A grower's guide to soil fundamentals

For millennia, soil has been the conventional growth medium for crop cultivation. Even as modern growing systems gain popularity, there will always be food grown outdoors and in soil.

Hydroponic systems and soil-less media can be expensive to set up and maintain, especially with the rising cost and scarcity of water in many areas of the world. Soil is therefore an accessible and viable option for commercial growers.

Successful growth in soil relies on growers to take good care of the soil and the organisms within it. This article is an introduction to soil as a growing medium to help you on that journey.

The following is an excerpt from the Bluelab Grow Book, written by Dustin P. Meador, Ph.D. and Leila Jones, Chemistry and Genetics graduate. You can download the full copy here.

Soil composition: What is soil made of?

The composition of soil can vary, but the basic components are air, minerals, water and a small amount of organic matter, all of which can foster a diverse microorganism community. The mineral components of soil are the sand, clay and silt particles contained within it.

The different chemical compositions of these components can impact the retention of nutrients in the soil and thus their availability to a plant. This is often referred to as the Cation Exchange Capacity (CEC) of the soil and is a way to quantify the fertility of the soil.

Clay and organic matter (sometimes called humus) are two common components that have a net negative charge and so can attract and hold positively charged ions, which includes most nutrient ions. This increases their availability to the plant within the soil, as they won’t be leached out and will assist the soil’s natural pH buffering ability.

Understanding soil pH for successful plant growth 

The pH (potential Hydrogen) of your soil plays a pivotal role in making these elements available to your plants. 

Soil’s ability to minimize shifts in its pH stabilizes the solubility of nutrient ions - if the pH is within the solubility range of those ions. However, different soils have different pH values due to their composition and this can impact the solubility of some nutrient ions.

Plants have adapted to differing conditions, but species unaccustomed to certain conditions struggle and have nutrient deficiencies. Blueberries and pine trees, for instance, grow best at a low pH and have no nutrient deficiencies.

Each species has a preferred pH range. It is thus advisable to measure the pH of your soil before planting to determine which plants will grow optimally. Conversely, if your crops aren’t growing well, measuring your soil pH may uncover an underlying factor that you can treat.

Soil moisture content (MC): When to water

The moisture content of the soil is the amount of water present in the soil and thus available to the plant. Measuring this ensures that the plant roots are sufficiently hydrated and not becoming waterlogged. This is particularly important as the physical properties of different growth media retain water differently and can cause issues if not monitored.

Measuring the moisture content of your soil will help you to decide how frequently your crop requires irrigation and feeding.

Soil EC (nutrient levels): When to feed or flush

An EC reading measures the level of nutrients that are available to the plant. It's a general measure, and can’t tell you whether you have a balanced nutrient composition. 

If you’re using organic nutrients, you may think you can’t quantify the available nutrient or nutrient levels using solution conductivity – because you may often get a low reading.

However, a low reading does mean a low level of nutrients available to the plants. It may be that the organic nutrients haven’t yet been broken down into an available form or that they have already been taken up by the plants.

How to test soil pH, MC (moisture content), and EC (electrical conductivity) of soil

Many methods are available to measure soil, like pour-through and extraction methods, but here at Bluelab, we advocate for measuring your parameters directly to save time, like Kawahara Nurseries in California. The team measures pH and EC to ensure premium-grade annuals, perennials, and edibles for sale. 

• Bluelab Soil pH Pen: Handheld device to measure the pH and temperature of your soil.
• Bluelab Multimedia pH Meter: Handheld device that comes with a multimedia pH probe to measure the pH of solution, soil or any growing media.
• Bluelab Pulse EC/MC Meter: Handheld device that measures moisture, EC and temperature directly in the root zone.
  
  

Top tip for measuring soil moisture, EC, and pH

Don’t take measurements from the surface of the soil. Most plant roots are lower. The pH at the soil surface may also be affected by accumulated salts, which may affect measurements.

Measure close to the root zone and at the same depth. This will give you a more accurate picture of the growing environment.

