Ash as fertilizer

Wood ash, a waste or a treasure trove

Wood ash, Charcoal ash or hard wood ash or Cow dung ash or just ash. Is it just a byproduct in our barbecue Owens or fire places and a nuisance to deal with or is it a treasure trove waiting to be opened? 

To find an answer to this question we need to  look into our traditional value system where everything had a second life. Our ancestors and still in many parts of country we use ash as a replacement of soap, be it to wash hands after doing ones business or washing utensils ash was the solution. Was it just because of scarcity of  resources that forced us to make use of something waste or was it based on strong scientific background?

Ash and soap are both alkaline and most bacteria cannot survive in this alkaline environment i.e on the surface of soap or ash. Ash is a very good source of lye (Sodium Hydroxide or historically Potassium Hydroxide). Hence Ash is a very good soap alternative and an disinfectant (please refer the list of references for use of “Ash as disinfectant and soap“).

The above is an example, that Ash is not just a waste to be disposed. But is it of any use in our modern gardens with host of expensive fertilizing components and pesticides with fancy names on them just a click away.

Let us try to examine this topic using various purviews

Defining Ash

Wood ash is the inorganic and organic residue remaining after combustion of wood or unbleached wood fiber. The physical and chemical properties of wood ash vary significantly depending on many factors. Hardwoods usually produce more ash than softwoods, and the bark and leaves generally produce more ash than the inner woody parts of the tree. On average, the burning of wood results in 6 to 10 percent ashes. When ash is produced in industrial combustion systems, the temperature of combustion, cleanliness of the fuel wood, the collection location, and the process can also have profound effects on the nature of the ash material. Therefore, wood ash composition can vary depending on geographical location and industrial processes. This makes testing the ash extremely important. 

Since wood ash is plant material, it contains essential nutrients for the plant growth, except Nitrogen and Sulphur as these are lost as gases during combustion process. If the ash were sourced where the combustion was complete even carbonates may not be present. This makes ash a concentrate of minerals and their oxides that can be readily absorbed by plants.


Typical Chemical Analysis of Ash

Ash is composed of many major and minor elements that trees need for growth (Table 1). Since most of these elements are extracted from the soil and atmosphere during the tree’s growth, they are common in our environment and are also essential in production of crops and forages.

Calcium is the most abundant element in wood ash and gives ash properties similar to agricultural lime. Ash is also a good source of potassium, phosphorus, and magnesium. In terms of commercial fertilizer, average wood ash would be about 0-1-3 (N-P-K). In addition to these macro-nutrients, wood ash is a good source of many micronutrients needed in trace amounts for adequate plant growth. Wood ash contains few elements that pose environmental problems. Heavy metal concentrations are typically low.

Presence of heavy metals like Cadmium, Mercury, Lead are low and are dependent on the source of wood. To negate the effects of Aluminium toxicity that may arise due to presence of Aluminium, it is advisable to use Agriculture grade gypsum with Ash. One can do it by mixing both the components in 50:50 mix.

Use of Ash as fertilizer

Taking forward from above discussion where we have seen that Ash (Wood ash or for that matter even the ash from Cow dung cakes) contain most of the vital macro and micro elements required for proper plant growth. 

In many areas Lime, Dolomite have been used as Liming agents since they provide vital micro elements to soil. But in many studies it has been found that plants have responded better to application of Ash as compared to that of Lime or Dolomite.

Ash or any other liming agent should only be applied to soils where the pH is about 7 or less. This application should be done in conformance the the local conditions of soil. If your soil is Sodic (damaged from saline water application) then it is better to avoid Ash application. In such a case you should first treat this soil with Gypsum or Cottonseed meal and make it more acidic or atleast neutral. 

Ash can be readily be mixed with farmyard compost, vermicompost, or any other fertilizer to boost the availability of micro nutrients in fertilizer.  Ash can be mixed while you are making compost, vermicompost or at the end of composting process. In my opinion if it is mixed with material to be composted in the initial stages it mixes better and nutrient availability is increased. Earthworms love Ash as it is alkaline and ash also solves the problem of bad smell that may arise from composting bin.

And yes it imparts a pleasant aroma to the final compost.

In a nutshell you do not need to spend huge money to buy expensive and fancy micro nutrients if you just have a source of humble ash.  

Plants that benefit from application of ash

Plants With Potassium Deficiencies

Plants require potassium for healthy flowering and fruiting. Use wood ashes as a soil amendment for plants suffering from potassium deficiencies. Potassium deficiencies appear in plants as a browning or discoloration of leaf edges. It is important to test the soil and rule out any other possible diseases, but if potassium is lacking, wood ashes add it to the soil. In addition to potassium, wood ashes contain other essential nutrients like calcium, magnesium and phosphorus.

