A useful post from the defunct Rural Network forum, by Dr Doug Edmeades.

Soil organic matter – what is it and where does it come from?

The importance of soil organic matter, or to use the colloquial term, humus, has been known for centuries – certainly long before the Organic Movement. Most text books will list the benefits of soil organic matter (relative to soil with little or none of it) as improved soil structure, water holding capacity, storage of nutrients and better heat absorption. Soil organic matter is also the home and food for the myriad of soil micro and macro organisms from earthworms and insects to bacteria, fungi and the tiniest protozoa.

We are blessed in New Zealand because our well developed pastoral soils, taken in the international context, contain large amounts of organic matter. This is a consequence of our temperate climate, and our clover-based, grazed pastoral system.

Soil organic matter (SOM) - humus - comprises the breakdown products of plant and animal (dung) material returned to the soil. The fresh plant material and dung returned to the soil is food (energy) for soil bugs which get to work in a sort of chain gang and break this material into increasingly smaller and more stable units, which are then often joined together (polymerised) into stable, large, complex organic substances. Humus is dark coloured and as a general rule the darker the colour and the deeper it extends into the topsoil the better the soil. It is this colour that enhances heat absorption.

In our grazed, clover-based pasture, carbon (the major component of soil organic matter) comes into the system from the atmosphere (as carbon dioxide) via the plant (photosynthesis), some of which goes into the soil as plant residues (from tops and roots) and some via the dung. The amount of carbon added from these sources is of the order of 1-3 tonnes per hectare per year annually. Losses occur from the animal because it breaths out carbon dioxide (respiration) and belches methane, and from the soil via the oxidation of organic matter.

The key point is this: if the sum of the inputs is greater than the sum of the outputs then carbon and hence organic matter is accumulating in the soil.

So, how should we manage our soils to ensure that this happens or that at least we are not depleting soil organic matter levels and hence jeopardizing soil quality and contributing to green-house gas emissions?

Soil organic matter – how should we manage it?

Several studies in the 1950s and ’60s showed that soil organic matter accumulated (inputs are greater than outputs) following pasture improvement (i.e. clover + fertiliser + animal). This accumulation continues for about 20-50 years and then reaches a steady state (inputs = outputs). The time required to reach this steady state, and the amount of soil organic matter present at steady state depended on the climate and the soil group. Generally, the wetter and warmer the environment the more soil organic matter.

Thus the management recipe was simple: clover (to add nitrogen), fertiliser (especially phosphorus, potassium and sulphur - PKS - to maximise clover growth), and the animal (to do the recycling) plus time equals more soil organic matter.

The situation under cropping is very different. Cropping exploits soil organic matter (outputs are greater than inputs) and this is especially so when the crop residues are removed. So the second management lesson in terms of soil organic matter management is: do not crop if you can help it! Or if you need to crop make sure there is a good rotation from clover-based, grazed pasture to crop and then back again. Green manuring or heaps of compost are also helpful. Civilisations have failed by not following this simple rule.

A number of more recent studies suggest we need to modify slightly our understanding of soil organic matter accumulation.

Tate (1997) compared the soil organic matter contents of 43 topsoils sampled first in the 1960s and again in 1992. He concluded that there was no change over this period. This is consistent with the idea that the soils were at a steady state with respect to soil organic matter accumulation, as discussed above. In contrast, Schipper et al (2007) reported an average decline (about 1% or 1 tonne SOM/ha) in 37 sites over a period of about 20 years.

How do we reconcile these studies – one suggesting no change the other indicating a small decline?

Other researchers have dug deeper into this apparent paradox and have reported results which indicate that soil organic matter can be reduced by:

Land-use intensification – management practices which increase pasture utilisation (e.g. better grazing management, increasing stocking rate, introducing irrigation) and hence reduce the proportion of plant material (litter) being returned to the soil and thus result in the steady state soil organic matter being reduced.

Changes in the quality of the litter returned to the soil – it is suggested that some of our newer management practices (and this includes all those listed above plus the introduction of new pasture cultivars and the introduction of fertiliser N) result in a change in the chemical composition of the litter returning to the soil allowing it to be more readily broken down in the soil and hence less is conserved in the soil organic matter pool.

Two points must be emphasised: First, even if the figures reported by Schipper are true, (i.e. a decrease in SOM at the rate of 1% per annum) there is no need for panic or alarm. As stated earlier, our developed pastoral soils already contain large amounts of soil organic matter.

Pastoral agriculture in New Zealand is not on the verge of collapse. I stress this point because of our propensity in this PC and environmentally sensitive age, to seize on and highlight the negatives, especially on environmental issues. Second, in the scientific sense, the possibility that modern management practices are depleting soil organic matter levels is somewhat speculative. It is an emerging issue and more science is most definitely required.

The above is a summary of a talk I presented to a special meeting of the NZIPIM. It is also the origin of a formal paper presented to the FLRC Conference (see Metherell, Edmeades and Ghani 2008, Massey University, Fertiliser and Lime Research Centre Workshop, February 2008) and an adaptation was published in the Fertiliser Review No 20.

Dr Doug Edmeades is a soil scientist of independent fertiliser consultancy AgKnowledge www.agknowledge.co.nz/