Poa annua...
This sounds too good to be true, but it is possible to convert playing surfaces from predominantly poa annua to
Effect a stress free change to fine grasses
predominantly fine grasses without stressing the grass - the solution lies in the life in the rootzone.
Why do different grass species colonise your rootzone?
Natural plant species progression has evolved so that bare ground is colonised first by annual seeding grasses, like poa annua and tap rooted weeds, and then progresses through early perennial grasses to fine grasses such as fescue and agrostis. In certain climates, with grazing animals or grass mowers, nature stops there, but plant species progression continues through shrubs to deciduous trees and, eventually, coniferous forest. Nutrition for plant growth is driven by soil biology, it is the life underground that makes nutrients available for growth. As the nutrient requirements for plant species above ground changes so does the microbial life underground. Most perennial grasses release about 50% of the energy they produce above ground in the stem and leaves to feed the above ground food chain. They also release about 50% underground by leaking proteins and carbohydrates as root exudates, roots and thatch. This energy feeds the underground food chain of bacteria, fungi, protozoa, nematodes, arthropods and worms, this chain is known as the soil food web .
Free Nutrient
Simply put, bacteria eat the proteins sugars and carbohydrates that leak from the roots, fungi eat the root exudates, but most live on the cellulose and lignin found in dead organic matter, or thatch. The bacteria and fungi get eaten by nematodes and protozoa which excrete ammonium, bacteria converts the ammonium to nitrate in the presence of oxygen and the cycle of growth and decay continues ad infinitum.
Mycorrhiza
In addition to the microbes in the soil there is another very important group of fungi that live inside
the roots of the grass. These fungi are called mycorrhizae, which means fungus root. Mycorrhizal fungi colonise the root system and form a marvellous beneficial association with most perennial plants. Mycorrhizal fungi extend their hyphae into the soil, produce enzymes that make phosphate and other nutrients available then transfer the nutrients, trace elements and water in available form to the grass plant, extending the effective root area many times.
The massive increase in the efficiency of
mycorrhizal grass and its root system means it can grow at double the rate of grass relying on fertiliser alone, which is why most new pitch and golf course grow ins now specify that the new rootzone must be inoculated with mycorrhizal fungi. This activity goes on all the time in unfertilised grasses, which explains why you never have to feed the rough on a golf course, and it is nearly always made up of perennial grasses. Poa annua puts its energy into producing seed and only about 25% into feeding its underground support system. Poa annua does not associate positively with mycorrhiza, so the soil food web that grows underneath seed producing annual plants is very different from the soil food web that lives under perennial grasses that are there for the long haul.
If you can manage the soil food web you can manage the type of grass you grow, and convert annual grasses to perennial grasses without the need for stressing the grass, players, management or greens and grounds staff.
Different soil food webs for different grasses
Bare soil is quite sterile as there is no thatch to feed fungi, and bacterial levels are quite low, so the plants that colonise bare soil have growth mechanisms that rely on the limited bacteria in the soil, not an active soil food web because it is not there. Poa annua puts most of its energy into producing seed and relatively little goes underground to feed the supportive food chain (which is why poa annua needs more fertiliser than other grasses).
As an annual plant it is designed to seed and die. When it does, thatch is formed and this is food for fungi and a more
complex soil food web can now form that supports perennial grasses. If you identify and measure the weight of bacteria and fungi found around the root system of poa annua, there is about ten times more bacteria than fungi and little or no mycorrhizal fungi in the root systems. However, if you identify and weigh the microbes around the roots systems of fescues or agrostis in healthy soil, you find approximately equal amounts of bacteria and fungi and mycorrhizal colonisation of the root system. Poa annua does not associate so readily with mycorrhizal fungi so, in addition to its own less developed root system, it also lacks the extended nutrient gathering system provided by mycorrhizae that is so beneficial to perennial plants.
Sports turf management versus nature
Why is it that, given free draining soils with enough water and organic matter, natural selection will invariably start with poa annua and progress to perennial grasses, while sports turf managers start with perennial grasses and progress to poa annua?
The answer is that, traditionally, sports turf soil is specified and managed in such a way that it becomes almost sterile, just like the soils designed to be colonised by weeds and poa annua. There are a number of reasons for these sterile conditions. Microbes eat carbohydrates and proteins but inorganic fertilisers are mineral salts. Ammonium sulphate has a salt index of about 70, ferrous sulphate about 72. This means that, for every 100kg of fertiliser you apply, about 70kg are mineral salts. No living cell can survive on a diet of salt, the plant and surrounding microbes suffer from osmotic shock - water is taken out of the cell.
The plant and soil needs more water,
which removes the spaces for oxygen, anaerobic conditions build up and fungi die, while bacteria start to metabolise anearobically reducing ammonium and sulphate to toxic ammonia and sulphide. As the fungi die thatch starts to build up which, in turn, holds more water, and a vicious cycle begins. Natural thatch degradation is extremely important for healthy turf. When thatch degrades it
Bentgrass, fertiliser, mycorrhiza
Bentgrass, fertiliser only
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