The determination of phosphate‐extractable sulphate in soil with an anion‐exchange membrane

Abstract

A method for extracting sulphate from soils using strips of a phosphated anion‐exchange membrane is described. The results obtained by this method are in good agreement with those obtained by extraction with Ca(H₂PO₄)₂solutions and the method has a number of practical advantages over the use of phosphate solutions. No charcoal treatment, centrifuging or filtering is required and the strips are reuseable. No organic interferences are encountered during the turbidimetric measurement of the extracted sulphate.     

Comparison of phosphate‐phosphorus and total phosphorus in DTPA extracts for assessing plant‐available phosphorus in soilless potting media

Abstract

Hakea francisiana and H. laurina were grown in soilless media based on pine bark, to which had been added one of the following phosphorus (P) sources: crushed bone, rock phosphate, calcined rock phosphate, sewage sludge, or sludge compost. Available P was assessed through extraction with unbuffered 2 mM DTPA. Similar regression equations between shoot P content and P in 2 mM DTPA extracts of the media at potting were obtained for both total P in the extract (determined by inductively coupled plasma emission spectrometry) and PO₄‐P. The difference between them was small compared with the variation caused by different rates of dissolution of P the various sources during the growing period. Extractants give only an approximate guide to plant P uptake when the medium contains sources that slowly dissolve during the growing period. Nevertheless, the data indicate that, irrespective of P source, the maximum P concentration in a 2 mM DTPA extract (1:1.5 v/v) of the medium that is tolerated by P‐sensitive plants is 3–4 mg/L. This is similar to the concentration found previously for superphosphate as the source of P.     

Increasing plant‐available phosphorus in an Ultisol with a yard‐waste compost

Abstract

Beneficial uses of a yard‐waste compost as a soil amendment and plant‐growth medium were evaluated on a highly weathered, acid soil (Ustic Kanhaplohumult), using corn (Zea mays L.) seedlings as a test crop. First, factors responsible for the soil's infertility were identified in a greenhouse experiment consisting of five phosphorus (P) rates (0, 75, 150, 300, and 600 mg/kg) with or without 1.0 g calcium (Ca)/g (2 tons Ca/ha) as gypsum (CaSO₄‐2H₂O) or calcium hydroxide [Ca(OH)₂]. At no or low P additions, severe growth restrictions (low dry matter production and shoot P concentration <0.10%) irrespective of the Ca amendments indicated that P deficiency was the most growth limiting factor in this tropical soil. Subsequently, P sorption isotherms were constructed for the soil, the compost, and soilxompost mixtures by equilibrating 1.0 g soil in 20 mL 0.001M calcium chloride (CaCl₂) containing various P concentrations. The results showed that in the unamended state, the soil supported only 0.01 mg P/L, the compost 9.5 mg P/L, and the mixtures containing 25%, 50% or 75% compost by volume 0.04, 0.06, or 0.10 mg P/L in the soil solution, respectively. Approximately 300 mg P/kg must be added to the unamended soil to maintain 0.20 mg P/L in the soil solution. Finally, effects of the compost amendment were studied by growing corn in various volumetric mixtures containing 0, 25, 50, 75, and 100% compost. Best growth was obtained when compost fractions were >75%, corresponding to approximately 0.20% P in the plant shoots and 40 mg/kg Mehlich‐1‐extractable P.     

Evaluation of anion exchange membranes to estimate bioavailable phosphorus in native grasslands of semi‐arid Northwestern Australia

