By George Giltner, Louisiana Master Gardener
Many discussions among gardeners, landscapers, and online blogs are about mulch. A good mulch beautifies the plant base, retains moisture, prevents weeds, allows gaseous exchange to the roots, prevents erosion, provides soil insulation, and adds nutrients to the soil. We are so fortunate in Beauregard Parish to have one of the best mulches under our feet, pine straw. However, an online computer search or a neighborly conversation always seems to bring up “the acidity issue with pine straw”.
Beyond opinions and bias are scientific facts and factors that can resolve these “acid discussions”. To a certain extent, whatever your position, you are correct. Yes, pine straw releases organic acids to the soil, but it will have little effect on your plants. Now let’s “look in the weeds” for the reasons.
Rain – Every time it rains, from the early beginning of time, the soil gets an acid wash. Gas compounds of carbon (CO2), sulfur (SO2), nitrogen (nitrates), and other substances have reacted with water to form acid rain. Our current pH (acid-base scale of O to 14) of rain is 5.6 here, which is mildly acidic. Rain’s acid water that is loaded with oxygen, is great for plant’s root systems. Oxygen allows for the roots and microbes in the rhizosphere to respire and grow, and to produce more acids, organic acids. The mildly acid nature of the water enables minerals to be available for plant absorption. Therefore rain contributes to a continuous supply of acids, which is overall more in quantity than a typical mulching of pine straw.
Soil – The soil in our area is classified as Ultisol with a composition of weathered clay, kaolinite. This aluminosilicate clay soil is acidic (pH 4.3 on my land), has a poor ability to hold mineral nutrients, and lacks organic matter. In order to grow crops, it has to be limed, fertilized, and benefits significantly with organic matter additions. So, correcting the soil pH to begin with, is a much larger acidity factor than addition of pine straw mulch. Also the pH will drift down with time, as the calcium of the lime is released from the weak mineral-holding kaolinite or goes into solution with rainwater leaching.
Amendments – Acid fertilizers whether synthetic or organic contribute to the total soil acidity. Most box store fertilizers like 10-10-10 are acidic. Acidic organic fertilizers like cotton seed meal are for use around blueberries, azaleas, and other low pH plants. Peat moss adds organic matter and lowers the pH of soil. Therefore these amendments can impact soil pH greatly, depending on application quantities and length of time (years) they have been applied.
Pine straw – Cell vacuoles of pine straw are loaded with tannic acids. These phenolic acids are antibacterial, anti-enzyme, antioxidant, and bind with insect proteins. Their function is to protect the needles from biological attacks. Some tannins and other organic substances that form acids are readily soluble in water, while others become soluble as they break down by slow fungal decay. A large factor in release of these acids from the needles is physical. If the waxy cuticle of the needle is damaged, water readily penetrates the interior needle, hastening release of tannic acids, cellulose and lignin by-products. Example: The mowing of pine needles destroys them in weeks, whereas whole pines needles take many years (as many as 10 years) to break down in the forest. The mower blades shred pine straw opening it up to water and microbes which speed up leaching of organic acids and decay.
Therefore the bottom line is, yes, pine straw is a good mulch. It will add some temporary acidity to your soil, but even these organic acids are eventually broken down by microbes. However the amount of pine straw acid contribution is very small in comparison to rain, amendments, and soil acids over a growing season. Monitor the soil pH with soil testing, correct acid levels with calcium compounds, add organic matter to aid in buffering the soil pH, and have great plants mulched with pine straw.