Soil micro-organisms provide an essential function in nourishing and protecting plants. They also play a crucial role in providing soil, air and water services that are absolutely critical to human survival. Understanding this linkage allows better nutrient management decisions.
This information here comes from an article published in Better Crops, developed by the International Plant Nutrition Institute at the beginning of this year by Mark S Coyne and Robert Mikkelson. To read the full article please click here.
Sustainable crop production is essential to a healthy and adequate food supply. At first glance a healthy crop reveals only the above ground plant; the roots that support the visible plant are seldom seen. However these plant roots grow in an incredibly complex environment which is teeming with billions of soil organisms, particularly bacteria and fungi, which play a crucial role in promoting health and maintaining an adequate supply of plant nutrients for crop growth.
As previous blogs have shown, we are nowhere near knowing all about what goes on within the soil and the details of all the interactions that take place. It is understood that plants modify their soil environment by exuding large amounts of carbon from their roots. This zone in the soil where the roots are becomes a hotspot for the biological populations that live in the soil. Adding carbon to the soil surrounding the roots leads to a huge increase in the number of micro-organisms living within and outside the roots.
As soluble carbon is released by roots, micro-organisms are stimulated and colonise the soil surrounding the roots. This can result in competition for nutrients because plants and microbes rely on the same essential nutrients for growth.
Below are some of the key interactions between soil microbes and plant nutrition in a diagram and explained in more detail underneath.
Nutrients are converted to plant-available forms
Living organisms have a crucial role in controlling the transformation of plant nutrients in soil. In most soils nitrogen, phosphorus and sulphur are present as various organic compounds that the plant can’t use. Micro-organisms play a crucial role in regulating the conversion of these organic pools into plant available forms. This conversion takes place through numerous mechanisms. Various management practices including tillage, irrigation, residue management, using manures and the addition of specific biological inhibitors or stimulants and innoculants can be used to influence these important microbial processes.
If these biological processes aren’t accounted for in soil / nutrient management, then excessive nutrient loss or plant nutrient deficiency with a significant reduction in crop yield / quality can result.
Nutrient recovery is enhanced
Nutrient recovery has an intrinsic relationship with mycorrhizal fungi. The small diameter of the fungal hyphae allows greater access to soil pores than roots alone which gives the plant better utilisation of water and nutrients and maintaining root sorption activity in older parts of the root.
Mycorrhizal fungi can increase the supply of various nutrients to plants (including Copper, Iron, Nitrogen, Phosporus, and Zinc) in exchange for plant carbon. The boost in Phosphorus uptake provided by Mycorrhizal fungi is especially important for crops with high P requirements. Mycorrhizal fungi can also release various enzymes to solubilise organic phosphorus and they can extract soluble P from the soil at lower concentrations than plant roots are able to do alone.
Nitrogen fixation is facilitated
Certain specialised symbiotic bacteria can fix atmospheric N2 into ammonium based compounds for plant nutrition. The most important of these organisms for agricultural plants are from the species Rhizobuim and Bradyrhizobium.
It is estimated that N2 fixation provides between 10 -20% of the N requirement for cultivated crops and between 25-40% of the entire annual reactive N in the world.
Improved soil structure promotes root growth
An often overlooked contribution of soil microorganisms to plant nutrition is their enhancement of soil physical properties. Good soil structure enhances plant root growth and results in greater extraction of water and nutrients.
Individual soil particles are bound into aggregates by various organic compounds (especially polysaccharides) released from soil microbes. Glomalin, which is a protein released by mychorrizal fungi and is in the green picture at the top of the page, sticks soil particles together and improves overall soil structure. The more aggregation in soil, the greater the porosity which often leads to greater soil aeration and water storage capacity.
Pathogens are controlled
There is a growing appreciation of the link between soil microbes and plant pathogen control. Soil bacteria that produce siderophones can deprive pathogenic fungi of iron. Various antibiotics have been identified on soil that can suppress pathogenic organisms. Rhizosphere organisms can compete with pathogens for attachment to the plant root and essential nutrients for growth.
There is still much more to learn about how soil micro-organisms improve the health of plant root systems and overall nutrient efficiency.
Effect of fertiliser on soil microbial communities
Any management practice has the potential to influence soil microbial communities in positive or negative ways.
Short term, the effect of addition of mineral fertilisers on biology will be negative and will cause temporary osmotic stress. Long term experiments with wheat from Rothamstead show that adding mineral fertiliser over a longer term did not significantly influence the diversity of the bacterial population or two genes specific to important N transformation.
Other literature has shown that mineral N fertiliser application was associated with an average 15% increase in microbial biomass and 13% increase in soil organic carbon, compared with unfertilised control soils. They found that increases in microbial populations were largest in studies with at least 20 years of fertiliser. But it also showed the importance of pH, with soils with an acid pH (less than 5) having a negative affect on biological populations.
Soil micro organisms interact intimately with plants to stimulate productivity by supplying essential nutrients in a soluble form. Healthy plants stimulate the microbial community of the soil through the root exudates they secrete and the organic residue they leave behind.