Abstract

Anion exchange membranes (AEMs) were used to assess the P status of semi‐arid sub‐tropical soils of high P sorption capacity from the Pilbara region in northwestern Australia. We determined the most appropriate procedure for using AEMs in these soils using a factorial of extraction ratios and shaking times and compared the method with extraction by water. Significantly more inorganic P (Pi) was extracted by the membranes (AEM‐Pi) than by water, and the amount extracted increased with extraction time but was generally independent of the extraction ratio. Maximum AEM‐Pi was 3.61 μg g^‐1 after eight hour extraction. The AEM procedure was compared with traditional extraction procedures using 0.5 M sodium bicarbonate (NaHCO₃) and 0.1 M sodium hydroxide (NaOH) to assess ability to detect spatial heterogeneity. The amount of Pi extracted decreased in the order: AEM>NaOH>NaHCO_3* The AEM method detected a significant effect of depth on Pi (P=0.0001), while the NaOH method detected both site and treatment effects (P<0.05). Inorganic P extracted by NaHCO₃ did not vary by site, treatment, or depth. Coefficients of variation were generally least using the AEM method. We recommend that studies of spatial and temporal dynamics of P on highly‐weathered soils in semi‐arid regions include measurement of both AEM‐Pi and NaOH‐extractable Pi.     

A new soil test method for the determination of plant‐available cadmium in soils

Abstract

A new soil test procedure using 1M NH₄Cl was developed for the extraction of plant‐available cadmium (Cd) from soils. Five grams of soil is weighed into a 50‐mL polyethylene vial to which 30 mL of 1M NH₄Cl solution is added. The soil suspension is then shaken on a horizontal shaker for 16 h at 25°C at 180 cycles per min. The suspension is then centrifuged at 2,500g for 5 min and the supernatant filtered through a 0.45 μm nitrocellulose filter under vacuum. Cadmium in the extract is then determined at 228.8 nm on a graphite furnace equipped atomic absorption spectrophotometer. A highly significant correlation was observed between the natural logarithm (In) of 1M NH₄Cl‐extractable Cd in soils and the Cd content in the grain of durum wheat (Triticum turgidum var. durutn L.) grown on the same soils (r = 0.974, p = 3.8 x 10^‐7). In comparison with several commonly used extradants, such as ABDTPA, CaCl₂, NH₄OAc, and NH₄NO₃, the 1M NH₄Cl‐extracted Cd from soils was found to be a better index of Cd availability.     

A simple manual method for the determination of phosphate‐extractable (ion‐exchange membrane) sulphate in soils

Abstract

A simple manual method for measuring phosphate‐extractable sulphate in soils using basic laboratory equipment is described. Sulphate is extracted by a phosphated anion‐exchange membrane which, after elution, is measured turbidimetrically as barium sulphate. The results obtained by the proposed manual method compare well with a continuous flow analyser method using a range of Pacific Island soils.     

Depletion of plant‐available silicon in soils: A possible cause of declining rice yields

Abstract

The phenomenon of yield decline in different rice ecosystems from many parts of the world has been reported to be associated with decreases in the effective nitrogen (N) supply from soil. However, many reports in the literature suggest that silicon (Si) is an agronomically essential element for sustainable rice production. Depletion of plant‐available Si in soils where rice is grown could be a possible limiting factor contributing to declining yields. In order to address this problem of yield decline or stagnation, it seems necessary to survey Si status of soils and rice grown in different ecosystems throughout the world and develop region‐specific integrated nutrient management systems that include the element Si.     

Colorimetric determination of phosphorus in citrate‐bicarbonate‐dithionite extracts of soils

Abstract

Existing colorimetric methods for determining phosphorus (P) in citrate‐bicarbonate‐dithionite (CBD) extracts from soils and sediments require pretreatments of extracts prior to the determination of the element. A method is proposed for the determination of P in CBD extracts without any chemical treatments. Maintaining the citrate concentration below 2.4 mM, maximum and stable absorbance develops in P‐Mo‐Sb system within one hour and remains stable for 20 hours. An evaluation of potential interference was made by adding varying amounts of P to CBD extracts over a range of extractable iron (Fe) and aluminum (Al) levels. Near 100 percent recovery of the added P suggests the applicability of the method to soils and sediments. Major advantages of the method are speed, simplicity, reliability and stability of the colored complex.     

Evaluation of some soils in South‐Western Nigeria for arable crop production

Abstract

Five representative soil profiles were excavated and examined along a toposequence at different physiographic points in Itagunmodi area of Southwestern Nigeria. The soil series identified were Ibule, Ijare, Owena, Itagunmodi, and Adio. The soils were characterized with physical, chemical, and mineralogical analyses. Ibule series was not suitable for yam and cassava production. Ijare series was marginally suitable for yam, maize, lowland rice, and cassava production. Owena and Itagunmodi series were highly suitable for yam, maize, and cassava production. However, the latter two series were moderately suitable for lowland rice. Adio series was only suitable for lowland rice production.     

