Soil testing and the Bio-Nutrient Cycle

by Julan White, Director, Fertile Farm.

To achieve sustainable land use and maintain abundant production, natural soil fertility must be a central goal for every farmer. Agriculture is now starting to catch up on understanding more of the science behind natural fertility, which is no longer defined on the chemical fertiliser company’s terms.

When soil testing, it is important to distinguish between nutrients that read low because there is not enough in the soil and between readings indicating that the nutrient is low in plant availability. This enables the focus of the fertiliser program to be on adding more of a particular nutrient to the soil so that at some time in the future you don’t run out, or to focus on releasing the already abundant source of a nutrient that is not in a plant available form. Often, the more mobile nutrients such as sulphur and boron are low in the top 30 cm but older annual plants or deep rooted perennial plants are able to access enough to avoid deficiency. Monitoring nutrient uptake by leaf analysis during the season is an important follow up to a soil test. Testing soil for available nutrients involves the lab using acids that simulate the conditions of the rhizophere (root zone). This differs from the actual root zone which contains a multitude of microorganisms that secrete enzymes which are more efficient at solubilising locked up nutrients than the mild pseudo plant acids used in the lab. This is often why a soil test can say a particular nutrient has inadequate availability but the plant growing in that soil still has adequate uptake. Plants voluntarily secrete about 40% of the sugars (energy) produced by photosynthesis out through the roots. Why would they do this? Microbes need carbohydrates, plants need nutrients. (It’s a bit like in some countries where the deal depends on a wink and the hidden cash under the table to get the job done). The symbiotic function of Mycorrhizal fungi (VAM) is well documented and even photographed. Water soluble fertilisers not only tend to tie-up mid season, but have a high salt index that burns out soil microbes such as VAM if applied at in row rates above 30 – 50 kg per ha. In the future VAM may be developed for different species in the same way rhizobia bacteria are used to enhance legume production.

The two main fuels of soil fertility are carbon and nitrogen. Carbohydrate and protein drives the life in your soil as it does in your self. The optimal balance of C:N in the soil is around 11:1 to balance the population of bacteria and fungi. High rates of Nitrogen fertiliser in a low carbon soil blooms the bacterial population which inevitably run out of soluble carbon. This creates an energy (carbon) drought for the fungi and other microbes resulting in the slowing of the mineralisation and enzyme production of other nutrients. Applying the entire nitrogen requirement (based on removal rates) to a plant is unnecessary and shuts down natural nitrogen fixation processes. Free living nitrogen fixing bacteria contribute significantly to total N supply. A local study demonstrated this mathematically by adding the total of all N inputs over a 4 year period in a non legume cropping program and subtracting all nitrogen removed in grain harvested showed about a 30% deficit – so where did the extra N come from? Some will say the air! Its 79 % N. This is true and is accessible to nitrogen fixing entophytes (TwinN) that colonise plants, but most of the N is in soil air pores that are accessible to species such as azotobacter. Compacted soils don’t mineralise as much N as well aerated ones.

Most soils (especially heavy soils) have an abundant supply of total nutrient. Without adequate populations of soil microbes that don’t have enough fuel to keep feeding hungry crops with plant available nutrient, the farmer will continue to be dependent on the chemical fertilisers. Chemical fertilisers are produced using scarce resources and have such a large carbon foot print that in a future carbon centric economy will become unaffordable without some form of offset.

Julian White

Director

3/26/2009 Prepared by Julian White, Independent Fertiliser Distributors pty ltd.