Soil Health Assessment - How it can help you
Soil health is the integration of chemical, biological and physical approaches for long-term sustainability of crop and animal production with minimum impact on the environment. Healthy soil should have good structure, unimpeded root growth, adequate drainage, air space, good water/nutrient retention and release, and a well-rounded microbial community.
Soil testing in New Zealand to date has focused on the chemical status but has failed to consider the physical and biological factors which are also imperative to soil health and maximising yields.
Soil Health Indicators – Biological
Soil microbes are essentially scavengers responsible for breaking down plant and animal remains in the soil and converting them into useful nutrients such as nitrate, phosphate, and potassium (NPK). Plants use these nutrients for growth. Soil microbes also keep pathogens in check by creating unfavorable conditions for their growth and filling their potential niches. In healthy soils, all these groups of organisms co-exist happily. When one group is affected their abundance will change - indicating unhealthy soil.
Nematodes are ubiquitous within the soil and can be characterised into seven groups based on their diet: bacterial, fungal, predator, insect, plant parasite, omnivore, and algae/moss feeders. The relative proportions of these groups are indicative of nematode community composition. A high proportion of bacteria and fungi nematodes infers high fungal and bacterial abundance and good conditions for plant growth. High proportions of plant parasitic microbes will infer a potential for significant crop damage.
Using DNA-based culture-free techniques is the future of understanding soil health as they allow us to survey the entire microbial community rather than only the tiny fraction that are culturable. Eurofins is a leading provider in next generation-sequencing technologies and currently setting-up commercial tests to determine soil nematode community using PCR.
There are several other bacterial and fungal groups that impact soil biological health. The actinomycetes, aerobic bacteria, fungi, and yeast are responsible for degrading organic matter into soil nutrients such as NPK. Nitrogen-fixing bacteria are responsible for providing a nitrogen source for plants and their levels indicate the requirement of nitrogen-based fertilizers. Low aerobic and high anaerobic bacteria indicate poor soil drainage and compaction.
Soil Health Indicators – Physical
Macroporosity is the amount of air space between soil particles. The recommended level for soil macroporosity (or air-filled space) in soil should be >10%. Macropores are important for air penetration into the soil. They are crucial for supplying oxygen to plant roots and aerobic soil microorganisms. Soil becomes compacted when these pores are lost and affects soil microbes and root growth.
Low macroporosity will reduce soil aeration and generally leads to an anaerobic condition (less oxygen). Soils with low macroposity will result in decreased pasture, crop yield, less clover growth and nitrogen fixation. On the other hand, extremely high macroposity is undesirable as this promotes water loss through rapid drainage and the leaching of important nutrients such as nitrate, sulphate, calcium, magnesium and potassium.
Low macroporisity is often caused by heavy machinery and livestock treading which is exacerbated under wet conditions. It is estimated by AgResearch that pasture production is increased by 10% for every 1% increase in macroporosity beyond the critical level of 10%.
Soil Health Indicators – Chemical
Organic Matter (OM), Total C, N and C: N ratio
High levels of Soil Organic Matter, Total C and Total N indicate fertile soil and infer good soil physical properties.
Nitrogen can be released from soil organic matter by the activity of soil micro-organisms. The conversion of organic forms of N into inorganic forms is termed mineralisation. Micro-organisms also incorporate mineral N into their own bodies for their own use.
Low C: N ratio (<25) promotes net mineralisation of N and is better for plant nutrition. Generally, pasture soils are higher in OM, total C and total N than arable or cropping soils. High C: N ratio (>25) promotes net immobilization.
Soil fertility can be improved by applying OM and rotating between crop and pasture. Nitrogen is the most limiting element for plant growth (particularly pasture) in general. If a soil has low total N plant growth may be inhibited. Higher soil N concentrations can be achieved by planting legume species such as clover in the pasture.
The amount of carbon and nitrogen that is tied up in all soil microbes also indicates soil fertility. Medium to higher levels indicates fertile soil with a prolific microbial community. Low levels indicate low microbial abundance and increased risk of nutrient leaching due to nutrients not being locked-up in microbial cells
Labile Carbon and Labile Nitrogen
An indication of soluble forms of soil organic matter, that is readily available to plants. High rating of labile C and N indicates greater availability of labile nutrients, such as N P K and S. Often labile C and N are strongly correlated with microbial activity and microbial biomass. Conversely, low levels indicate an infertile soil. This can be amended by fertiliser application and over time will consequently build up organic matter in the soil.
The hot water soluble carbon & nitrogen tests (HWSC & HWSN) are a good indicator of labile carbon and nitrogen. The HW extract contains other labile nutrients such as sulphur and fractions of organic phosphorus, which may also be analysed to assess organic fertility of the soil.