Healthy Soils - Basics


Soil Basics & diagnosis

On this page

Is my soil healthy?

Quantifying the benefits of Soil Organic Matter

What’s my soil?

Sampling soils

Soil labs - example laboratories where you can send your samples for analysis

Interpreting soil test results

Healthy Soils

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)

In summary, a healthy soil will increasingly:

  • provide the nutrients needed (which involves its Cation Exchange Capacity (CEC)),
  • have a good pH
  • have active soil flora and fauna,
  • hold water and makes it available,
  • be free of contaminants,
  • have good soil structure for greater water infiltration and good aeration for healthy root growth (See video USDA) ,
  • provide protection against erosion,
  • be free of soil chemical and physical barriers,
  • be free from crusting,
  • provide a medium for good crop emergence and plant growth, etc..

Take the course

Healthy Soils (UC SAREP & San Luis Obispo)

Additional reading

What is a healthy soil (SAREP)

Learn about soil chemistry and soil physics through Soil Doctor (Bell)

This site provides practical tips to help diagnose, build and protect your soil. 

Quantifying benefits of Soil Organic Matter (SOM)

SOM soil soil organic matter

General benefits

Increased Soil Organic Matter (SOM) can bring lots of benefits to soils including enhanced nutrient and water retention, enhanced micronutrient availability and better soil structure, increased moisture infiltration and increased microbial activities, etc.. We quantify here some of the more widely cited (and 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% SOM increases CEC by 250 to 400 meq/100 g (Moore 1998)
  • Check this CEC calculator to see the (approximate) CEC for your soil and the effect of changing SOM.
  • 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 this Article)

Water holding capacity

  • 1% SOM 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 would be 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?

  • While you can import compost, manures, etc. for small scale gardens, increasing SOM at a commercial a scale can be tough.
  • There is around 2 million pounds of soil in a typical plough layer, (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%, you need to add somewhere between 100 to 200 tons per acre of residue.

See this paper for a nice simple example of the challenges and reality of increasing SOM.

What’s my soil

soil web map

Soil web (T O’Geen)

A remarkable map resource showing soil types throughout California with detailed information on soil characteristics and potential uses.

Sampling and Monitoring How do I sample and monitor my soil? (where to find a lab, how to sample soils, how to interpret a soils test) See SAREP web

Soil sampling

Soil sampling Fact sheet (D. Geisseler & W.R. Horwath)

Soil contaminants and when to check for them Fact Sheet


Sampling Orchard soils (Doll & Sanden)

Video 1: FAQs: Sampling orchards: Who, Where and When (4:46)

Video 2: Soil Sampling Equipment (5:21)

Video 3: Taking Samples in the Orchard (6:55)

Video 4: Preparing your Samples for the Lab (3:35)


Soil sampling for nitrate

Sampling for nitrate Fact sheet  (D. Geisseler & W.R. Horwath)

Quick Field Test Video (Lundy)


Soil sampling for pests and diseases

See UC Integrated Pest Management (UC IPM) Find the specific disease and look for “field evaluation”

Where can I send my soil sample?

Soil Laboratories

(list from CDFA + UCMG) (Needs to be checked)

What do my soil test results mean?

Interpreting soils data

(UC ANR Small farms)