soil testing question

woohoomanApril 22, 2014

I posted this in the tropical fruits forum but I'm double posting because I really need some good feedback on this one. So, Please excuse.

Just got a Kona Sharwil Avocado tree.

I've read that they like it around a ph of around 6 and I'm pretty sure I'm up there around 7 or more, so I'm going to get it tested professionally.

From the pic I drew with my fine artistic skills in MSPaint, here's a cross section of the plot. The DG and Clay is native and the top fill is 40 year old top fill that been amended many times off and on over those 40 years. Where should I take the samples from since the tree will be growing about half and half in the native and the fill? How deep should I take from? Should I take a bit of the native and a bit of the fill and mix together equally half by volume or weight prior to sending off for testing?



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My experience is with growing blueberries, at pH around 4.5, in Wisconsin, where the native soil has pH around 7.6. To avoid damaging the roots of the blueberry shrub, I take soil samples from the top 2 inches. I am doing soil pH tests once a year, on all of our shrubs. The pH tends to rise, over time, because we also have alkaline well water, so any irrigation done tends to raise the soil pH. If you are going to water your tree using water from a well, or supplied by the city, it would be helpful to know the pH of this water. However, if you are able to collect rainwater, and use this for watering your avocado tree, the rainwater should help keep the pH down around 6. If you are going to use agricultural sulfur to lower soil pH, it is OK to lightly scratch the soil surface, put down the sulfur granules, and cover this with mulch. The sulfur does not have to be dug in deeply, in order to work. The soil bacteria that consume the sulfur are able to find it when it is scattered on the surface. The acid that is slowly released will tend to drift down into the soil.

    Bookmark   April 22, 2014 at 9:42PM
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Yeah. Thanks eric. My muni water is at 8.1 so I'll be doing the sulfur thing. I just want to know exactly how much I should be adding. So, as accurate of soil samples is paramount at this point imo. Just not sure which way to go, sample the fill, sample the native or mix both and sample?


    Bookmark   April 22, 2014 at 10:16PM
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Personally, I would sample the fill and the native soil separately, each to a 6" depth and get them tested separately. Then I'd take the average pH value or perhaps a value that lies between the average and that of the native soil. If there is lime in the native soil as you are implying, it will gradually raise the pH of the fill anyway.

Since you're dealing with such a small area, you could also apply iron sulfate. It's water soluble and faster than sulfur.

    Bookmark   April 22, 2014 at 11:23PM
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johns.coastal.patio(USDA 10b, Sunset 24)

Maybe I am too lackadaisical, but I'd just presume that this is Southern California, and an avocado will grow like a weed ... the Fuerte I planted up in the San Gabriel valley did, on a scrape lot, with just occasional deep watering. I see now that the Fuerte is also supposed to have slightly acid soil (5.0-7.0).

    Bookmark   April 23, 2014 at 9:55AM
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johns.coastal.patio(USDA 10b, Sunset 24)

(My mom still lives there, and I had Fuerte avocado this morning on my breakfast burrito ;-)

    Bookmark   April 23, 2014 at 10:00AM
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Most every soil testing lab will tell you to go to a depth of 4 to 6 inches and collect 15 to 20 core samples from the total area. Mix those core samples together and take out about a pint of that to send in. Remove any stones and large bits of organic matter.

    Bookmark   April 23, 2014 at 11:05AM
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Thanks all.


    Bookmark   April 24, 2014 at 12:40PM
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