Take John Doe, for example (name changed for privacy considerations). John loves to garden and he's justifiably proud of his accomplishments. He's taken a couple of spots in the woods and in 30 years (that's right, three oh) turned the soil from clay to a loamy humus by adding leaf mold, compost, and yes - wood ashes. In fact, he added a LOT of wood ashes. As the years went by he dumped all of the wood ashes his wood stove produced over winter onto his garden spots. Didn't dig them into the soil or spread them thinly over the surface. Just dumped them. In spring, satisfied he'd done the right thing he tilled the ash into the ground, and planted his garden. In the fall after harvest he tilled again, and the cycle repeated. All winter long he threw wood ashes on his garden plots. As the years went on the production of his acid loving plants - potatoes, sweet corn, radishes, declined and eventually they stopped bearing. Radishes bolted and never made globes, sweet corn grew and grew but only produced tiny 3 to 4 inch ears that never set one kernel, potatoes shrank in size until all he got were wee ones not worth the planting.
Someone, no doubt with the best of intentions, told him wood ashes provide essential nutrients to the soil. He seized onto that information with gusto - and made the erroneous assumption that if a little is good, a lot must be better.
John's garden suffered from a surfeit of riches. He didn't test but amended routinely, and when he finally did test was told when the results were explained to him that his numbers were right where they were supposed to be with a pH somewhere north of 7.6. No one thought to inquire into any problems he might be having so he accepted the explanation without question and the next year his crops were just as dismal as they had been the previous year. (Note: his testing was done at a commercial laboratory, NOT through the UW testing lab.)
So what happened? The simple answer is - too much of a good thing. Over application of wood ash had made John's soil alkaline, making nutrients like phosphorus, iron, zinc and postassium chemically tied to the soil and as such unavailable to his crops, as well as adding salt to the soil making it an uninviting places for his plants. The resulting problems were inevitable.
Excerpt from Fertilizing Your Garden from Oregon State University Extension
Wood ash is a good source of potassium. You can use ash as a fertilizer and liming material on vegetable gardens, flower beds, lawns, and most shrubs. It’s particularly useful on acid soils low in potassium. The fertilizer value of wood ash depends on the type of wood burned. Generally, hardwoods weigh more per cord than softwoods and yield more ash per pound of wood. Their ash also contains higher percentages of nutrients. Ash from a cord of oak meets the potassium needs of a garden 60 feet by 70 feet (4,200 square feet), and ash from a cord of Douglas-fir, a garden 30 feet by 30 feet (900 square feet). Both contain enough calcium and magnesium to reduce soil acidity slightly.
Elements in ash are water-soluble. Therefore, always store ash in a dry place until you’re ready to use it.
Apply ashes evenly, and if possible mix them into the soil. Never leave ashes on the surface in lumps or piles. If ashes are concentrated in one place, excessive salt leaches into the soil, creating a harmful environment for plants. Spreading wood ashes in a thin layer over your lawn is a safer application than to use them in your garden, where they are more likely to be concentrated in the soil. Be aware of several precautions if you use wood ashes:
- Don’t apply wood ashes to acid-loving plants such as blueberries, rhododendrons, and azaleas.
- You may have problems with potato scab, a fungus disease, if you use ashes where you grow potatoes.
- Don’t add fertilizer containing nitrogen in the form of ammonium immediately after adding wood ash; the presence of ash can result in the loss of ammonia.
- Don’t add fresh ashes to newly germinated seeds.
- Some types of ash are not safe to use on plants, including coal ashes, ashes from lead-painted or chemically treated wood, and ashes from fireplaces or incinerators where trash has been burned.
- If the potassium soil test value is above 600 ppm, do not apply wood ashes for 5 years.
So what is the correct pH for my acid loving plants?
Very Acidic Soil - Few vegetables thrive in very acidic soils, with a pH between 4.5 to 5.5. Radishes, sweet potatoes and potatoes are good choices, though. If you have room, consider planting blueberry bushes, as well. These bushes thrive in acidic soil and make attractive landscaping plants, in addition to providing flavorful fruit.
Moderately Acidic Soil -Most vegetables will grow in moderately acidic soil, although yields may be decreased for crops that require a lot of calcium and magnesium, such as tomatoes and peppers. Try endive, parsley, tomato, corn, carrot, rhubarb, celery, squash and cucumber if the pH of your soil lies between 5.5 and 7.0.
Mildly Acidic Soil - Most vegetables prefer a slightly acidic soil with a pH between 6.0 and 7.0. Lettuce, asparagus and brassicas, such as broccoli and cauliflower, prefer a slightly alkaline soil pH. Unless your soil is very acidic, though, these plants will probably perform well in spite of a low soil pH
Things to consider:
Send a soil sample to the local county extension office for a soil test analysis. The analysis provides detailed information about your soil, as well as recommendations for amending it. Acidic soil can be modified by adding lime; however, unless your soil is very acidic, this might not be necessary. Often, improving soil texture and drainage through the addition of compost and manure is enough to counteract any negative effects from a low pH.
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