Costs of fertilizing citrus groves with compost or chemicals

SINCE the early 1990s, several thousand acres of commercial citrus groves in Florida have been supplementing their traditional fertilizer applications with composts. The data obtained from these programs provide a basis for cost comparisons.

Today, a typical chemical nutritional program for Florida citrus costs approximately $188/acre/year. This breaks down as follows: Ammonium nitrate $47.76 Triple superphosphate $7.83; Muriate of potash - $25; Magnesium oxide - $13.39. The total for primary and secondary nutrient materials - $93.98. The cost for blending, transportation and taxes for fertilizer adds up to $17.25. The average acre cost for minor elements is $25. Liming due to acidification of soil by chemicals is calculated at $18. Three spreadings cost $8/acre/trip - or $24. Spray application of minor elements is $10. The total for a chemical nutrition program is $188.23/acre/year.

Annual surveys by university agronomists support the above costs involved in maintaining the nutritional health of commercial citrus groves. One of the most important factors not spoken to in the above fertilizer program is the efficiency of uptake of the nutrients by the citrus trees. University studies in the field have shown that the relative efficiency of applied chemical (water soluble) fertilizer under commercial conditions rarely exceeds 50 percent. The primary loss pathways are leaching by rain, runoff due to rain, irrigation and volatilization, and ultraviolet degradation.

An equivalent citrus nutritional program using compost and a fertilizer approach not only adequately nourishes the plants and grows the crop, but also improves the quality of the microenvironment in the grove. Here is how a typical compost/materials program stacks up:

Compost materials at 2.5% nitrogen, 2.5% phosphorus plus minor elements at $30/ton times three tons/acre/ year = $90; Muriate of potash = $25.

Delivery is included in the cost of compost; no taxes apply to the compost; no blending of materials is needed.

Three applications of materials per acre per year (two tons/acre of compost in the spring, one late summer, and a single application of potash in the spring) = 3 trips at $8/trip = $24.

Liming reduced to every third year due to less acidification = $12.

Total cost of compost/materials program/acre/year = $151!

Although less total nutrients are applied per acre per year in the compost program (150 pounds of nitrogen versus 200 pounds of chemical), it has been shown that the slow-release characteristics of compost increase its efficiency to about 80 percent of the applied amount.

So 200 pounds of chemical fertilizer at 50 percent efficiency equals 100 pounds of delivered nutrients; 150 pounds of compost nitrogen at SQ percent efficiency delivers 120 pounds of nitrogen to the trees and soil microfauna.

The supply of good quality compost in the state of Florida is very limited - perhaps 100,000 tons/year are produced. There are more than 850,000 acres of commercial citrus groves, most of which would dearly love to save $37/acre/year in production costs. It can be seen that only 33,000 acres could benefit from the compost materials program - 3.88 percent of the total. - Mike Litvany

[Author Affiliation]

Mike Litvany is with Nutri-Source, Inc., based in Orlando Florida. Nutri-- Source is a compost broker providing compost marketing services for the municipat facility in Sarasota (about 20 DDD cubic yards (cy)/year), Reedy Creek Improvement District - Disney World (40,000 cy/year) and Comp-Lete Food inc. of Nicotee (60,000 cy/year). (Conversion rate from volume to weight for the compost averages 2.5 cubic yards to one ton.)