If you’re looking to start a raised garden bed in your yard, there are several things to do. One of these important tasks is to decide what you are going to use to fill the planter box with. These containers, after all, are quite large. So, if you were to fill them solely with soil, you’ll need a whole lot. The cost of buying that much soil can add up quickly. This is why many people look to other items that will add some bulk to their beds without costing them a fortune.

However, there are many items that you should avoid adding to your raised bed. Choosing the wrong material can lead to drainage issues, as well as prevent your plants from getting the right nutrients, contaminate edible plants, and more. Ahead, we’ve put together a list of some of the items you’ll want to stay away from when deciding what to use to fill your raised beds. Take some time to read through this list so that you don’t inadvertently make a mistake by choosing a material that will stand in the way of your plants being healthy and growing to their full potential.

After you’ve finished building or installing your raised beds, you may be feeling a bit overwhelmed when you see just how much space there is that needs to be filled before you can start planting your garden. It is understandable how you might assume that digging up some dirt from your yard and using it to fill each box could save you money and wouldn’t threaten the health of the plants. The dirt is obviously free. Plus, it must be good for plants if it is currently growing grass in your yard, right? Unfortunately, this is a false assumption that may end up threatening the health of your future plants.

While you might consider adding a little bit of dirt or soil from your garden to the base of the container (especially if it’s good-quality topsoil with organic matter), you don’t want to use it to fill the whole thing. Depending on the site in your yard where you remove the dirt, you could be adding subsoil. Subsoil is not considered fertile. While it will have a few beneficial microbes, their populations are going to be far lower than what’s present in compost or rich garden soil. Moreover, if you simply opt to fill the containers with pure dirt, it’s also likely that the soil structure will be less than ideal. Depending on the soil type where you live, the dirt could be very sandy. This would cause water to drain from the beds too quickly, leaving them dried out not long after rainfall or a shower from the hose. The other side of the spectrum is also possible — the dirt may be very heavy and compacted. Such soil is also not good for plants, because their roots have a hard time growing, and water doesn’t seep through the dense soil as well to really support the plants.

So, if pure dirt is out because it lacks sufficient nutrients and won’t allow for optimal drainage, then surely compost will be a better choice, right? When you’re looking for the best type of soil to fill raised garden beds with when growing vegetables, amending the soil with some compost can certainly be beneficial. However, using only compost to fill the container is not advisable. While compost has key nutrients that can support growth, those nutrients are very concentrated.

Ammonium, magnesium, sodium, and potassium, for example, are heavily concentrated in compost (especially those made using manure). When plants are grown solely in compost, exposure to such high levels of these nutrients can prevent them from properly absorbing the other nutrients needed for healthy growth. For example, if you try to grow plants in compost with a heavy concentration of ammonium, they may not absorb sufficient levels of potassium, magnesium, or calcium.

So, while compost can enhance soil fertility, it should only be used to amend the existing garden or topsoil. Before planting, it will also be important to test the soil to make sure it will provide the optimal growing conditions for your plants. One thing you should test is the pH. If too much compost was added, the pH may be too high (over 7.0) for most vegetables and other plants to grow. If the pH is too high, lowering it can take time. First, avoid adding more compost. You can also lower the pH over time by using nitrogen-only fertilizers, repeatedly adding sulfur to the soil, or acidifying the water you use to irrigate the beds.

You’ve probably heard about the fertility of manure and might see this as the ideal opportunity to tap into this rich fertility source and support your plants. However, there is one important mistake to avoid when using manure in your yard or garden — you must never use fresh, un-aged manure. If you’re growing a fruit or vegetable garden, one of the biggest reasons you’ll want to stay away from fresh manure is because of the risk of contaminating your crop with E. coli. Fresh manure is also not ideal for flowers and other non-edible plants, too. Horse manure may have too many weed seeds mixed in with it, as horses don’t fully digest the foods they eat. Poultry manure can be problematic as well, due to its higher salt content, which could damage your plants.

