Investigating the response of soil and vegetable crops to poultry and cow manure using ground and satellite data

Based on the massive production of cow and poultry manures, farmers in Saudi Arabia are moving towards the application of organic fertilizers in their farms. Therefore, the present work was conducted to study the response of soil and selected vegetable crops to poultry and cow manures, using ground data and Landsat-8 and Hyperion images. The studied vegetable crops are cabbage, cauliflower, broccoli, and lettuce. A total of 100 t ha^?1 organic manures were applied as a pre-planting treatment. A 12.5 ha field in Tawdeehiya Farms, 200 km southeast of Riyadh, was earmarked for this study. The field was divided into sectors cultivated with the above-mentioned vegetable crops. Soil characteristics, including the soil pH, the electric conductivity (EC), the nitrogen (N), the phosphorus (P) and the potassium (K), were examined before the application of manures and 25 days after the transplanting process. Observations on crops chlorophyll content, number of leaves, the diameter of merchantable products and yield were also investigated. Furthermore, the relationship between the crop performance and yield was investigated through the satellite images generated vegetation indices (VIs). This study revealed the better performance of poultry manure compared to cow manure in terms of development and production parameters of the experimental crops. Dynamics of the chlorophyll content across the crop growth period revealed that all the tested crops responded significantly (R² = 0.69; P = 0.001) to the poultry manure treatments. Among the tested crops, the chlorophyll content, curd or head sizes and crop yields were quite better in poultry manure applied plots. The investigation of crop yield was significant with poultry manure (R² = 0.64; P = 0.001) than cow manure (R² = 0.57; P = 0.001) using the OSAVI and mNDVI, respectively.     

Availability of iron,manganese, zinc and phosphorus in submerged sodic soil as affected by amendments during the growth period of rice crop

Summary Effect of amendments, gypsum (12.5 tonnes/ha), farmyard manure (30 tonnes/ha), rice husk (30 tonnes/ha) and also no amendment (control) on the availability of native Fe, Mn and P and applied Zn in a highly sodic soil during the growth period of rice crop under submerged conditions was studied in a field experiment. Soil samples were collected at 0, 30, 60 and 90 days of crop growth. Results showed that extractable Fe (1N NH₄OAC pH 3) and Mn (1N NH₄OAC pH 7) increased with submergence upto 60 days of crop growth but thereafter remained either constant or declined slightly. Application of farmyard manure and rice husk resulted in marked improvement of these elements over gypsum and control. Increases in extractable Mn (water soluble plus exchangeable) as a result of submergence and crop growth under different amendments were accompanied by corresponding decreases in easily reducible Mn content of the soil. Application of 40 kg zinc sulphate per hectare to rice crop could substantially raise the available Zn status (DTPA extractable) of the soil in gypsum and farmyard manure treated plots while the increase was only marginal in rice husk and control plots indicating greater fixation of applied Zn. Available P (0.5M NaHCO₃ pH 8.5) behaved quite differently and decreased in the following order with crop growth: gypsum>rice husk>farmyard manure>control.     

Effect of gypsum,pressmud, fulvic acid and zinc sources on yield and zinc uptake by rice crop in a saline-sodic soil

Summary The application of fulvic acid to a saline-sodic soil augmented the solubility of zinc by thousands fold. Zinc fulvate when applied at levels equivalent to that of zinc sulphate was more effective in enhancing diffusion of zinc in the soil. Application of gypsum, zinc sulphate and fulvic acid significantly increased dry matter yield and uptake of zinc by rice crop in a saline-sodic soil. Application of gypsum with pressmud or with fulvic acid and zinc sulphate resulted in significantly higher yield and zinc uptake than in other treatments.     

Seventeenth century organic agriculture in China: I. Cropping systems in Jiaxing region

The cropping systems of seventeenth century traditional organic agriculture in the Jiaxing region of eastern China required about 2000 hr of labor per hectare for rice production. Rice and related grain crops were produced employing only human power. The input was about 200 times that for most mechanized grain production today. The charcoal or fossil energy input to produce simple hand tools accounted for only 1–2% total energy in the crop systems. Organic wastes including manures, pond sediments, and green manure crops supplied most of the nutrients. Rice yields, ranging as high as 6700–8400 kg/ha, were similar to some of the highest yields today. The energy output/input ratio ranged from 9 for compost-fertilized rice to 12 for green manure-fertilized rice production. These ratios were 2–10 times higher than most mechanized rice production systems of today. Knowledge of the crop and soil system enabled the early Chinese farms to maintain high crop yields and sustain highly productive soils.     

