Soil Life Index

The productive capacity of soil depends largely on the quantity and diversity of soil life. PhD student Nicole Schon is making an inventory of selected fauna in pastoral soils sampled from a variety of long-established research farms that have a known management history. The result will be an index of soil biological, activity using soil invertebrates, that linked the biological activity to the soil processes that provide the provisioning (e.g. supply of nutrients, water) and regulating (e.g. flood mitigation, green house gases) services of a resilient and productive soil. The index will give farmers and their advisers a benchmark to compare their own soils with and perhaps indicate ways in which changed management could enhance soil life and hence sustained production and (hopefully) profit.

Soil is the engine room of the farm, and the power of the engine can be gauged by the quantity and quality of soil biological life. Generally speaking, the more life in the soil the more productive it is.

The total weight of fauna in a hectare pastoral soil can be many times greater than the livestock that the pasture will feed. Fostering soil biology has long been a major focus of organic and biological producers, but it has become a background issue for many conventional farmers. That is likely to change with the knowledge that many intensely farmed dairying soils are coming under increasing pressure from the liveweight loading associated with intensive livestock grazing systems. Biological capacity to sustain pores that help the soil breath and drain water are critical. There is also an indication that some of our intensive pasture systems are losing carbon. With the increasing interest in sequestering more atmospheric carbon into soils to gain carbon credits the role soil fauna play in the initial incorporation and mixing of surface plant litter and dung into the soil becomes another reason for the increased interest.

With AgMardt support, PhD student Nicole Schon is putting together an index of the main fauna groups found in a selection of pastoral soils along with indications of soil condition and functions. She has sampled a series of long-term sheep and dairy pastoral research sites around the country and characterised what was living in the soil in terms of earthworms, mites, springtails and nematodes.

There were considerable differences between sites, and Nicole says that the intention is to match soil characteristics to productivity and determine what might be contributing to the differences.

What is there and what is missing may have implications for soil functioning. For example, if you have a high earthworm count that is good for soil functioning; but if the deep burrowing earthworm species is missing then the influence of the biology is restricted to the topsoil, because you don't have vertical burrows, says Nicole.

Other factors in the index include a measure of what food is available for the soil animals in terms of the amount of litter and dung being returned to the soil. If there is an adequate food supply, but little biology there must be another factor limiting life. Habitable pore space provides an indication of where they can live. If there is no pore space they are obviously going to have trouble surviving, so this is an additional thing that will need to be looked at in the index as well. The last ingredient in the index is the number, weight and stocking rate of livestock. This provides a measure of the level of disturbance the soil fauna might face as a part of day to day life.

Nicole sampled both dairy and sheep trial sites at Ballantrae in the Manawatu sheep and beef at Whatawhata and dairy in the Waikato and Taranaki, and irrigated and dry land sheep systems at Winchmore in Canterbury along with several organic farms in the Manawatu and the organic farmlets at Ballantrae. For earthworms counts she collected 15 cm cores from each paddock; for the mites and springtails she collected 5cm cores and 2.5cm cores for nematodes. Back at the lab she counted earthworms and used several extraction techniques so that the smaller animals were visible under a microscope.

Earthworms are easy to count but the rest are more difficult because although you can see mites and springtails with the naked eye if you know what you are looking for they are usually well hidden within the soil, says Nicole.

I collect the mites and springtails in alcohol and identify them under a microscope. Nematodes are less than100 microns in width so I suspend them in water and identify them under the microscope so that requires a bit more specialist ability.

Earthworms comprise the largest biomass of soil fauna and play a critical role in nutrient cycling and maintenance of soil structure so their numbers and types give important information about the soil. Mites have a hard exoskeleton and need soil pores for survival. Nicole has found that they are often low in abundance in dairy pastures, an indicator of soil compaction. Nematodes are useful because they help to identify the balance between fungi and bacteria in the soil. Also, while plant-feeding nematodes stimulate root growth, high numbers will decrease pasture growth.

The index will comprise a list of soil invertebrates and what processes they contribute to within the soil. It will give figures for what is typically a low abundance and high abundance within soils so farmers will be able to compare the figures from their soils.

Nicole hopes to complete the index by mid 2010 and publish it for use by farmers and their advisers. Since the samples come from well established and managed farms with known history and from a range of soil types throughout the country the index should provide a basis for comparison with other farms in these areas.

I see the data being used by farmers to examine their soils and see what soil fauna are missing and relate that to what kinds of processes the missing fauna contribute to in their soil and how they could stimulate missing populations, she says.

It could be a pore space problem, for example, and helping the natural processes within the soil may correct that and improve productivity without necessarily increasing fertiliser.

AgResearch Senior Scientist Dr Alec Mackay whose team has been looking at soil fauna and soil services management, says that more attention should be given to soil life.

Our farming sector has spent a lot of time and effort on the above-ground aspects of pastoral systems and these are now highly engineered, with attention paid to livestock breeds, crop and grass choice, fertiliser and irrigation. In contrast, the size and composition of life below the ground has often been given little consideration, he says.

In fact in most pastoral soils the earthworm introductions have been accidental. What we hope to achieve is to add soil bioengineering to the list of technologies that are used by pastoral farmers to deliver greater agricultural output and enhance farm profitability.

Earthworms are the most obvious of the soil organisms and play a critical role in soil productivity. Having a range of species is important for soil health different species live occupy different parts of the soil profile. Some consume dead litter and incorporate it into surface layers, some live shallow within the turf and burrow laterally, and others live deep and burrow meters down into the soil. Their tunnels improve aeration and drainage and provide channels for plant roots.

All earthworms excrete vermicasts that improve soil fertility by providing plant-available nutrients. Vermicasts can amount to 30 tonnes/ha/year and so represent a considerable turnover of soil material.

Worm counts taken on some soils in the 50s, again in the 80s and more recently show a significant decline from a high of 45+ per 200mm cube, probably reflecting the greater intensity of farming. A worm count or 35 or above is considered preferable, but many soils have fewer. The introduction of earthworms to soils deficient in them has led to improvements in pasture production of 10-30%.

The species of economic importance to pastures are all European and were introduced by early settlers along with plant material or in the soil used as ships ballast.

Although there are about a dozen such species only two or three are commonly found under pastures. The most common is the grey worm Aporrectodea caliginosa that lives in the turf a few centimetres below the surface.

There are many native earthworms but they are found mainly in their natural habitat under native forest or dense scrubland.