zn up to 14 days and thereafter, the former slightly increased and the latter continued to decrease. Addition of amendments decreased soil solution pH in the order gypsum, pyrites and farmyard manure (FYM) and increased Zn concentration in the order FYM, gypsum and pyrites. The values of Zn potential (pZn + 2pOH) were within the range of pKsp for Zn(OH)₂-Zn²⁺ (aq) from 16 to 84 days of submergence in Ghabdan soil and from 9 to 42 days of submergence in Langrian soil, where later it shifted to ZnCO₃-Zn²⁺ (aq) system. Addition of FYM, pyrites and gypsum shortened the period of predominant existence of Zn(OH)₂-Zn²⁺ (aq) system to 40, 30 and 12 days in Ghabdan soil and 30, 20 and 6 days in Langrian soil respectively. After these periods the system was saturated with respect to ZnCO₃-Zn²⁺ (aq) except gypsum treatment where Zn-soil (unknown solid phases) -Zn²⁺ (aq) system controlled the solubility of Zn after 38 and 28 days of submergence of Ghabdan and Langrian soil respectively.     

Critical manganese deficiency level for soybean grown in Ustochrepts

A greenhouse study with 15 soils, having a range in DTPA extractable Mn, was conducted to determine the critical deficiency level of Mn in Ustochrepts for predicting response of soybean to Mn application. Soil application of 10 mg Mn kg^–1 soil significantly increased the dry matter yield in deficient soils. Soil Mn was significantly related with Bray's per cent yield (r = 0.72^**) and Mn uptake (r = 0.75^**). Both graphical and statistical models of Cate and Nelson indicated the critical level to be 3.3 mg kg^–1 soil of DTPA extractable Mn. Critical Mn deficiency level in recently matured terminal leaflet blade at V6 growth stage in soybean plant was 22.0µg g^–1 dry matter. The predictability of soil and plant critical Mn level was 87 per cent.     

Efficiency of fall-applied urea for barley: Influence of date of application

Fall application of N fertilizers is often inferior to spring application for increasing yields of spring-sown cereal grains. The objective of this study was to determine the influence of date of application on efficiency of fall-applied N. Fall application dates were related to recovery of fall-applied N as mineral N in soil in spring, and related to yield and N uptake for spring-sown barley. Urea at a rate of 50 or 56 kg N ha^–1 was incorporated into the soil to a depth of 10 cm. There were 2 or 3 application dates in the fall and one in the spring at sowing. Linear regression indicated recovery of fall-applied N as soil mineral N in spring increased from 30% with urea added on 19 September to 79% with addition on 6 November, but the predictability was low (r = 0.54^**). Increase in grain yield, expressed as relative efficiency of fall- versus spring-applied N, was only 23% on 19 September but rose to 76% by 6 November (r = 0.68^**). Results were similar for N uptake in grain. Other approaches to predicting the relative efficiency of fall- versus spring-applied N for yield increase were based on fall soil temperature at 5 cm depth, instead of fall calendar date. Soil temperature on the day of N application gave inferior correlation (r = –0.55^**), but the use of number of days from application to first day of 0°C soil temperature gave a fairly close correlation (r = –0.77^**). Soil degree-days accumulated from application to first day of 0°C soil temperature gave a similarly close correlation (r = –0.78^**). In all, the efficiency of fall-applied urea was markedly increased by delaying the application into the late fall; and calendar date, number of days or soil degree-days from application to soil freezing all predicted the efficiency fairly well.(Contribution No. 599)     

Potential for nitrogen mineralization in central Alberta soils

Incubation experiments were conducted to determine the relationship between N mineralization potential of soils and yield or N uptake of barley grain. In addition, the effect of soil type and soil depth on N mineralization potential was investigated. In an experiment with 39 cultivated surface soil samples varying in organic C from 1.5 to 8.6%, the amount of mineralized N (as determined by the incubation method of Stanford and Smith, 1972) ranged from 34 to 111 mg N kg^?1 over a 12-week period but the correlation coefficient between mineralized N and soil organic C was only 0.49^**. Mineralized N was not correlated with grain yield or N uptake (r = 0.29 or 0.32, respectively), but there was a fairly close correlation between soil NO₃-N at sowing and yield (r = 0.79^**) or N uptake of barley grain (r = 0.82^**). Combining soil NO₃-N at sowing and mineralized N on incubation did not improve correlation. In the other experiment with just two soils, the mineralized N sharply decreased with increasing soil depth.     

