iann wrote:That fertiliser looks fine to me. Is the 98ppm total dissolved solids, or 98ppm of each main nutrient? If 98ppm of each you could even go more dilute with every watering.
Nitrogen in soluble fertilisers basically comes in three forms: nitrate; ammonium; and urea. Nitrate is half of a salt and always comes with something else, sometimes another major nutrient and sometimes even ammonium. If you don't want to read further, you could just go with the simple rule that a mix of forms will cover most eventualities. I looked on my Chempak labels and noted that basically the 12.5-25-25 and 20-20-20 have about the same amount of nitrate and ammonium (and some urea), with the extra nitrogen in the 20-20-20 mainly coming from urea.
Nitrate ions are highly soluble and mobile in soil so they are subject to leaching, so you should consider any nitrate form nitrogen to be gone by the next time you water. Nitrates are taken up rapidly by roots, are highly mobile inside the plant, and can be utilised very quickly. This means you get a very rapid greening effect from nitrate fertilisers, but an excess can quickly lead to weak over-lush growth.
Ammonium is usually applied as half of a salt, or occasionally as the gas ammonia but not in a packet of soluble fertiliser. Ammonium ions are more tightly bound to soil particles so they can last longer between waterings, but are still short-lived compared to other nutrients. Ammonium is readily taken up by roots but must be converted by the plant before it can be used so it acts slightly slower than nitrate, on a timescale of days rather than hours. Plants differ in their preference for nitrate or ammonium ions, but the vast majority will take either. Ammonium-based fertilisers tend to have the most acidifying effect in soil, and nitrates the least. The worst situation is when ammonium ions are converted to nitrates, which is the acidifying process, and then the nitrates leach out of the soil.
Urea acts even more slowly than other forms of nitrogen fertiliser, but is relatively persistent in soil. It is either converted by soil bacteria into ammonium, or taken up quite slowly by the roots. Urea is popular for high nitrogen applications because of the persistence, low danger of burning, the highest nitrogen content of any fertiliser (except anhydrous ammonia which is a highly toxic gas), and ease of use on bulk scales. It is less popular in soluble fertilisers for basically the same reasons, but is often the basis of slow-release fertilisers.
Ian's discussion on the 3 Nitrogen sources for cacti came as a totally new experience to me today, so I'll bring this into the mix as I examine the issues here.
Per Darryl's advice, the dilution with Dyna-Gro 7-7-7 is 1 teaspoon per gallon of water. He did the math, and it works out to 98 ppm for the total ingredients in that fertilizer. According to his experience, the dilute fertilizer-water solution is used to water cacti every time during the growing season. I'll have to determine watering frequency by temperature conditions, although at the height of summer I believe it'll be once a week with good drenchings as appropriate by species requirements. My part of L.A. is close enough to the coast that any heat waves won't be as long or as frequent compared to the more inland areas. Also, with the exception of a few plants, my cacti will be going over to a decomposed granite/pumice mix when I'm ready for the new growing season.
EDIT -- I need to correct this. The formula for ppm is 7/100/768 x 1000000 = 91. That works out to 91 ppm for total N, 91 ppm for P, and 91 ppm for K. If I dilute the Dyna-Gro 7-7-7 at a rate of 3/4 tsp. per gallon of water, the ppm for NPK will be 68, 68, and 68.
I have two options I need to think about re. pros and cons. The first option is the Dyna-Gro 7-7-7. On the pro side, Ian confirms that Darryl's choice is a good one. However, thanks to Ian's understanding about the importance of a nitrate-ammonium-urea balance, I'm concerned if the lack of urea in Dyna-Gro could become a long-term problem as he describes. This is where it would help me to know if the ammoniacal and nitrate combination with Dyna-Gro overcomes this problem by fertilizing less frequently, or it's not even a problem at all for cacti.
My other option would be to find a liquid fertilizer with a nitrate-ammonium-urea mix. All I can seem to find are either nitrate- or urea-based fertilizers. However, to be honest the only fertilizer I use is urea-based, and it's the controlled-release fertilizer I purchased from the California Cactus Center when I started my new collection last June. Trying to insert fertilizer pellets down through the gravel top dressing of my cacti thrills me not at all, so it'll have to be a liquid fertilizer for me. The CCC does have one, and I'll ask them if their liquid uses a nitrate-ammonium-urea mix. If they do, I'll get the specs and run them past our forum members (Ian especially!) to see if it's the right stuff. If not, hopefully someone here can recommend something good we can find here in the U.S. (I'm pleased to say that when it comes to the "urea free" cult, you can include me out. I'm not interested in fads, so I only care about results if they're backed up with some reliable science.)
Many thanks to Darryl and Ian for helping me out here. I really do appreciate the information resource we have on the forum, and I wish we had the Internet about 40 years ago!