G. S. Sekhon

Nitrate pollution of groundwater from farm use of nitrogen fertilizers — A review

Abstract The fears that the use of fertilizer nitrogen on farms is contributing considerably to nitrate pollution of groundwater have increased in the past few years. Investigations have indicated that nitrate is accumulating in the shallow groundwaters of some irrigated areas with intensive agriculture using fertilizers. In certain areas, natural geologic deposits of nitrate contribute a large percentage of nitrate leached to groundwater formations. Soil organic matter, animal wastes and plant residues also contribute, but their relative inputs are difficult to determine. The amount of fertilizer nitrogen leaching as nitrate below the root zone and the stability of nitrate in the unsaturated zone and in aquifers are the factors that determine the extent of nitrate pollution of groundwater from fertilizer N. The amount and distribution of rain and irrigation affect the leaching of nitrate below the root zone. However, exactly to what extent nitrate leaching occurs is determined by the amount of water percolating down the profile, which in turn is affected by growing plants. Vegetative cover is the most important factor affecting nitrate leaching by utilizing water (as transpiration) and fertilizer nitrogen. Nitrate in the unsaturated zone and aquifers is generally stable because there is insufficient supply of oxidizable carbon for denitrifiers to utilize. But the possibility of such an occurrence is not ruled out. For computing nitrogen application rates which can ensure both optimum crop yields and permissible nitrate leaching loss, the fertilizer efficiency factor needs careful consideration. Maximizing the efficiency of fertilizer nitrogen can reduce the risk of nitrate pollution from fertilizers.

Interaction of zinc with other micronutrient cations

SummaryEffect of copper on zinc absorption by wheat (Triticum aestivum L. variety WG 357) seedlings and its translocation within the plant was studied in a nutrient solution culture using Zn65.Zinc absorption was increased linearly with time within the limits studied (upto 80 minutes). It decreased, however, with increasing concentration of copper in nutrient solution. Plotting of the reciprocals of rates of zinc absorption vs zinc concentration showed that copper concentration in the nutrient solution inhibited zinc absorption, and this inhibition was competitive. Copper decreased only the absorption of zinc but not its translocation from roots to shoots. re]19750219

Effect of clay, organic matter and CaCO3 content on zinc adsorption by soils

SummaryZinc adsorption was studied in suspensions of six soils of different physicochemical characteristics in dilute ZnSO4 solutions. At low concentrations, Zn2+ adsorption was described by the Langmuir adsorption equation. The calculated Langmuir adsorption maxima were related positively to clay and carbonate content and negatively with organic matter content of soils. Multiple regression analysis revealed that zinc adsorption maxima can be predicted with good precision from information in soil survey reports.When the added Zn2+ exceeded the adsorption maximum, the solid phase of zinc controlling its concentration in solution was either zinc hydroxide or carbonate so long as soil carbonates were present. The values of zinc potential also indicated that soils retain Zn2+ more strongly than Zn(OH)2 or carbonate.

Nitrate pollution of groundwater from nitrogen fertilizers and animal wastes in the Punjab, India

Abstract Fifty-seven water samples were obtained from wells situated on cultivated farms and near village settlements in the Ludhiana and Hoshiarpur districts of Punjab (India). Water samples collected in June and September, 1975 were analysed for nitrate. Probability plots indicated that 90% of the well-water samples may contain less than 45 mg NO 3 − /l, the upper safe limit. The nitrate content of well water near villages was significantly higher than in the cultivated area. Animal wastes appear to be the major contributor in the village environment. The nitrate concentration of well water decreased significantly with depth to water table, but correlated positively with amount of nitrogenous fertilizers added per unit area per year. Soil profile samples taken in June, 1975 from farms in the immediate vicinity of the sampled wells were analysed for nitrate and water content. The amount of NO 3 − contained in the soil profile down to 2.10 m depth correlated significantly with the nitrate concentration of well water in September, thus confirming that nitrates tend to reach the water table during the rainy season (July–September). The projected geometric mean nitrate concentration of ground water (computed on the expectation that the nitrate content of the saturated zone would equal that of the 1.50−2.10 m soil layer) was found to be 35.6 mg NO 3 − /l, many times higher than the observed mean of 1.88 mg NO 3 −1 /l.

Some measures of reducing leaching loss of nitrates beyond potential rooting zone

SummarySeven sites in two long-term fertility experiments progressing at PAU Farm Ludhiana were selected on the basis of fertilizer treatments they were receiving. Soil samples were obtained upto 225 cm depth at 15 cm interval and nitrate was estimated from them by phenol disulphonic acid method. In the first experiment, to each of the three sites, equal amount of N was applied. When phosphorus and potassium were added at the rate of 26.2 kg P/ha and 24.9 kg K/ha, there was little NO3--N left in the profile for leaching, and where no P and K was added, lot of NO3- was left in the profile unutilized. Graphs for P13K25 treatment were in between the two extremes. Perhaps by balanced fertilization roots become proportionately efficient absorbers and little amount of nutrients is left, which is not absorbed. In the second experiment, supply of NPK to all the three treatments was increased or decreased from the recommended dose in a proportionate manner. This resulted in a nitrate distribution pattern similar to that of control treatment where no N was applied and thus strengthened the case for balanced fertilization.

