Introduction
My quest to write this article was born out of desire to help farmers and gardeners mitigate challenges experience stemming from the use of sawdust. The use of sawdust on farm is not bad in itself but the manner it is put to use. Sawdust is an inexpensive material that could be put to great use on the farms for many purposes but it has its downside. Sometimes back, I planted local leafy vegetable called “Ugu” (African pumpkin) and I dug hole around each transplanted seedling stand and mulched with sawdust to conserve moisture. After a week, I noticed the seedlings weren’t growing further and many were having their leaves turned yellowish. This is obviously a sign of Nitrogen deficiency! “I have applied NPK 15:15:15, so what could be the problem I said?” The answer came many weeks later after I began to research what the problem was. So, it was actually the sawdust used with intention of conserving moisture since this was done during the peak of dry season.
For the readers’ benefits, reading through this article would now give you a better way of putting sawdust to use if need be without producing adverse effect. Let us take a look at the pros and cons of using sawdust. You may want to read: Why you should use single growing medium for veggies.[i][/i]

Demerit of using sawdust

High in carbon: If you plough sawdust into your soil especially using high sawdust-soil ratio, plant would find difficulty in growing there for a year or more. Pure wood materials like sawdust and wood shavings are very-high in carbon and this carbon will absorb all of the plant-feeding Nitrogen in your soil in the process of decomposing. Though, after it does decompose, the soil would be far rich in soil humus and organic material but before then, using it is tantamount to digging your own plant graveyard. This is exactly what happened to my ‘Ugu’ talked about above; seedlings were growing fine but immediately after mulching the transplants with sawdust, growth stopped and yellowing of leaves commenced followed by seedling death. For gardeners, it’s even unsafe to compost it. As I often pointed out, the best compost is made by combining carbon-rich "dry brown" material, like shredded Fall leaves, with "wet green" sources of nitrogen, like grass clippings and kitchen waste. Sawdust is a 'dry brown' material, but it's a much more highly concentrated form of carbon than leaves therefore its use should be discouraged. If there is any reason to be used as mulch, it must be combined with high Nitrogen source materials like blood meal, poultry waste etc. but to what extent is the stress if you can find alternatives.

Sources of sawdust: One major problem with the use of sawdust is; not knowing the source. You walk up to a sawmill market, pack the sawdust into a bag and up you go to your farm to put it to use. Waoh! Do you bother asking from which wood was the sawdust processed? Even if you asked, would you have gotten any answer anyway? Who cares since you are getting the sawdust for free! The rule of thumb is; if you must use sawdust, don’t use pressure treated wood, old railroad ties or other toxic wood in any form. Example of toxic wood is walnut wood. Black walnut—the type most often used in woodworking contains juglone, a naturally occurring compound that stunts the growth of (or just plain kills) many other plants, especially tomatoes and other popular backyard crops. Although the concentrations are highest in the roots, there is some juglone in every part of the tree, and compost made from black walnut sawdust might send some of your most prized plants to their grave early. I probably had an hunch that the one I used for my ‘Ugu’ was from wicked black walnut wood.
You may want to read: Understanding the world of ISFM/IPM to turn around the fortune of your garden

Merit of using sawdust
That I have underlined all the negative sides of sawdust does not mean it has no good side. The use to which it is put determines the side its falls; whether with good or bad outcomes.
Use in broadcasting tiny seeds: If you are into planting of local leafy veggies like ‘ewedu’, amaranthus or you wish to plant onions, carrot, lettuce directly on the field (not advisable though), you may want to use sawdust as a filler mixed thoroughly with the seeds then broadcast on the field. Using this would ensure that the seeds aren’t closely packed on the soil therefore ensuring enough spacing between plant stand after germination. Picture: mixing ewedu with sawdust

Amend soil: As an option, you may want to add small amounts of sawdust to your soil to increase organic matter and improve its texture. This is because sawdust is very slow to decompose; it works especially well in moist, heavy soils like clay, where soil amendments tend to break down quickly. After final decomposition, a soil containing decomposed sawdust is very rich. You may as well pack thoroughly decomposed sawdust for this purpose.
Cover farm walk-way, mulching & Prevent erosion: If you have a small to medium farm size or a garden and controlling weeds seem to be your foremost headaches, then sawdust, the villain is at your rescue especially if you can get the one from walnut tree. Sweep it between the cracks and crevices of concrete sidewalks and in between stepping stones to help prevent weeds from popping through or even spread on the walk way. In this form, it also would help to prevent erosion and watching away of top soil. Eliminate existing grass and weeds to expose the soil. Apply a thick layer of sawdust and tamp it firmly into place. Walked-on sawdust compacts quickly. You would need to refresh your pathway every few years.
Growing medium: Who says sawdust can’t be used as growing medium? If you have ever considered growing your own mushrooms, sawdust can make a good growing medium. Unlike green garden plants, mushrooms lack chlorophyll and rely on other organic materials for their food. That it does not have chlorophyll means it won’t need Nitrogen anyway. In the forest by nature, logs work well for this. In the garden, you can use a mixture of sawdust and woodchips. However, you must note that this is not only the key to success in growing mushrooms, successful cultivation requires monitoring temperature and light.
Repel slugs: Are you an organic farmer? Sawdust can help... If you really enjoy this article, visit http://www.hybridveggies.com/2017/04/why-and-why-not-use-of-sawdust-in-your.html and also read about other educating articles.