How does compost improve soil fertility?

The organic matter in compost is negatively charged. When we add the compost to the soil, it will increase the soil capacity to hold positive ions. Many positive ions in the soil are macro-nutrients that plants need, such as calcium (Ca2+), magnesium (Mg2+), sodium (Na+) and potassium K(+) and nitrogen in ammonium form (NH4+).

The capacity of holding positive ions is called CEC
The organic matter in the compost is negatively charged so it holds positive ions.

In soil science, we use Cation Exchange Capacity (CEC) to measure the soil ability to hold positively charged ions :

  1. CEC means the fertility of your soil. The unit is usually expressed in meq/100g. The bigger, the greater.

  2. Sandy soil has the lowest CEC, which suggests low fertility. To increase the CEC, you can add clay and compost full of organic matter to the soil.

  3. Increasing 2~5% of organic matter in your soil will largely increase soil CEC.

Soils with a low CEC are more likely to develop deficiencies in potassium (K+) and magnesium (Mg2+). Plants who lack potassium will often wilt even being watered since potassium regulates the stomata. Magnesium is the center of the chlorophyll molecule. When plants have magnesium deficiency, chlorophyll can't be made and photosynthesis stops. The symptoms are that leaves start to lose their green color in between the leaf veins while the veins remain green. Usually older leaves show the sign first.

Magnesium and potassium are natively abundant elements on earth. When your plants show a deficiency, it means the CEC in your soil is wrong. Remember, three things will affect soil CEC :

1. Soil pH

CEC changes with soil pH. When pH is at 3.5 to 4.0, CEC will be the lowest. CEC will increase as soil pH increases, so it's a common practice to measure CEC at a pH of 7.0 at lab. When your soil pH is low, it will have other issues with nutrient absorption.

Compost will buffer soil pH, keeping it in a good range. When you apply ammonium (NH4+) to the soil, bacteria convert it to nitrate, the process will lose 3 Hydrogen cations (H+) and make the soil more acidic, lowering the pH. The organic matter in the soil will absorb the H+ and thus prevent the pH from getting too low. This is how organic matter buffers the pH.

The humus in the compost is negatively charged so it can hold positive ions like potassium, hydrogen, calcium, magnesium and sodium
Compost buffer the soil pH by absorbing the Hydrogen cations

2. Soil organic matter content

Organic matter is negatively charged so it will hold more cations onto the surface, thus increasing CEC. The organic way to increase CEC is adding compost. This process takes a long time but reliably improves the soil CEC.

3. Soil type

Clay and silt particles are negatively charged so they absorb cations. Sand has 3~5 meq/100g yet clay loams have 20~50meq/100g.

To find out CEC in your soil, having a soil test is the best way. If you have a CEC lower than 10 meq/100g, it's too low. Amend the soil with clay or clay loam is recommended.

Soil Texture Typical CEC
Light-Colored Sand 3-5
Loam 10-15
Silt Loams 15-25
Clay and Clay Loams 20-50
Organic Soil 50-100

Source: Ohio State University Soil Fertility Lab, 2019

It's important to have a steady supply of compost together with rich organic matter. The compost will increase the soil fertility by improving the important soil characteristic- CEC. To have compost, I found three practical ways, they both have pros and cons.

1) Buy compost from a farm

Pros: Usually not expensive, ready to use.

Cons: You don't know the ingredients and if it's pathogen-free? In the production process, compost has to reach 75 degrees Celsius to kill the pathogen or weed seeds. If you don't know the ingredients and how it was made, you are risking bringing pathogens and weeds to your garden.

2) Make it yourself

Pros: You have control of the ingredients and process, cost is low

Cons: Take lots of time and energy and there is a learning curve, might be bitter.

3) Use iComposteur

Cons:  You will have a set up cost of €599 but your purchase is Risk-Free. 

Pros: Compost will be ready in 14 days,  reduce your food scraps up to 40%, success guarantee !