Transformation and movement of zinc in an alkali soil and their influence on the yield and uptake of zinc by rice and wheat crops

Summary In the summer of 1980, a field experiment was started to evaluate the direct and residual effect of applied zinc (as zinc sulphate) on the yield and chemical composition of rice and wheat grown as crops in sequence, on an alkali soil. The treatments comprised six rates of zinc 0, 2.25, 4.5, 9.0, 18.0 and 27.0 kg ha⁻¹ applied either only once to the first crop, or repeated to each successive crop in a split plot design with 4 replications. Gypsum at 14 t ha⁻¹, was applied uniformly to all plots. The results show that with respect to increase of yield and available zinc content of soil, an application of 2.25 kg ha⁻¹ zinc frequently to each crop was better than a single high dose. A major portion of the applied zinc accumulated in the 0 to 10 cm soil layer; the movement of zinc to lower layers was negligible. Zinc applications increased the concentration of exchangeable < complexed < amorphous sesquixoides-bound zinc > crystalline sesquioxide-bound zinc fractions. Amorphous sesquixoides bound the major portion of the applied zinc compared to other fractions. Exchangeable and amorphous sesquioxide-bound zinc fractions contributed significantly more to zinc uptake by rice, than the other fractions. DTPA extracted zinc more readily from exchangeable and complexed fractions than from sesquioxides. Application of zinc increased the DTPA extractable zinc and hence zinc uptake by plants.     

Growth of Vetiveria zizanioides and Phragmities australis on Pb/Zn and Cu mine tailings amended with manure compost and sewage sludge: a greenhouse study

The Lechang lead/zinc (Pb/Zn) mine and Dabao Shan copper (Cu) mine are located at the north of Guangdong Province in southern China. The residual tailings were permanently stored in tailings ponds which required revegetation to reduce their impact on the environment. A greenhouse study was conducted to evaluate the feasibility of using Vetiveria zizanioides (vetiver) and Phragmities australis (common reed) for the reclamation of Pb/Zn and Cu mine tailings and to evaluate the effects of organic amendments using manure compost (11.00, 22.03, 44.05 and 88.10 t/ha) and sewages sludge (11.00, 22.03, 44.05 and 88.10 t/ha) on the revegetation of these tailings. The results revealed that the applications of manure compost or sewage sludge not only increased N, P and K concentrations, but also decreased DTPA-extractable Pb and Zn contents in Pb/Zn tailings and DTPA-extractable Cu contents in Cu tailings. For Pb/Zn mine tailings, application of sewage sludge increased the yields of both species (highest yield at 44.05 t/ha), but not manure compost. For Cu mine tailings, application of manure compost (highest yield for both species at 44.05 and 22.03 t/ha for vetiver and common reed accordingly) or sewage sludge (highest yield at 22.03 and 44.05 t/ha for vetiver and common reed accordingly) increased the yield of both species. In general, vetiver achieved a higher yield when compared with common reed, under the same treatment. Plant tissue analysis showed that application of manure compost and sewage sludge could significantly reduce Pb uptake and accumulation, but not Cu in both vetiver and common reed.     

Mineralization of copper, manganese and zinc from rock mineral flour and city waste compost for efficient use in organic farming

Restricted supply of micronutrients is a common constraint for plant growth worldwide, especially in organic farming systems where nutrients supply to crops mostly depends on the mineralization of native soil organic matter, decomposition of applied manures and crop residues. A laboratory incubation study was therefore conducted to investigate the potential release of copper (Cu), manganese (Mn) and zinc (Zn) from the rock mineral flour (RMF) and city waste compost (CWC) as compared to inorganic micronutrient fertilizers for 140 days. Release of the micronutrients from RMF and CWC showed different trends. The results showed that about 4.6% of Cu added as RMF was released irrespective of the quantity of the RMF applied. However, Cu release from CWC increased from 0.7 to 3.5% as the amount of compost added was increased. Copper recovery from copper sulphate was 98%. Manganese release from RMF decreased from 114 to 103% as the RMF level was increased, while the corresponding decrease in Mn release from CWC was from 14 to ?3%. Manganese recovery from manganese sulphate was 100%. Zinc release from RMF increased from 5.8?15.5%, with an increase in the amount of RMF applied, while no Zn was released from CWC. Recovery of Zn from zinc sulphate was 98%. These results show that RMF and CWC could be used to meet Cu, Mn and Zn requirements of organically grown cereals. The results of the investigation have general applicability in organic farming.     

