Master Tips from a Master Gardener (6/15/17)

Bill Koellner · Wednesday, June 14, 2017
I've long cautioned gardeners not to include anything other than very small quantities of wood ash in a compost pile—a little of this highly alkaline material goes a long way. But those of us with naturally acidic soils can (and should) use much larger amounts on our lawns and gardens—as a substitute for lime. What are our soil pH values? Recently, I had 80 acres in south West Liberty tested, and the pH was between 6.4 – 6.7, which is slightly acidic. In a similar manner, we tested soil near our home and it was 7.2 pH. So the first thing you need is a soil test. All soil tests report a minimum of pH as well as other nutrients — primarily the measure of your soil's acidity or alkalinity. These tests are available at Iowa State University, or in some cases from a local farm chemical provider. The center of the pH scale is the number 7, or neutral. Most plants prefer soil to be a little acidic, around 6.5, for good growth. Some of our most popular plants require a more acidic soil such as azaleas, rhododendrons, blueberries, mums, marigolds, oak, pecan and sweet potato, (4 to 5.5 range), to name a few. Keep your alkaline ashes far away from these.

Burning fouls the air, adds greenhouse gases we don't need, and wastes lots of nutrients. Consider piling up your "garden/yard" compost, especially with lots of leaves in the mix.  Composting enables gardeners to dispose of large quantities of leaves and clippings in an efficient and cost-effective way. Iowa State University Extension horticulturists state that the nutrient content of composted leaves is very small. The levels of nutrients vary somewhat from species to species. However, the nitrogen content of composted leaves on a dry basis usually varies from 0.5 to 1.0 percent. Potash amounts are in the same range. Phosphate amounts are around 0.1 percent. Increasing the organic matter content of the soil is the main benefit of incorporating composted leaves into the soil. If you burn wood, only burn good quality hardwood ashes and don’t use ashes from BBQ grills, cardboard, plywood, painted, or pressure treated wood. Using softwoods have a lot less value, but you shouldn't be burning soft wood in a stove or fireplace anyway. The best information is found in Alberta's department of Agriculture, Food and Rural Development.

Typically, ash from good quality hardwoods contains potassium; at least 3% by weight. Also known as potash, potash improves root health and strengthens the very cellular structure of plants, helping them resist all kinds of stresses. In addition, wood ash also contains lots of micronutrients, and, on average, 15% calcium. Ashes can improve the structure and tilth of soils. But they must be used with care, as all that calcium makes them very alkaline, ranging between 9 to 13, and they can change your soil's pH.

I suggest being cautious and applying the same amount of ash as lime was called for but avoiding possible alkalinity problems. Much easier to add a little more rather than exceed, you can add more later. The best time to add is in the fall, which, of course, is also when you have the least ashes. Ideally, save up this year's ashes for fall use. Otherwise, try and spread them over winter. Composting fireplace ashes allows the lye and salt to be leached away. Not all wood ash fertilizers are the same. If the fireplace ashes in your compost are made primarily from hardwoods, like oak and maple, the nutrients and minerals in your wood ash will be much higher. If the fireplace ashes in your compost are made mostly by burning softwoods like pine or firs, there will be fewer nutrients and minerals in the ash.

The salt in the wood ash will kill bothersome pests like snails, slugs and some kinds of soft bodied invertebrates. To use wood ash for pest control, simply sprinkle it around the base of plants being attacked by soft bodied pests. If the ash gets wet, you’ll need to refresh the wood ashes as the water will leach away the salt that makes wood ashes an effective pest control. Another use for ashes in the garden is to change the pH of the soil. Wood ashes will raise the pH and lower the acid in soil. Because of this, you should also be careful not to use wood ashes as fertilizer on acid loving plants like azaleas, gardenias and blueberries.

Wood ash has a very fine particle size, so it reacts rapidly and completely in the soil. Although small amounts of nutrients are applied with wood ash, the main effect is that it is a liming agent. The average ash is equivalent to a 0-1-3 (N-P-K). The chemical makeup varies with the type of wood burned. Hardwoods produce three times as much ash per cord as do softwoods.

Calcium and potassium are both essential to plant growth. Calcium is needed for root development, strong cell walls and protein formation in the plant. Potassium is an important catalyst in photosynthesis and is essential for the movement of sugars, seed formation, protein synthesis and the use of nitrogen in plants.

Wood ash should never be applied to areas where potatoes will be planted as ash can promote potato scab. For most garden soil, 20 pounds (about a 5-gallon pail) per 1,000 square feet can be applied safely each year. That equals about 6 pounds of ground limestone applied to the same area.

The best time to apply wood ash is in the spring when the soil is dry and before tilling. In compost piles wood ash can is used to maintain a neutral condition, the best environment for microorganisms to break down organic materials. Sprinkle ash on each layer of compost. This is especially good if you have oak leaves or pine needles in your compost heap.

Summary when handling wood ash:

• Protect yourself as you would if you were handling household bleach or any other strong alkaline material. Wear eye protection, gloves and a dust mask.

• Do not use ash from burning trash, cardboard; coal or pressure-treated, painted or stained wood. These materials contain potentially harmful chemicals. The glue in cardboard contains boron, an element that can inhibit plant growth if applied in excess.

• Do not scatter ashes during windy periods.

• Do not mix ash with nitrogen fertilizer as ammonium sulfate or ammonium nitrates or urea. These fertilizers lose their nitrogen as ammonia gas when mixed with high pH materials such as wood ash. For a lawn, wait at least a month after wood ash is applied before putting down a nitrogen fertilizer to allow for the soil to reduce the alkalinity of the wood ash.

• Never leave wood ash in lumps or piles. Concentrated piles of wood ash causes excessive salt build-up in the soil through leaching and can create a harmful environment for plants.

• Don’t use it around acid-loving plants such as blueberries and azaleas, or on potatoes, which get scab disease if the pH is too high.
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