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Pine needles in the compost

Q: Do pine needles affect the acidity of a compost pile?

Christie Lemon, Hopedale, MA

Decomposing organic materials cause an increase in acidity but over time, the compost returns to neutral. Decomposing organic materials cause an increase in acidity but over time, the compost returns to neutral. Photo/Illustration: Allison Starcher

A: Lee Reich, contributing editor and author of Weedless Gardening, replies: Pine needles do indeed create acidic conditions in compost piles and soils, as do most other organic materials, but there is no need to exclude pine needles—or any other organic materials—from a compost pile. The acidic conditions created by pine needles are only transitory. As organic materials decompose, they typically cause an initial decrease in pH (increase in acidity), but over time, the pH rises so that the acidity of the composted material becomes near neutral.

Even if pine needles did not equilibrate to a near neutral pH, or if you wanted to avoid that initial burst of acidity, you could still use them in your compost pile with good effect. Just sprinkle on some limestone, the same material used to neutralize acidity in soil, as you build the layers in your compost pile. Besides making the compost less acidic, limestone can improve the “feel” of the finished compost, making it less sticky. Don’t be too heavy-handed with limestone on a compost pile because it wastes nutrients and causes odors as nitrogen is converted to gaseous ammonia. Also, you would not want to add limestone to compost destined for use under plants like rhododendrons and azaleas, which enjoy acidic soils (pH 4 to 5).

Although pine needles will not cause problems with acidity in the long term, they do break down relatively slowly. The reason for the slow decay is that the needles are covered with a waxy layer that resists bacteria and fungi, and, like other fallen leaves, they have an excess of carbon relative to nitrogen. The process could be speeded up by shredding the needles, thereby offering bacteria and fungi greater surface area at which to “chew” away. Decay can also be hastened by adding a nitrogen-rich supplement to the pile, like succulent, young plants in the form of weeds, thinnings from the vegetable or flower garden, and kitchen waste, as well as sprinklings of a seed meal, such as soybean meal.

From Fine Gardening 93, pp. 75