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Soil pH, moisture content, and soil contamination are among the several reasons why might soils rich in organic matter not be fertile. We discuss them all in this post.
Regardless of whether you grow plants for fun or profit, it is natural for you to worry about the quality of your soil. Managing soil fertility is key to optimizing plant growth and yield.
To improve soil fertility, growers and horticulturists often resort to using composts, fertilizers, and manure. This increases the organic matter levels in the soil, which serves a range of purposes.
However, high organic matter levels don’t always translate to high soil fertility.
why might soils rich in organic matter not be fertile
Briefly, it’s because soil fertility also depends on factors like moisture content, pH levels, and soil organism activity besides organic matter levels.
Organic matter is only one of the many components of soil. While it is one of the most important components, it is also perhaps the most misunderstood component.
We’ve broken down the answer in detail in this post.
Here’s a quick overview of the purpose that organic matter serves in soil fertility and subsequently plant growth.
Role of Organic Matter in Plant Growth
Understanding the role of organic matter in soil fertility will give you the insight you need to manage soil fertility effectively. Organic matter serves the following purposes:
- Nutrient Storage and Supply: Organic matter acts as a reservoir of nutrients in the soil. As microbes decay the matter, several insoluble minerals like nitrogen and sulfur are made available to the plants in the soil. Nutrients from organic matter are released mainly in the summer due to increased microbial activity. For this reason, summer crops benefit the most from these nutrients.
- Soil Aggregation: The arrangement of various particles such as sand, clay, rocks, and the like in the soil is called its aggregation. Adding organic matter to the soil facilitates aggregation by clumping the particles together. The improved soil structure offers a range of benefits, including better aeration and permeability and decreased compaction and surface crusting due to heavy rain.
- Water Retention: Organic matter has sponge-like properties and can hold up to 20 times its weight in water. Furthermore, organic matter releases most of the water to plants that absorb it out of the soil. In contrast, clay holds a lot of water but does not release it to plants. This is why mixing organic matter in the form of compost and manure into sandy soil makes sense. It increases its water holding capacity.
- Erosion Prevention: Many growers don’t know that organic matter also helps prevent the erosion of the nutrient-rich top layer of the soil. The Universal Soil Loss Equation suggests that adding as little as one percent of organic matter to the total weight of the soil can reduce erosion by upwards of 20 percent. This is a result of the stable aggregates (clumps of particles) that the organic matter helps form in the soil.
Besides offering these advantages, organic matter helps regulate soil temperature, and its buffering nature aids with pesticide and herbicide residue management.
Organic matter is a necessary integrant of soil, and in the right quantities, can greatly improve plant growth. However, by itself, organic matter in soil isn’t of much use to plants. Beyond a certain point, adding organic matter will not improve soil fertility, even if you buy a quality fertilizer and use one of the best fertilizer spreaders to spread it.
It is not the only factor that influences soil fertility – which is why might soils rich in organic matter not be fertile.
Other Factors That Influence Soil Fertility
Besides the quantity of organic matter, six factors influence soil fertility:
- The soil’s pH level has to be just right – it cannot be too acidic or too basic. A high pH level removes the usable nutrients from the soil, and too low a pH level can make the nutrients in the soil toxic for plants. According to a 2018 study, a pH level between 5 and 6 is considered optimal for plant growth. If you find that your soil is too basic, you can read our post on how to make soil more acidic.
- Moisture content: The amount of moisture in the soil can greatly influence its fertility. The nutrients in the soil are made available to plants through the moisture in the soil and not by solid soil matrices. For this reason, soil with higher moisture content is considered more fertile.
- Soil texture and components: Particles of different sizes and types clump together to maintain the structure of the soil. Soil with high clay content can retain more nutrients since clay acts as a reservoir for calcium, magnesium, and potassium. Getting more technical, soils with high clay content tend to have a high Cation Exchange Capacity (CEC). Conversely, if the CEC is low, it indicates that the soil can easily lose its nutrients due to leaching.
- Bulk density: The soil’s fertility is heavily influenced by how tightly the particles are packed together. If the soil is compacted, the plant’s roots cannot penetrate through the soil and absorb water and nutrients. This hinders plant growth even if organic matter and nutrient content in the soil is high.
- Soil organism activity: Microbes like archaea, actinomycetes, algae, and larger fauna such as earthworms and mites improve nutrient availability in the soil by breaking down organic material into organic matter. These organisms also aid in improving soil structure and subsequently improve water retention.
- Soil contamination: A high concentration of salt or pesticides in the soil can impede plant growth and sometimes even repress growth completely. A high metal content can also hamper growth.
For healthy plants to grow on the soil, the conditions have to be just right. Issues like water logging, extremely high or low pH, lack of aeration, and shortage of nutrients all influence the health of the plants and crop yield.
How to Determine if Your Soil is Not Fertile
Soils that are darker in colour are considered to be fertile and rich in organic matter. Also, soils with a loamy texture are considered more fertile than soil with a sandy texture.
But looks can deceive, and this holds for soil as well.
Your soil may be deficient in nutrients if you notice your plant(s) showing any of the following signs:
- Early or late maturity
- Extremely poor initial growth
- Pale, brown, or yellowing leaves
- Abnormal or restricted root growth
- Poor appearance, taste, firmness, and moisture content of fruit/vegetable
However, these symptoms may also be a result of plant diseases or pest damage. Therefore, you will need to closely examine the plant’s leaves, stem, and roots to determine the right cause of the symptoms.
How to Improve Soil Fertility (The Right Way)
While adding organic matter may help improve the nutrient content of the soil, there are other effective ways to improve the soil’s condition:
- Cover crops: Planting legumes, grasses, and other herbaceous species annually is a great way to restore nitrogen in the soil and rebuild the soil’s biology. These crops can be used as material for grazing, composting, and mulching. Some horticulturists prefer ploughing cover plants into the soil to let microbes act on them directly. Besides improving fertility, cover crops keep the moisture locked into the soil and help prevent erosion.
- Mixed cropping/balanced crop rotation: Sowing different crops across the same field is called mixed cropping. It helps manage soil nutrition and prevents erosion. If mixed cropping isn’t possible, you could consider growing a suitable sequence of crops annually so your soil is never fully depleted of nutrients.
- Avoid tilling: While tilling can aerate the soil, it can also lead to the topsoil blowing away with strong gusts. Loose topsoil also easily washes away in the rain. Using a digging fork is a great way to aerate the soil without promoting erosion. However, since it takes longer than tilling, it is only suitable for smaller fields and gardens.
- Feeding soil organisms: Providing the soil’s fauna with organic material increases their activity in the soil and improves soil structure and fertility.
- Liming: A low soil pH can make plants severely nutrient-deficient. Liming is a great way to bring the pH up to the optimal 5-6 pH range. The amount of lime you need to apply depends on the pH you want to achieve, which depends on the plant you want to grow. An Oregon State University study specifies ideal pH levels for various crops. You will need to apply lime once every three years, at least six weeks (ideally eight weeks) before sowing. Using agricultural lime with at least 80% CaCO3 content is the right way to go. If your soil is magnesium deficient, use Dolomitic lime instead. We discuss what lowers soil pH in depth in this post.
Building organic matter in soil is a long-term process that doesn’t always improve soil fertility. Numerous other factors influence soil fertility, and now that you’ve read this post, you know why might soils rich in organic matter not be fertile.
Your next step should be picking an appropriate method listed above to improve your soil’s fertility. You will then be one step closer to growing healthier plants that provide greater yields.