There is a simple answer to this question. They are equally important. Simply throwing manufactured fertilisers on poorly-managed soil does not automatically guarantee large increases in production, unless all the ‘building blocks’ are there.
With the post-World War 2 oversowing of poor-quality, unfertilised pastures with lime and superphosphate, pasture production typically trended upwards for up to 5 years before a plateau is reached, while soil organic matter (60% of which is carbon) can continue to increase for another 10 years before equilibrium state is reached. In many areas, the only recognised ‘biological’ input by man appeared to be inoculation of clover seed.
Clover Under Increasing Threat…
However, with clover under increasing attack by new pests and diseases, and made less competitive with grasses by increased use of fertiliser N, there is increasing indirect evidence that more and more farms – dairy farms in particular – are in a state of soil organic matter decline, and deteriorating soil biological activity and structure. Farmers have become trapped into coping with these effects by massively increasing their use of urea fertiliser N, and placing increasing reliance on bought-in feed to maintain and increase total farm production. Deterioration in physical soil characteristics, especially compaction and poor drainage, are becoming increasingly obvious on dairy farms.
The important question then becomes, how do you fix it?
The important question then becomes, how do you fix it? What is the best way of increasing soil organic matter content and biological activity, thereby maintaining healthy pastures (and crops). The truth is, no one really knows, because the type of long-term research needed to answer it was killed off by Government research-funding reforms in the early 1990s. Very few long-term sites, such as those at the Winchmore Irrigation Research Station, where Dr Nguyen and I both worked in the 1970s/80s, remain. Moreover, very little research dollars are being put into using these sites to examine how soil C and N storage can be increased, especially under dairy farming. Instead we now see a plethora of short-term industry-run trials with a wide range of products, ranging from ‘silver-bullet’ cures such as humic acid through to shotgun mixes of dozens of trace elements. These have invariably been too short-term and too lacking in scientific rigour to provide much more than a little anecdotal information.
We will only ever get to the truth by following through various methods and inputs over 5 years at least on closely monitored farms with known histories of soil C and N levels. Essentially, we need to go back to the basics and determine what types of desirable biological processes and organisms are diminishing, and what inputs and management strategies will best sustain them.