• The availability of essential plant nutrients
• The activity of microorganisms in the soil
• The solubility of some phytotoxic elements. For example, aluminium, iron, and, manganese have greater solubility at pH values below 5.5. Sodium levels may become excessive when pH is above 8.5.

Soil moisture and organic matter also affect a plant's ability to take up nutrients from the soil. Before nutrients can be used by plants, they must be dissolved in the soil solution. Soil pH is an indication of the acidity or alkalinity of the soil and is measured in pH units. The pH scales goes from 0-14 with pH 7 as the neutral point. Soil pH can be altered through the application of elemental sulfur or lime (calcium carbonate). Application of lime or elemental sulfur is not an instant fix for soil pH. Depending upon the soil properties, it make take months or even a year to measurable changes in pH to occur after an amendment has been added to the soil.

Soil pH and Interpretation

Best Range for Most Crops

Acidic, neutral, or alkaline (basic) soil is determined by the concentration of hydroxyl ions OH- and hydrogen ions H+ in the soil. Neutral soil has an equal combination of both hydroxyl and hydrogen ions; alkaline soil has more hydroxyl ions; and acidic soil has more hydrogen ions. From pH 7 to 0, the soil is increasingly more acidic, and from pH7 to 14, the soil is increasing more alkaline. pH stands for "power of hydrogen". Since the pH scale is logarithmic, one unit of change represents a tenfold change in the pH. This means that a soil pH of 6 has 10 times more hydrogen ions that a soil with a pH of 7, and a soil with a pH of 5 has 100 times more hydrogen ions that a soil with a pH of 7.

Biological processes such as nitrogen fixation and decomposition of dead plant matter is optimal when the pH is close to neutral. The availability of nitrogen, phosphorus, and potassium is best when the soil pH is between 6.5 and 7.0.

Acid soils may be found in areas with sandy soils, high rainfall, and high levels of organic material. The weathering of granite and sandstone rocks also results in acidic soils. Limestone may be added to raise the pH of acidic soils. It does this by removing hydrogen ions from the soil.

Alkaline soils may be a result of arid conditions, soils high in sodium or parent rock material that is rich in calcium carbonate (CaCO3). Elemental sulfur may be added to alkaline soils to lower the pH. Elemental sulfur forms sulfuric acid (H2SO4) when it reacts with oxygen and water in the presence of soil microbes. This lowers the pH." (Source: Learn About Ag)

Soil pH Neutral

(Source: Learn About Ag)

Long Term Soil Enrichment Project: at Home or School

Primary Middle Secondary

Australian Curriculum General Capability: Personal and social capability

Australian Curriculum General Capability: Numeracy

Australian Curriculum Cross Curriculum Priorities: Sustainability Priority

1. You are going to create a "No Dig" Garden and measure the results!

Firstly, you are to select two patches where you are going to create two gardens. Ideally these gardens would be side by side. They are to be identical except for the soil enrichment you will carry out on one garden.

The Hypothesis is that with enrichment the soil characteristics will change and the resultant plant growth will be different (if all other key markers remains the same).

2. Test the soil from both gardens. They should be both the same initially. Record the results and the date.

• pH (you will need to get litmus paper from Science Teachers and record the pH from 0 - 14)
• Look at the friability of the soil - is it full of compost/humus? Clay? Sand? What does it smell like? How much water is in the soil? (do the squeeze test - dripping, wet or dry)
• How quickly does a cup of water go through this soil - measure the time it takes and record the result.
• Depth of soil (humus layer or dark layer) in cms.

3. In the second bed/garden, you are to create a no dig garden. Here are the instructions from Gardening Australia:

"Just like compost, you need a good amount of dry materials -like straw, Lucerne and dried leaves and thinner layers of high nutrient green leafy weeds and manure.

• [Costa] starts with rock dust straight on top of his sandy soil. This helps to hang onto any nutrients that leach out of the bed.
• Cover with layers of newspaper, at least 10 pages thick to smother the weeds and wet down
• A layer of grass clippings goes over that - this is full of nitrogen.
• Top this with a thick layer of dry leaves about 200mm deep or whatever you can rake up in your street [or garden]!
• Water in with 1 tbsp of molasses dissolved into a 9L watering can. The molasses feeds the microbes that will help to break all of the materials down.
• Add 150mm-200mm layer of Lucerne.
• Sprinkle with a layer of chicken manure and water in with diluted molasses.
• Continue to add layers of Lucerne and Manure (cow or chicken) 3-4 times to reach desired height (The pile will sink by about 20% as it breaks down).
• Add a loose layer of mulching straw to 150mm.
• Top with 100mm of home-made compost.
• Mulch the bed with straw."

No dig garden lasagna! (Source: Blackfly Gazette)

4. Plant in seedlings of pea or beans. Each week measure the growth of the peas or beans in each garden. Graph the results for 10 weeks.

5. Re-measure the soil - pH, friability, and depth.

Which garden bed produced the best results regards: plant growth?

If you want even better results, you should try using "worm juice" from a worm farm each week.

6. Describe how you felt about your "no dig" garden. Discuss with a partner.

Soils in Schools - Teacher Resources

Primary Middle  Secondary Teacher

Australian Curriculum General Capability: Personal and social capability

Australian Curriculum General Capability: Numeracy

Australian Curriculum Cross Curriculum Priorities: Sustainability Priority

Teacher

Look at Soil Science Australia Teacher Guides [need to scroll down]

Local copy: Soil Science Careers - Years 7 - 10 - PDF

Local copy: Soil in food chains: Year 7 Science - PDF

Local copy: Water Contamination from run-off: Year 7 Science - PDF

Local copy: Soil in the urban environment: Years 10 - 11 Chemistry - PDF