Extraction of phosphorus,potassium, calcium,and magnesium from soils by an ion‐exchange resin procedure

Abstract

A procedure for the simultaneous extraction of phosphorus, potassium, calcium and magnesium from soils, by an ion‐exchange resin procedure applicable to large‐scale advisory soil testing, is described. The important steps are the disaggregation of soil by shaking in water during 15 minutes with a glass marble, the transference of the elements from the soil to a sodium bicarbonate treated mixture of anion and cation exchange resins during a 16‐hour shaking period, the separation of the resin from the soil by sieving and extraction of the elements from the resin.

The results of resin extractable calcium, magnesium and potassium were comparable to the results of these elements extracted with 1M NH₄OAc, to calcium and magnesium extracted with 1M KCl, and to potassium extracted with 0.025M H₂SO₄.

For phosphorus the resin extractable values were not comparable to the results obtained by the former routine method, based on the extraction with 0.025M H₂SO₄. The results of resin extractable P presented closer correlation with cotton response to phosphorus application in 28 field experiments (r = 0.85**) as compared with 0.025M H₂SO₄ extractable P (r = 0.68**), and also with P uptake by flooded rice in a pot experiment with eight lowland soil samples (r = 0.98**), as compared with extraction with 0.0125M H₂SO₄ in 0.050M HCl, for which the correlation was not significant. The reasons for the superiority of the extraction of P with the described procedure are discussed.     

Short‐term evolution of phosphorus in an ultisol

Abstract

Evolution of residual phosphate was monitored as function of time on an Ultisol without cropping in a field trial at Sembawa Rubber Research Station, South Sumatra, Indonesia. Three treatments were imposed and either triple superphosphate at a rate of 600 kg ha^‐1 containing 21.54% phosphorus (P) or lime at the rate of 4.14 tons ha^‐1 was applied. During the 20 months of incubation, extractable phosphate as evaluated by hydrochloric acid‐ammonium fluoride (Bray‐I) declined in all treatments. The decrease in the P fertilized plot was more pronounced in relation to the control and the limed soil. Only 56% of P added to the soil was initially recovered as extractable P. This percentage was 30–40% in limed soils. No significant pH difference was recorded for all treated plots except for limed soils. An increase of 1.3 pH units was initially obtained by liming, but the pH dropped to about 0.9 unit after 20 months of incubation. In addition, total P in arable layer remained fairly constant. According to the recoveries of P added referring to 4.3 P 100 g^‐1, half‐life of a single P dose was estimated at 31 months for a P fertilized soil and almost threefold delay for combination of P fertilization and lime application, which was obtained from the following equation:     

Extractability of added phosphorus in short‐term equilibration tests of Portuguese soils

Abstract

As soil phosphorus (P) availability changes with soil type and time after fertilizer application, use of a P availability index, “F”; (fraction of added P remaining available after a given time), was evaluated for 28 Portuguese soils as a function of soil P extractant (Egner, Fe‐oxide strip, Mehlich 3, and Olsen). The F index values, based on a short‐term P recovery after 2‐h and 24‐h soil‐P equilibration periods, were related to longer‐term F values determined after a 90 day soil‐P equilibration of soil and P (r=0.87 to 0.98; P<0.001). Soil sorption was related to F values (r=0.73 to 0.88; P<0.001). The results suggest that the 2‐h and 24‐h equilibration procedures could be used as a rapid prediction of P fertilizer availability in Portuguese soils widely ranging in P sorptivity.     

