A Beginners Guide to Soil Fertility

In order to grow, plants extract nutrients from the earth. Unless these nutrients are regularly replenished, your topsoil will gradually lose its fertility and your plants will struggle to grow.

In this guide we discuss the importance of soil fertility, what to look for in your soil, and how to improve your soil in ways that have a minimal negative impact on the environment.

Recommended: Read our 5 step guide to starting a CSA written by a successful startup CSA owner.

Soil Fertility in Organic Farming

When using the word organic, we are essentially talking about soils. In conventional farming systems, synthetic, water soluble fertilizers are used to feed plants with nutrients. But in organic systems, properly managed soil can feed the plants by itself.

It is very important to be mindful of your soil’s health in terms of nutrient availability, soil structure, pH, and so on. Otherwise, you’ll end up with poor yields and your CSA will struggle to stay in business.

Factors that Determine Soil Health

There are several factors to look for when testing your soil fertility:

Soil structure (tilth)

The tilth of soil refers to the soil’s physical condition. This is indicated by the following:

• How easy it is to till
• Whether or not seeds can easily germinate
• How easy it is for roots to penetrate the soil

The quality of soil tilth can be increased over time with proper amendments and soil management strategies.

Recently, more farmers are turning to a no-till approach for managing soils. In no-till production, the goal is to reduce the amount of soil disturbance. This will be expanded upon later on.

Soil pH

This refers to the levels of acidity and alkalinity found in soil. This is measured in pH units that range from 0 to 14. Soil pH ranging from 7-0 are increasingly more acidic, and from 7-14 are increasingly more alkaline. The soil’s pH level affects its ability to make nutrients available to plants, and so it must be carefully monitored. Some plants, such as blueberries, prefer more acidic soils (a lower pH), while certain plants do better in more alkaline soils (a higher pH). Generally speaking, a pH of around 6.5-7 is perfect for most plants.

If plants are showing signs of nutrient deficiencies, this may be the result of a high or low pH level. Soil pH can be easily changed using organic amendments, such as mulch, compost, manure, or cover crops.

Nutrient availability

When it comes to plants, there are three essential nutrients:

• Nitrogen
• Potassium
• Phosphorous

Three secondary nutrients:

• Calcium
• Magnesium
• Sulfur

And 6 micro-nutrients:

• Iron
• Manganese
• Zinc
• Boron
• Molybdenum
• Copper

In organic systems, micro-organisms break down amendments such as compost or cover crops and make these various nutrients available for plants to uptake. Therefore, there is a saying in organic agriculture: “Feed the soil, not the plants.”

When we say feed the soil, we’re really talking about feeding the micro-organisms in the soil. These organisms make up the ”organic matter” of the soil. Organic matter refers to the percentage of soil which is living organisms. You want as much organic matter in your soil as possible for good soil health. The percentage of organic matter in your soil can be increased by means of proper amendments, and by reducing soil disturbance with a no-till or minimal tillage approach.

Soil Testing

In order to really know what's going on below the surface, you should conduct soil tests regularly, at least once or twice a year. Soil samples can be sent to the local ag-extension office, and you should receive your results within a couple weeks. These results will show where you have nutrient deficiencies or surpluses in your soil.

To test your soil, take a sample from 6 inches below the soil surface using a spade or a soil probe. In order to get accurate test results, take multiple samples from the area you are testing then mix them together. You can then put the mix into a plastic bag and send it to the lab at the local ag-extension office. Be sure to talk to the lab about their requirements and follow their instructions on how to properly collect the soil samples. For instance, some labs require that the soil be dry. Also, if you have compost on top of a bed, avoid mixing the compost into the soil sample.

You can also test your compost separately, as this will let you know how your compost will affect your soil. When sending compost to the lab, let them know that it is compost and not soil.

When you receive your soil and/or compost test results, the lab will give guidance on how to properly amend your soil.

Mason Jar Soil Test

If you want a quicker, simpler soil test to do at home, you can try a mason jar soil test to determine what type of soil you're working with. Knowing specifically what your soil is made of will help you learn the best ways to work with that soil, or amend it.

This test will help you figure out what percentages of clay, silt, and sand are in your soil. The ideal percentages are:

• 20% clay
• 40% silt
• 40% sand

If you have a large area where you want to plant, you may want to take samples from several areas across your property. 

To start, you just need a shovel or hand spade, a large mason jar with a tight-fitting lid, and some water. That's it!

1. Fill your mason jar halfway with the soil you want to test
2. Fill most of the rest of the jar with water, but leave an inch of air at the top, so the soil can be shaken up
3. Secure the lid of the mason jar tightly. Shake the jar for a couple of minutes
4. Place the jar somewhere it won't be disturbed for 24 hours so the layers can settle
5. After at least 24 hours, the particles will have settled into three layers, with clay on top, silt in the middle, and sand on the bottom. Measure the depth of each layer in inches and then measure the total depth of all the soil
6. Divide the depth of each separate lyer by the total depth. Then, multiply by 100 to get each percentage

You'll know if you want to amend your soil if your percentages aren't within 10% of the "ideal" ratio. 

Sandy soils will need some more organic matter added to it, like compost. Clay heavy soils are often seen as "bad" but some plants do well in clay because of how it retains moisture and nutrients. Too much moisture may lead to root rot, which is why you might want to add organic matter and/or earthworms to your soil if you fear there's too much clay for what you want to plant. 

Organic Soil Amendments

There are a number of things you can add to your soil to enhance nutrient content and improve soil tilth.

Mulch

Mulch is used to suppress weeds and can be made from of a number of things, including compost. Some common types of mulch used in organic farming include straw, hay, and wood chips.

The benefit of using compost as mulch is that it adds a lot of readily available nutrients to your soil and can be tilled in without causing nitrogen leaching.

Compost

Compost is decayed organic matter that is used as plant fertilizer. It can be made from plant matter, manure, wood chips, and more.

Manure

Manure can be used to amend soil, but it needs to break down before being directly applied to plants. Wet manure can carry pathogens which cause bacterial disease to crops, and this is another reason to either compost the manure first or add it to the soil months before you intend on growing in the area being amended.

Some great animal manures that are commonly used in organic farming include chicken, horse, sheep and cow manure.

Cover Crops

Using cover crops is a great way to add organic matter to your soil while also improving tilth and reducing input costs. Cover crops, or green manures, are plants which are grown for the purpose of being recycled into the soil as an amendment. Some other benefits of using cover crops is that they protect the soil from solar, wind and water erosion and they choke out weeds.

There are many different varieties of cover crops, and your choice will depend on the intended use. Some cover crops fix nitrogen in the soil, while others are especially effective at combating weeds.