Population Size 
The total number of individuals of a single species within a defined area.
Potential Evapo-Transpiration (P E-T)
The potential, rather than actual, amount of water that might be lost to the atmosphere, by a plant and its soil environment, if an adequate amount of water was continuously available.
Potential Transpiration
The amount of water lost by a surface with a complete coverage of green vegetation, which also has an adequate, non-limiting, supply of water within the soil profile in which the roots are present.
Power Take Off
This provides a means of transferring power from an engine to drive an implement. A power source, such as an engine, is connected to an implement, such as a tracked mounted mower, by means of a connector, a PTO. A tractor PTO is usually located at the rear of the tractor, is a small shaft of about 150mm in length and rotates at 540 RPM, with 6 splines. Other rotational speeds can be achieved, e.g. a 21 spline 1000 rpm shaft. When the PTO is not in use the shield (i.e. the guard) must be angled on the horizontal and a cover also attached to enclose the splined shaft.
Precision
How close repeated measurements are to each other. For example, a soil pH meter used in the field might record measurements of 6.3 to 6.5, so the degree of precision is good, with the range of measurements being narrow. If the true pH value is 5.5 then it is fair to say that the field test equipment, whilst being precise in its measurements is not very accurate, being only of general use. However, if a field piece of equipment is precise and the true value is known then a fairly simple readjustment of data can help provide much more useful information when other soils are tested. If, for exam
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Predator
An organism that seeks out and attacks a target pest. This might, for example, be a bird feeding on earthworms, or a fox seeking out rabbits.
Preventative Control
The use of a pesticide which is applied prior to the onset of any weed, pest or disease incidence. The aim being to prevent, or at least reduce, an outbreak. The main use of preventatives within turf management is probably in the use of systemic fungicides.
Primary Data
Prokaryotes
Organisms that are usually unicellular (sometimes multi-cellular), that lack a membrane-bound nucleus, mitochondria, or any other membrane-bound organelle: These types of cells are found in organisms within the Monera and Protista Kingdoms; the earlier classifications being called Archaea and Bacteria. Compare with Eukaryotes.
Proteins
A large molecular nitrogenous organic compound consisting of one or more amino acids. These form structural parts of plants, as well as forming enzymes.