You bought some potting soil from your favorite gardening shop or online retailer. However, you might have heard that potting soil might be improved with the addition of sand? Is this true? If so, how much sand and of which type should be added? For this and more keep reading!
Should you add sand to a potting mix for your indoor herbs? Good quality potting soil, in general, does not need the addition of sand as it presents drainage and structure adequate for most herbs. However, sand can improve the plant growth in case: 1) the planted herbs prefer mainly dry soil or 2) the potting soil is too dense.
Hence, in general, you are good to go with your potting soil without bothering in adding sands. However, this is not always the case. The herbs you are growing might be dry-soil lovers. Your potting soil might be too dense. Hence, how to know if you are in one of those cases and what to do? This article has you covered, just keep reading.
Sand In Potting Soil: Not Necessary Except in Two Cases
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Sand is, among the three types of soil (sand, silt, and clay), the one with the coarsest particles. For more details on soil types, you can read one of my previous articles.
This means that sand presents the highest drainage capability. As explained here, this is not a good thing for your herbs. Indeed, if forced to grow exclusively in the sand, any herb will definitely suffer. This is because they would not have time to “drink” water as it will quickly pass through the soil due to its very low holding capability. However, if sand is added to a medium (like regular potting soil), it will increase its drainage ability. This is the crucial point in adding sand to a potting mix.
First Case: Good quality potting soil (as this one) has adequate drainage, and so it does not need sand as discussed in the 2 aspects of the best potting soil. However, some potting soil might be too dense, and so sand might be needed. How do you know if this is the case?
A trick is to adapt a well-known drainage test performed by expert gardeners for outdoor soils, for potting ones. What I suggest is to water thoroughly a small pot containing potting soil only (so to avoid damaging the plant). If the water takes more than 1-2 minutes to pass through the pot and escape through the drainage holes, then the soil is definitely too dense and needs sand.
Second Case: Another case in which you need sand is for those herbs that do require a drier potting mix with higher than usual drainage capability. The following herbs, also based on scientific research and gardeners opinions, are the most commons for which a sandy soil is ideal:
Adding Sand: How Much and Which Type
How much sand you have to add? Well, to be precise, does not exist a single answer to this question as the amount of sand strongly depends on the drainage features of your starting potting soil. However, as a general rule of thumb, many gardeners recommend 1 volume of sand for each 4 volume of potting soil. If you are afraid in your soil becoming dry, you can also try with 1 volume of sand every 5 volumes of potting mix. This will put you on the cautious side.
How do you need to sand to your potting mix? The sand cannot be added on top of the potting soil but should be mixed throughout to give the drainage capability you require. Hence, just place the potting soil and the sand in a large container.
Then mix them either by hand or with anything at your hand (I do use a wooden spoon). The potting mix so prepared (well, enhanced) can be used for your herbs.
Which type of sand do you need? Yes, there is a large variety of sand out there in which you can actually find available. Here a list with some suggestions weather is suitable or not:
- Beach sand: if by any (lucky!) change you live close to a beach, and perhaps you are tempted to grab a bit of sand for your potted herbs. This is a bad idea. Indeed, such sand, constantly washed by salty water, has a high salt content. This is a problem as salt can severely damage your herbs’ roots.
- Building sand: this type of sand, also note as C-33 sand, is adopted to create concrete. The opinions regarding the use of such sand as potting soil addition are diverse. Some recommend it due to its very coarse structure. Others, on the opposite, warn against its use due to the presence of chemicals that can damage the plants. Moreover, this sand is known to contain silica, a substance correlated with severe chronic lung disease.
Another aspect to keep in mind is the high lime content (an extremely acidic material) in this sand type. This is another negative aspect as it can reduce the potting mix pH and so damaging your herbs (here an article explaining the effect of a wrong pH on herb growth).
Hence, I would again skip this sand as well. The only real benefit is its price, quite low compared to other sand types. However, this should not be a real advantage as you only need to buy a small amount of it.
- Play sand: this is most of the time a safe material (just avoid the one with silica). However, I would not recommend it as a way to enhance the potting mix. Indeed, it is way finer than builder or beach sand, and so its drainage ability is limited.
Moreover, it might (if of “higher quality”) be beach sand, with the salinity problem previously discussed. Furthermore, this sand has very rounded particles. This is not good news as it can make your soil very compact (imagine having a jar full of spherical balls pressed in it).
- Horticultural sand: this is the best sand you can choose for your potting mix. It is designed for gardening applications. It does not present any salinity issue and its particles are irregular in shape (also called sharp sand for this reason), differently from the beach and play sand. It is more expensive than other solutions.
However, this is because they are looking for outdoor applications. In this case, as several hundreds of kg might be needed, the cost is an essential factor. However, with a few dollars for indoor applications, you can find a good quality small bag of sand (like this good one in Amazon), still a very reasonable price and sufficient for many small pots.
Sand VS Potting Soil
For those of you already familiar with one of my previous articles, they might already know that there are a few key differences between potting soil and sand. I will summarise them here:
- Soil VS Non-soil: sand is a soil type while potting soil is more often than not a non-soi mix as it does not have trace
- Inorganic VS Organic: sand comes exclusively from the erosion (from air and water) of rocks that are broken down in small (sub-millimeter) particles after thousands of years. Hence, this is not organic matter. On the other hand, potting mix very often contains organic matter (like compost, for more info you can check this article) to enhance both nutritional content and structure
- One Medium VS Multiple: Sand is one medium per definition. Potting mix is obtained by different types of mediums (generally not soil) such as compost, perlite, peat, fertilizer, etc (as explained in this article).
- Nutrients-Poor VS Nutrients-Rich: sand is usually nutrient-poor. Indeed, due to the large particle size, it has a limited capacity to retain nutrients. This is because nutrients typically stick to the surface of the particles. And because larger particles create, in overall, a lower particle surface, you will end with a more moderate nutrition content in the soil.
- Too much drainage VS controlled drainage: Sand presents, due to its coarser particles, a limited ability to retain water, making unsuitable even for those herbs that prefer dry soil.
On the other hand, potting mix is designed (here for the potting mix recipe) to have an optimal water-holding capability thanks to a balanced blend between high holding water material (like peat moss) and increase-drainage medium (like perlite).
Is concrete sand the same as builder sand? No, concrete sand is coarser, with bigger and more irregular in shape particles than builder sand. Hence, concrete sand is a better choice (once verified it does not contain any dangerous chemical) when added to potting mix.
Can herbs be grown in sand? It would be challenging to grow herbs in a 100% sand soil for the excessive drainage and lack of nutrients. However, a few herbs (such as lavender and salvia) can survive in mainly sandy soil (sandy loam).