Mid North Coast Microherders

Caring for the little things in our soils

Kempsey Landcare David Hardwick Soils Workshop - part 2: Chemistry of soil. 3 July 2015

If I'm a commercial farmer, I want to be making money. Just saying I've got a good level of soil biological fertility isn't enough.

You can't buy soil structure in a bottle. You have to manage soil to get good structure, managing things like grazing, biological activity, planting of deep-rooted pasture like daikon radish.

If you want nutrients to cycle you have to have organic matter: some as humus, some cycling.

A lot of carbon breaks down and does up into the air as CO2. If the carbon is underground, it is more likely to be broken down and stay in the soil.

A good vigorous root system is the key.

Q. What is a good level of organic matter to have in your soil? Q. Can a sandy soil have as much organic matter as a clay soil?  No. Clay can have about 5%; sandy less.

Photosynthesis meets rocks. Combine to give top soil.

Rocks: made up of minerals.  Rocks weather to break down - rain, cracking, frost, root action,...

Depending on what type of rock you start with you end up with different chemicals (different proportions of different chemicals).

Rock breaks down to sand, silt and clay particle sizes. The parent rocks breaks down to different textures and different proportions of sand: silt: clay.

Organic matter modifies it a little.

Most texture comes from the parent rock.

Texture affects: water infiltration, tillage, ...................................

Different minerals have different levels of nutrients.

Generally heavy clay soils have higher fertility and beach sand low fertility.

Soil testing is not 100% accurate science.

Some nutrients are there but locked up. Some are soluable and available. It's an estimate.

Look at estimated total phosphorus (in Soil Analysis readout).

Q. Is the phosphorus in the Total Nutrient Levels in soil available to the plant? Q. What enterprise do I want to run? Q. Is this soil suitable for this type of enterprise? Q. What is the soil fertiliser history?

The Lockyer Valley soil profile will be mostly the rock type is started from. (Lockyer Valley has very good soil).

Say I'm out to buy a truck for the farm. Q. What are the types of questions I'd ask? Q. What capacity do I need? "Engine capacity" Q. What condition is it in?

Look at Total Aluminium, and Total Iron in the Soil Analysis readout. It is natural for them to be high.

Young basalts are usually quite high fertility.

Iron, O2, aluminium and silicon are the highest occurring elements on the planet.

Clays are made up of Aluminium.  Most of the Al is locked up.

Zinc - a trace element; quite good in granite soil. Sulphur - also proportionately more.

Sometimes the heavier soils can be proportionately low in sulphur because sheet basalts have a lot of air exposure and the sulphur volatilizes into the air.

Clay has a particle size of less that 2 microns.

Clay is important because it is a colloid.  Examples of colloids are milk, silver colloid, aerosols. Colloids are tiny particles homogenously suspended throughout a medium of either water or air.

Clay has an electrical charge - a negative charge. This is very important for soil.

Clay has a massive surface area for the size of the particle it is, and it is a charged surface, so positively charged elements are attracted to it.

Clay contributes to the colloids in the soil.

Most humus is also a negatively charged colloid (some can be +vely charged).

Clay and humus give soil its charge that attracts nutrients. The positively charged nutrients are called cations (cat - ions).  

The CEC - Cation Exchange Capacity - of the soil a measure of how much of this positive nutrients to negative colloids is going on in the soil.

The other thing that influences the size of any charge is the ph. As the ph goes up (in number towards the alkaline end of the scale), the charge can increase. Acid ph (lower end of the scale) reduces charge.

Some soil types have almost no charge - just to complicate things!

Over a ph of 7, ph doesn't influence charge any more.

You can have an alkaline soil with high charge.

In sandy soils, the charge mostly comes from humus. In clay soil, the charge mostly comes from clay.

On the clay colloid, as minerals break out of the total, the CEC is the active area of the mineral exchange in the soil.

**Most nutrients are locked up (calcium, phosphorus, nitrogen, ..), but the high charged stuff is what is coming and going and available.

How much calcium I've got often depends on what weathered out of the rock.

Positively charged nutrients include:

calcium * magnesium * potassium * aluminium * hydrogen * sodium * zinc manganese = dominate in the soil. The positively charged nutrients like to sit on the highly negatively charged clay/humus. This total collection of stuff is called a cation.

**The minerals in the cation are the very active soil zone and they are available to the plant.

Clay is the most weathered you can get.

On a garden level, you can buy in clay to add to your soil. On an agricultural level it would break the bank. ie. Clay is valuable!

Around Kempsey, the soils are mostly quite young alluvial soils broken down from the basalts in the ranges behind. Quite nutritious.

If one soil particle has a high CEC, and another soil particle has a low CEC, each can have the same proportion of different minerals attached to it, but the total quantity of minerals attached will be higher on the high CEC particle.

Defn: Acidity means you have hydrogen on the colloid. Q. Is acidity good or bad? A. Depends on what you are growing.  Sugar copes with acidity. Fruit trees not keen on it.

Sodicity - sodium comes from the parent rock. Sodic soil not good.  In sodic soil, structure collapses, nothing grows. There is high exchangeable sodium.  Reduce the sodium by adding gypsum. Sodic soils: Best thing is DO NOT DISTURB. Get the organic matter up. Sugar can can grow in soils with up to 30% sodium soil.

Exchangeable aluminium is related to acidity. It can be locked back up by adding lime, or dolomite. Exchangeable aluminium is only toxic if it is more than 10%.  Sugar can can cope with up to 50%. Tea copes with up to 80%!  Most aluminium is locked up, not exchangeable.

**In tropical soil, calcium gets leached out. End up with a lot of hydrogen, making the soils go acid. Adding lime (CaCO2 - calcium carbonate) sorts this out, by the Ca reacting with the hydrogen H in the colloid, lowering the number of hydrogen molecules attaching to the colloid, thus raising the ph (becomes more alkaline), and also adding calcium Ca to the colloid.

Q. Does liming last forever? A. No. Soil reverts to its "natural" balance, revert to type they came from the parent rock.  We can balance soils, but we need to keep at it.

Organic matter helps to balance soils too.

Dolomite contains Ca too, and also some magnesium Mg.

The parent rock plus the farming history go together to make a unique soil profile.

Organic matter - quantity and quality. Q. Is it green or brown? Green breaks down quicker - it has more nitrogen N in it. It is more digestible. Wood chips (brown) - a lot will slow the soil nutrient cycling right down (not desirable - want soil nutrient cycling to be happily chugging along).

Soil needs a balance with protein.

C: N - Carbon: Nitrogen ratio needs to be right. In a compost heap aim for C: N of 25 to 30:1 Q. What does this mean in terms of brown stuff: green stuff?

Need enough nitrogen but not too much or it goes too fast. The carbon (organic matter) will break down really quickly and go off up into the air as CO2.

Soils 'hit the wall' with constant too high nitrogen once they run out of carbon.

Salinity measures the salt in the soil, not the sodium on the clay. There are various salts (chloride, and some other types). Too much salt dries plants out because they can't take in salts.

Q. Do you want a "12 tonne truck"  (total capacity) with a "V8 engine" (carrying capacity CEC) or something else? No point deciding how much fuel to put in the tank until you've bought the right vehicle first!

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