The role of microorganisms in soil

Organic nutrients, like compost, are a degradable matter that can be broken down. They come from a source that was once alive. They are added to soil to provide slow-release nutrients. Decomposition is essential as the primary particles are too large to absorb. They need to be broken down by microorganisms into a readily available ionic form that plants can recognize and take up. The microorganisms consume the carbon present in the organic matter and then release the essential nutrients for the plant.

Just as in hydroponics, it is the soil water that carries the dissolved nutrients to the plant root zone. However, instead of readily available nutrients being directly provided in the irrigation water, the nutrients are released from decomposing matter such as compost or converted from unavailable chemical forms through microorganism activity.

These microorganisms play an essential role in maintaining and improving soil fertility as well as assisting plant growth. An example is the process of nitrification, where atmospheric N2 is converted into ammonium ions (NH4+) by bacteria, making this essential nutrient available to the plant.

Microorganisms are always present in the environment and so will also be present in the growing environment. Microorganism communities will become established in any growth media nutrient solutions and the root zone and roots of cultivated plants. These organisms are very sensitive and any change to their environments, such as pH, the addition of fertilizers and water levels, can impair their performance.

"Take good care of your soil and the organisms within it and you’ll create a symbiotic ecosystem that can be relatively self-sufficient and requires minimal added nutrients."

Dustin P. Meador, Ph.D. Executive Director of the Center for Applied Horticultural Research

Industrial malpractice and careless use of fertilizers have damaged much arable land, exhausting its essential nutrients and depleting the microorganism community. Where the natural soil ecosystem has been severely damaged, it must be built up again and healed before crops can obtain their essential nutrients.

It’s also worth noting that many decomposable fertilizers derived from organisms at the top of the food chain - such as animal manure, guano and fish emulsion - and even minerals like rock phosphates can contain accumulations of heavy metals.

Repeated applications will cause these to build up over time. Heavy metals are usually toxic and, when present, can be absorbed by plants and found in their produce.

Microorganisms commonly present in soil

Microbes can colonize every part of the growing environment, from the substrate to the nutrient solution. They play important roles in breaking down organic matter to release nutrients, fertilizing the soil, recycling nutrients, assisting nutrient uptake and disease prevention. Some microbes can be pathogenic, but completely sterilizing your growth environment can do more harm than good.

Aim to maintain a balanced microbial environment.

• Algae: Increase soil fertility by becoming organic matter when they die. Help increase water retention in the soil. Carry out photosynthesis and release oxygen underground, thus aerating the soil.
• Fungi: There are many different species of fungi. Some form a mutualistic relationship with plants and attach to their roots to protect them and assist in nutrient uptake. Here, they receive carbon (their food source) from the plant and, in return, transport nutrients to the plant. Some fungi decompose organic matter into acids, carbon dioxide, and fungal biomass.
• Actinomycetes: A group of bacteria that play a part in the biological control of insect pests and disease.
• Protozoa: Play an important part in soil fertility.
• Bacteria: Decompose organic matter and release nutrients by processing them from unavailable to available forms. Many different varieties of bacteria use the carbon content of the soil as an anchor and food source.

• Nematodes: Some nematodes (small worm-like animals) are parasitic and feed on algae, plant roots, fungi, bacteria, and other nematodes. By digesting their food source, they assist and accelerate the decomposition of organic matter and the release of nutrients.
  
  
  

Some species of microbes can produce a variety of substances that promote and protect plant growth, including hormones, auxins, gibberellins, and antibiotics.

Hydroponic systems using soil-less media have also been found to contain the same microbes as soil and assist plant growth and control disease.

Wrapping up

There is a lot to learn about soil as a growing medium, and the nuances of your own plants and crops will determine your success.

Once you start to understand soil composition and the role of pH, moisture, and EC, you’ll be well on your way to growing better plants in soil.

Still curious? Here’s some further reading:

Comparison of Land, Water, and Energy Requirements of Lettuce Grown Using Hydroponic vs. Conventional Agricultural Methods. (National Library of Medicine, US Government.)

The microbiology of soil and of nutrient cycling. (Ohio State University.)