There are some basic assumptions about growing plants by which turf managers and farmers alike have operated. We first assumed that plant nutrition is a relatively easy process. That if we add chemicals at the right time, the right place and in the right amount, we can supply all the nutrients plants require and at the rates plants require them. However, we were wrong. Our second assumption was that we understood plant nutrition well enough to supply these needs. We know now that plants are much more complex than we originally thought.
We had presumed that plant growth requirements were placing just enough nitrogen, phosphorus and potassium for the whole of a plant’s growth in the soil at one or a few times. We had forgotten that plants require many other nutrients throughout their growth cycle. Before we decided that we had plant nutrition figured out, plants thrived without our help for millions of years. We started selectively breeding plants for more color, more fruit, bigger and more abundant flowers, etc. for our benefit. Plant physiology and plant nutrition did not change through this process.
Plants obtain almost all of their nutrients through the help of beneficial organisms working in the soil around the plant’s roots. This is referred to as the “soil foodweb.” These organisms are beneficial by supplying nutrients in plant-ready form, retain nutrients in the soil and not allowing them to leach, compete with, inhibit and consume disease causing and plant parasitic organisms, decompose plant residue, toxic materials and pollutants that kill plant roots, form soil aggregates that improve water infiltration, root penetration and the water-holding capacity of soil and improve plant quality and increase the nutritional and aesthetic value of plants through the above listed benefits.
So what are beneficial organisms? This group is comprised of certain bacteria, fungi, root-feeding nematodes and arthropod herbivores. Fixing bacteria and fungi immobilize nitrogen, potassium, sulfur, calcium, magnesium and other soil nutrients in their own biomass. Once nutrients are immobilized, they are transferred into plant-available forms through the process of mineralization. This allows the plants to take in these nutrients at the time and place they need it. All of these compete with pathogens for food. In doing this they either inhibit or kill off the pathogens, lessening the potential for plant injury caused by diseases. Also, the beneficial organisms influence soil structure by helping to produce soil aggregates, soil pores and channels. These aid in root penetration and water infiltration.
If the beneficial organisms are killed, we lose these benefits. Pesticides, herbicides and fertilizers (with a high salt index) can directly impact their populations. Generally, all organisms, beneficial or pathogenic are affected. However, there are some basic differences in their makeup which can hurt the beneficial organisms even more. Pathogens tend to have short life cycles (they usually kill their hosts), produce many offspring (increased odds at finding a host) and have a wide genetic variability (so a response to a change in the host’s defenses can be overcome). Pathogens can adapt quickly to their environments and can develop a resistance to chemicals. On the other hand, beneficial organisms don’t work the same way. They have long, complex life cycles, have fewer offspring to compete for food and the habitats they prefer get divided among other competing organisms. They need time to recover from chemicals. There defense is to compete. When pesticides, herbicides and fertilizers are used in small amounts with applications spread out, they are not very detrimental. Beneficial organisms require time to return and control pathogens.
We also need to understand the effects of cultivation on the soil. Every time the soil is disturbed during cultivation, some of the soil aggregates are broken allowing for organic matter to mix. This allows for bacteria to predominate, as compared to fungi. This tends to drive the soil pH more alkaline. As bacteria dominate the major form of nitrogen will be nitrate due to the nitrifying bacteria. Nutrients will be pulled from the soil by the turf not being replaced. This will start to reduce the bacterial populations. With reduced numbers of fungi and bacteria, the cycling of nutrients such as nitrogen, phosphorus, magnesium, etc. drops. Traditionally, these nutrients are replaced by manures, organic fertilizers, and compost.
Once synthetic fertilizers became available, they were easier to spread and had more concentrated nitrogen. The response was reliable and everything seemed to go well for the plants and the microorganisms. However, we assumed that “if a little is good, more is better.” Fertilizers in high concentrations started killing the beneficial microorganisms due to salt content. Salt removes water from a microorganism causing death. Once synthetics were increased and natural fertilizers were decreased, there was a downward spiral in soil health. Organic matter decreased and all populations of all beneficial organisms began to decline. Our dependency on synthetics increased. The more fertilizer used, the more beneficial, pest suppressive organisms died and damage to plants increased.
Next came quick fix petroleum-based pesticides. These helped control insect pests, fungal diseases and weeds. However, we don’t know what the long term effects of these products are on the soil. We do know these products stay in the soil, so pesticide resistance can develop in the pathogens. Natural biological control was lost due to these applications. Beneficials were killed off along with pathogens. Next, methyl bromide came along. Through continued applications of methyl bromide on crops and plants, there are now resistant strains of fungal diseases, nematodes and insects. Methyl bromide totally wipes out the soil foodweb. There are no beneficial organisms left in the soil to retain nutrients. Excess fertilizer cannot be retained in the soil causing yet another problem with leaching into the groundwater.
How can we get soil health back and reestablish the soil foodweb? Diversity in the food resources provides for a diversity in beneficial organisms. Use of organic materials is the easiest way to build up a population of beneficial organisms and to strengthen them. They naturally will compete with pathogens, and in most cases, will win. There may be times a chemical pesticide or fungicide may need to be used in moderation. We need to pay more attention to beneficial organisms in the soil and reduce our dependence on synthetic, short-lived solutions to turf health. When the foodweb is in balance, the plants will fully utilize nutrients available in the soil and ultimately, the plant will become the best it can be.