Effects of Management Practices on Nematode and Fungus Populations and Cucumber Yield

Three crops of cucumber were grown in succession in beds by use of trickle irrigation, plastic film mulch, and soil chemical treatments over a 17-month period, including a fallow winter season. Total yield for the three crops was highest (1208 quintals/ha) in film-mulched plots treated with MBR-CP, and next-highest in film-mulched plots treated with DD-MENCS (1094 quintals/ha); total yield was only 456 quintals/ha in film-mulched control (untreated) plots. Yield in untreated film-mulched plots was 256% of that in untreated unmulched plots (178 quintals/ha). Plant growth and yields were greatest when populations of nematodes and soil-borne fungi were suppressed to very low levels. The residual control by soil treatments lasted longest on Meloidogyne incognita and Fusarium solani.     

Effect of zinc-amended poultry manure and zinc sulphate on the growth and uptake of zinc by corn (Zea mays L.)

Summary A greenhouse experiment conducted to study the effect of different levels of Zn supplied through Zn-amended poultry manure and ZnSO₄ on corn (Zea mays L.) indicated that both the sources significantly increased the dry matter yield and uptake of zinc. The zinc amended poultry manure at all levels of zinc application was more effective than ZnSO₄ in this respect. The percent zinc in corn derived from the fertilizer and percent utilization of applied zinc from the Zn-amended poultry manure was higher than that from ZnSO₄ when applied at equivalent levels.     

Responses of Wheat Yield,Macro- and Micro-Nutrients,and Heavy Metals in Soil and Wheat following the Application of Manure Compost on the North China Plain

The recycling of livestock manure in cropping systems is considered to enhance soil fertility and crop productivity. However, there have been no systematic long-term studies of the effects of manure application on soil and crop macro- and micro-nutrients, heavy metals, and crop yields in China, despite their great importance for sustainable crop production and food safety. Thus, we conducted field experiments in a typical cereal crop production area of the North China Plain to investigate the effects of compost manure application rates on wheat yield, as well as on the macro-/micro-nutrients and heavy metals contents of soil and wheat. We found that compost application increased the soil total N and the available K, Fe, Zn, and Mn concentrations, whereas the available P in soil was not affected, and the available Cu decreased. In general, compost application had no significant effects on the grain yield, biomass, and harvest index of winter wheat. However, during 2012 and 2013, the N concentration decreased by 9% and 18% in straw, and by 16% and 12% in grain, respectively. With compost application, the straw P concentration only increased in 2012 but the grain P generally increased, while the straw K concentration tended to decrease and the grain K concentration increased in 2013. Compost application generally increased the Fe and Zn concentrations in straw and grain, whereas the Cu and Mn concentrations decreased significantly compared with the control. The heavy metal concentrations increased at some compost application rates, but they were still within the safe range. The balances of the macro-and micro-nutrients indicated that the removal of nutrients by wheat was compensated for by the addition of compost, whereas the level of N decreased without the application of compost. The daily intake levels of micronutrients via the consumption of wheat grain were still lower than the recommended levels when sheep manure compost was applied, except for that of Mn.     

Potential mineralization of nitrogen from organic wastes to ryegrass and wheat crops

Two-pot experiments with ryegrass and wheat plants were conducted in a Cambic Arenosol to test the reliability of N fate predicted by incubation experiments previously performed, with the same soil, to assess potentially mineralizable nitrogen from six organic wastes (municipal solid waste compost, secondary pulp mill sludge, horn meal, poultry manure, solid phase from pig slurry and composted pig manure). Two treatments, corresponding to 80 and 160 kgN/ha were tested, with or without mineral N fertilization. Experimental data obtained in the pot trials was consistent with nitrogen net mineralization trend observed in the aerobic incubations with all the wastes tested. Values of potentially mineralizable nitrogen (N(0)) from the equations obtained by model fitting, to the incubation data, were well correlated to ryegrass and wheat N uptake. Poultry manure was the most efficient N supplier to crops.     

