Predicting Plant Phosphorus Requirements for Hawaii Soils using a Combination of Phosphorus Sorption Isotherms and Chemical Extraction Methods

Phosphorus (P) is an essential nutrient for plant growth and reproduction. One of the tasks of soil testing is to identify whether the soil P level is sufficient to meet crop requirements, and if not, to provide an estimate of the quantity of P that must be added for good growth of a given crop. Data for 12 soils (11 series) from Hawaii were used to develop correlations between critical P concentrations in soil solution derived from P sorption isotherms with P extracted with Mehlich 3, Olsen, or modified Truog solutions. Extractable P, in turn, was correlated with P fertilizer requirements. Critical P levels in soil solution reported in the literature for various crops ranged from 0.005 mg L^?1 for cassava (Manihot esculenta) to 0.30 for lettuce (Lactuca sativa) and to 1.6 for nonmycorrhizal onions (Allium cepa). The P buffer coefficient, defined as the ratio of fertilizer P added to extractable P, averaged 2.2, 4.2, and 8.6 for the modified Truog, Olsen, and Mehlich 3, respectively. Phosphorus requirements for certain crops could be estimated by the following steps: (i) obtaining (possibly one time only) soil solution P levels via P sorption isotherm for a given soil (series or family), (ii) identifying the critical soil solution P for a given crop from the literature, (iii) regressing soil solution P against extractable P, and (iv) establishing relationships between extractable P and fertilizer P.     

Soil Organic Carbon,Biochar, and Applicable Research Results for Increasing Farm Productivity under Australian Agricultural Conditions

The conversion of vegetative biomass waste to biochar (biologically derived charcoal) is a source of carbon (C) that can be used to increase the level of soil organic C (SOC) in agricultural soils. This review collates available research into the effects of biologically derived C species with respect to the direct and indirect effects on agricultural productivity and their potential for use in Western Australian agricultural systems. There is a growing requirement to quantify the effect of specific biochar applications for agroecological purposes and to verify biosequestered C for climate-change-mitigation activities. This work provides a review and assessment of safe biochar application rates and examines the present levels of scientific uncertainty surrounding the efficacy and reliability of applying biochar to soils in relation to crop productivity.     

Tillage Effects on Dryland Soil Physical Properties in Northeastern Montana

We evaluated the effect of no tillage (NT) and conventional tillage (CT) on soil penetration resistance (PR), bulk density (BD), gravimetric moisture content (MC), and saturated hydraulic conductivity (Ks) during the fallow phase of a spring wheat–fallow rotation. The study was conducted on two soils mapped as Williams loam at the Froid and Sidney sites. Soil measurements were made on 19 May, 23 June, and 4 August 2005 at the Froid site and on 6 June and 8 July 2005 at the Sidney site. Tillage had no effect on either soil properties except on the PR at Sidney. However, soil PR, MC, and BD varied significantly with depth regardless of tillage and location. Further, soil PR and MC varied with the date of sampling at both locations, and PR generally increased with decreased MC at all soil depths. Soil Ks was slightly influenced by tillage at both locations.     

Gross fluxes of nitrogen in grassland soil exposed to elevated atmospheric pCO2 for seven years

Plant response to increasing atmospheric CO₂ partial pressure (pCO₂) depends on several factors, one of which is mineral nitrogen availability facilitated by the mineralisation of organic N. Gross rates of N mineralisation were examined in grassland soils exposed to ambient (36 Pa) and elevated (60 Pa) atmospheric pCO₂ for 7 years in the Swiss Free Air Carbon dioxide Enrichment experiment. It was hypothesized that increased below-ground translocation of photoassimilates at elevated pCO₂ would lead to an increase in immobilisation of N due to an excess supply of energy to the roots and rhizosphere. Intact soil cores were sampled from Lolium perenne and Trifolium repens swards in May and September, 2000. The rates of gross N mineralisation (m) and NH₄⁺ consumption (c) were determined using ¹⁵N isotopic dilution during a 51-h period of incubation. The rates of N immobilisation were estimated either as the difference between m and the net N mineralisation rate or as the amount of ¹⁵N released from the microbial biomass after chloroform fumigation. Soil samples from both swards showed that the rates of gross N mineralisation and NH₄⁺ consumption did not change significantly under elevated pCO₂. The lack of a significant effect of elevated pCO₂ on organic N turnover was consistent with the similar size of the microbial biomass and similar immobilisation of applied ¹⁵N in the microbial N pool under ambient and elevated pCO₂. Rates of m and c, and microbial ¹⁵N did not differ significantly between the two sward types although a weak (p<0.1) pCO₂ by sward interaction occurred. A significantly larger amount of NO₃⁻ was recovered at the end of the incubation in soil taken from T. repens swards compared to that from L. perenne swards. Eleven percent of the added ¹⁵N were recovered in the roots in the cores sampled under L. perenne, while only 5% were recovered in roots of T. repens. These results demonstrate that roots remained a considerable sink despite the shoots being cut at ground level prior to incubation and suggest that the calculation of N immobilisation from gross and net rates of mineralisation in soils with a high root biomass does not reflect the actual immobilisation of N in the microbial biomass. The results of this study did not support the initial hypothesis and indicate that below-ground turnover of N, as well as N availability, measured in short-term experiments are not strongly affected by long-term exposure to elevated pCO₂. It is suggested that differences in plant N demand, rather than major changes in soil N mineralisation/immobilisation, are the long-term driving factors for N dynamics in these grassland systems.     