If you do want to use manure in your garden beds, there are appropriate ways to do so. You can either compost it or give it at least six months to age to mitigate the threats mentioned above. Generally speaking, composted manure is the better choice of the two. While aged manure is less likely to have weed seeds (as long as it has been heated by the sun to a temperature of at least 140 degrees Fahrenheit), some pathogens may still be present. If you’re going to use aged manure, reserve it only for the containers where you do not plan to grow edible plants. While composted manure will have lower nitrogen levels than those found in fresh, it is still much less risky to use and will have a much lower ammonium content to avoid potential fertilizer burn. However, don’t forget that you still should only use composted manure to amend the soil — not to fill the entire garden bed — as noted above.

You may have heard of people burying logs in an effort to beef up their raised beds to reduce the amount of soil they’ll need to fill the remaining space. However, while this may sound like a good, money-saving option, there are a few things to know before adding logs to a raised bed. Unfortunately, it isn’t going to be the solution you were hoping for, as using logs to fill in excess space can be problematic for a few reasons. One of the most important reasons is because of the potential harm they can cause to the soil. Even if you add a high-quality soil to start, as the wood decomposes, it’ll start pulling nitrogen out of it. You want those nitrogen levels to remain consistent to support the growth of your plants, so you’re taking a real gamble with their health. Eventually, the logs will be fully decomposed and will stop extracting nitrogen, but you really can’t predict whether nitrogen deficiency will be a problem for a few months or a few years.

There is one potential caveat/area where additional research is needed. If you are using hügelkultur, a garden-building technique from Europe that grows plants over decomposing buried logs, nitrogen depletion may be less of a problem. While more studies are needed to evaluate this growing technique, and particularly, its effect on nitrogen levels for the plants, some initial trials suggest that the plants are not deprived of nitrogen due to the fungi that are present. Compared to bacteria, the fungi hold onto the nitrogen for less time and only take it from smaller portions of the soil.

Rocks are another item that might sound like a good filler option to bulk up your raised container and help you keep soil costs down. You may even think that they’ll offer drainage benefits by allowing water to more easily seep out of the soil. However, you might want to avoid putting gravel and rocks in a raised bed.

First, despite what you might think, rocks do not actually enhance drainage. If you use them to line the base of the bed, it can result in water building on top of them. If water can’t drain, your plants could become waterlogged, suffering from problems like root rot (this will be of particular concern for those deep-rooted plants, which will be likelier to stay wetter due to the build up of water along the bottom of the planter box). Moreover, the use of rocks can also keep beneficial organisms, such as earthworms, from accessing the soil in the bed.

Sawdust is inexpensive and viewed by many as a good material to help bulk up the soil in their garden beds. With how much soil is needed to fill raised beds, it can seem like a welcome solution to keep costs down without threatening the health of the plants. Sawdust is, after all, only very fine shavings from wood. However, don’t jump to conclusions so quickly. Using this material isn’t the best choice if you’re striving for a healthy garden.

First, as we covered earlier, decomposing wood pulls nitrogen from the soil to aid in the process as it breaks down. Decreased nitrogen levels are not ideal for plant growth, particularly the roots, so you may find that your flowers, fruits, and vegetables just don’t grow to their potential. Beyond this potential threat, using sawdust from treated timber may pose health concerns. Chemicals that are unsafe for consumption are often used when treating timber. So when the timber is turned into sawdust, those chemicals will still be present and could seep into your plants.

If you have a container garden or some potted plants that are thriving, you might decide that filling your raised beds with the same potting mix will help them see the same results. However, potting mix is not something that you should use to fill a raised bed. Let’s start with a quick review of what these soil mixes are composed of. They typically consist of a mix of peat, perlite, and compost, making them almost entirely an organic material. This is ideal for potted plants, because it holds moisture well and is nutrient rich.

