I would have bought it, as it is the highest rated for accuracy, but you do not mention its use for gardeners.

The best that you can do with any of the three elements is to get rather crude measurements, i.e., "high, medium, low," by visually comparing the color or opacity (for K) developed in the cuvette to a reference card that is included with the kit. So, in order to have an idea which of those relative levels is adequate for a plant species, you would have to have different beds with different levels of those elements in which that single plant species is growing for comparison. In spite of that somewhat limited ability to distinguish different availabilities of the elements, the chemistry is convenient and very easy to use. Be careful to follow the instructions and to use distilled water for extracting the soil (otherwise, you need to analyze a "blank" of your water source with no soil added in order to see if the water is rich in any of the three elements). It is important to be consistent in the time allowed for color development in order to ensure that results are repeatable.

need to test water for hydroponics

I suspect there is a way to use this for hydroponics but have not found anything to say how. It appears that the commonly used tests for hydroponics are ph and TDS or EC. Note that TDS and EC measurements are two different ways of measuring the same thing so you don't need both. The TDS (total dissolved solids) or EC (electrical conductivity) is a general gage of the quantity of nutrients in the water. The ph is important because if the ph is wrong the plants will not absorb the nutrients. That said amazon does sell inexpensive ph, TDS and EC meters that are easy to use. You should get reference solutions to calibrate these meters. (be careful not to contaminate your reference solutions with your nutrient solutions!) If you are looking to do custom nutrient solutions or replenishment of individual nutrients some of the nutrients can be measured using aquarium test kits but some of those test kits do not have a range that is fitting for hydroponics. I purchased the LaMotte N-P-K soil test kit for soil based gardening but I also do some hydroponics. I have not yet found tests (or a way to use this test kit) to measure the individual hydroponic nutrients.

How many of each test can be performed with this kit?

50 tests of each of N, P, and K can be performed (measuring in comparison to a color card as "Low, Medium, or High"). BUT there are only enough extraction tablets for 50 extracts. So, the presumption is that you do N, P, and K tests on each extract. If you only want to do N or P, before disposing of extracts, then you'll be limited to 50 tests (the total number of soil extractions that you can perform).

How long do the test chemicals last, do they go bad?

If stored in high humidity (e.g., Miami summer weather) in spite of the foil packaging, they will not last a year. But, if kept in an air conditioned room (or even in a freezer), they should last a year or more, although there may be a diminution of potency. The best way to check is to have KNOWN high concentrations of N, P, and K, so that you can see if you get full color development. Freezing a soil sample that analyzed as "high" for all three elements when the chemicals were fresh should work to provide such a check (i.e., "positive control").

Manufacturer	LaMotte
Part number	5880-6
Item Weight	4.54 g
Product Dimensions	14.48 x 13.46 x 8.64 cm; 4.54 g
Item model number	5880-6
Item Package Quantity	1
Batteries Included?	No
Batteries Required?	No

LaMotte - N-P-K Soil Test Kit

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