Soil and water terms.

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Acidity, exchangeable - amount of acidity which has to be neutralised to raise the pH a given amount. Sometimes used to estimate how much lime to apply to raise pH.

Alkalinity - measure of bicarbonates under pH 8.3 & includes carbonates, hydroxides over pH 8.3.

Bicarbonates - a measure of alkalinity for water if pH is under 8.3. Sometimes called the buffering capacity of water. By my reckoning this is the least understood but one of the most important aspects of water. I've just written a PhD thesis on this so I'll give y'all a quick rundown on what's important and what's not. And remember to say you saw it here first. Just say some water quickly lost Carbon dioxide (like bore water when it reaches the surface). The pH drops because H+ ions dissappear to counteract the loss of CO2. Why? because the equilibrium equation

[H+][HCO3-] / [H2CO3 (aqueous)]

has to stay balanced. What happens to the H+? It came from water molecules when the carbon dioxide was dissociated so goes back there. What happens to the bicarbonate? Some goes back with the H+ to form some replacement CO2. Some recombines with Calcium ions which we assume are present in the water to form Calcium carbonate. I haven't seen it but some people say that when CO2 gets used up quickly when there are a lot of algae photosynthesizing in the water, some whiteish deposits of CaCO3 can be seen. Where do the Calcium ions come from? Well when Carbon dioxide dissociates it forms some H+. This dissolves some Calcium carbonate leaving free Ca2+ ions in the water. There are a couple of interesting implications of this. The so called CO2 - bicarbonate equilibrium (equation above) can get out of equilibrium! How? When more CO2 is added or taken away than can be compensated quickly. Or when some acid is added or maybe the water is in an area where there is limestone and it is continually using up H+ ions. This is where you can be a water sleuth.

Say you are investigating the ecology of a dam or creek (we are in Australia remember). By doing some sneaky calculations it is possible to work out if the water is suffering from too much or too little CO2 or H+. Then you can look for reasons. Perhaps it is obvious eg algae are using up CO2 during the day and producing excess at night. The pH will go up and down over 24 hrs. For those interested you can check out my program which does these calculations for you and helps find the causes. Email me for latest details if you are interested.

Bulk density - gms of dry soil per cubic cm. If we want to be able to say how much of a particular nutrient is available on a per hectare (or acre) basis then we need to know the bulk density so we can do the proper calculations.

Cyanobacteria - little critters known to cause blue-green coloured blooms in rivers and fish ponds. They can produce toxins so there is the problem. They usually occur if there are too many nutrients in the water but this may be only part of the picture.

Daphnia. Checkout my new Daphnia page.

Daphnia sp. possibly carinata.

EC - electrical conductivity - measures dissolved salts. See the discussion on the salinity page.

Hardness - a way of saying how much calcium & magnesium is present in water. Generally a bit of hardness is better. It offsets the effect of having sodium present and it helps prevent corrosion.

meq - milliequivalent. A good way to record the amount of a nutrient present in soils or water because the contribution of each nutrient can be compared directly. If two nutrients have the same ppm value say sodium and potassium there will be more sodium charges (each sodium has 1 positive charge) than potassium charges because sodium atoms are lighter than potassium atoms. The meq value for sodium will then be higher than the meq value for potassium. Now some nutrients have two charges eg sulphate and others 1 charge like nitrate. If a soil test measured the same amount of each nutrient (in ppm) then sulphate would have the larger meq value because sulphate contributes two charges for each 1 charge which nitrate contributes.

meq% - milli equivalents percent. A convenient way to express nutrient levels in soils. It means milli equivalents of a nutrient per 100 gms of soil.

mS/cm - milli Siemens per cm. Another way to express conductivity measurements specially for hig values. 1 mS/cm equals 1000 ÁS/cm.

ORP - Oxidation - Reduction electrical Potential. In all natural environments like a compost heap, dam water, soil etc an electrical potential exists. Usually in fresh compost, swamp sediments septic tanks etc the potential is low. In running streams and cropping soils the potential should be relatively high. Here's an example of what this means. In soils where you would want to grow something its better to have ammonium turn into nitrate for plants to use. This will only happen if the potential gets to a certain level. In well aerated soils this happens. In wet soils nitrate tends to be tuned into ammonium. Cereal farmers know that wheat goes yellow if it is too wet. This is the technical reason. Of course low potential is not always bad. Wetlands are used to remove nitrates and other nasties because they have low ORP values. ORP values in soil and water tests are very useful because they indicate which substances (including nutrients) will possibly be formed and which may disappear.

pH - Hydrogen ion concentration. 1 very acid, 7 neutral, 14 very alkaline. pH is widely quoted but little understood - see Bicarbonate above.

ppm - parts per million. This is the same as mg/kg or even mg/l.

Salinity - a broad term often means sodium chloride but loosely used to describe dissolved salts.

TDS - Total Dissolved Solids. Can be measured directly or estimated from EC. Check out the salinity page for details.

uS/cm - micro Siemens per centimeter. A standard way to report conductivity measurements.



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