Posts: 13778
Joined: Tue Jan 01, 2013 8:32 am
Location: Hawaii, zone 12a 587 ft elev.

soil tests on my mygardens are back

I did my last soil test 3 years ago, so it was time for a check. I still don't have the recommendations yet. But I generally know what to do and it helps that my friend is a soil scientist and can can steer me in the right direction

Yr Location pH P K Ca Mg soil type
2013 Wahiawa 7.6 212 472 7577 529 Heavy
2016 Wahiawa 7.7 153 292 8668 635 Heavy

Recommended: sulfate of ammonia (1.91 lbs/ 100 sq ft split into 3 application and organic compost. I added Peat moss (1.5 bales or about 4 inches over a 15x15 sq ft area. and 1/4 cup sulfur with 1/3 of the sulfate of ammonia to lower the pH.
Calcium and Mg are higher and in the high range. P and K have dropped. K is now in sufficient range. P and K were in very high ranges before. P has dropped from very high to high and calcium and magnesium have increased into the high range.
pH remains alkaline.

2013 Mililani 6.4 2154 452 3137 352 Heavy
2016 Mililani 6.0 1620 283 4399 816 Heavy

Recommended 1.53 lb/100 sq ft split into 6 applications of Urea.
The pH of 6.0 is actually the expected value on my soil test. The phosphorus is still in the very high range but it is no longer off the map. It has moved in that direction since I have been fertilizing primarily with sulfate of ammonia and the only source of other nutrients would have been the compost and residual citrus food from the potted plants. The acid fertilizers do bring the pH down.
The addition of compost in both gardens seem to be raising the calcium and magnesium levels in both gardens while decreasing the phosphorus and potassium. I had also reduced the amount of total sulfate of ammonia I had been using which would also affect the calcium and magnesium levels

The higher magnesium levels may be why my soil is harder to work even though it is deep and drains well.

I am going to add more of the alkaline compost that is made by the local composting facility is very alkaline with a pH 8.16. It should improve soil tilth and cec and keep the pH from dropping more. I have to see if I can find Urea in smaller bags than 50 lbs. ... eet(2).pdf

Greener Thumb
Posts: 1216
Joined: Sat Jul 18, 2015 1:10 pm
Location: Zone 8A Western Washington State

I am just wondering about how you do your soil samples. Do you take a sample from one spot and assume that it us the same for the rest of the dirt? Do you take more than one sample and mix them together? How many samples do you mix together?

Posts: 13778
Joined: Tue Jan 01, 2013 8:32 am
Location: Hawaii, zone 12a 587 ft elev.

I take several samples about 2 ft apart in a zigzag pattern. The Wahiawa plot is about 15x15 and the Mililani garden is 8x16 (oval)
I dug a hole with a trowel about 6 inches deep and removed the loose soil on the surface inch or two and took a vertical scraping from each hole. I put everything in a bag, mixed it up and took about 2 cups of each sample and placed it in a labeled ziploc and took it to the UH extension office which sent it to the soil lab in Manoa. It took a couple of weeks to get preliminary test results and I just received the recommendations yesterday (about 3 weeks total). I have a soil scientist friend so when I got the test results and before the recommendations came out I asked him for advice on my plan to use peat moss and sulfur for the Wahiawa plot. He agreed that peat moss was a better choice than alkaline compost. I did not understand why the calcium and magnesium was rising. He said probably because I have been using less sulfate of ammonia but more likely because the manganese levels are high which is common in oxisols. Manganese is not measured on the soil test. K potassium has dropped and that is one of the cations that affect the calcium and magnesium levels. The most important thing was the calcium to magnesium ratio. It did explain why the Mililani garden is so hard.when it is dry. Oxisol is pretty much weathered red clay.
I am going to use the alkaline compost in MIlilani to improve tilth, cec and bring up the pH, but I asked the extension if I could use potassium nitrate for fertilizer. I am still waiting for an answer.

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