The Underground Currency
Think of the soil in commercial terms for a minute. It’s not as out there as you might think. Cation nutrients are the currency in which soil deals, and soil colloids are the traders. As a couple of American researchers put it, “the first order of business for soil colloid is to hold nutrients – nutrients that can be traded off as the roots of a plant demand them”. Because soil colloids come from clay and organic matter, there are clay colloids and there are humus colloids. Both have been broken down as far as they will go.
Laboratories usually report cation exchange capacity – the energy of the clay and the humus – in terms of milliequivalents. Think of this in similar terms to an electrician measuring electricity in volts and amperes, or a physicist measuring magnetic energy in ergs and joules. It all comes back to principle of positives and negatives. Negative attracts positive. Cation nutrients (which are positive) are attracted to and held on the soil colloids (which are negative), and remain free to move in the soil solution or water. A milliequivalent represents the amount of colloidal energy needed to absorb and hold to the soil’s colloid. For instance, in the top 17-18 centimetres of a 0.4-hectare (one acre) block of soil, the colloid is holding on to 182kg of calcium, 109kg of magnesium, 355kg of potassium – or, simply, 9kg of exchangeable hydrogen.
That’s the big picture, but what does it all mean for individual farmers?
It all comes back to soil analysis, and the need for you to know where your soil is.
Because the clay and humus that make up soil colloids carry negative charges, you need to use the right type of fertiliser. You need to use a fertiliser with a positive charge. Fertilisers with a negative charge will not be attracted to and held by the colloid, and you’ll be wasting your time and money.
Calcium and magnesium from lime compounds have this positive charge. So does sodium and hydrogen (in gas form). Negatively charged elements (called anions), such as nitrogen, phosphorous and sulphur, do not hold to colloids.
In addition the more colloids in the soil, the more negatives there are to attract the positively charged elements (cations). But there is always a saturation point. The soil can hold only so much fertiliser – put on too much, and some will be lost.
Colloids are also very mobile. Because they are the merest pieces of clay and humus (imagine broken-down particle of dust or talcum powder) they are extraordinarily susceptible to erosion.
If you could collect the dust the wind has shuffled around, you would find it has the highest fertility of any part of the paddock. When water or wind is at work, it always moves the most fertile part of the soil first. As a result, soils can not only be torn down, but also built up. Which explains why soil quality and fertility can vary so much in even in a small area. Remember that you cannot see any of this with the naked eye. But, while these colloids are microscopic, they form the bottom line in this positive-negative transfer (trade). They govern most of the chemical reactivity that goes on in soil.
The first thing you need to do for your land is to get a detailed analysis to measure the amount of clay and humus in the soil. You need to know your soils’ cation exchange capacity – a measure of your soil’s capacity to exchange nutrients.
The result will tell you a lot about its capacity to hold nutrients such as calcium, magnesium and ammonia nitrogen. It has a strong bearing on the quantity of nutrients needed to improve their relative levels in the soil.
This knowledge will help pinpoint the amount of fertiliser needed to get the right nutrient balance. It will also tell you about your soils’ capacity to retain fertiliser.
In other words, this analysis arms you with the information to use the correct fertiliser for your soil – and to use in the most effective quantities. The bottom line for you is; whatever you spend you know it will be effective.