Testing Nutrients and pH [San Pedro Cactus Wiki]

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====== Testing Nutrients and pH ======

When using synthetic fertilizers its very important to understand the concentration of nutrient levels in your feed solution and soil to prevent excess buildup and toxicity. With or without synthetic nutrients, its important to understand the properties of the water you use in your home and garden including its TDS and pH. Its also important to understand your soils pH, what effects that might have, how to check it, and how to correct it. For all of this, you need to know how to test.


=====TDS and pH=====

TDS stands for total dissolved solids. It can be measured in PPM (parts per million) or EC (electroconductivity). PPM is not a consistent way to measure since there are multiple standards and the meters just digitally convert EC to PPM behind the scenes anyway, so it's best to use EC. The units of EC are milliSiemens per centimeter (mS/cm) or microSiemens per centimeter (μS/cm). 1.0 mS/cm equals 1000 μS/cm. You don't need fancy expensive meters - a cheap TDS meter that measures EC from amazon and 100 pH strips can be had for $20, will be accurate enough for our uses here, and will last years.

=====Testing fertilizer feed solution=====

Lets start by measuring the TDS of a mixed fertilizer feed. First, measure a sample of the pure water used to mix the fertilizer. If it reads over 0.35 mS/cm you should look into a filter and a water test… Next, measure the prepared fertilizer solution and subtract the result of the starting water. So for example: Fertilizer solution test of 2000 μS/cm minus water testing at 200 μS/cm equals a fertilizer concentration of 1800 μS/cm. I recommend using scoops to measure your fertilizer, counting how many it takes to hit your target, then you don't need to test your feed every time.

A value of 0.7/700 is suitable for first time fertilization of baby seedlings.

A value of 1.8/1800 is a typical label rate standard application.

A value of 3.5/3500 is very strong and borders on nutrient toxicity.

For pH, aim for 5.5-6.5 which will allow the most optimal nutrient availability. pH out of range causing nutrients to be unavailable is known as lockout and will cause health issues quickly. The pH of the mixed fertilizer solution will change over time as the solution sits. I recommend testing it right before you use it, 3 to 12 hours after mixing it. Letting it sit with a bubble aerator to oxygenate it as it gently mixes is a nice touch.

=====Testing soil=====

The easiest way to test the TDS and pH of soil is to collect runoff water after fertilizing in a specific and repeatable way, called the pour through method, then test it. You will first fertilize your plants to about 15% runoff. Then, 30-120 minutes later, you will place the pot in a collection container and add additional fertilizer until you collect 50 mL of runoff. This typically requires adding about 120 mL of fertilizer solution for a 1 gallon pot but will vary depending on your substrate mix. Alternatively, if you can collect 50 mL of runoff by tipping the pot about half an hour after fertilizing, this is acceptable also. Testing the runoff will give you an accurate picture of the TDS and pH of your soil.
I recommend you test your soil every couple waterings when you start a new fertilizer regimen until you can figure out a good schedule for how frequently to fertilize vs flush. Keep the feed strength at your target. If your soil’s TDS starts dropping or climbing, pour more fertilizer solution through the pot when you fertilize. Once you have this figured out, you will only need to test your runoff on occasion just to make sure everything is still in order.