Not all fertilizers stay in the soil. Leaching is the process of contaminants washing out of soil and into waterways. This isn’t only wasteful to your pocketbook and harmful to your garden crops, but it can also be extremely damaging to local ecosystems.
Instead of staying in plant root zones, most leached nitrogen (and other compounds like phosphorus and potassium) flows into waterways, contributing to devastating algae blooms and dead zones throughout the Mississippi River Basin and Gulf of Mexico. To prevent environmental damage, we need fertilizers to stay in the soil where plants can use them.
Sandy soils in high-rainfall areas have the greatest risk for leaching, but any dirt can be susceptible if it has poor structure, damage from tilling, or a lack of organic matter. Let’s explore how to prevent leaching in your garden and keep fertilizers where they belong.
The Short Answer
Prevent nutrient loss by using organic matter and avoiding overwatering.
Leaching is when water flushes fertilizer nutrients and other substances out of the soil. Excessive rainfall or irrigation can “flush” nutrients out of plant root zones, moving these contaminants into groundwater, streams, rivers, and lakes. This phenomenon is a major cause of algal blooms and the Dead Zone in the Gulf of Mexico. It also dramatically reduces garden yields, stripping soil of its fertility and wasting money used on fertilizer products.
You can prevent leaching by:
- Avoiding excess fertilizer or overfeeding
- Swapping synthetic nitrogen fertilizers for organic, slow-release fertility
- Increasing organic matter with compost, manure, mulch, and cover crops
- Ensuring constant plant cover by keeping living roots in the ground
- Never leaving bare dirt in the garden
- Using mulch, such as leaves, straw, or compost
- Avoiding overwatering
- Reducing or eliminating tillage
- Using perennial groundcover in pathways
- Plant perennial herbs, shrubs, and trees
The Long Answer
Not everything you put in your soil will stick around for long, and adding fertilizer does not mean nutrients will stay in the ground for your plants to uptake. This is because the belowground ecosystem is constantly changing with exposure to water, air, minerals, roots, and microorganisms.
Water is particularly powerful because it has the ability to wash materials out of the soil, draining those soluble chemicals or minerals into groundwater, streams, and rivers. A discharge of water-soluble fertilizers and chemicals can be incredibly harmful to aquatic ecosystems and water reservoirs that humans rely on for hydration. In order to prevent contaminated waterways and keep plant nutrients in your garden, you must understand what causes leaching and how to prevent it.
The Problem With Leached Nutrients
Excess nutrients and moisture can lead to severe ecological issues.
Excess nutrients, poor soil structure, and a lot of moisture are a recipe for ecological disaster. When water rushes through degraded ground, water-soluble fertilizers and chemicals are flushed away with it.
Fertilizer pollution is most notably linked to the Dead Zone in the Gulf of Mexico. This is where an accumulation of nitrogen and phosphorus runoff has concentrated in the ocean, causing an overgrowth of algae. When algal blooms form on the surface, they rapidly use all the oxygen in the water, causing a hypoxic (low-oxygen) zone in the water below. As a result, vast amounts of fish, coral, and aquatic mammals die.
However, there are even more problems associated with contaminants that get washed out of the dirt. When all these minerals and contaminants flow into the groundwater, plants are left without fertility in their root zone and local waterways become polluted with soluble nutrients. As you can imagine, this creates loads of problems for humans, plants, and the rest of the ecosystem.
The biggest problems caused by leaching include:
- Loss of crop productivity
- Nutrient deficiencies in plants
- Low soil fertility
- Worsening erosion
- Economic loss from ineffective fertilizers
- Waterway contamination
- Drinking water pollution
- Algae blooms
- Aquatic dieoff
Causes
Poor soil structure and moisture lead to nutrient loss.
Leaching is caused by a triple threat of issues:
- Excess nutrients (especially synthetic soluble fertilizers)
- Poor soil structure (particularly barren, heavily disturbed, sandy ground)
- A lot of moisture (rainfall or irrigation)
When these issues converge, it causes nutrients to escape our gardens and leave plants deficient of many nutrients. Nitrogen, potassium, and phosphorus are the most commonly leached macronutrients. However, micronutrients like calcium, magnesium, and boron may also leach.
Many plants struggle with deficiencies of these nutrients because the soil has a harder time “holding on” to them. Water-soluble minerals easily dissolve into pockets of water in the soil solution, and they can get flushed out over time, or when lots of water is poured into the ground at once.
Other major causes of leaching include:
If there is a lack of decomposed plant or animal matter (humus), the soil will struggle to hold onto nutrients. Organic matter like compost is negatively charged, which means it attracts positively charged nutrients like a magnet. When these nutrients don’t have a “magnet,” they float into moisture pockets, making them more vulnerable to washing away.
Clay particles are also negatively charged, which means they attract and hold positively charged cations like nitrogen, potassium, calcium, and magnesium. If the ground is very sandy, or lacks clay and organic matter, then it has a mostly net positive charge. Like magnets, the positively charged soluble nutrients will fail to “attach” to other positively charged particles.
When garden beds are left barren for prolonged periods, the exposed soil becomes extra vulnerable to leaching. Without roots, cover crops, or ground cover to cover the surface and hold the particles in place, water erosion washes away soil and any nutrients that are in it.