Variation of plant‐test phosphorus for individual lupin and wheat tops

Abstract

Two field experiments measured variation of phosphorus (P) concentration in dried tops (plant‐test P) of individual lupin (Lupinus angustifolius cv. Merrit) and wheat (Triticum aestivum cv. Cadoux) plants collected from random locations in experimental plots treated with different levels of superphosphate. Plant‐test P for the individual tops varied by between 4 to 65% of mean values. Coefficient of variation for the 10 individual plants per plot varied from 10 to 24%. For each plot, mean plant‐test P for the 10 individual tops were similar to values for bulk samples of 30 plants collected at random within the plot. It is concluded that a bulk sample of 30 lupin or wheat plant tops need to be collected from uniform areas in farmer paddocks to provide a representative estimated of the current P status of the crop.     

The role of fertilization on phosphorus stratification in no‐till soils

Abstract

No‐tillage induces the stratification of soil nutrients because of the return of crop residues to soil surface, fertilization and the lack of soil mixing. In this research we have attempted to develop a phosphorus (P) balance on soybean, to study the relative importance of the causes of P stratification. An experiment was performed on a Typic Hapludoll located in mid Buenos Aires province, Argentina. The treatments were fertilized and unfertilized. Soybean biomass and P concentration in grains, stubble and roots were determined. In both treatments the P stratification was produced by the enrichment of the surface layer and the impoverishment of the deeper layers. In the non‐fertilized plots the soil lost P (7.5 kg P ha¹) meanwhile in the fertilized plots (20 kg P ha^‐1added) the soil gained P (6.6 kg P ha^‐1). The accumulation of plant residues alone is enough to redistribute P in soils, but fertilization was the main factor in P stratification.     

Desorption of soil phosphate by anion‐exchange resin

Abstract

Kinetics of phosphate desorption from twenty soils by anion‐exchange resin were not satisfactorily explained by either film diffusion (or ion exchange mechanism) mechanism or particle diffusion mechanism. Eut the phosphate desorption process was adequately explained by a two‐constant rate equation. The two constants, the coefficient term (i.e., rate factor) and the constant term (i.e., solution P initially) were significantly correlated with amorphous Al; rate factor increased and solution P decreased with the increase in amorphous Al among the soils. Thus, influence of amorphous Al in P availability from soils must be considered.     

Anion‐exchange membrane,water, and sodium bicarbonate extractions as soil tests for phosphorus

Abstract

Three techniques were evaluated as soil P tests for western Canadian soils: anion‐exchange membrane (AEM), water, and bicarbonate extraction. The AEM, water, and bicarbonate‐extractable total P represented novel approaches to compare to the widely used bicarbonate‐extractable inorganic P (traditional Olsen) soil test. In a range of Saskatchewan soils, similar trends in predicted relative P availability were observed for AEM, water extraction, bicarbonate‐extractable total P, and bicarbonate‐extractable organic P. Correlations between soil test values revealed AEM and water‐extractable P to be most closely correlated, consistent with the similar manner of P removal in the two tests.

Phosphorus availability, as predicted by the tests, was compared to actual P uptake by canola and wheat grown on 14 soils in a growth chamber experiment. P uptake by canola was highly correlated with AEM (r² = 0.86–0.90), water (0.87 ‐0.94), and bicarbonate‐extractable total (0.91) and inorganic (0.92) P. Uptake of P by wheat was not quite as highly correlated with test‐predicted values: AEM (r² = ‐0.73–0.78), water (0.72–0.77), bicarbonate total (0.82), bicarbonate‐inorganic P (0.75).

The similarity in coefficients of determination among test methods indicated nearly identical abilities of the tests to predict soil P availability in the range of soils examined. The AEM and water extractions, unlike bicarbonate, are largely independent of soil type and may prove superior when a wider range of soils is being tested. Bicarbonate‐extractable total P and water‐extractable P suffer limitations in analytical simplicity and cost. In testing for P alone, AEM was considered superior to the other methods due to low cost, simplicity, independence of soil type, and high correlation with plant uptake.     

The determination of ammonium and chloride by an auto‐analyser for the measurement of cation exchange capacity of soils

Abstract

An auto‐analyser method has been developed for the simultaneous determination of NH₄ ⁺ and Cl^‐ in Ca(NO₃)₂/KNO₃ extracts of NH₄Cl treated soils for cation exchange capacity measurements. The method gives satisfactory agreement with manual titration procedures.     

