Soil Fertility 101
Healthy, fertile soil is the foundation of our food system. Fertile soil provides the nutrients that plants need to grow and become the productive mature plants that feed us. Soil is much more than just dirt. It’s a living ecosystem full of microscopic and larger organisms that convert decaying organic matter and minerals into nutrients that plants can take up through their roots.
A few years ago I enrolled in a course series on growing nutrient dense food. The course was taught by Dan Kittridge, founder of the Bionutrient Food Association. The course focused on soil biology, mineralogy and energy dynamics. It took place on a farm that followed the practice of growing nutrient dense food. It was fascinating to learn about the interconnected system of soil and plants and see the difference improved soil fertility made on the host farm’s crops through the growing season.
The workshop series covered soil testing, the benefits of cover crops and how to calculate necessary amendments based on your soil test results. I followed the recommendations and feel that by increasing my soil fertility, I had a more abundant harvest. My plants looked healthier and seemed to be more resistance to disease and pest pressure. People are healthier and less susceptible to illness when they get enough nutrients and the same is true for plants.
Organic matter is any plant or animal material that goes through decomposition. Compost and manure are examples of organic matter. In addition to providing food for soil organisms, organic material improves the water holding capacity of soil. Gardeners and farmers use crop rotation, animal and green manures, fertilizers, compost and cover crops to improve soil fertility.
Nutrients are broken into two main categories; macro- and micro-nutrients. The three macronutrients are often referred to as N-P-K. They are:
Nitrogen (N) – necessary for green leafy growth.
Phosphorus (P) – important for vigorous root growth along with fruit and seed formation.
Potassium (K) – essential for overall growth and disease resistance.
Micronutrients are used in smaller quantities than macronutrients by plants for optimal health. The micronutrients are:
Nitrogen is the only non-mineral nutrient. The other nutrients are formed by slow weathering of rock formations. Nitrogen in the air cannot be directly used by plants. Certain bacteria in the soil take nitrogen from the air for use in their cell structure. When these bacteria die, their nitrogen becomes available for use by plants. Other bacteria that have a symbiotic relationship with legumes (peas clover, beans, etc.) remove nitrogen from the air and store it in nodules in their roots. After the legumes die and decompose, other plants can use this nitrogen. Legumes are nitrogen fixing plants.
Compost and organic fertilizers are often used as a source for mineral nutrients. Rock dust can also be applied. As it weathers, its minerals become available for plants. My soil was low in several micronutrients and I added rock and dust from my local quarry as part of my soil fertility improvement process.
I highly recommend getting a soil test to see which nutrients your soil has inadequate levels of. Most state university extension services offer reasonably priced soil testing services. Often, they will also help you interpret the results. Not only is it a waste of money to randomly add fertilizer to your soil, excessive amounts of certain elements may be as detrimental as having too little.