Plant-directed bioaccumulation of limiting elements
Fertile soils the world over show similar compositions and ratios of nutrient elements. We can call the ideal balance of nutrients the “window of life”. Even when the geology is poor in certain elements needed by plants (and consequently animals), the topsoil is nevertheless closer to the ideal window of life than either the geology or the subsoil.
Put another way, the essential nutrients for life seem to accumulate in topsoil in exactly those ratios that are needed by life. How could this happen?
The answer is bioaccumulation. This is fancy word for the fact that vegetation communities efficiently recycle limiting nutrients through the litter layer (the organic or O horizon). Before an old leaf even falls off the plant, much of its soluble nutrient content, such as potassium, has already been leached into the topsoil. When the leaf finally falls its less-soluble nutrients are gradually released by the decay process.
Immediately below the litter layer are feeder roots awaiting the shower of nutrients raining down upon them. They do not take up the useless excess of abundant elements or those they do not need, but rigorously absorb those nutrients in limiting supply.
Through this process of organisms taking what they need and letting go of what they don’t need, limiting nutrients are gradually accumulated, and the topsoil moves towards the ideal nutrient ratios.
Figure 1 presents some actual measurements made in a Sydney soil derived from clay/shale geology. In this case the geology (Pleistocene marine shale) is deficient in calcium and phosphorus but has abundant sodium and magnesium. The figure shows that calcium and phosphorus have been strongly bioaccumulated in the topsoil. Sodium and magnesium, on the other hand, are at high levels in the subsoil but are diminished in the topsoil.
Organic matter also bioaccumulates in the topsoil, where it is variously called humus, leaf litter, mulch, compost, thatch or leaf mould. It remains in the surface where it provides water and nutrients.
Figure 1. Bioaccumulation of limiting elements.
Thus from dross the living community creates soil: the plants alter soil properties in a way that actually improves their growth. The longer this goes on in a stable community, the closer the soil chemistry comes to the ideal window of life and the better the plants grow.
We can see this in Table 1. Notice that the soil under the higher-status vegetation community (tall closed-canopy Eucalyptus saligna forest growing in gullies) has much higher levels of bioaccumulated nutrients than the lower-status community (sparse, open Eucalyptus gummifera woodland growing on ridge tops).
Table 1. Soil conditions related to status of vegetation.
|
Determinant |
Eucalyptus gummifera open woodland on ridges |
Eucalyptus saligna tall forest in gullies |
|
pH |
4.45 |
5.82 |
|
Total P in topsoil (ppm) |
170 |
469 |
|
Na (kg/ha) |
94 |
443 |
|
K (kg/ha) |
430 |
1217 |
|
Ca (kg/ha) |
470 |
5032 |
|
Mg (kg/ha) |
765 |
3925 |
|
Al (kg/ha) |
4538 |
420 |
|
Ca/Mg (mass ratio) |
0.61 |
1.3 |
Source: McColl JG. 1969. Soil–plant relationships in a eucalyptus forest on the South Coast of NSW. Ecology 50(3): 354–362.