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Help reading a soil test

Posted by dyess_gardener 9 (My Page) on
Wed, Oct 2, 13 at 7:19

I got my soil test back today. I'm not sure what the ideal range of each element is. Any one care to educate me?

PH (1:1 Slurry). 6.88
% organic matter. 1.13
Phosporus ppm. 27.88
Potassium ppm. 48
Calcium ppm. 3100
Magnesium ppm. 43


Follow-Up Postings:

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RE: Help reading a soil test

For each element, along with your stated results there should have been a "critical level" cited. The value will depend upon how the test was conducted (method), and the calibration or correlation levels for specific crops or growing uses on soils typical of your state or region.

In general you want to have the critical level for each element available from your soil, without significantly overdoing things. Of the things you had tested, P is one that can be a problem if present in appreciable excess. For the others there's a pretty wide range of tolerance. Was your test done by a private lab? Most State labs also include specific fertilizer recommendations based upon your results, state soil characteristics, and what you plan to grow.

Your page says you're in Guam and mentions container garden. But with the low soil OM (1.13%) I suspect this isn't from containers? If you were in TX, I would say you might be a bit low in P (CL= 50 ppm) depending on method details, very low in K (CL = 175 ppm), and maybe a wee bit low in Mg (CL=50 ppm). pH looks good for general planting, and if this were soil in the ground you would like also like to increase the OM. But none of that may be appropriate for your location or the methods used for your tests.


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RE: Help reading a soil test

  • Posted by nil13 z21 Mt. Washington L (My Page) on
    Wed, Oct 2, 13 at 11:07

Guam? Did you test for sodium?


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RE: Help reading a soil test

I had it tested at the University of Guam. What I typed up is the only thing on the page that I got back. The test was supposed to cost $10 but they wouldn't let me pay when I dropped the sample off. Maybe I'm supposed to go back to U of G and pay to get the rest of the results?

I do a container garden but I have recently acquired a 21x20 plot for a traditional garden and that is what the soil test was done on.

I could look at the soil and tell it was low on organic matter so I have added a truck load of horse manure to the soil since the soil sample was pulled.

No sodium test was conducted.


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RE: Help reading a soil test

The fact that they didn't provide critical levels tells me they probably don't have calibration studies for Guam soils. I don't believe there is much in the way of large scale ag on Guam, so it wouldn't surprise me they don't have the levels.

If you can find out the extraction (Mehlich III or other, especially for P) and determination methods (ICP vs. colorimteric), we could probably give you a better idea of where you may be short.

If you haven't looked already you might check out the Guam Soil Series from the same people who did your test - see link below. From a quick perusal, all the soils appear to be quite high in OM. Many are very shallow over bedrock or limestone. Your analysis seems closest to the Kagman series, but your OM is much lower.

I think, especially after the manure, the thing you need to bring up is the K level - the rest should be pretty good for general veg culture. What do you plan on growing?

Here is a link that might be useful: Guam Soils

This post was edited by TXEB on Wed, Oct 2, 13 at 20:55


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RE: Help reading a soil test

Soil pH is quite good. A range of 6.2 to 6.8 has been found to be what most plants need to grow well and is where most soil nutrients are most available.
Phosphorus around 50 ppm.
Potash around 20 ppm
The Calcium to Magnesium ratio is somewhat difficult to pin down since most research looks at yields and not plant health. Plants need both in some ratio to properly utilize the other. Your soil test would te4ll me that your soil lacks adequate levels of Magnesium.
Soil organic matter is very low. That should be in the 6 to 8 percent range.


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RE: Help reading a soil test

Good info guys.

I plan to grow tomatoes, bell peppers, beans, corn, okra, not sure what else at this point. I would like to be ready to plant by December and I plan on going with organic fertilizers.


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RE: Help reading a soil test

In the high CEC soils of TX using Mehlich III extraction and ICP determination, the CL for K is 175 ppm, P is 50 ppm, and Mg is 50 ppm.

Given your analysis you need to add ~ 0.5 lbs of potassium and about 0.10 lb of phosphorus per 100 sq feet to approach the TX critical levels. Those are based upon the elements (K and P), not the K2O or P2O5 stuff used in fertilizer analysis.

As far as organics, a lot of that will depend on what you can get locally. You probably got some of all with the primary and secondary nutrients with the manure. How much depends upon the state of the manure and how much you added.