Evaluating iron‐impregnated paper strips for assessing available soil phosphorus

Abstract

Iron (Fe)‐impregnated filter paper strips (Pi) have been proposed as a method for measuring available soil phosphorus (P). A well‐defined Pi method has not yet been developed and Pi strips are often prepared with different filter papers and procedures. A study aimed at arriving at a consistent Pi method is thus needed. Four types of Pi strips, prepared with the two most widely used papers, Whatman No. 50 and 541, following a procedure that incorporates improvements both proposed in the literature and made in our laboratory, were evaluated for P extraction capacity and error. Two of the best strips, which are significantly different in P extraction capacity, along with the Mehlich 1 (0.05M HCl and 0.0125M H₂SO₄) and the Olsen method (0.5M NaHCO₃, pH 8.5) were further evaluated in a greenhouse experiment involving eight soils planted with corn (Zea mays L.). Results indicated that strips prepared with both Whatman No. 50 and 541 were appropriate for P extractions as long as strips were washed with deionized water after treatment with ammonium hydroxide (NH₄OH). At room temperatures the strips probably contain both hydrous Fe hydroxides and oxides in both crystalline and amorphous forms. Pi P was well correlated with Olsen P and P uptake in all soils, indicating that Pi is generally applicable in diverse soils. No obvious advantage was found for the Pi with respect to the Olsen method. Both the Pi and the Olsen method were better extractants with respect to the Mehlich 1, which was ineffective for extracting P in calcareous soils. Extractable P by Mehlich 1, Olsen, and Pi all correlated highly with accumulated plant available P estimated by eight sequential crops in the greenhouse. However, none of the methods could account for all the variation in plant P removal.     

Evaluation of Soil Tests for Plant‐available Mercury in a Soil–Crop Rotation System

A reliable soil test is needed for estimating mercury (Hg) availability to crop plants. In this study, four extraction procedures including 0.1 M hydrochloric acid (HCl), 1 M ammonium acetate (NH₄OAc) (pH 7.0), 0.005 M diethylenetriaminepentaacetic acid (DTPA), and 0.1 M calcium chloride (CaCl₂) (pH5.0) were compared for their adequacy in predicting soil Hg availability to crop plants of a rice–cabbage–radish rotation system. The amounts of Hg extracted by each of the four procedures increased with increasing equilibrium time. The optimal time required for extraction of soil Hg was approximately 30 min, though it varied slightly among the four extractants. The amounts of Hg extracted decreased with increasing soil/solution ratio, and a soil/solution ratio of 1:5 appeared to be adequate for soil Hg availability tests. The amounts of Hg extracted increased in the order of NH₄OAc < CaCl₂ < DTPA < HCl in silty loam soil (SLS) soil, and the order was NH₄OAc < CaCl₂ ≈ DTPA < HCl in yellowish red soil (YRS) soil. Significant positive correlations among the four extractants were obtained in SLS soil. In contrast, the correlations were poor in YRS soil, especially for HCl. There were significant correlations between concentrations of Hg in edible tissue of three plants and the amounts of soil Hg extractable to the four extractants for soil–rice system and soil–radish system, but not for soil–Chinese cabbage system. The 0.1M HCl extraction overall provided the best estimation of soil‐available Hg and could be used to predict phytoavailability of Hg in soil–crop systems.     

Is air-drying of soil samples an appropriate step in determining plant available potassium for corn?

Potassium (K) fertilizer recommendations are mainly based on air -dried soil samples which can lead to over- or under-estimation of plant available soil K. Three on-farm trials were conducted in North Dakota and Minnesota to determine the variation of soil test-K between air-dried (K_Dry) and field moist (K_Moist) soil samples. The differences between K_Dry and K_Moist decreased exponentially as soil K increased, but increased linearly with increasing soil moisture. Soil drying influenced the plant available soil K-test value, producing higher K values compared to the moist soil K. It is unclear based on these initial experiments which method might produce a more predictable K critical value to aid in directing K application for corn in this region.