So, if potting mix is so ideal for containers, shouldn’t you be fine to use it for other garden structures? If you try, things will look great at first. That rich and loamy soil will support the growth of heavy plants. However, as time passes, the mix will slowly start to break down — remember, it is almost fully organic. This process can take several years, but within 10 years, that once fluffy and rich mix will have turned into something nearly unrecognizable: a dense and mucky silt with poor aeration — precisely what you don’t want your plants growing in.

If this leaves you wondering why the mix works so well with plants, but is a poor choice for the garden beds, then you are overlooking one essential detail. With container gardening, you remove the old soil and replace it with fresh potting mix at least every few years, eliminating the potential for it to break down enough to transform into something so undesirable for gardening. You won’t be doing this with your raised beds. Moreover, you’ll end up spending several times as much to purchase enough potting mix for the beds. Instead, you should aim for the soil you use to fill the beds to have a composition of no more than 50% organic material.

You may be considering repurposing some old and unusable materials from the home or garden to bulk up your raised beds. Some people seek out materials such as old plastic bottles, styrofoam packaging, or broken plastic pots to add to the base layer of the bed. There are also several pool noodle hacks for the home and garden, including one that involves adding pieces of cut pool noodles to the base of the planter or using them to line the edges. In theory, these ideas are designed to minimize the amount of soil that you’ll need. However, they are not a good choice if you’re looking for a way to help the plants in your garden bed thrive — and especially if you are growing fruits and vegetables.

Using any plastic (and yes, that includes pool noodles) to fill your garden bed can threaten the health of the plants themselves, as well as the health of anyone who consumes fruits or vegetables grown in the soil. As plastics degrade, they will leave behind microplastics in the soil. These microplastics may be absorbed by the plants (though more research is still needed to fully understand the potential harm this poses). They can also cause other issues, such as reducing the fertility of the soil by disrupting nutrients, like nitrogen and phosphorus, and the habitats of microbial life. Moreover, plastic fillers aren’t ideal even from a purely practical standpoint. Imagine how frustrating it will be each season when you’re turning the soil in the bed to get it ready, but you just keep churning up bits of plastic.

Newspaper is often recommended for use with garden beds. It is considered a “brown” material, meaning it can contribute carbon to the soil. Creating a layer of newspaper along the base of the container can help prevent weeds from breaking through into the bed. Plus, since the material will decompose, it will eventually break down and incorporate into the soil — there won’t be any need to try to dig it out of the bed in the future. However, while there is nothing inherently wrong with using newspapers, you should be selective about which sheets you add to the raised planter.

Some newspaper pages may be printed in color. Sheets with colored ink should not be used when lining the base of the bed. Certain colored inks can be toxic to the earthworms, who you certainly don’t want to harm. Additionally, you’ll want to stick to only using true newspapers, not any other types of mailers or the glossier pages from a magazine. Some of the inks used for these pages contain heavy metals, which could contaminate the soil.

If you follow gardening websites and some of the latest tips, then you likely know that there are several ways to use leftover coffee grounds in your yard and garden. The grounds are rich in nitrogen, as well as other beneficial nutrients like phosphorus. If you’re a regular coffee drinker and have been saving your leftover grounds for some time now, you may see them as an excellent option to reduce how much soil you’ll need to purchase for the beds without compromising the health of the plants.

In reality, however, using too many coffee grounds to try to bulk up the soil can be detrimental to your plant’s health. When non-composted coffee grounds are added to soil in large quantities, they can create a barrier in the soil, preventing the plants from accessing all the moisture they’ll need. Moreover, the nitrogen in the non-composted grounds won’t deliver the benefits you think it will, since it will become tied up by the microorganisms in the soil instead of actually reaching your plants. Adding so much caffeine to the soil may also interfere with the ability of some plants to grow properly.

If you want to use coffee grounds in your raised bed, it is best to compost them or combine them with fresh compost first. Doing so will prevent them from forming that moisture-blocking barrier that can be so detrimental to the growth of your plants.