Hi GC, Long time lurker, first time poster. I have 4 different strains all the same age, 21 days from germination. Some are going ok & some aren't as the photos depict. I'm thinking Nitrogen and/or Potassium deficiency based on the info I've found here. Any ideas? The pics are the pairs of each strain and you can certainly tell the difference. Big Bomb THC Bomb Sugar Haze Aurora Indica
you have a magnesium deficiency. the last pic is an extreme magnesium deficiency. epsom salt (magnesium sulfate) works great. it is cheap and organic. use 1tsp/gal. before you add that, though, check to make sure your pH is in correct range. the pH of your soil shouldnt be below 6.4. when you get lower than that number, magnesium isnt absorbed as well by plants. your pH looks a little bit low.
Thanks for that. Gives me an avenue to follow. Can I use a water pH kit to test the soil? Or should I take a sample of the water coming out of the soil to gauge?