THE EFFECT OF SOIL PROPERTIES ON ADSORPTION AND DESORPTION OF ZINC BY ALKALINE SOILS

The adsorption of Zn by seven alkaline soils of Punjab, India followed the Langmuir equation. At high applied concentrations, however, Zn was probably precipitated as hydroxide. Sequential desorption of adsorbed Zn with IM KC1 and 0.05M Cu(CH3COO)2 provided a measure of its exchangeable and chelated forms. Adsorption-desorption parameters have been computed by analyzing the data according to Langmuir equations. The influence of soil properties on the adsorption-desorption of Zn has been illustrated by correlating them with Langmuir coefficients. Significant zero order correlation coefficients exist between Sm, Rm1, or Rm2 and clay, organic carbon, CaCO3, or CEC. The retaining powers of clay and organic matter are predominantly due to their CEC. Although clay functions independently, CaCO3 and organic matter interact with one another for adsorption of Zn.

The effect of soil characteristics on the zinc-copper interaction in the nutrition of wheat

A pot culture experiment was conducted on 11 soils differing in their physicochemical characteristics to study the occurrence of Zn-Cu interaction in relation to plant growth and Zn and Cu concentrations of wheat. The nature of the interaction was found to vary with the soils and the amounts of applied nutrients. The plant growth showed antagonistic interaction between Zn and Cu when one of the two elements was added in low amounts. However, when the two elements were applied in large amounts, their concentrations in the plants increased but the plant growth was adversely affected. The applied Zn generally failed to affect the uptake of native or low amounts of applied Cu whereas the applied Cu decreased the uptake of native zinc in five out of 11 soils. The DTPA extractable amounts of Zn in experimental soils were not affected signicantly by Cu application and vice versa. The fact that the application of Zn or Cu may not affect or increase the uptake of each other may depend upon the soil characteristics and the rates of their application.

LEACHING OF NITRATE IN CALCAREOUS SOILS AS INFLUENCED BY ITS ADSORPTION ON CALCIUM CARBONATE

Adsorption of the nitrate ion on calcium carbonate and its leaching in calcareous soils were examined by equilibrium and elution techniques. Nitrate ions adsorbed on the surface of CaCO3 fitted the Langmuir model well at an equilibrium concentration of 40 ppm NO3−. Sulphate ions reduced adsorption of nitrate. Data for elution of surface-applied nitrate from laboratory soil columns, when plotted in the form of elution curves and semi-log plots, indicated interactions of nitrate with the soils. The elution curves had long trailing portions due to desorption of nitrate. The length of the trailing portion of a curve was determined by the amount of nitrate adsorbed which in turn seems to depend upon the total surface area of CaCO3. Sulphate ions when present in the displacing fluid seem to desorb nitrate ions from the surface of the CaCO3, whereas chloride ions have little or no effect.

Efficiency of Mussoorie rock phosphate as a source of fertilizer phosphorus to guar ( Cyamopsis tetragonaloba ) and groundnut ( Arachis hypogaea )

In two field experiments, Mussoorie rock phosphate with or without pyrite, farmyard manure or straw was compared with concentrated superphosphate when applied to guar and groundnut. The treatments included rates of 10, 20 and 30 kg P/ha and were located on a soil low in available phosphorus. Mussoorie rock phosphate was 44–52% as efficient as concentrated superphosphate. The incorporation of Amjhore pyrite, farmyard manure or straw in the soil with rock phosphate increased the effectiveness of Mussoorie rock phosphate. Extractability coefficient ratios obtained upon laboratory incubation of soil with concentrated superphosphate and rock phosphate correlated fairly well with the availability coefficient ratios obtained from measurements with plants in the field experiments.

Evaluation of nitric phosphates differing in water solubility of their phosphorus fraction

Three nitric phosphate fertilizers with 30, 60 and 70% of their phosphorus in watersoluble form were evaluated relative to superphosphate and rock phosphate on neutral to alkaline soils. The test crops were wheat, paddy and berseem. Linear grain yield and dry-matter yield responses were obtained up to 100 kg P 2 O 5 /ha for wheat and berseem, respectively. Quadratic response functions were fitted to paddy grain yield. Yields of the various crops were well related to the water solubility of the P fraction in different sources. Availability coefficient ratios were calculated for each fertilizer. These indicated that the minimum water solubility of the phosphorus fraction in nitric phosphates required for high crop yields is dependent on the nature of the crop, on soil characteristics and on the method of application which together determine the shape of the response curve. Band placement of sources with higher water solubility of their P fraction (70% or more) resulted in fertilizer efficiency similar to wholly water-soluble sources. Nitric phosphates of 30 and 50% water-solubility were inferior to water-soluble sources when applied broadcast. However, where curvilinear responses were obtained, low water-solubility sources (30 or 50% water-solubility) performed better at higher than at lower rates of application.