The objectives of the investigation were to evaluate no tillage and tillage by plowing with respect to yield of tobacco and corn plants and to compare uptake of P by corn plants from surface and mixed applications of superphosphate. A further aim was to determine the relationship between P uptake by oat plants and inorganic P fractions of three Virginia soils which received varying amounts of superphosphate.

During 1963 corn plants were taller on no tillage- and plowed-P fertilized plots than their respective checks, 50 days after planting. Later in the season, 88 to 104 days after planting; height of corn plants on no tillage- and plowed-P fertilized plots were not significantly different than their corresponding checks. From these data it was concluded that P-fertiliser applied to the surface of the no tillage treatment and mixed with soil of the plowed treatment increased growth of corn plants early in the season.

Studies with _p32 indicated that surface applied P was more available to corn plants than broadcast-mixed P during the early portion of the 1963 and 1964 growing seasons. Higher yield of corn grain in 1963 from the no tillage treatment was attributed to the higher uptake of surface applied P. During the middle and later portions of the two seasons, surface and mixed applied superphosphate were equally available to corn plants. Because soil temperature and moisture content were not significantly different for the plowed and no tillage treatment it was concluded that these two variables did not influence corn yield.

Higher uptake of surface applied P was explained on the basis of less fixation due to less soil-fertilizer contact. This explanation was supported by investigations which showed negligible downward movement of surface applied P and a higher amount of difficulty soluble P compounds in soil of the broadcast-mixed P treatment. It was pointed out that moisture, which does not penetrate deeply into soil, favors plant uptake of surface applied P due to a higher amount of P in solution from surface applied P than broadcast-mixed P.

Burley tobacco plants grown on Wellston loam were taller on no tillage than plowed plots early in the growing season. The difference in height was attributed to the higher available water content of the no tillage treatment. Content of N in tobacco leaves at harvest time was lower for plants grown by no tillage. Smaller plants late in the growing season and lower yield for the no tillage treatment were attributed to N deficiency. Presumably, surface applied N (no tillage treatment) supplied less N to plants than mixed N (plowed treatment). The two methods of fertilizer application resulted in equal contents of P and K in tobacco leaves.

In a greenhouse investigation, P fertilization increased yield of oat plants on Huntington silt loam and Lloyd clay loam which contained 8 and 26 ppm. of available P (Mehlich test), respectively, but not on Lloyd clay loam which contained 96 ppm. For the soils on which oat plants responded to P fertilizer, uptake of applied P was higher from the Huntington than Lloyd soil whereas the soil test indicated higher amounts of available P for the Lloyd soil. It was concluded that the available P content of the Huntington soil (pH 7.8) was underestimated due to neutralization of the extracting solution.

Higher amounts of Al-P and Fe-P than Ca-P were present in the Huntiugton (pH 7.8) than two Lloyd soils (pH 6.9 and 7.0), 28 days after application of S amounts of superphosphate. These data show that considerable amounts of Al-P and Fe-P may form in alkaline and neutral soils. Amounts of Al-P present in the fertilized soils correlated significantly (r = 0.93**) with P uptake by oat plants. From these data it was concluded that Al-P compounds, formed shortly after application of superphosphate, supplied the major portion of P absorbed by oat plants.