Healthy Soils - Basics

Soils 101 - take the fun soils test

What are the benefits of soil organic matter?

Where can I send my soil samples for analysis?

What do my soil test results mean?

Take the soils quiz

6 (+1) fun questions to test your basic soil knowledge

What is Soil?

Soil is the loosely arranged mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants. (From USDA. Want more? See definition of soil)

Is my soil healthy?

Currently, there are no agreed criteria for what makes a “healthy” soil. Rather, (virtually) every soil will benefit as we increase soil organic matter (SOM), enhance aggregation and soil aeration. As SOM increases we will likely get healthier soil microflora (good bacteria, fungi and microbes) and fauna (especially worms).

Healthy soils - an overview? Web (USDA)

Glossary of soil terms Web (USDA)

Is my soil stable?

Soil stability test video (NRCS) (1:13)

Healthy soils

A healthy Soil will

provide the nutrients needed (which involves its Cation Exchange Capacity (CEC)) (From Advise Labs)
have a good pH (often given as 5.5-7.0)
have active soil flora and fauna
hold water that is available (from Noble RI)
are free of contaminants (Read more at the Soil Science Society of America)
have good soil structure which helps with greater water infiltration and good aeration for healthy root growth (video from USDA) ,
provide protection against erosion. In addition to good soil coverage by mulch or compost, good structure with increased infiltration (see above) helps.,
are free of soil chemical and physical barriers,
are free from crusting, (From Corangamite)
provide a medium for good crop emergence and plant growth, etc..

What about potting soil versus garden soil?

Potting soil refers to a manufactured product; consisting of variable amounts of materials such as peat, composted bark, sand, perlite or recycled mushroom compost - although many other ingredients can be used and the proportions vary hugely. Many mixes have their pH adjusted and some may contain small amounts of fertilizer. Despite the name, little or no actual soil is used in potting soil - in large part as it is considered too heavy (or dense) for growing many houseplants

Take an on-line soils course

Healthy Soils (UC SAREP & San Luis Obispo)

Additional reading

What is a healthy soil (SAREP)

Visit Soil Doctor (Bell): practical tips to help diagnose, build and protect your soil.

What are the benefits of soil organic matter (SOM)?

General benefits

Increased Soil Organic Matter (SOM) can bring lots of benefits to soils including enhanced nutrient and water retention, enhanced micronutrient availability, better soil structure, increased moisture infiltration and increased microbial activities, etc.. We quantify below some of the more widely cited (and sometimes mis-cited) approximations.

Cation Exchange Capacity (CEC)

CEC is a measure of how many cations - positive ion elements like Calcium and Magnesium - can be retained on soil particle surfaces.
Each 1% Soil Organic Matter increases CEC by 250 to 400 meq/100 g (Moore 1998)
Check this CEC calculator to see the (approximate) CEC for your soil based on texture and Soil Organic Matter.
What does it mean? An increase of 1 meq/100 g would mean an increase for each of the following of about
- Calcium (Ca⁺⁺) - 400 lb/acre, or
- Magnesium (Mg⁺⁺) - 240 lb/acre, or
- Potassium (K⁺) 780 lb/acre, or
- Ammonium (NH₄⁺) - 360 lb/acre
- (Data from Fundamentals of Soil Cation Exchange)

Water holding capacity

A 1% increase in Soil Organic Matter increases soil moisture retention by 3-5% (Sources FAO and U Sydney)
This number seems impressive when quoted in gallons, namely: around 7,000-12,000 gallons per acre (some figures quote up to 20,000 gallons per acre)
While obviously helpful, the increase is very approximately somewhere between 0.3-0.4 acre-inches. Note: This is retention. So not all the moisture would be available to the plant (maybe around half would be available water).

What's the but?

The "But" is that while increasing Soil Organic Matter is a highly desirable objective, it can be very difficult to achieve.
While you can import compost, manures, etc. for small scale gardens, increasing SOM at a commercial scale can be tough.
There is around 2 million pounds of soil in a typical plough layer (about 7"), (where most of the SOM will accumulate). That means a 1% increase in SOM requires (about) 20,000 lbs of new SOM.
Many factors (temperature, moisture, soil texture, C:N rations, microbial populations etc.) affect conversion of residues and compost to SOM. However, as a very gross estimate, conversion of green manures, residues, etc. to SOM runs somewhere from 10-20% (Source SARE).
As such, to increase SOM by 1% (around 20,000 lbs SOM), you need to add somewhere between 100 to 200 tons per acre of residue.

Challenges and reality of increasing SOM (paper - Penn State)

What’s my California soil?