Phosphorus Mass Balances for Successive Crops of Fertilised Rainfed Rice on a Sandy Lowland Soil

Raising and sustaining rice yields in the rainfed lowlands requires an understanding of nutrient inputs and outputs. On sandy lowland rice soils, managing phosphorus (P) supply is a key factor in achieving increased yields and sustainable production. Phosphorus inputs, rice yields, and crop P uptake were used to quantify P requirements of rice: together with results on soil P fractions, P balance sheets were constructed over five consecutive cropping seasons on a sandy Plinthustalf near Phnom Penh, Cambodia. Grain yields ranged from 665 to 1557 kg ha⁻¹ with no added P. Average yields increased significantly with P fertiliser application over five consecutive crops by 117, 139 and 140% when the phosphate fertiliser was applied at 8.25, 16.5 and 33 kg P ha⁻¹, respectively. Without added P fertiliser, a net loss of 1.2 kg P ha⁻¹ per crop was estimated with straw return and 2.0 kg P ha⁻¹ per crop with straw removed from the field, whereas, with added P fertiliser, there was a net P gain in the soil of 5.6 or 9.5 kg ha⁻¹ per crop when straw was removed and returned to the soil, respectively. After one crop, the addition of P fertiliser significantly (P < 0.01) increased recovery in all soil P fractions. Across five successive crops, repeated application of 16.5 and 33 kg P ha⁻¹ rates resulted in progressive P accumulation in the soil, especially a labile NaOH–Po pool, but had no effect on yields and P uptake of rice. By contrast, 8.25 kg P ha⁻¹ per rice crop was generally adequate for grain yields of 2.5–3.0 t ha⁻¹ and to maintain soil P pools.     

Critical levels of Mn in coarse textured rice soils in India for predicting response of barley to Mn application

Green house studies of 20 soils, having a range in DTPA extractable Mn, were made to determine the critical deficiency level of Mn for predicting response of barley to Mn application. Soil Mn was significantly related with both Bray's per cent dry matter yeild (r = 0.70^**) and Mn uptake (r = 0.65^**). Soil application of 25 mg Mn kg^–1 soil significantly increased yield. Both graphical and statistical models of Cate and Nelson indicated the critical level to be 2.05 mg kg^–1 soil of DTPA extractable Mn. The critical Mn deficiency level in 45 day barley plants was 18.6 mg kg^–1 dry matter. The predictability of soil and plant critical Mn level was 91 and 80 per cent respectively.     

Relative performance of chelated zinc and zinc sulphate for lowland rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Field experiments were conducted during the wet seasons of 2005–2006 and 2006–2007 on an Aeric Endoaquept (pH 7.2) to study the relative performance of chelated zinc [Zn ethylene diamine tetra-acetic acid (EDTA)] and zinc sulphate (ZnSO₄) on the growth and yield of rice (cv. IET 4094). The diethylene triamine penta-acetic acid (DTPA) extractable (available) Zn concentration in soil and total Zn content in dry matter of rice increased initially up to 28 days of crop growth when Zn was applied as a single basal source, being greater with chelated Zn compared with ZnSO₄ application. The highest mean Zn uptake by rice grain and straw was found to be 209.2 and 133.8 g ha⁻¹, respectively, in the treatment T₇ (1 kg Zn ha⁻¹ as Zn-EDTA at basal). The mean filled grain percentage, thousand grain weight and number of panicles m⁻² were highest with 90.4%, 25.4 g and 452, respectively, in treatment T₇ where 1 kg ha⁻¹ Zn as Zn-EDTA was applied. The highest yield of grain and straw was 5.5 and 7.3 t ha⁻¹, respectively, in treatment T₇, resulting in a 37.5 and 43.1% increase in yield over that of the control during both the years.     

Efficiency of nitrogen fertilizer use under rainfed maize and irrigated wheat at Kadawa,northern Nigeria

Field trials were conducted at Kadawa, northern Nigeria, during 1975–77 to study the efficiency of nitrogen fertilizer use under maize (Zea mays L.)—wheat (Triticum aestivum L.) rotation; the study also examined the impact of continuous N use on some soil properties. Grain and straw dry matter yields, grain N content, crop N uptake and whole plant N concentration of wheat at different growth stages increased significantly with increasing levels of N application. Per cent increases in mean grain yield of N treated plots over control were 77, 131 and 141 for maize and 195, 308 and 326 for wheat at 60, 120 and 180 kg N per ha levels, respectively. The calculated N rates for maximum yield were 177.5 and 164.0 kg N per ha for maize and wheat, respectively. Short-term beneficial effect of dung on maize yield was ascribed to its additional N supply. Urea and calcium ammonium nitrate (CAN) were equally good for both maize and wheat; full and split N application gave no significant difference in yield. The values for mean fertilizer N recovery over all the crops were 64, 58 and 44% respectively, at 60, 120 and 180 kg N per ha levels.Nitrogen application at the highest rate (180 kg per ha) reduced the soil pH significantly in the top 40 cm of the soil profile. The magnitude of soil acidification at levels of N below 120 kg per ha was not appreciable in this study. High N application also depleted the soil of its cations at differential rates. Other factors such as N source, time of N application and addition of dung along with N fertilizer did not have much influence on the rate of short-term soil acidification due to N fertilizer use.     

