Everywhere in the world, before the intervention of man and fertiliser, soil, vegetation and animal systems evolved naturally to make the most efficient use of the rate of nutrients becoming available from parent materials, assisted by the local climatic conditions. Except during extreme events such as flooding or earthquakes, losses of nutrients to the outside environment were exceedingly small. Animals died and the nutrients they contained were returned to the soil during their decay.
Then civilisation came along, and imposed totally new systems, where considerable quantities of nutrients are being removed in produce, and, to maximise production, plant-available forms of nutrients are kept at very high levels in the soil through the use of fertiliser. Unfortunately, at these high levels, nutrients are prone to loss to the atmosphere (in the case of N), and, in the case of all nutrients, in run-off to waterways and in leaching.
New Zealand Experience
Weathering of the soil’s parent material provides some nutrients, but only some. From research at the Winchmore Irrigation Research Station in Mid-Canterbury and elsewhere, it was known that weathering of potassium minerals in the soil provided enough K to maintain even intensive irrigated sheep grazing. However, when land was converted to dairy farming, that equilibrium changed, simply because of the high levels of K going off the farm in milk. So potash (and more trace elements) had to be supplied, although not as much as on the North Island volcanic ash soils.
Dr Bert Quin’s thinking about fertiliser and the maintenance of soil fertility in general was profoundly influenced by his involvement, from 1974 to 1982, with the long-term rates of superphosphate trials at Winchmore commenced by Russell Lobb in 1954, and long-term national comparisons of superphosphate and RPR designed by Dr Quin, and coordinated by him from 1982-87.
The long-term trial at Winchmore showed that long-term, irrigated pasture production without P fertiliser inputs was only 35% of that achievable with inputs of maintenance P, S and lime, whether as SSP or RPR. With fertiliser, soil organic matter (of which 60% is carbon) accumulated over time before reaching a new, far more biologically active plateau, particularly in terms of earthworm numbers, and maintained a high sheep-grazing intensity, without any need for biologically-stimulating amendments and very few trace elements.
A farm that cannot ‘afford’ maintenance fertiliser for more than 3 years is essentially uneconomic…
Maintaining Equilibrium
It is important to note that this was a ryegrass-clover system, with no input of fertiliser N. However, production with this system is self-limiting. As soil organic N fertility rises, grasses become more competitive with clover. As it falls, the opposite occurs. A natural equilibrium in soil organic matter content and biological activity is established. If fertiliser and lime are with-held, pasture production begins to decline – within 1 to 4 years depending on the initial soil fertility level – by about 5% of the previous year’s production on average. A farm that cannot ‘afford’ maintenance fertiliser for more than 3 years is essentially uneconomic.