A. 5. What do different plants need?

In early succession, soils are strongly bacterial-dominated, because soils have not built to the point where the fungal foods, or the structure in the soil will allow them to grow as well as the bacteria.

The first organisms that enter a sterile natural parent material (sand, silt, clay alone) are the photosynthetic bacteria. In the aerobic atmosphere, these will be photosynthetic Cyanobacteria, for example. Before the Earth’s oxygen atmosphere developed, the first bacteria to exist, and the first to colonize parent material in anaerobic conditions are the photosynthetic purple or sulfur bacteria (see work on mid-ocean rift bacteria).

These first photosynthetic bacteria release waste compounds, and in succession, bacteria that could use these products evolved, while today, bacteria that use these products have to reach the material, through “taxi-cabs” of various kinds, such as wind, human movement, birds, snakes, mice, or insects for examples, carrying microbes on their feet. Every day, additional species of bacteria may arrive, depending on the distance to a source of inoculum and the ability of bacteria to disperse.

Many bacteria are really bad at dispersing. They just don’t grow in places where bird feet, or earthworms, or spiders come by. If they aren’t picked up by something, they don’t get around very well. Especially when people are doing so much to destroy the normal set of organisms in soil or on plant surfaces, dispersal to new soils is not happening adequately. When insecticides are used, insect dependent dispersal is just about nil.

Early successional species of bacteria, or fungi, and disease-causing bacteria and fungi, have better mechanisms for dispersing. To stay alive, to maintain the species, they have to be able to find new places to grow. So, the organisms that arrive first in our agricultural soils are the diseases. They are better at dispersing than any other kind of bacteria or fungi.

As the bacterial populations increase in number, and as the number of species increases, because soil structure improves, the kinds of organic materials improve, the habitats in soil increase. And so more kinds of bacteria can find a place to live, grow and reproduce. Diversity builds. But all the nutrients will be tied up in the bacteria.

Bacteria don’t mineralize nutrients all by themselves. They can’t. There is no evidence that bacteria die of old age in the soil. Bacteria will become dormant, go-to-sleep, when conditions become too poor for continued growth. They don’t die unless a disturbance occurs that kills them. Otherwise, they have mechanisms for surviving tough times.

Why do microbiologists say that bacteria mineralize? First, we need to understand what mineralization means. When protein is converted into carbon dioxide and ammonium or nitrate, that is mineralization. More generally, conversion of an organic material into mineral forms (carbon dioxide is a mineral form of carbon, and nitrate or ammonium are mineral forms of nitrogen) is mineralization.

What about when rock is solubilized? Rock is a mineral. You can’t mineralize something that is already a mineral. Typically rock P is turned into an organic form, through the action of bacteria or fungi, and on occasion root acids, and incorporated into the biomass of these organisms. When the bacteria or fungi or plant are eaten, phosphate can be released, and since phosphate is a mineral, that would be mineralization.

But the take-home message is that soil scientists often “black-box” the processes going on in soil, and minimize the activity of the biology. They often do not take the time to learn that bacteria perform functions, but not others. Fungi do some metabolic functions, but not all things. It is critical to know what each group does. It is probably important to know what different species, or group of species do as well. Our ability to measure that level of information is limited. But differentiating bacteria from fungi is critical. They do very different things.

And up to this point in succession, fungi haven’t even appeared on the scene.

At a certain point, when there is enough bacterial biomass to maintain protozoan numbers, protozoa will arrive, grow, reproduce, and maintain their populations over time. Nutrient cycling is finally present in this soil.

And now, plants can survive in this soil. But not just any plant, the plants that occupy this stage in succession are things we all would agree are weeds. The word weed has been used too generally in recent times, so let’s agree that by weed, we mean something that requires high nitrate levels, poor soil structure, that produces huge numbers of seeds that disperse far and wide. We all would agree, that’s a weed. And when bacterial and protozoan numbers are very out-of-whack, then the protozoa over-eat their food resources, mineralize high concentrations of N, P, S, etc, and help out the weeds that require these pulses of nutrients to germinate, and to set the stage to allow them to out-compete the later successional plant species, the things we humans like to eat.

Nematodes also arrive during this time, bacterial-feeders of course, since fungi aren’t present yet in high enough levels to support fungal-feeders. Root-feeding nematodes may arrive and survive, since weeds are present. Since few competitors of the root-feeders are present, if they do arrive, they can have major impacts on weeds.

The “support matrix” is now actually soil. Soil requires life to be present, not just sand, silt and clay fractions of the mineral or parent material. Organic matter is required in order to have the organisms performing their functions in the soil. Soil structure is being built, diversity is increasing, but it is all at a very early stage of development. Plant production will improve with time, but at this stage, the plants in the system are mostly weeds. But weeds leave litter on the soil surface, contribute root biomass to the soil, as well as root exudates. So, more and more food resource is made for the bacteria.

Fungi that arrive now have a food resource to utilize, to begin to build their communities, using the cellulose the plants provide. Building soil, slowly, slowly.