Boron uptake and toxicity in wheat in relation to zinc supply

Zinc deficiency may enhance B absorption and transport to such an extent that B may possibly accumulate to toxic levels in plant tops. Therefore, a screen house experiment was conducted to investigate the effect of B levels (0, 2.5, 5.0, 7.5 and 10 mg B kg^–1 soil) as influenced by Zn levels (0, 10 and 20 mg Zn kg^–1 soil) on DM yield of wheat tops and tissue concentration and uptake of B, Zn, Cu, Mn, Fe, Ca, Mg, K and P. Application of B decreased the dry matter yield of wheat significantly at all levels of Zn. Conversely, increasing levels of Zn increased the wheat yield significantly. The application of B increased the tissue concentration and uptake of B by wheat plants more in the absence than in the presence of Zn application. Consequently, concentration of B in wheat plants decreased with increasing levels of Zn application to the soil. This decrease in tissue B concentration was not only due to increased growth of wheat plants. Zinc application appears to have created a protective mechanism in the root cell environment against excessive uptake of B, as evidenced by the reduction of B uptake in Zn treated plants. The uptake of Mn, Mg and P decreased while the uptake of Cu, Fe, and K by wheat plants increased with Zn application. Whereas, the uptake of all nutrients (Cu, Fe, Mn, Ca, Mg, K and P) decreased significantly with the application of B. However, this depressive effect of B on nutrient uptake was less marked in the presence of applied Zn.     

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.     

Effects of different levels of chemical Nitrogen (urea) onAzolla and Blue-green algae intercropping with rice

Application of higher levels (60 and 90 kg N ha^–1) of nitrogen fertilizer (Urea) inhibited the growth ofAzolla pinnata (Bangkok) and blue-green algae (BGA) though the reduction was more in BGA thanAzolla. Inoculation of 500 kg ha^–1 of freshAzolla 10 days after transplanting (DAT) in the rice fields receiving 30, 60 and 90 kg N ha^–1 as urea produced an average of 16.5, 15.0 and 13.0 t ha^–1 fresh biomass ofAzolla at 30 DAT, which contained 31, 31 and 27 kg N ha^–1, respectively. The dry mixture of BGA (60%Aulosira, 35%Gloeotrichia and 5% other BGA on fresh weight basis) inoculated in rice field 3 DAT at a rate of 10 kg ha^–1 showed a mat formation at 80 DAT with an average fresh biomass of 8.0, 5.8 and 4.2 t ha^–1 containing 22, 17 and 12 kg N ha^–1, respectively with those N fertilizer doses.Application ofAzolla showed positive responses to rice crop by increasing the panicle number and weight, grain and straw yields and nitrogen uptake in rice significantly at all the levels of chemical nitrogen. But, the BGA inoculation had a significant effect on the grain and straw yields only during the dry season in the treatment where 30 kg N was applied. During the wet season and in the other treatments performed during the dry season no significant increase in yields, yield components and N uptake were observed with BGA.The intercropping ofAzolla and rice in combination with 30, 60 and 90 kg N ha^–1 as urea showed the yields, yield attributes and nitrogen uptake in rice at par with those obtained by applying 60, 90 and 120 kg N ha^–1 as urea, respectively but, the BGA did not. The analysis of soil from rice field after harvest showed thatAzolla and BGA intercropping with rice in combination with chemical fertilizer significantly increased the organic carbon, available phosphorus and total nitrogen of soil.     

Root growth and phosphorus and potassium uptake by two corn genotypes in the field

In a field study P and K uptake by two corn (Zea mays L.) genotypes which differed in root growth was investigated. The effect of differences in root growth on P and K uptake was assessed using a mechanistic-mathematical model which describes nutrient uptake by growing plant roots in soil. Nitrogen was applied at 0 and 227 kg ha^–1 to Pioneer 3732 and B73xMo17 corn grown on Raub silt loam (fine-silty, mixed, mesic Aquic Argiudoll) and at 227 kg N ha^–1 to these two genotypes on Chalmers silt loam (finesilty, mixed, mesic, Typic Haplaquoll). Root growth and P and K uptake by the two corn genotypes was measured 31, 47, 75 and 91 d after planting on the Raub and 31, 47, 61 and 75 d after planting on the Chalmers soil.Root growth and P and K uptake by B73xMo17 was greater than that of Pioneer 3732 on N-fertilized Raub soil. On Chalmers soil the difference in root growth between the two genotypes resulted in an increase in K but not P uptake. The higher soil P level of the Chalmers appears to have offset possible differences in P uptake due to root size. There were no differences between the two genotypes in either the percentage of roots with root hairs, or the density or length of root hairs. Phosphorus and K uptake calculated with the simulation models for both corn genotypes on both soils over each of three growth periods agreed with observed P (Y = 0.68X + 1.71; r = 0.944^**) and K (Y = 0.88X + 15.52; r = 0.928^**) uptake. Differences in P and K uptake between B73xMo17 and Pioneer 3732 resulted primarily from the difference in root growth in the topsoil. A high correlation was found between root surface area and P (r = 0.893^**) and K (r = 0.928^**) uptake by both corn genotypes on both the soils.Journal paper No. 10,316 Purdue Univ. Agric. Exp. Stn., W. Lafayette, In 47906. Contribution from the Dep. of Agron. This paper was supported in part by a grant from the Tennessee Valley Authority.