Perhaps you are attempting to create your own soil, or you are considering to improve your current one. Loam soil and compost are two terms that you might have heard of, and you wonder what they actually mean. Here an article based on my experience, and on one of many gardeners, that will provide all the information you need to know.
What is the difference between loam and compost? Loam is a mixture of inorganic components of sand, silt, and clay. It might also contain organic matter. On the other hand, compost is not soil, but 100% decomposed organic matter. This is reflected in significant differences between their physical and chemical properties.
Hence, loam soil and compost are two totally different things considering their origin (organic and inorganic). To discover the main differences in terms of physical and chemical properties, keep reading.
Differences: Origin, Texture, and Nutrients
The best way to understand the differences between loam soil and compost is to refer to their physical and chemical properties. Without getting into details (for more info here) a physical property is everything related to the soil structure (like porosity and capacity to retain water). On the other hand, acidity and nutrient content are classified as chemical properties.
Loam soil is essential in the gardening world. It is often defined as the best gardening soil. Indeed, it is a balanced combination of the three types of soils. Consequently, it holds the benefits of each one of them: the drainage and airflow of sand and the high nutrient retention capability of silt and especially of clay.
Compost is obtained by 100% (specific) organic matter decomposed in particular conditions. This means that, from a bunch of food scraps (like eggshells, fruit peel), you can have after a few months a totally broken down dirt-like (but it is not dirt) material.
On the other hand, loam soil is the combination of mineral materials like sand, silt, and clay that have originated, in millions of years, from rocks. You can also find decomposed organic matter, however this, not the main component.
Loam soil is not one type of soil an umbrella of soils (like sandy loam, silt loam, clay loam, etc…) depending on the proportion of sand, silt, and clay. The only thing such soils have in common is that sand and silt are dominant (and often in the same proportion) compared to clay (the finest material among the three).
The best way to understand the differences between loam soil and compost is to refer to their physical and chemical properties. A physical property is everything related to the soil structure (like porosity and capacity to retain water) while a chemical property is referred to its acidity and nutrient content. For more info, you can read my article here.
Different Physical Properties
For simplicity, I will refer to a balanced sand-slit-clay combination as quite commonly used among gardeners. Even if the mixture is perfectly balanced (like, for instance, a 40-40-20%), the below still applies.
Moreover, this is a description of what compost and loam soil look like once opened from their package (I am assuming you just bought them). This because the level of added moisture (by watering), and age can affect their aspect, making the comparison of not much sense.
Color: Compost presents a dark brown (close to black) color. Darker color indeed is in general correlated with a higher carbon (hence organic) content. Loam soil, due to its higher sand concentration, is still brown but slightly lighter in color.
Moisture: Due to its 100% organic origins, compost has a higher humidity level (up to 70%) compared to loam soil.
Texture: On your hand, compost will feel like a moist crumbling medium. If you press, it will keep that shape to crumble a few seconds later. It is not smooth like sand, and you will find pieces of partially decomposed material (such as wood in commercially available compost).
Loam soil has a slightly more dense and smoother texture if watered to reach the same level of moisture of compost. When pressed, it keeps its shape, but it still crumbles when you move your fingers through (not like clay that would stay compact and very dense). This is due to the presence of silt and clay.
Drainage and Water Retention: Compost has, in general, lower drainage than loam soil and a significantly higher water retention capability (it acts like a natural sponge). This is not a negative aspect as it can release water to the roots when needed.
Different Chemical Properties
Nutrients: Compost, due to its organic origin, presents a higher concentration of nutrients than loam soil, mainly nitrogen and carbon. The exact proportion changes depending on the type of organic matter it comes from. Generally, nitrogen content oscillates from 2 to 4% of its weight with slightly less phosphorus and potassium (up to 2%). Compost has up to 20 times more carbon than nitrogen (as organic matter is mainly carbon-based). To know the nutrient content in a medium there are cheap and inexpensive way as detailed in the Best nutrient testers for beginners article.
Loam soil generally has a way lower nutritional content, and the one present might not be readily available to the plants. That’s why, when creating a gardening soil, it is always suggested to add compost to loam soil.
pH: compost pH can vary significantly from 4 to 8 depending on the organic material used (wood, food scraps, manure, etc…), on the level of oxygen during the decomposition process, and on time required by the decomposition process.