Plant maintenance and environmental stress. Summarising the effects of contrasting elevation,soil, and latitude on Quercus ilex respiration rates

Aims

Rice (Oryza sativa L.), wheat (Triticum aestivum L.) and common bean (Phaseolus vulgaris L.) are major staple food crops consumed worldwide. Zinc (Zn) deficiency represents a common micronutrient deficiency in human populations, especially in regions of the world where staple food crops are the main source of daily calorie intake. Foliar application of Zn fertilizer has been shown to be effective for enriching food crop grains with Zn to desirable amounts for human nutrition. For promoting adoption of this practice by growers, it is important to know whether foliar Zn fertilizers can be applied along with pesticides to wheat, rice and also common bean grown across different soil and environmental conditions.

Methods

The feasibility of foliar application of zinc sulphate (ZnSO₄.7H₂O) to wheat, rice and common bean in combination with commonly used five fungicides and nine insecticides was investigated under field conditions at the 31 sites-years of seven countries, i.e., China, India, Pakistan, Thailand, Turkey, Brazil and Zambia.

Results

Significant increases in grain yields were observed with foliar Zn/foliar Zn?+?pesticide (5.2–7.7 % of wheat and 1.6–4.2 % of rice) over yields with no Zn treatment. In wheat, as average of all experiments, higher grain Zn concentrations were recorded with foliar Zn alone (41.2 mg kg^?1) and foliar Zn?+?pesticide (38.4 mg kg^?1) as compared to no Zn treatment (28.0 mg kg^?1). Though the magnitude of grain Zn enrichment was lesser in rice than wheat, grain Zn concentrations in brown rice were significantly higher with foliar Zn (24.1 mg kg^?1) and foliar Zn?+?pesticide (23.6 mg kg^?1) than with no Zn (19.1 mg kg^?1). In case of common bean, grain Zn concentration increased from 68 to 78 mg kg^?1 with foliar Zn alone and to 77 mg kg^?1 with foliar Zn applied in combination with pesticides. Thus, grain Zn enrichment with foliar Zn, without or with pesticides, was almost similar in all the tested crops.

Conclusions

The results obtained at the 31 experimental site-years of seven countries revealed that foliar Zn fertilization can be realized in combination with commonly-applied pesticides to contribute Zn biofortification of grains in wheat, rice and common bean. This agronomic approach represents a useful practice for the farmers to alleviate Zn deficiency problem in human populations.

     

Build up and depletion of soil phosphorus and potassium and their uptake by rice and wheat in a long-term field experiment

Summary In a field experiment initiated at the Central Soil Salinity Research Institute, Karnal in 1974 involving rice wheat cropping sequence and NPK fertilizer use on sodic soil (pH 9.2, ESP 32.0), an attempt was made to evaluate the available P and K status of the soil and their uptake by the crops during 1982–83 and 83–84.Application of P to either or both the crops significantly enhanced the yields of rice and improved available P status of the soil. Wheat yields remained unaffected. Fertilizer N reduced P content in rice but increased P uptake in crops and considerably brought down available P to a level (4.5 ppm) where rice plants showed reduced tillering and phosphorus deficiency. Application of K did not affect the yield of either crop but enhanced its available status in soil and uptake by the crops. Contribution of the non-exchangeable K towards total potassium removal was about 93% in the absence of applied K which decreased to 87% with the use of K. Application of K to both crops resulted in lesser uptake from non-exchangeable form as compared to its application to either crop. Laboratory studies carried out on soils of the experimental plots showed that cumulative K release measured after five successive extractions was higher in K-treated soils as compared to untreated ones. The major difference was only in the first extraction representing the exchangeable K after which release became independent of the available K of the soil.     

Vegetation Response to Lime and Manure Compost Amendments on Acid Lead/Zinc Mine Tailings: A Greenhouse Study

Land disturbed by mining in China is a serious problem and lead/zinc (Pb/Zn) mine tailings constitute the majority of the metal mine tailings produced in Guangdaong Province, China. A greenhouse study was therefore conducted to evaluate the effects of lime (40, 80, 120, and 160 t/ha) and manure compost (50 and 100 t/ha) amendment on the revegetation of the Pb/Zn mine tailings using Cynodon dactylon (Bermuda grass) and Agropyron elongatum (tall wheatgrass). The results showed that a combination of lime and manure compost amendment together with deionized water leachating was able to increase pH, reduce electrical conductivity and diethylenetraminepentaacetic acid (DTPA)‐extractable concentrations of Zn and Pb in tailings. Using 80 t/ha lime amendment with the supplement of fertilizer or manure compost was able to effectively improve germination of both C. dactylon and A. elongatum. The highest dry weight yields were obtained in tailings receiving 80 t lime/ha and 100 t manure compost/ha for both plant species. Plant tissue analysis showed that lime amendment at 120–160 t/ha reduced Zn accumulation in both shoot and root of C. dactylon. However, this trend was not observed for Pb.     