As the ratio of fungi to bacteria change, the pulses of nutrients are smoothed out, and true weedy species lack the ability to out-compete the later successional plants. The early successional grass species, like Bromus, and in addition, the brassicas, the mustards, kale and cole crops do best in these soils. They develop a food web that feeds fungi a little, but mostly bacteria. But on their residues and litter, fungi get more and more food, and eventually, the soil shifts to more fungal. Not yet fungal-dominated, but more fungal relative to what came before.

And the plant species shifts, to plants that need more fungal biomass – about 2 times more bacteria than fungi, for the ryegrass related species. Some bunchgrasses, some Bermuda grass here, though many Bermuda grass species do better more bacterial. Most vegetable crop species – tomato, potato, celery, sudan grass, soybean, etc make the balance of the biology in the soil about 0.5 ratio of fungi to bacteria around their roots.

And their roots, and litter residues grow more fungi than bacteria, and again, eventually, the ratio shifts, and with it the plant species. A soil with a ratio of equal fungi to bacteria, with biomass levels above 150 ug, selects for row crops and grasses, such as corn, wheat, barley, rye, fescue, bluegrass, etc.

But again, these plant species put more fungal foods into the soil, and the soil, slowly but surely, becomes fungal-dominated. As that threshold passes, shrubs, bushes, early successional trees win in the competition for soil nutrients. Soil pH generally shifts as well. In western soils, the soil organic matter builds, soils become more and more fungal. Late success ional systems are strongly fungal-dominated. Typical ratios of fungi to bacteria observed in systems requiring few or no inorganic chemical inputs are:

Bacterial-dominated plants (most row and vegetable crops)

Fungal-dominated plants

How can soil organism dominance be changed? Most simply, feed the organisms that are low in number.

Bacterial Foods are: Green, high in easy-to-use sugar and nitrogen, e.g., green grass clippings, cover crops, especially legumes, molasses, compost teas, compost made with green material (high in N and simple sugars) and manures (high N) Be careful to tie up all the manure N by adding enough plant material, or plants may be burned.

Fungal foods are: brown plant materials high in cellulose, lignin, and tannin, e.g., woody fibrous materials, like straw, sawdust, compost made with woody material and small amounts of manure. Enough N needs to be present to start decomposition, but not encourage bacterial growth. Time is needed to reach the N-release stage before putting this material on plants, or planting into such material. At first, the fungi growing on organic matter high in carbon take up N from the soil, which can stress plant growth.

Those soil organisms that make N available for plants are predators of (or eat-) fungi or bacteria. Low numbers of protozoa (bacterial-feeders), nematodes (both bacterial feeding and fungal-feeding,) earthworms, or micro arthropods can be enhanced by improving their food, i.e., bacteria or fungi. Interaction of bacteria and their predators (e.g., protozoa and bacterial-feeding nematodes), or fungi and their predators (e.g., fungal-feeding nematodes and micro arthropods), produce as much as 80 % of the plant-available N that occurs in soil.

If bacterial or fungal foods, i.e., cover crops, residues, or compost can’t be obtained or grown, commercial products are available to “ jump start” fungi or bacteria. If spreading organic matter is difficult, a water extract of the compost can be prepared and this “tea” can be applied through the irrigation system. Compost tea can be bacterial or fungal, depending on the compost and addition of fungal foods, unless rock dust or kelp is added.

When and how to add fungal or bacterial food
Both should be placed on the surface of the soil, although if greater bacterial activity is desired, mix lightly into the top few inches of soil. Realize that plowing and compacting kills many soil organisms, and soil structure is destroyed.

A mineral crust may then develop, decreasing water infiltration, water-holding capacity, and soil structure is destroyed. The benefit from mixing food into the soil and growing a burst of bacterial biomass may not offset all the detrimental results of the tillage. During periods of optimal moisture and temperature (spring, and fall in some areas), bacterial-food should disappear rapidly, within 2 to 9 weeks if soil organisms are diverse and healthy. Fungal-food takes a bit longer, 6 to 16 weeks for a healthy Foodweb to do it’s job. If the original plant material remains identifiable, after a month or more of warm weather, some part of the Foodweb is lacking and needs to be added.

Monitor soil and compost organisms on an annual or semi-annual basis to make certain the right numbers and functions are present. Monitoring in the fall allows assessment of appropriate management through the fall into the spring when it is easiest to improve the fungal community. Organic matter additions, either as compost or as cover crops, decompose rapidly during the winter, even under the snow, if the soil Foodweb is healthy. Soil organisms growing in organic matter produce metabolic heat and will increase soil temperature in the spring. As some cover crops decompose, inhibitory compounds can be produced to so the initial flush of decomposition needs to be finished before planting the next crop. Checking the soil Foodweb in the spring allows assessment of whether beneficial are growing, or problems are developing. Assessing the Foodweb in mid-crop growth is also useful, especially for VAM colonization, pathogens, or pest problems. However, it is difficult to do much to help the current crop at mid-season, and this information is more useful determining what management is needed for the next crop.

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