Do you suspect that your potted plants aren’t growing as well as they used to? Or, if you’re like me, do you ever wonder if that bag of potting soil that’s been sitting around for a few years now in your backyard is still good?
If so, you might also be wondering how long your potting soil will keep its nutrients.
Potting soil will retain adequate levels of nutrients for 6 to 12 months. After 1-2 years, it is advisable to replace it with fresh potting soil to restore the nutrient levels, reset the pH levels, and remove any anaerobic bacteria buildup in the soil.
To better understand why this happens, let’s take a look at the causes and contributing factors that impact nutrient loss.
What causes potting soil to lose nutrients?
Nutrient loss occurs naturally over time when they are consumed by your plants and bacteria in the soil. Obviously, plants use nutrients to grow and develop. At the same time, bacteria consume nutrients as they break down organic matter.
Another way to think about nutrient loss is through the agricultural term: soil erosion. In agriculture, soil erosion happens when water flows over the surface of the land and carries nutrients away with it, either through runoff or sediment.
In the case of potting soil, water running through the soil over time can wash away nutrients. Assuming your potted plants have drainage holes in their pots (which they should), some nutrients will wash out of the soil over repeated cycles of watering.
In fact, research has shown that 20% of nitrogen fertilizers leach into surface and groundwaters. The 20% figure tells us that a sizable chunk of your nutrients will wash away over time if it’s being watered.
What affects the rate of nutrient loss?
As you might expect, there are a number of factors that affect the rate of nutrient loss in potting soil. But a general rule of thumb you can use is to expect the decrease in nutrients to have a noticeable effect on your plants after about a year or so.
While there isn’t a way to crunch some numbers and come up with an exact date when your potting soil runs out of nutrients, I can point you to a few controllable factors that are known to increase the rate of nutrient loss.
The table below shows various rates of nutrient export coefficients (higher number = faster nutrient loss) for various crops and soil types.
This table gives us three factors that impact the rate of nutrient loss:
- Type of soil used
- Type of plant or crop being grown
- Type of nutrient being considered
The type of soil used can impact the rate of nutrient loss in a number of ways. For example, soil that is high in organic matter and clay will generally have a higher nutrient retention capacity than more sandy, low-organic-matter soils. This is because organic matter helps to hold onto nutrients and make them available to plants.
The type of plant or crop being grown can also affect the rate of nutrient loss. Some plants are heavier feeders than others and require more nutrients to grow. As a result, these plants may deplete the soil of nutrients at a faster rate than plants that are not as demanding.
The type of nutrient under consideration can also impact the rate of nutrient loss. Some nutrients are more prone to loss through leaching and erosion than others.
Nitrogen, for example, depletes 5-10x more quickly than phosphorus according to the table above. Nitrogen is more likely to be transported in runoff than phosphorus because of greater mobility in its dissolved forms in water.
The rate of nutrient loss also depends on availability of other nutrients, the presence of soil organisms, and the overall health and growth of the plants or organisms involved.
An overabundance of nitrogen fertilizers, for example, can disturb the biology of the soil and overall reduce the quality of soil in complex ways. It can kill off bacteria and fungi that facilitate a plant’s nutrient uptake, causing a net loss in effective nutrient content in the soil.
How you store your potting soil will also impact how long the soil will keep its nutrients. Soil stored in opened bags that have been exposed to the elements will start to lose quality after about a year.
Soil stored in unused, unopened bags will start to lose quality after 1-2 years. Unfortunately, untouched soil will also eventually lose nutrients due to the buildup of anaerobic bacteria and mold which consume nutrients in the soil, especially nitrogen, carbon, and sulfur.
How do you prevent nutrient loss over time in potting soil?
My recommendation is to simply replace your potting soil every couple of years or so. This will ensure that the quality of your soil will remain high and that your plants will have access to an adequate level of nutrients needed for them to grow and thrive.
Another option I would recommend is to add organic matter to your soil, such as compost or worm castings. Increasing the organic matter in your soil can help improve its structure and its nutrient holding capacity.
The other option you could consider is to add fertilizer to replenish the nutrients. Be aware, however, that this may not resolve other issues with using aged soil, including increased acidity from decomposed organic material, build up of anaerobic bacteria, or the fact that your plant might have outgrown its pot.
Although fertilizers are certainly the most convenient option, one possible downside of using fertilizer is that its effect is only short-lived. Generally, fertilizers will only have an effect for at most a few months and will require regular create applications to sustain the nutrient levels.
In case you aren’t aware of this already, you should avoid over-watering whenever possible. Too much water can cause nutrients to leach from your soil. I recommend only watering your plants only when the soil is dry to the touch (but not too dry).
Hope that helps!