Effect of Nitrogen Fertilizer Rate and Harvest Season on Forage Yield, Quality, and Macronutrient Concentrations in Midland Bermuda Grass
Bermuda grass [Cynodon dactylon (L.) Pers.] is a major forage for grazing and hay production in the southern United States. The objectives of this study were to determine effects of nitrogen (N) fertilization rate (0, 112, 224, 336, and 448 kg ha−1), split spring and summer applications of N at the 224 and 448 kg ha−1 rates, and harvest periods (spring and summer) on forage yield, crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), total digestible nutrients (TDN), and concentrations of phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca in Midland Bermuda grass. Data were collected from 2002 to 2008 as part of an ongoing, long-term soil fertility experiment in southern Oklahoma. Repeated measures analysis of these long-term data showed that forage yield responses to N rate varied with year and harvest time with up to 2.5-fold yield differences among years. Nitrogen fertilization increased CP, TDN, and macronutrient P and Mg and decreased ADF and NDF. Crude protein was increased by ≥50%, and ADF and NDF dropped by up to 25% with the greatest N rate. In general, split N applications did not affect forage yield but produced low-quality forage compared to single N application in spring. Split application of 448 kg N ha−1 gave forage with CP, TDN, ADF, and NDF similar to the Bermuda grass receiving 336 or 448 kg N ha−1 as a single application. Spring forage had better forage quality than summer harvests. While N fertilization increased forage Mg and P concentrations by more than 50% during both spring and summer, it had no effect or slight increased K and Ca concentrations. In the southern Great Plains, despite the weather-dependent variability in forage yield of Bermuda grass, N application increase forage quality.
Potassium availability in relation to soil moisture
SummaryIn order to study the influence of soil moisture on the availability of potassium a simulation model was used. The model is designed to describe the transport of a nutrient from the soil to plant roots and its distribution around a root. From a pot experiment, the measured K uptake of onion plants, grown in soil under different moisture levels, agreed satisfactorily with the calculated K uptake. The model is therefore regarded as a valid means of quantifying the dynamics of K in the soil around plant roots.Calculations from a loess soil have shown that decreasing water content resulted in- a strong decrease of K transport from the soil to the root,- a faster decrease of the K concentration at the root surface and therefore- increasingly steep gradients of the K concentration around the root
With the root density found in this experiment the K concentration of the moist soil (θ∼ 0.4) decreased almost equally in the total soil volume whereas in the dry soil (θ ∼ 0.1) not much change occurred in the middle between two roots.Therefore, the rate of K uptake per unit of root decreased much faster in the dry than in the moist soil. Calculations for sandy and loess soils, which have different water tension curves, have shown that the availability of K in the sandy soil is much more sensitive to changes in water tension than in the loess soil.The simulation technique can thus be used to analyzed the influence of single factors on the availability of K and to estimate the extent of this influence.ZusammenfassungUm den Einfluss des Wassergehaltes des Bodens auf die Verfügbarkeit von Kalium zu untersuchen, wurde ein Rechenmodell angewendet, das den Transport eines Nährstoffs vom Boden zur Wurzel und dessen Verteilung in der Umgebung der Wurzel beschreiben soll. An einem Gefässversuch mit Zwiebelpflanzen bei unterschiedlicher Bodenfeuchte ergab die Rechnung eine befriedigende Übereinstimmung mit der gemessenen K-Aufnahme der Pflanzen. Daraus wird geschlossen dass das Modell realistisch genug ist, um auch die K-Dynamik im wurzelnahen Boden zu quantifizierenSolche Rechnungen haben an einem Lössboden gezeigt, dass abnehmendem Wassergehalt- der K-Transport aus dem Boden zur Wurzel stark abnimmt,- dei K-Konzentration an der Wurzeloberfläche rascher sinkt, und daher- zunehmend steilere K-Konzentrations-Gradienten in Wurzelnähe enstehen.
Bei der gegebenen Wurzeldichte sinkt die K-Konzentration des feuchten Bodens im gesamten Volumen nahezu gleichmässig ab, während sie im trockenen Boden in der Mitte zwischen zwei Wurzeln nur wenig abnimmt. Die K-Aufnahmerate pro Einheit Wurzel sinkt daher in trockenem Boden viel rascher als in feuchtem Boden ab. Rechnungen an Sand- und Lössböden, die sich durch ihre Wasserspannungskurve deutlich unterscheiden, zeigen sinngemäss, dass ein Sandboden in seiner K-Verfügbarkeit auf Änderungen der Wasserspannung viel empfindlicher als ein Lössboden reagiert.Die Modellrechnung ermöglicht es demnach, die Wirkung einzelner Faktoren der K-Verfügbarkeit zu erkennen und in ihrem Ausmass abzuschätzen.
THE EFFECT OF SOIL MOISTURE AND RUBIDIUM CONCENTRATION ON DIFFUSION AND UPTAKE OF RUBIDIUM-86
An autoradiographic technique was used to study the effect of soil moisture and Rb concentration on Rb⁸⁶ diffusion and uptake by corn plants. Changes in soil moisture influenced the rate of diffusion, rate of root elongation, and the incidence of root hairs. The self-diffusion of Rb⁸⁶ was linearly correlated (R² = 0.89) positively with soil mositure, Rb concentration, and interaction of these two factors. The correlation of Rb⁸⁶ uptake from the soil was positively correlated with Rb⁸⁶ diffusion values approximated from measurements made of the depleted areas about the roots. When the root diameter used in the calculation included the length of the root hairs, the r² was 0.70 and when the root hairs were not included the r² was 0.56. The correlation of uptake with the values of Dp/b from self-diffusion studies gave an r² of 0.99. Uptake by roots without root hairs was linearly correlated (r² = 0.98) with soil moisture, Rb concentration, and their interaction. Since the uptake relationships were similar whether or not the roots possessed root hairs, it was concluded that moisture's effect on rate of diffusion was of greater significance in its effect on Rb⁸⁶ uptake than was its effect on root hair incidence in these experiments.