Fate of Salmonella enterica serovar Typhimurium on carrots and radishes grown in fields treated with contaminated manure composts or irrigation water

Three different types of compost, PM-5 (poultry manure compost), 338 (dairy cattle manure compost), and NVIRO-4 (alkaline-pH-stabilized dairy cattle manure compost), and irrigation water were inoculated with an avirulent strain of Salmonella enterica serovar Typhimurium at 10(7) CFU g(-1) and 10(5) CFU ml(-1), respectively, to determine the persistence of salmonellae in soils containing these composts, in irrigation water, and also on carrots and radishes grown in these contaminated soils. A split-plot block design plan was used for each crop, with five treatments (one without compost, three with each of the three composts, and one without compost but with contaminated water applied) and five replicates for a total of 25 plots for each crop, with each plot measuring 1.8 x 4.6 m. Salmonellae persisted for an extended period of time, with the bacteria surviving in soil samples for 203 to 231 days, and were detected after seeds were sown for 84 and 203 days on radishes and carrots, respectively. Salmonella survival was greatest in soil amended with poultry compost and least in soil containing alkaline-pH-stabilized dairy cattle manure compost. Survival profiles of Salmonella on vegetables and soil samples contaminated by irrigation water were similar to those observed when contamination occurred through compost. Hence, both contaminated manure compost and irrigation water can play an important role in contaminating soil and root vegetables with salmonellae for several months.     

‘On-farm’ seed priming with zinc in chickpea and wheat in Pakistan

A series of on-station trials was implemented between 2002 and 2006 to assess the response of wheat (Triticum aestivum L.) and chickpea (Cicer arietinum) to zinc (Zn) added by soaking seeds (priming) in solutions of ZnSO₄ before sowing. Wheat seed was primed for 10 h in 0.3% Zn and chickpea for 6 h in 0.05% Zn. Seed treatments increased the seed concentration in wheat from 27 to 470 mg/kg and in chickpea from 49 to 780 mg/kg. Priming wheat seeds with 0.3% Zn significantly increased the mean shoot dry mass, Zn concentration and Zn uptake of 15-day-old seedlings relative to non-primed controls and seeds primed with water alone. Using 0.4% Zn further increased shoot Zn concentration but depressed shoot dry mass to the level of the non-primed control. In seven trials, mean grain yield of wheat was significantly increased from 2.28 to 2.42 t/ha (6%) by priming with water alone and to 2.61 t/ha (14%) by priming with 0.3% Zn. Mean grain yield of chickpea in seven trials was increased significantly from 1.39 to 1.65 t/ha (19%) by priming seeds with 0.05% Zn. The effect of priming chickpea seeds with water was intermediate (1.49 t/ha) and not statistically separable from the non-primed and zinc-primed treatments. Increased grain yield due to priming in both crops was associated with increases in total biomass but there was no significant effect of priming on harvest index. In addition to increasing yield, priming seeds with Zn also significantly increased grain zinc concentration, by 12% in wheat (mean of three trials) and by 29% in chickpea (one trial) and the total amount of Zn taken up by the grain (by 27% in wheat and by 130% in chickpea). Using ZnSO₄ to prime seeds was very cost-effective, with net benefit-to-cost ratios of 75 for wheat and 780 for chickpea. An erratum to this article can be found at     

集约化养殖禽畜粪中主要化学物质调查

对广东省集约化养殖场61个禽畜粪样本进行调查的结果表明,鸡粪和猪粪的总N、P、K含量均明显高于传统养殖,鸡、猪和鸽粪的P/N比高于一般作物的P/N比;3种禽畜粪总盐分含量分别为49.0、20.6和60.3 g·kg^-1,盐分组成以K和Na的硫酸盐和氯化物为主;鸡粪Cu、Zn和As的平均含量分别为107.5、366.6和21.6 mg·kg^-1;猪粪分别为765.1、1128.0和89.3 mg·kg^-1;鸽粪分别为56.1、210.9和2.9 mg·kg^-1.这3种禽畜粪的Pb、Cd和Cr含量均很低,在未检出至12.0 mg·kg^-1之间.按不同肥料重金属限量标准,鸡粪和猪粪重金属超标以Cu、Zn和As为主,其中Zn的超标最为普遍.     

