Phosphorus Availability and Response of Tomato to Phosphorous Fertilizer in Calcareous Soils

This article by Yuncong Li appeared in the
"Vegetarian Newsletter," August 1999.

Calcareous soils that contain a large amount of calcium carbonate (usually from 1 to 100 percent CaCO3 equivalent) are common in Florida (ed. note: and also in Texas). Calcium carbonate can occur in the surface soils naturally or as a result of land preparation (plowing, bedding, etc.). Soils also can be calcareous through over-liming or long-term irrigation with calcium-carbonate enriched ground water. Calcareous soils induce an array of nutritional problems for crops, and Phosphorus (P) is one of them.

Application of P fertilizer is important for vegetable production on calcareous soils. However, most growers apply too much P fertilizer for their crops. Over-fertilization leads to unnecessarily high production costs, may decrease yield and quality, and poses a risk to the environment.

A two-year field experiment was conducted in a commercial vegetable field on a typical Krome very-gravelly loam soil in Miami-Dade County during 1997-1998.

Dry fertilizer was applied in 2 bands along the top of the bed at three rates of P (37,63,100 percent of the grower rate, equivalent to 96, 163, 260 lb P2O5/ac) in the 1997 growing season, and at 4 rates of P (0, 70, 140, and 280 lb P2O5/ac) in the 1998 growing season as triple superphosphate with 6 replications. All of the treatments received the same amounts of N and K as dry and liquid fertilizers. 'Sunbeam' tomato plants were transplanted in a single row in the center of each bed, with 20 inches between plants. Tomatoes were harvested three times at mature-green stage. Total number, total weight, and color of fruit from each plot were recorded. Soil and leaf samples were also analyzed for P.

The results showed that phosphorus fertilization increased AB-DTPA extractable P in the soil, but did not affect the concentration of leaf P, yield, and quality of tomato, with the exception that the quantity of red fruit at the time of the first harvest (1997) was increased slightly. It was also found that there was no response to bean, Malanga, potato, and sweet corn to P fertilizer during three-year experiments on calcareous soils in Miami-Dade County.

Phosphorus removal via the harvested fruit usually accounts for less then 38 lb P2O5 for 1,000 cartons of tomato. A large portion of the applied P remains in the soil. It is important to know the availability of remaining P in calcareous soils. In 1998, surface soil samples (0- to 6 -inch depths) were collected from 6 typical vegetable fields in south Miami-Dade County. Soil samples were extracted sequentially with water, AB-DTPA, and the mixture of nitric acid and hydrochloric acid, to determine water-soluble, plant-available, and residual P in soils. Average concentrations of water-soluble P in soil samples collected from various vegetable fields were 1.2 ppm, and ranged from 0.87 to 1.69 ppm. Water-soluble P is available to the crop; however, this type of P is also subject to leaching out of the root zone through excessive irrigation or heavy rainfall. Concentrations of AB-DTPA extractable P in these soils range from 46.4 to 94.8 ppm, with a mean concentration of 70.9 ppm. AB-DTPA extractable P is plant-available, and highly correlated to the uptake rate by the crop-grown calcareous soils. Acid-extractable P in soils represent the P residue in soils that is not directly available to plants. About 95 percent of total P in 6 soil samples were in residual form. Concentrations of residual P ranged from 1,123.8 to 1,877 ppm, with a mean concentration 1,404.4 ppm.

Phosphorus fertilizers applied in calcareous soils are fixed through adsorption and precipitation. In 1999, a P sorption and desorption experiment was conducted with 24 soil samples collected from natural lands, vegetable fields, and tropical fruit groves. Adsorption was the dominant reaction at low P concentrations, and P precipitated with calcium carbonate at high P concentrations.

Results from this study indicate that soils from vegetable fields which were saturated with P, and excessive P applied as fertilizer, often precipitate and become less available to crops. The desorption rate from vegetable soils is higher than the desorption rate from natural soils, because of high initial soil P in those soils.

In summary, large amounts of P are accumulated in most cultivated calcareous soils from fertilizer application. No P fertilizer application is necessary for calcareous soils with high available P levels. Growers should conduct a pre-fertilizer soil analysis to determine supplemental P fertilizer rates.