From the microbial point of view, soil is a nutritional desert. In contrast, soil adjacent to roots (the rhizosphere) is relatively nutrient rich because, depending on plant species, age and environmental conditions, as much as 40% of the carbon that is fixed in the leaves and moves into roots, is lost to the soil in the form of exuded materials, mucilage, shed cells or cell-wall material. It is not surprising, therefore, that the root zone harbors great microbial activity. Populations of bacteria and fungi in the rhizosphere are enormous, ranging from 106 – 1012 cells per gram of soil. These rhizosphere residents, once considered as passive bystanders of the root environment, are now known to affect plant health, development and environmental adaptation, both beneficially and detrimentally. Biological and metabolic activities in the root-soil interface are one of the most important factors controlling plant health and productivity.
The interactions between microorganisms and plants can be categorized into fourteen activities that impact on plant growth (Lynch, 1990). Of these, seven are beneficial to plant growth, five are neutral or variable depending on environmental or physical conditions, and only two (infection and phytotoxicity) are harmful. The loss of carbon to the root therefore is the manner by which plants lure beneficial agents. What is a beneficial organism? This is not that easy to determine but organisms such as Rhizobia, which fix nitrogen and mycorrhizae, which facilitate nutrient uptake, are clearly recognized. Less well known associations also illustrate the potential impact of plant-microbe associations on plant performance. Pasture grasses are colonized by endophytic fungi of the Acremonium spp. Plants colonized by the endophyte grow taller and faster, have higher levels of resistance to microbial pests, insects and herbivore foragers, and have much higher tolerance to drought and other environmental stresses. As a result, infected plants displace non-infected plants in natural settings. As the fungus is seed-borne, even the seeds are protected from predation. Unfortunately for grazing animals, the endophyte produces several toxins which when ingested cause disease known as ryegrass staggers. The fungus has become commercially utilized for improving the resistance of turf grasses to attack by soil insects as well as for enhancing survival of turf during drought-like conditions.
Increases in agricultural productivity in the coming decades will be derived mostly from a greater understanding of soil and its ecology. One can provide to soil all the nutrition needs of a plant but if the root system is diseased, the plant cannot take advantage of it. Soils which are depleted in their organic matter, also lose the diversity of microorganisms. Conventional fertilizers replenish NPK, but they fail to deliver carbon, the energy source needed to maintain a healthy and diverse soil microbial population. Plant pathogens are not great competitors in soil, but they are very good survivors, particularly in soils with low levels of microbial activity. Increases in disease pressure forces managers to use chemicals to control disease. This often brings about further reductions in populations of soil microorganisms. Such microbial “vacuums” can lead to a rebound in the pest population and greater losses than those originally targeted. Managers then become locked into a perpetual cycle of chemical usage. Chemicals will continue to be an indispensable component of plant protection but organic amendments offer a viable strategy that will reduce the frequency of, or perhaps even the need for, such crisis intervention.
Organic amendments, such as animal and green manures, were used traditionally by growers toenhance soil fertility and to manage soilborne pathogens. The use of organic products however, was forgotten in the chemical era. We have been studying how such products impact on the population of soil microorganisms, specifically those of indicator soilborne plant pathogens. Of the numerous products tested proteinaceous products derived from the animal processing industries such as Nature Safe, were found to be the most consistent in their effects for increasing soil microorganism populations while at the same time reducing the populations of a spectrum of plant pathogens including bacteria, fungi and nematode species. These products have excellent batch to batch consistency and quality. We have found that 1) organic amendments control a spectrum of diseases and pests in all kinds of cultivated settings, 2) organic amendments increase soil microorganism populations; these organism are a reservoir of nutrients, 3) reduced populations of disease causing organisms persists for several years following application and 4) microorganisms and soil characteristics are critical for reducing pathogen viability and make the effects soil specific.
We are currently testing how Nature Safe products impact the health of turf and agricultural ecosystems. An experiment utilizing a randomized block design with seven treatments replicated three times was set up at the Ohio State University in collaboration with Dr. Mike Boehm. Dr. Boehm monitored the turf diseases and vigor while we examined soil nutrient composition and microorganism populations. The results from these experiments are preliminary. Nature Safe provided excellent turf vigor. Microorganism populations in the rhizosphere were marginally higher than with other treatments, but the type of organisms recovered were very different than that found with other treatments. A large number of distinct bacteria and fungal species were found to become established in and on the root system of turf from organically treated plots. These organisms will be characterized as to biological activity in the coming months. Adding Nature Safe to a fine sandy loam soil from a potato farm at 1% (w/w) caused bacteria to increase by 24 fold within 48 hours after incorporation and by 76 hours, the increase was over 40 fold (from 50 million to 2.1 billion per g soil). At 0.1% w/w (8 lbs./per 1000 sq. ft.) bacteria levels increased by 6 fold by 48 hours. NPK fertilizer (15-15-18) at the equivalent rate slightly decreased bacteria numbers. the higher populations of bacteria in organically treated soils were maintained over a 40 day time period. The energy tied up in these microorganisms should beconsidered as a slow release fertilizer and as a biological buffer against attack by plant parasites.
Bacteria is isolated from the turf rhizosphere were tested for their effect on plant growth using a potato nodal explant bioassay. In this assay a bacterium species is applied to potato cuttings which are then grown in tissue culture (Lazarovits and Nowak, 1997). The assay identified both detrimental and beneficial organisms exist in the root zone. These organisms will be identified so that their populations can be tracked in soils following different treatments. Our objectives are to ensure that formulations produced by Nature Safe lead to optimal increases in population of beneficial microorganisms while decreasing those deemed inhibitory. We envision that one day custom organic amendments will be the primary method used by managers to sustain plant health in a multitude of diverse agroecosystems.
Lynch, J.M. (Ed). 1990 The rhizosphere. John Wiley, Chichester U.K.
Lazarovits, G. and Nowak, J. 1997. Rhizobacteria for Improvement of plant growth and establishment. HortSci. 32:188-192.