Large amounts of rainfall, such as big storms, can cause tremendous leaching. In regions with regular rainfall, like the tropics, you will often notice heavily leached nutrient-poor soils with a red color. The red hue occurs because rainfall washes away most of the minerals, leaving behind iron oxides.
Anyone who has raised houseplants likely faced overwatering issues at some point. When too much irrigation flows through a pot, it carries away minerals with it out of the pot, leaving plants deficient. The same problems with excess irrigation can occur in lawns and gardens, except the water flows into the ground and nearby streams or rivers.
Rototilling and aggressive disturbance harm the belowground structure by degrading organic matter and reducing the diversity of particles. Without the intricate structures and microbial “glues” to hold soil together, minerals are more prone to washing out.
How to Prevent It
All complicated soil science aside, preventing leaching is actually super simple. Whether you have an edible raised bed garden, flower pots, ornamental beds, or a lawn, these steps can ensure nutrient retention and less pollution from your landscape.
Improve Soil Organic Matter
Add organic matter to enrich the soil and retain nutrients.
The best thing you can do for the belowground ecosystem is add organic matter! Compost is called “black gold” for a reason. Decomposed plant and animal materials enrich the dirt with vital microorganisms that help create structure below the surface. This includes elaborate clay matrixes, fungal “glues”, and bacterial habitats that actually help hold onto important minerals in the root zone.
Sandy and degraded soil is the most vulnerable to leaching. You can use compost, peat moss, manure, mulch, and leaf litter to rapidly improve the structure and keep minerals “locked in” with surrounding particles. This will also improve overall plant health and water retention, meaning less drought stress and higher yields!
Avoid Overfertilizing
Use less fertilizer to prevent excess and nutrient loss.
Nobody can eat nonstop. If there is too much fertilizer, your plants may not have the time or energy to uptake it and use it. Overfertilizing leads to high levels of unused fertility that can easily get flushed out. When adding fertilizer products, it’s much better to air on the side of caution. Feed less than you think. You can always add more later, but you can’t take it out.
Avoid Synthetic Quick-Release Fertilizers
Choose slow-release organic fertilizers to reduce water contamination.
Rapid-release synthetic fertilizers are highly water soluble and plant available. This means they are extra prone to leaching! Organic gardeners already avoid synthetic products to ensure the health of their garden ecosystems, but here is another reason to avoid these fertilizers: they can severely contaminate waterways!
Synthetic nitrate pollution from quick-release fertilizers is a global pollution issue. Research shows that less than 50% of the applied synthetic nitrogen is actually used by plants. The rest becomes pollution in waterways, which kills wildlife and harms our freshwater resources!
To avoid contributing to this issue, choose slow-release organic fertilizers. These products are not as rapidly available to plants, and they are less water soluble. This means that they require microbial decomposition to gradually become available over time. Instead of megadosing plants with a ton of nutrients at once, slow-release fertilizers provide long lasting nourishment that sticks around in the soil profile.
Use Mulch
Mulch protects soil from leaching while retaining moisture and enriching nutrients.
Straw or leaf mulch is very important for protecting the surface of garden beds. You never want to garden naked! This doesn’t have anything to do with what you wear—it’s about your soil. Never leave it naked or barren!
Mulch is like a protective layer that buffers the fragile dirt from intense sunlight and rainfall. This organic material helps hold particles in place while slowly decomposing and enriching the soil with more stable nutrients that won’t wash away.
Avoid Overwatering
Check soil moisture before watering to avoid oversaturation issues.
Excessive irrigation can cause many issues, including leaching, root rot, compaction, and fungal diseases. If the ground is oversaturated, you may notice yellow, droopy, or sick plants that really don’t want to sit in soggy beds.
The solution is to always check the moisture before watering. If the ground is already wet a few inches down, you can let it dry out another day or two before irrigating again. Ideally, the soil should never feel wetter than a wrung-out sponge. Soggy ground rapidly flushes away fertilizers.
If you often forget to turn off your irrigation, invest in a timer or try out a different system that will prevent you from saturating an area. While we cannot control the rain, we can control how much moisture comes out of the hose or irrigation lines!
Prevent Runoff
Prevent runoff by covering slopes with mulch or wood chips.
Runoff is closely linked to leaching. It occurs when flowing water forms miniature-streams and tributaries through your garden or walkways. The easiest way to prevent this is by watching the slope of your garden, and designing accordingly. Keep your pathways covered, instead of leaving bare dirt. As a bonus, your boots will get less muddy.
If you notice a major slope or an area where dirt and moisture are accumulating, use wood chips, mulch, or rock to slow the flow of water in that direction. Slower flow means less rushing movement through the landscape. This can help keep nutrients in the area longer, allowing plants to uptake them before they leach out.
Key Takeaways
Most organic, no-till, and regenerative growing methods naturally help to reduce leaching. If you want to protect waterways and ensure your expensive fertilizers are used by plants, remember to:
- Minimize tillage and disturbance
- Use organic slow-release products
- Avoid synthetic quick-release fertilizers
- Avoid overwatering
- Add compost and organic matter
- Use mulch
- Keep pathways covered
- Enrich sandy soils
