Organic gardeners are all about rich, dark soil. Compost is the black gold of the gardening community. Everything seems to center around nutrient-dense organic matter.
So, doesn’t that mean we should just replace the soil with compost? Or keep adding organic amendments and fertilizers? Is there such a thing as soil that is too rich?
Yes, soil can be too rich. Organic matter should only make up about 5% of the soil, or else some nutrients may become toxic, and it may be challenging to maintain a balanced ecosystem.
Like compost, organic matter gives the soil a healthy, dark, crumbly texture while providing valuable nutrients. But you can have too much of a good thing.
So, where’s the line between healthy and toxic? How can you tell if your soil is too rich?
What is Rich Soil?
Soil is made up of minerals, organic matter, living organisms, gas, and water.
Rich, healthy soil is 3%-6% organic matter, has a dark, crumbly texture, and holds water and gas at equal ratios.
Most lawns and landscapes can thrive in soil that has 2%-3% organic matter, as long as it is irrigated properly. Flower and vegetable gardens should have a slightly higher organic matter content, but should not exceed 6% in most cases.
Here’s the confusing part: soils can either be mineral or organic.
- Mineral soils are soils with a clay, silt, or sand parent material.
- Organic soils are soils with an organic parent material, like peat soils.
Organic is a confusing term, but the basic definition is anything that is or was living. However, it has been used to define growing methods that work with nature as opposed to harming nature.
So, nearly all of our produce, organic or otherwise, is grown in mineral soils. Organic fruits and veggies are not grown in organic soils.
Organic soils are extremely rare, many are protected wetlands or peat bogs, and only a small number of plants can tolerate the soil environment in organic soil.
Organic soils are formed when lots of plant material is collected and very, very slowly decomposed in an anaerobic environment. They are generally found in cool, wet climates, and can hold up to 400% of their weight in water.
When growers refer to their soil as organic, what they mean is that they are managing their soil in a way that works with nature, as opposed to working against nature. In this regard, organic soils are soils that are managed using sustainable, environmentally-responsible methods.
What does all of this mean?
It means that rich soil refers to a mineral soil that is high in organic matter, not a soil that is truly organic or made entirely of rich, organic material.
So, perhaps the question we should be asking is:
Can a Mineral Soil Have Too Much Organic Matter?
There are three different ways excess organic matter can harm your soil:
1. Nitrogen Burn
Nitrogen goes through a process, known as the nitrogen cycle, that turns organic nitrogen into ammonia, and then ammonia into the plant-available form nitrate.
During the composting process, the nitrogen cycle happens at the center of the compost pile. This is what causes the pile to heat up, release funny odors, and eventually break down into nitrogen-rich compost.
If you add plant material that is raw, or has not completed the composting process, then you will overload the soil with organic nitrogen.
This will result in large amounts of organic nitrogen being converted into ammonia in the soil. At first, this will restrict the amount of available nitrogen to surrounding plants, causing a deficiency. Then, as the organic nitrogen breaks down, the ammonia and sudden availability of plant-available nitrogen will result in nitrogen burn.
Nitrogen burn can kill roots and scorch leaves. Eventually, the soil will return to a balanced nitrogen cycle, but in the meantime, adding green, or hot, materials to your soil will harm established plants.
The most common ingredient that causes nitrogen burn is fresh manures.
2. Nutrient Toxicities
Plants use eighteen different nutrients. Macronutrients are nutrients that are used in large quantities, while micronutrients are nutrients used in small quantities.
Some nutrients, like nitrogen, leach out of the soil easily because they do not form strong bonds with soil particles. This is also what makes it easy for plants to access them and pull them up into the root system.
Other nutrients, like iron, can build up in the soil because they form strong bonds with soil particles. This bond can make them difficult for roots to absorb, but this is not a problem since they are needed in much smaller quantities.
Adding excessive amounts of compost to your lawn or garden may result in nutrient toxicities, which can be difficult to overcome, because these nutrients do not leach out of the soil easily.
Toxicities can be compounded by pH levels that are too high or too low, which can change how much of a nutrient is available to plants.
Nutrients that can become toxic in acidic soils (<6.5) are:
Nutrients that can become toxic in basic soils (>7.5) are:
The nutrients in your compost will depend on the ingredients in your compost, but if you add compost high in certain nutrients to acidic or basic soil, you may end up with toxicities.
Compost made from recycled plant material will be much higher in nitrogen than phosphorus or other nutrients, which is a good thing, since plants use more nitrogen than any other nutrient. However, compost based on manures will have an equal amount of nitrogen and phosphorus, which can lead to phosphorus toxicities.
3. High Salt Content
Excessive compost applications can result in high quantities of soluble salts. These salts are a byproduct of having too much sulphur, nitrogen, phosphorous, and other nutrients. So, nutrient toxicities and high salt content go hand-in-hand.
Salt kills plants. You may notice excessive salt content in runoff that settles into a low spot and dries out. Excess salt will look like white streaks along the soil, and cause leaf burn or dead patches of grass.
Salts can be leached out by irrigating heavily, but not quickly. Try to water thoroughly without causing runoff. This may take a few weeks before you have leached out most of the harmful salts, depending on the climate and the soil structure.
Just remember, nitrogen leaches easily, so washing away salts will also wash away nitrogen.
How Much Compost is Too Much?
Most lawns benefit from adding a ¼”- ½” compost layer each year. Clay soils or cool-season grasses may need more, while loamy soils and warm-season grasses may need less.
If you’re establishing a new lawn, you can add 3”-6” of compost over the lawn area before planting, and then add a thin layer of compost each year.
Landscapes and gardens can handle thicker compost layers each year, but you should not exceed 3” of compost unless you are using an intensive, high-production growing method.
In general, it is better to add slightly less compost and focus on managing the soil with proper irrigation and aeration schedules.
Remember- organic mulches add organic matter into the soil, too. If you add a 3” layer of wood chips on top of a 3” layer of compost, you may develop nutrient or pH imbalances.
How much compost you add also depends on the plants you’re growing. If you have a native plant landscape that thrives in poor, rocky soil, then a soil with 5% organic matter may be too rich in this scenario.
This video offers an excellent explanation and demonstration of this principle:
How To Fix Soil That is Too Rich
What if you’ve already added too much compost, and you’re seeing signs of toxicities or imbalances?
First, stop adding compost.
Second, do a soil test (link to Amazon).
Your soil test should show you what nutrients are to blame, if any, and then you can adjust as necessary.
The easiest way to fix soil that is too rich is to plant plants that like rich soil and wait it out. Plants will keep pulling up nutrients, and after a few years, your soil will return to normal.
Can soil be too rich? Yes.
Is it a big deal? No.
Soils that are too rich for one plant may be perfect for another plant. If you believe you’ve added too much compost, adjust your landscape or garden to compensate, and after a year or two, you should have a happy, healthy foundation to work with.