Loam soil has a pH that changes depending on the proportion of sand, silt, and clay deployed. However, in case of a balanced mix, it oscillates from 4 to 6. Remember though that in case of highly unbalanced loam soil, the pH can be quite different (for instance, a sandy loam can have a pH as low as 1-2). For information you can refer to the significant effect that pH hace on herb growth article.
Which Compost Can You Buy In Shops?
Both compost and loam soil are commercially available in your local gardener shop or even in large retailers. On Amazon for instance you can find around a dozen options under slightly different names such as “Super Compost”, “Organic Compost”, “Compost Crumbles”, with at least 4-5 different brands.
If you do not like online retailers, also Walmart is a valuable option with (apparently) even more brands to look for.
Regarding loam soil, you will unlikely find something on Amazon or Walmart. This is because, for gardening purposes, retailers are way more interested in selling a combination of sand, silt, clay (that is not necessary loam soil, although typically it is) with the addition of nutrients, fertilizer, compost, pesticide, or/and other media or substances. This is because this provides a soil ready to go in the garden while loam soil alone is not sufficient.
Indeed, if you type “gardening soil” or “topsoil” on Amazon and you check the product description, very often you will find something on the line:
Topsoil has a balanced clay, silt and sand composition with plenty of organic matter providing good structural stability under all weather conditionsTopsoil descrption found on Amazon
The above is an exact extract from a real topsoil product highlighting that this can simply be a loam soil plus nutrients.
Both compost and loam are sold in large bag oscillating from 3 to 50lb in weight. The price can vary greatly depending on: 1) the brand, 2) the usage (topsoil or gardening soil), 3) the origin (manure compost or food scrap compost), and 4) the presence or not of chemical (organic is way more expensive these days). You can find topsoil between $5 for a few liters to $70 for a 50lb top-notch quality bag.
Regarding, compost, as something that should be added to the soil to improve its quality, especially on Amazon, can be found in smaller bags of 10-20lb (rather than 50lb) with prices oscillating from $26 to $50 per pack.
Surprisingly, in the UK, I never found soil and compost in large chains like Tesco, or Sainsbury, and they definitely not online. Aldi and Asda do sell soil and compost. However, you need to be careful on the customer review to be sure the quality is what you want. Otherwise, you can always refer to your closest garden center.
Can I plant my Herbs in Loam Soil or Compost Only?
Both compost and loam soil alone are inadequate for herb growth. Indeed, loam soil has a great structure but insufficient nutrients, while compost does not provide the minerals and structure stability plants need.
Bad Idea #1: Compost Only
The earth-like and beautiful structure of compost might be so tempting that you might think to use only compost for your potted plants. Please, do not do it. Many people made the same mistake and ended up with dying plants in a couple of weeks. This because the water retention capability, although excellent, might be a double cut if the whole medium is a sponge. Moreover, as said before, compost is lighter than loam soil, so unable to provide adequate stability to your herb.
Another point against a compost only medium is that plants strive with a balance of a dozen nutrients that are not all present, in the required amount, in the compost.
Bad Idea #2: Loam Soil Only
This soil (I am not talking here of the commercially available garden soil, but only of purely loam soil) is not enough for plant growth. It just lacks both nutrients and presents an insufficient water retention capability.
Can I produce Loam Soil And Compost at Home?
Compost is currently produced by thousands of people around the world in the comfort of their house. Indeed, if correctly done, composting (the act of creating compost) is relatively straightforward. It just requires attention to what is decomposed (some food scrap like animal and dairy-related should be avoided) and patience. The process indeed usually requires a few months (a few shortcuts can be applied if you need).
Loam soil, on the other hand, is not that easy. Indeed, it does require minerals (that can be easily found in a specialized garden center) and also hummus/compost or any other form of nutrients.
Moreover, you need to remember that loam soil is mainly used for outdoor applications. I did not found so far anyone suggesting such soil for indoor gardening. Indeed, for indoor gardening, it is way better to rely on a non-soil potting mix as described in this article in which compost is one of the ingredients.
Does the composting process attract pests? If done incorrectly, the composting process can attract pests such as flies, large insects, and even rats.
Is topsoil and loam soil the same thing? No, topping soil do present sand, silt, and clay but their proportion might not match the loam soil and, more importantly, garden soil presents also another medium to improve pH, nutrients and other chemicals to prevent pests that loam soil does not have
Interesting video on water-based testing of different soil type