STUDIES OF PHOSPHORUS MOVEMENT IN VERY SANDY SOILS

The movement of soil phosphorus is very complex; this is a result of the ability of each phosphate ion to form a multitude of different compounds with different solubility product constants that are dependent on the environmental conditions that exist. To gain a better understanding of the chemical behavior of soil phosphorus in coarse-textured soils of North-Central Nebraska, a series of 4 laboratory experiments with disturbed soil columns were conducted. The objectives of these experiments were: (1) To determine the effect of vertical-downward flow on the pH of the leaching solution; (2) to determine the effects of ratios of the soil particle fractions from a Valentine sand on the amount of P, Ca, Mg, K, and Na in the leachate; (3) to determine the effects of the leaching rate on the amount of P leached; and (4) to examine the concentrations of several ions at various soil depths in soil columns during leaching. The bulk soil used in all experiments was a Valentine sand (Typic Ustipsamment) from Holt County, Nebraska. Two replications of the experiments were conducted using disturbed soil columns under vertical-downward flow conditions for distilled water and a 25 ppm P solution. The leachates were collected and analyzed for phosphorus, calcium, magnesium, potassium, sodium, and for pH. When water was leached through a soil column, the pH of the solution decreased due to the dissolution of CO(,2) in the leaching solution. After the leaching solution leaves the soil column the loss of CO(,2) from solutions caused the pH of the leachate to increase. Similar changes in pH also occurred for the 25 ppm P leachates. The rate of pH increase of the leachates was dependent on the rate of diffusion of CO(,2) from the leachate to the atmosphere. The total P leached by distilled water did not increase as the percent of fine sand increased. This resulted from greater P precipitation as the percent of fine sand increased probably due to higher concentrations of Al and Fe. This occurred even though the extractable P increased with the increase of the percent of fine sand. When the 25 ppm P solution was applied to the soil columns, more P was leached from the columns as the percent of fine sand decreased because of less precipitation of Al and Fe phosphates. Greater amounts of Ca, Mg, K, and Na were leached from columns as the percent of fine sand increased. These cations tended to follow the wetting front, with 20 percent or more of the total amount occurring in the first leachate collected. The leaching rate of distilled water and a 25 ppm P solution had no effect on the total amount of P leached. Since there were no differences, the time of contact of the water or solution with the soil was probably not a factor at these leaching rates. The movement of P in a soil column was probably controlled by a series of precipitation-dissolution reactions. The P precipitate that formed was probably Al and Fe phosphates due to their low solubility in an acid environment. Greater amounts of Mg and K were leached by the P solution because of the cation exchange and dissolution that occurred with Ca. The amount of Na leached by distilled water and the P solution was the same