Studies on the Availability of Soil Phosphate to Plants

Khan, Sana Ullah (1992) Studies on the Availability of Soil Phosphate to Plants. PhD thesis, University of Glasgow.

Abstract

The work of this thesis is concerned with the study of some aspects of phosphorus dynamics in soils varying in physico-chemical characteristics. Chapter 1 presents a general introduction and short review of some aspects of phosphorus behaviour in soils. The n.ain emphasis has been given to phosphate availability to plants. Chapter 2 deals with the analytical techniques used in the present study. A short description of the sites of soil sampling has been given. Some physico-chemical properties of the soils have also been described. Chapter 3 is about the P status of 14 soils. Six conventional procedures were adopted to measure all types of extractable phosphate in the soils studied. The effectiveness of various extractants was assessed through their relationship with one another. All of the extractants showed better relationship in slightly acidic soils than in slightly alkaline soils. Bray-I (acid ammonium fluoride) extracted the highest P in the former soils, but its performance was extraordinary poor in the latter soils. Bray-I, acetic acid and anion exchange resin extractable P was not significantly correlated with other extractants in the soils of above pH 6.2. However, their relationship with other extractants was much better in soils of pH below 6.2. Olsen 0.5M sodium bicarbonate extractable P was significantly correlated with extractable P of sodium acetate (r=0.81***), ammonium acetate (r=0.75**) and anion exchange resin (r=0.73**). Anion exchange resin, which is regarded as being analogous to plant available P showed more or less similar behaviour with other extractants. Acetic acid, sodium acetate and ammonium acetate had very highly significant correlation among themselves in all soils. The phosphate extracted by various extractants depends on the relative amount of calcium, iron and aluminium phosphate in a soil and the nature and composition of an extractant. Phosphate adsorption study of 14 soils has been described in Chapter 4. Adsorption isotherms of all soils were made with standard P concentration points of 0.5, 1, 5, 10, 20, 30, 40, 50, 70 and 100 mug P cm-3. All the soils showed different behaviour towards P adsorption. The P adsorption data conformed well to the Freundlich equation in most of the soils, while only in some soils to the Langmuir equation. Langmuir P adsorption maximum and bonding energy constant for all soils were calculated. P adsorption maximum was significantly related to pH (r=-0.63*) and oxalate extractable Fe (r=0.82***) and A1 (r=0.61*). When a few points at the top for the Freundlich plot or at the bottom for the Langmuir plot were excluded o from calculation, the R2 values were greatly improved. Chapter 5 is concerned with the detailed studies of P adsorption and desorption in 3 soils. Keeping in view the errors assessed in the initial P adsorption and desorption studies, a series of experiments were run and errors were measured. By adopting the improved experimental technique, the P adsorption/desorption isotherms were in better order. However, the top point at the adsorption and desorption isotherms did not meet as they should. The cause of the gap between the two points was investigated through the effect of time of shaking and soil sterilization. It was observed that P adsorption increased with time. There was no significant instantaneous P desorption except in Dunlop A soil. All the soil sterilants affected the P status of soils except sodium azide and toluene. Chapter 6 deals with the pot experiment on ryegrass (Lolium perenne). The effect of plant growth on extractable and adsorbed P and enzymic activities in 5 soils is discussed. The results showed significantly highest dry matter(tops) yield in Darvel A and Midelney (grass) soils. P content of leaves was highest in the Midelney soil. The NaHCO3 extractable P before planting was significantly correlated with the dry matter (tops) yield (r=0.90*) and with P uptake (r=0.89*). The P uptake in leaves was also significantly correlated with the dry matter yield (r=0.91*). The extractable P by 3 extractants in grassed soils was significantly reduced than nongrassed soils and in some soils before planting. The effect of plant growth was observed on P adsorption isotherms. The regression analysis of the Langmuir equations obtained from P adsorbed before planting, non-grassed and grassed soils showed significant differences only in Midelney (Grass) and Dreghorn A soils. The plant growth also revealed variable effects on enzymic activities in before planting, grassed and non-grassed soils. In chapter 7 a brief account of the experimental results and the general conclusions were made. Further development of this work is also suggested.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: T H Flowers
• Keywords: Plant sciences, Soil sciences, Biogeochemistry
• Date of Award: 1992
• Depositing User: Enlighten Team
• Unique ID: glathesis:1992-75838
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 17:55
• Last Modified: 19 Nov 2019 17:55
• URI: https://theses.gla.ac.uk/id/eprint/75838