Phospho‐molybdo blue color intensity, its relation to P concentration in solution, and color stability are greatly influenced by the solution acidity and matrix. Our objectives were to: find the acid‐stability plateau for the ascorbic acid‐Mo blue complex and its application to colorimetric P analysis; identify the stability of the calibration graphs for Soltanpour‐Schwab, Watanabe‐Olsen, and Bray‐Kurtz P‐1; and study the stability of the mixed and color‐developing reagents. Standards containing 0.0, 0.2, 0.4, 0.6, 0.8, 0.9, 1.0 and 1.1 mg P L−1 were prepared across a range of acidity from 0.0 to 0.6 M H2SO4. The acid‐stability plateaus were between 0.17 and 0.28 M and 0.17 and 0.46 M H2SO4 when absorbances were obtained after 10 min and 24h, respectively, after adding the color‐developing reagent. The blue color was stable with time for P concentrations up to 0.30 mg L−1 for AB‐DTPA and 0.80 mg L−1 for other extracts. The mixed reagent was stable for 16 wk and the color‐developing reagent was stable 2 and 14 d for the open and sealed‐cold conditions, respectively.

Information on Cl vs. SO4 salinity effects on alfalfa (Medicago sativa L) dry matter yield (DM) and cation‐anion balance is limited. Consequently, we compared Cl and SO4 salinity effects on shoot DM and ionic balance for Archer and Ladak varieties of alfalfa. A modified, flowing Hoagland solution, buffered with CaCO3, was the control: electrical conductivity (EC) = 0.7 dS m‐1. Chloride or SO4 salts of K, Ca and Mg were added to the control to get iso‐EC solutions (2–11 dS m‐1). Shoot cations (Ca, Mg, K, and Na) and their sum (C), anions (Cl, SO4, NO3 and H2PO4) and their sum (A), and DM were measured. Organic anions (C‐A) were calculated in mmolc kg‐1. The calculated hydroponic osmotic potentials (π) were from 1.4 to 1.7 times lower in Cl than in iso‐conductive SO4 solutions. As EC increased, DM decreased equally for both varieties in iso‐conductive Cl and SO4 solutions. Solution π decreased, shoot H2PO4 declined below its critical level of 65 mmolc kg‐1, shoot C stayed constant, shoot A increased; and therefore, shoot C‐A decreased. The shoot C‐A was lower in SO4 solutions. In Cl solutions shoot Cl exceeded the toxic level of 282 mmolc kg‐1. The DM was correlated positively with π, shoot H2PO4, and C‐A, and negatively with shoot Cl and SO4. We conclude that (i) iso‐conductive Cl or SO4 salinity depress DM equally, but isoosmotic SO4 is more depressive; (ii) the yield declines are probably due to any one or a combination of low water potential, toxic shoot Cl and possibly SO4, and deficiencies of shoot P and organic anions; (iii) neutral organic solutes were probably responsible for osmotic adjustment, since total ionic charges (2C) stayed constant; and (iv) P deficiency occurred despite high solution P, due to Cl or SO4 competition in saline environments.

No abstract

Weathering, pH changes, and biotic interactions through geologic times have created a gradation of easily released (labile) to strongly stabilized (resistant) P pools. While methodologies for fertility indices and labile P are well established, methods for quantifying less labile P, such as the occluded and resistant or residual P, are less well established. We reexamined existing methods for these pools, and suggested new procedures to improve their precision and timeliness. Specifically, we compared two methods for reductant‐soluble (occluded) P, and two existing and a new procedure for the resistant P. Occluded P was difficult to reproduce from a sequential extraction procedure because of problems associated with molybdate blue reaction, which required extra molybdate or persulfate oxidation to minimize citrate interference. However, use of inductively coupled plasma (ICP) spectroscopy eliminated this problem. Measurement of P by ICP for surface and occluded P in the total free Fe oxide pool was more quantitative and reproducible [average coefficient of variation (CV) = 5%] than in the sequential extraction (average CV = 7%) procedure. Results for the proposed resistant P method (total soil P minus acid‐extractable P in an ignited sample) approximated the two sequential extraction procedures (total soil P — total acid‐ and base‐extractable organic and inorganic P) and is easier, and more reproducible. Both procedures for all soils except the Molokai showed essentially the same amounts of occluded P. An average of about 26% of the total soil P (TP) was resistant, with the more weathered Cecil soil containing about 50% resistant P.

Information on dry matter accumulation and nutrient uptake pattern for proso millet (Panicum miliaceum L.) is very limited. Therefore, a project was initiated to measure dry matter accumulation and N, P, K, Ca, Mg, S, Fe, Mn, Zn, and Cu uptakes in whole plants, stalks, leaves and heads of proso millet during the growing season. The two cultivars, Cope and Dawn, were field grown on an Aridic Argiustoll that had optimum nutrient and water availability. Plant samples were collected at weekly intervals starting 30 d after planting, and continuing throughout the growing season. Nutrient concentrations, dry matter accumulation, and mineral uptakes were very similar for both varieties. Whole plant concentrations of most nutrients decreased from emergence to maturity, except for Ca, Mg, and Mn which increase with time. From anthesis to maturity, dry matter accumulated rapidly at a rate of 0.5 Mg/ha/d, tripling during this period. Heads at maturity accounted for 55% of the total plant dry matter, which is higher than for wheat (Triticum aestivum L.) or grain sorghum (Sorghum bicolor L.). Proso millet accumulated N and P during heading like grain sorghum, which is more efficient than corn (Zea mays L.). Dry matter accumulation and nutrient uptake patterns are illustrated during the life cycle of this plant.