Penetrant 
A wetting agent which reduces surface tension and allows water to penetrate a soil and move through the soil profile. These types of wetting agent can help as a first stage of soil-water improvement, reducing some repellency conditions which contribute to water logging, and which is then followed up by the use of residual wetting agents which have a longer residence time within a soil, thereby also helping to retain water better, especially within sandy soils. A penetrant is a surfactant.
Penetrant Wetting Agent
A substance which reduces surface tension and allows water to penetrate a soil and move through the soil profile.
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Perennial
A plant that lives for more than two years. There are woody perennial and herbaceous perennials.
Woody perennial
These are trees and shrubs that have persistent vegetation that is above ground. Woody perennials can be either deciduous or evergreen. Examples include:
Evergreen
• Holm Oak (Quercus ilex).
• Viburnum tinus.
Deciduous
• Spiraea japonica.
• Hawthorn (Crataegus monogyna).
Herbaceous perennials
These plants often have their vegetation die down and over winter as an underground organ, such as a bulb, corm, rhizome or tuber, with examples including:
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Permanent Wilting Point
The point at which a plant will wilt and not recover. The measure is usually given as minus 15 bar suction. This means a plant cannot exert enough suction to extract water from its surroundings, because water is held more strongly by the soil. Subsequently the plant will wilt and die.
Pest Threshold
A threshold level above which unacceptable damage or harm will occur to a turfgrass surface. Setting a threshold for pest (which is a generic term for diseases, animal pests and weeds) infestation is part of the approach taken in Integrated Pest Management. This can help to reduce unnecessary usage of pesticides, but also to focus a grounds manager’s attention on making best use of physical resources and technical knowledge.
Levels of pest infestation below the threshold can be tolerated, although this doesn’t mean that the extent of the infestation and damage is ignored, but actively mon
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pH [2]
pH is a measure of the soil acidity and the scale ranges from 0 to 14. It is measured as the negative (decimal) logarithm of the hydrogen ion concentration of a solution. With the pH scale being logarithmic (base 10) each pH unit is 10 times the value of the previous value; thus pH 5 is 10 times more acidic than pH 6; whilst pH 5 is 100 times more acidic than pH 7.
Turfgrasses will grow over a wide range of pH levels, although optimum growth will take place over narrower parameters.
The more acidic soils with a pH of less than 5 will favour the fine fescues and browntop bent grasses.
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Phenotype
The collective characteristics of a plant, or organism. This consists of genotype and morphology, and the influence of environmental factors (the latter being non-inheritable).
Photosynthetically Active Radiation
The range of light with a wavelength that is from 400 to 700 nanometres which is used by green plants in the process of photosynthesis. Chlorophyll a and chlorophyll b capture light in the blue and red spectrums. Other pigments, the Carotenoids consisting of Carotene and Xanthophyll, are highly active at 450 to 500 nanometres, and typically produce the respective orange and yellow colours of autumn leaves. The main reason for this is that they do not decay as rapidly as the green colour of leaves. The term is frequently abbreviated to PAR.
Phototropism
The growth of plants towards a light source.
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Phyla