Long term effects of manure, charcoal and mineral fertilization on crop production and fertility on a highly weathered Central Amazonian upland soil

Application of organic fertilizers and charcoal increase nutrient stocks in the rooting zone of crops, reduce nutrient leaching and thus improve crop production on acid and highly weathered tropical soils. In a field trial near Manaus (Brazil) 15 different amendment combinations based on equal amounts of carbon (C) applied through chicken manure (CM), compost, charcoal, and forest litter were tested during four cropping cycles with rice (Oryza sativa L.) and sorghum (Sorghum bicolor L.) in five replicates. CM amendments resulted in the highest (P < 0.05) cumulative crop yield (12.4 Mg ha⁻¹) over four seasons. Most importantly, surface soil pH, phosphorus (P), calcium (Ca), and magnesium (Mg) were significantly enhanced by CM. A single compost application produced fourfold more grain yield (P < 0.05) than plots mineral fertilized in split applications. Charcoal significantly improved plant growth and doubled grain production if fertilized with NPK in comparison to the NPK-fertilizer without charcoal (P < 0.05). The higher yields caused a significantly greater nutrient export in charcoal-amended fields, but available nutrients did not decrease to the same extent as on just mineral fertilized plots. Exchangeable soil aluminum (Al) was further reduced if mineral fertilizer was applied with charcoal (from 4.7 to 0 mg kg⁻¹). The resilience of soil organic matter (SOM) in charcoal amended plots (8 and 4% soil C loss, mineral fertilized or not fertilized, respectively) indicates the refractory nature of charcoal in comparison to SOM losses over 20 months in CM (27%), compost amended (27%), and control plots (25% loss).     

From Flooded to Aerobic Conditions in Rice Cultivation: Consequences for Zinc Uptake

Scarcity of water causes a shift from flooded to aerobic conditions for rice production in zinc deficient areas in Northern China. This shift alters soil conditions that affect zinc availability to the crop. This paper concerns the effect of aerobic compared to flooded conditions on crop biomass production, grain yield and zinc content. A field experiment was done with six rice genotypes (Oryza sativa L.) grown on a calcareous soil, both with (23 kg Zn ha⁻¹) and without Zn fertilization. Sampling was conducted at tillering and physiological mature stage. Zn concentration in the shoots was significantly lower at both stages in plants grown in the aerobic field. At maturity, Zn uptake, biomass production, grain yield and Zn-harvest index [grain Zn/(shoot + grain Zn)] were lower under aerobic cultivation. Rice genotypes including aerobic rice and lowland rice differ in degree of response to low Zn supply. A twofold difference was found among aerobic genotypes in grain yield and Zn uptake. Also Zn-harvest index varied significantly. Zn application affected neither grain yield nor grain Zn content, although it significantly improved biomass production in both systems in most genotypes. These results demonstrate that introduction of aerobic rice systems on calcareous soils may increase Zn deficiency problems.     

The importance of initial exchangeable ammonium in the nitrogen nutrition of lowland rice soils

Summary The importance of initial exchangeable soil NH ₄ ⁺ in nitrogen nutrition and grain yield of rice was studied in a number of representative lowland rice soils in the Philippines. The initial exchangeable soil NH ₄ ⁺ +fertilizer N plotted against nitrogen uptake by the crop resulted in a highly significant linear relationship (R²=0.91), suggesting that the presence of exchangeable NH ₄ ⁺ in the soil at transplanting behaved like fertilizer nitrogen. The correlation between N fertilizer rate and N uptake by the rice crop was relatively poor (R²=0.73). On the other hand, relative grain yield was more closely correlated with the initial exchangeable soil NH ₄ ⁺ +fertilizer N than with fertilizer nitrogen applied alone. These results indicate that the initial exchangeable NH ₄ ⁺ in the soil contributed substantially to the nitrogen uptake of the crop.Critical nitrogen levels in the soil defined as the initial exchangeable soil NH ₄ ⁺ +fertilizer N at which the optimum grain yield (95% of the maximum yield) is obtained, varied from 60 to 100 kg N/ha in the wet season and from 100 to 120 kg N/ha in the dry season for the different fertilizer treatments. The results further suggest that the initial exchangeable soil NH ₄ ⁺ should serve as a guide in selecting an optimum nitrogen fertilizer rate for high grain yields.