Relative Efficacy of a Drinking‐Water Treatment Residual and Alum in Reducing Phosphorus Release from Poultry Litter

Abstract

Amending poultry litter with alum (aluminum sulfate) effectively reduces soluble phosphorus (P) concentrations, but the practice can be expensive. Aluminum (Al)‐based drinking‐water treatment residuals (WTRs) can be obtained free of charge and are enriched in Al hydr(oxides) that make them efficient P sorbents. Substituting Al‐WTRs for alum would be a cost‐effective practice to reduce soluble P in manures when compared with alum‐only use. The research studied the reductions in soluble P, Al, and total organic carbon (TOC) concentrations in suspensions prepared by mixing variable Al‐WTR and alum rates (0 to 25% by weight) with poultry litter. Suspensions were maintained at pH of 6.5 during the sorption step, and allowed to react up to 50 d, without shaking. On a per‐mole of oxalate‐extractable Al basis, the Al‐WTR was nearly as effective as alum in reducing P release. Increasing mixed alum/WTR mass loads resulted in greater soluble P reduction, simply due to increased molar Al/P ratios. Contact time did not significantly influence soluble P reduction. Two significant advantages of Al‐WTRs compared to alum indicated by this study are cost effectiveness and significantly less release of dissolved Al. Soluble Al and TOC concentrations were least for suspensions with the lowest soluble P levels, suggesting that P was removed from solution as an organo‐Al‐P amorphous phase. The amount of P desorbed from the mixtures decreased to <1% with increasing molar Al/P ratios ≥1. Results suggest that Al‐WTR application with or without alum can reduce soluble P in poultry litter; however, field validation of the amendment effectiveness is needed.     

Effect of soil pH and N fertilization oh uptake and fractions of Ca and Mg in two barley cultivars

Abstract

A study has been made of the amounts of Ca and Mg removed from dry matter of young barley plants by sequential extraction with water, M NaCl, 0.4 M acetic acid and 0.6 M HCl. Two contrasting cultivars were examined and other factors included the effects of the soil pH in which the plants were grown and the form of N‐fertilizer used. The distribution of total Ca and Mg and the different fractions between young and old tissues was also examined.

Water and NaCl between them removed the bulk of the Ca and Mg (between 80 and 90%) and broadly, where uptake of total Ca or Mg increased with increase in soil pH, there was little change in the percentage of either cation appearing in the fractions, indicating a constant removal by the extractants, irrespective of content.

Only about a third of the total Ca was found in the upper half of the plants, against over 60% in the case of Mg and the same distribution was found for the percentages of the water‐soluble and the exchangeable fractions.

Small differences between cultivars and between N forms in respect of fractions and their distribution in the plant were found, but these did not affect the overall general pattern.     

Effect of soil pH on plant growth and nodulation of cowpea

Abstract

A study was conducted to evaluate the effect of soil pH on rhizobium inoculation, plant growth and nodulation of cowpea (Vigna unguiculata). Both inoculated and non‐inoculated seeds of the cultivar ‘California Blackeye No. 5’ were grown in the greenhouse in plastic pots with growth medium being a Norfolk sandy loam (Fine, loamy siliceous, thermic, Typic Palendult) soil under different pH levels. Both soil pH and rhizobium inoculation significantly affected root length, plant height, nodule and pod number per plant. Within the pH range of 6.6 to 7.6, these growth parameters generally were at their maximum, decreasing above or below this pH range. Non‐inoculated plants produced some nodules, indicating failure of the methyl bromide to totally destroy all residual soil rhizobta before inoculation treatment.

The inoculated plants produced more seeds and the increased number of nodules of treated plants was directly related to increased seed weight. Since nodule number was highest at the approximate pH range of 6.6 to 7.6, this range was considered optimum for nodulation of cowpea by this strain of rhizobium under greenhouse conditions. At pH 7.5 and above, roots tended to be more fibrous and nodules were generally smaller in size.     

Bio-organic Amendment of Udic Ustochrept Soil for Minimizing Yield Decline in Sugarcane Ratoon Crops under Subtropical Conditions

The effects of subsequent sugarcane ratooning on soil quality and the crop yields under four treatments [an absolute control (T0), application of recommended dose of nitrogen (N)–phosphorus (P)–potassium (K) (T1), application of sulfitation press mud (SPM), a sugar factory by-product (T2), and SPM along with Gluconacetobacter diazotrophicus (Gd, T3)] were evaluated for 7 years. In the control (T0) and NPK-fertilized (T1) plots, an increase in soil compaction (5.4%), decrease in infiltration rate (6.04%), lower microbial activities, and increased soil phenolic contents (72.4%) rendered the nutrients unavailable, leading to significant declines in the crop yields at the rate of 5.47 Mg ha^?1 y^?1 and 4.67 Mg ha^?1 y^?1, respectively. The crop yield declined from 53 kg ha^?1 in plant crop to 18 kg ha^?1 in the sixth ratoon crop under the absolute control. The rates of yield decline, however, were minimized in SPM (T2) and SPM + Gd (T3) plots to 3.54 and 3.51 Mg ha^?1 y^?1.     

The effect of gamma‐radiation on the release of carbon dioxide from fresh soil

Abstract

The release of CO₂ from fresh soil at medium moisture was examined for 14 days after the application of gamma‐radiation over the range 0.025 ‐ 10 Mrad. All doses stimulated the release of CO₂ compared with non‐irradiated soil, but there was no extra yield of gas between 4 and 10 Mrad. Rapid evolution occurred during irradiation and over the next 24 hours, but towards the end of incubation both irradiated and untreated soil produced CO₂ at similar rates.

Studies to elucidate the origin of CO₂ indicated that the contribution from enzymes was predominant up to 2 Mrad, but at 10 Mrad, 45% of the gas could be formed by radiolytic decarboxylation of soil organic matter. Consequently, heavy irradiation of soil cannot stop production of CO₂, and if high concentrations do interfere with the application of radiation to specific soil research investigations, the gas should be displaced or allowed to diffuse from the sample.