Rose, Jasmine, Lemon, Hibiscus, Pomegranate, Guava etc. all benefit from ash application.

Plants in Overly Acidic Soils

As wood ash has a liming effect on the soil, you can use it on the lawn, on ornamental flowers, plants and shrubs as well as in the garden to reduce soil acidity. Some plants thrive in neutral to alkaline soils, as opposed to overly acidic soils. For example, garden plants like artichokes, tomatillos, greens like collards and arugula, and brassicas like broccoli require alkaline soil for optimal health.

Because wood ash raises the pH of your soil, always test the soil to ensure that it does not become overly alkaline.

Apply ash judiciously with plants that love acidic soil. You may still apply ash in amounts that do not significantly increase the pH of soil.

Plants Overrun by Pests and Disease

Use wood ash on alkaline-loving plants to keep various pests at bay. Cooled, untreated wood ashes directly from a fire and applied as mulch, or wood ashes mixed into compost, are useful around cabbage and onion plants to keep away root maggots. Wood ash mulch or compost also keeps slugs and snails from overrunning alkaline-loving flowers and ornamental plants.

In some cases, raising the pH of the soil with wood ashes is helpful in disease elimination. Brassicas like broccoli and cauliflower are commonly affected by club root disease. Raising the pH of the soil with wood ashes helps to eliminate club root.

Considerations and Application

Always use caution when choosing to implement wood ash in the garden landscape. Do not use wood ash if the soil pH is 7.0 or higher, as the addition of wood ash further increases the soil pH. While some plants thrive in alkalinity, overly alkaline soils cause damage to other plants. Avoid using wood ash if there are already high amounts of potassium in the soil.

Do not use wood ash on plant leaves or directly against the base of plants, or around new plant roots and seedlings during germination, as it has the potential to burn plants. To treat diseases, increase potassium or increase soil alkalinity, use wood ash at the rate of 1/2 lb per shrub, 10 to 15 lbs per 1,000 square feet of lawn, or 1/2 lb per square yard of garden space.

Use of Ash in fertilizer blends

Ash can be a very contributing ingredient in making fertilizer blends. As already discussed ash is a treasure trove of micro nutrients and is fast releasing fertilizer. If mixed in correct proportions and with suitable base ingredients ash can do wonders. 

Also Ash can be used to replace banana peels in fertilizer blends. This is possible as both Banana peels and Ash have almost identical nutrient profile except for organic carbon content which is missing in Ash.

How to make liquid fertilizer with ash?

Since many minerals in ash are soluble in water, Ash makes a very good candidate to make your very own liquid fertilizer with no Nitrogen. 

To do so follow following simple steps;

  1. Remove any non powdery remains or impurities like nails etc. that may be present in Ash. You can choose a fine mesh to achieve this.
  2. Fill Ash in a cheese cloth/empty tea bag or make a small potli with any cloth. In first run try to use approximate 1 cup of ash.
  3. Now fill a bucket with approximate 5 liters of water and soak this potli in it for 3-4 days. Cover the bucket and place it in a warm/sunny area of the garden.
  4. After the duration remove the potli. The residue in potli will be mostly Calcium and most of the Potassium, Phosphorus and other minerals will be already dissolved. You can now mix this residue in potli with garden soil where no plantation is done or save it to mix with composting mix.
  5. The resultant water can be further diluted as per requirement or be stored for future use. It is advisable to use Distilled water from AC drain to make this liquid fertilizer as water from AC drain is an aggressive solvent and has capacity to dissolve most of the minerals in ash.
  6. If you do not have AC you may use normal tap water, you may want to preheat tap water before soaking Potli in it.
  7. To avoid making Potli fine filtered ash may be soaked in water directly and the container can be sealed and stored. 
  8. This solution can be used as hydroponic fertilizer as well.

Extract Potassium Hydroxide (KOH)/Potash from Ash

To make the potash, its solubility is useful as it can dissolve in water, whereas the insoluble other compounds will not. So by soaking the ash in water overnight the potash will all dissolve into the water. Then the solid waste can be filtered out. However, you may choose not to use filter paper,  instead leave the ash mix and wait for the solids to sink to the bottom. You can then decant off the water and potash solution.

Once we have a rough solution of potash and water,  then just separate the potash from the water. The easiest way to do this is to evaporate the water and leave the potash as a crystal. To do this  place the solution in a pan and placed it on the stove. To get the best crystals you should not boil off all the water and instead should stop heating once a few small crystals started forming and let the solution slowly evaporate and let large crystals form. However, it already has many impurities in it so may not make perfect crystals.

Once the pan is cooled  scrape the crystals out. You can use the crystals in this form, but it is relatively impure potash and so might not work perfectly. To gain purer crystals you can redissolve them in water and filter again to get rid of the last insoluble compounds, and then recrystallize the potash properly.

Further Reading