The balanced soil concept (e.g., Ca-Mg ratio) based up cation saturation ratios has been repeatedly disproven as having an bearing on plant productivity or health. So long as you have sufficient levels of both (you have plenty of Ca and are extremely close on Mg)and nothing extremely absurd (you don't), and the soil pH is in the proper range (your's is) plant's will get what they need.


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RE: Help reading a soil test

I'll endorse the Mehlic III test as it gives the Cation exchange capacities and percent saturation. It's essential because otherwise you do not have ratios of Ca to Mg and desired % P and K. The ideal Soil by Astera is a good Handbook and from the questions here it would help greatly to understand these questions. We need to know what's in the soil and how to balance minerals to get nutrients into our foods.


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RE: Help reading a soil test

On cation ratio and balance, I suggest the following:

1. From the University of MN, which concludes:
"The optimum soil cation ratio concept, developed about 50 years ago, has been incorporated into some fertilizer recommendation philosophies in various ways. Recent field evaluations of this concept, however, show that the ratio of cations has no impact on the response of crops to Ca, Mg, and K in fertilizer programs. The optimum cation ratio concept has a major disadvantage in that even if the ratio of cations in the soil is considered to be optimum, a nutrient deficiency may still exist. A sufficient supply of available cations in the root zone is the most important consideration in making economic fertilizer recommendations"

2.From Iowa State University, in which the following will be found:
"Various greenhouse and field trials indicate that crop productivity is not influenced by ranges from less than 1:1 to more than 25:1--ratios outside of what is normally measured in soils. Plants also play a role in Ca and Mg uptake and exclude excess Ca or Mg at the root surface."

3.From the University of KY, a summary of studies comparing different basis for fertilization.

4.This review by Kopittke and Menzies in which it is stated "Our examination of data from numerous studies (particularly those of Albrecht and Bear themselves) would suggest that, within the ranges commonly found in soils, the chemical, physical, and biological fertility of a soil is generally not influenced by the ratios of Ca, Mg, and K. The data do not support the claims of the BCSR, and continued promotion of the BCSR will result in the inefficient use of resources in agriculture and horticulture."

5. This study out of Iowa State that compared catio stauratio ratio with sufficiency, and concluded the following:
"Soil fertility treatment did not affect crop yield to the extent of achieving statistical significance. The small yield differences that were observed cannot be positively attributed either to the effectiveness of the cation ratio philosophy or to what the sufficiency approach would class as excessive fertilization.
Fertilizer and lime costs averaged $9.27 per acre per year greater with the cation ratio (CR) approach than the SLAN (sufficiency level of available nutrients). This is before application expenses and assuming local sources for calcitic limestone. The expense outweighed the possible yield advantages at conventional grain prices. Assuming organic prices, only soybean demonstrated a yield increase of greater value than the input cost; overall, the three-crop organic system was less profitable using the CR approach. "

This post was edited by TXEB on Thu, Oct 3, 13 at 18:45


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RE: Help reading a soil test

You want to find a lab that will test for soil pH and major nutrients, P, K, Ca, and Mg. That will need to be a Canadian lab since sending soils across international borders is greatly frowned upon. My understanding is that AgCanada does not do soil tests, but they will supply a list of labs that will.


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RE: Help reading a soil test

While calcium to magnesium ratios may not be critical to plant nutrition, I have read that ratios too high in magnesium can make clay soil very sticky and dry out very cloddy.


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RE: Help reading a soil test

wayne - there has been research in constructed clay soils where calcium and magnesium were incorporated in various ratios relative to each other varying from 100% Ca to 100% Mg. Those works showed that when Mg dominates the clay exchange sites that the fine clay particles do not flock or aggregate as well. Further, when irrigated via a rain simulator the Mg-rich clays initially absorb more water, swell more, then self seal reducing the infiltration rate. It is explained as the effect of the larger radius of a hydrated Mg ion compared to a hydrated Ca ion. The Mg ends up keeping the fine clay particles further separated and non-aggregated. Excessive sodium has a similar effect in clay soils.

The weight ratio of Mg to Ca (think ppm in soil) would need to be ~1/2X to have equivalent exchange binding capacity. Beyond that, Mg begins to dominate. I don't know of any natural, arable soils in North America where that's the case.

A side effect of excessive or very high Mg levels in soils is that it tends to displace and mobilize K, leading to a potential loss of and deficiency in soil K.

My suspicion (read SWAG) is that soil organic material is a great moderator of the effects.


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