Up to now we’ve looked at some of the main physical factors in the soil at the Gembrook horse property. Now its time to look at the other main nutrients and the soil’s fertility.
Soils have an ability to loosely hold nutrients to prevent leaching and to make them slowly available to plants. This applies mostly to the positively charged nutrients like calcium, magnesium and potassium. This is called the Cation exchange capacity and it depends on the type of clays but is higher in good structured soils like loams. Because organic matter also holds these same nutrients the amount of organic matter affects CEC.
This soil has a CEC of 17.76 meq%. This value is around mid range with sandy soils at around 1 meq% and organic soils around 30 meq%.
Nutrients are assessed in terms of absolute level, relative proportions, percent of nutrient holding capacity and levels compared to non nutrients and acidity.
Major nutrients weight for weight in the soil.
| Nutrient | Result | Recommended range | Level | |
| ppm | ppm | |||
| calcium | 1859 | 1200 – 2000 | good | |
| magnesium | 297.5 | 150 – 300 | good | |
| potassium | 400.7 | 180 – 300 | high | |
The overall level of the major nutrients calcium, magnesium and potassium is good but a look at the makeup of the exchange capacity tells a different story.
| Nutrients as proportion of (CEC) with comparison to non nutrients | ||||||
| Nutrient | % of nutrient | Recommended | Level | |||
| holding capacity | range | |||||
| calcium | 52.3 | 60 – 70 | low | |||
| magnesium | 13.8 | 12 – 20 | ok | |||
| potassium | 5.8 | 4 – 6 | ok | |||
| Non nutrients | ||||||
| sodium | 1 | < 4 | ok | |||
| exchangeable acidity | 27 | 13 – 20 | too high | |||
The proportions (of CEC) of calcium and magnesium are low. At the same time, exchangeable acidity proportion of CEC is very high. So the soil has a too high proportion of acidity taking the place of more desirable nutrients. Its like saying, the soil has the capacity to hold more nutrients but they are not there. Instead, their place is being taken by acidity.
For similar soils, the higher the exchangeable acidity, the lower the pH.
Another way to look at this is to say that in this soil desirable nutrients (plus some low levels of sodium) occupy 73% of available capacity. This figure is called the base saturation. The base saturation represents the proportion of nutrient holding capacity CEC that is actually occupied by desirable nutrients. Around 80 – 87 % is considered desirable. The non desirable proportion is exchangeable acidity.
Sulphur was also measured in this soil. The result was 17.6 ppm. Around 10 – 40 ppm is considered desirable so the sulphur level was OK.
To change the pH of the soil some of the exchangeable acidity will need to be neutralized. This has to be done gradually, usually by applying lime. As the lime neutralizes the acidity, calcium and magnesium (if using dolomite lime) slowly occupy more of the exchange capacity and will slowly raise the pH by increasing the base saturation.