How does the P-tester work?
Phosphorus (P) is an essential macronutrient for all plants, however if the availability of P in soil is insufficient  to cover the needs, P deficiency results in reduced growth and low harvest yields of grains, seeds or fruits.
To ensure that plants are adequately supplied with P, a large number of soil and plant analyses are performed every year, where P is extracted from the soil or leaves, and then analyzed in a laboratory. These analyzes are expensive, and  typically it takes several weeks before the results are available and action can be taken to correct a deficiency. Moreover, the results of the analyses are only useful if you know how to interpret them properly - and this is not always an easy and straightforward task.
With the P-tester, you can take measurements on plants directly in the field, and in seconds determine whether the nutritional status of P is adequate. The handheld device is calibration free and transmits the measurements directly to your smartphone (iphone). Using easy understandable graphics, you get instant answers on whether the plant is well supplied with P (green), on the border to be P deficient (yellow) or if the plant lacks P and require fertilization (red).
The P-tester is based on a novel principle, which measures on the bioactive pool of P participating in the biophysical processes of photosynthesis. Here, P has a very important function in the formation of the molecule ATP, which is used in a cascade of biochemical processes that fix CO2 in the air and convert it into carbohydrates. If P is not present in the optimal concentrations, key processes in photosynthesis are affected, which specifically and with high sensitivity can be measured by the P-tester. The process measured is the same for all higher plants, and therefore there is no need for correction and calibration in order to adapt the measurement to a specific plant species.
The P-tester can measure much more than just P deficiency  
Plant growth and vitality can be affected by many factors such as drought, cold, heat, pathogens and various nutrient deficiencies. If plants are stressed by one or more of these factors, it affects the ability of plants to utilize sunlight in photosynthesis. In addition to measuring the effects of P deficiency on photosynthesis, the P tester also measures the ability of photosynthesis to utilize light (photons) and it measures if the energy transfer between the many processes of photosynthesis is ideal or inhibited by one or more stress factors. These measurements are carried out in parallel with the P measurement, and results are presented in a way that makes them easy to interpret.

The P-tester measures the following vitality parameters:
Photosynthetic Efficiency (PE): In a non-stressed plant, regardless of plant species, the value of PE ranges between 0.75-0.85. If the value is less than 0.75 it indicates that light is not used optimally in photosynthesis and plant growth is weakened as a result. If PE remains reduced over time, the plant will become susceptible to diseases, develop leaf symptoms (necrosis) and crop yield and quality will be reduced. There are many stressors that can potentially reduce photosynthetic activity and the measurement provides no information about what is wrong - it only indicates that something is wrong. If the value is less than 0.75, a systematic approach to identify the cause of plant failure must be implemented and initiatives must be taken to restore vitality. PE is a very useful and sensitive parameter that provides the opportunity to follow plant vitality and optimize the growth conditions, and is one of the strongest parameters, the plant grower has available to optimize productivity.

Plant vitality-Index (PI): This parameter represents a measurement of the energy transfer between the two photo systems (photosystem II and I) in plants. The PI value of a healthy plant will always be larger than 2.5 - If the value is less, it indicates that the plant is exposed to stress. Often, the PI value follow the photosynthesis efficiency (PE - see above), but PI has been shown to be more sensitive to certain kinds of stress, such as drought and heat. Therefore PI is a useful additional parameter to PE, in order to optimize plant productivity.