Soil Testing Part Two: Extension Office Results

In a previous post, Soil Testing: A Good Idea,  I used a three dollar test to give a better idea of what was in the soil and how I could work within that framework to better the soil and increase the organic matter in the soil.   As helpful as it was, you get what you pay for.  A three dollar test gave me a color chart and colored dye results.  It was cool to watch, but deriving any detailed results was lacking immensely.  It was if I was asking someone directions and they gave me directions using signs like, “Take the road with the broken tractor and the old truck.”  I have a general direction but I could be misled quite easily. I wanted something like a GPS that would tell me exact directions.  Luckily, I found out that the local Ag Extension office in Weslaco was sending soil samples to their testing office for free.  (Found out upon arrival, however, that if you don’t have a farm number you would have to pay 10 dollars for shipping.  Normal costs can range from 10-75 dollars.  Check with your extension office to see if they have a small producer or home garden discount.)

The soil test itself is a little more intense than the three dollar test.  You need at least a gallon bucket that has been sanitized with no leftover soil.  It needs to be washed and scrubbed out so as to not contaminate the sample.  This also needs to be done to anything that will touch the soil, e.g. shovels, trowels, etc.

Once the instruments have been sanitized, you are ready to collect the sample. Since I am probably speaking to people who have less than 5 acres of land the amount of soil sites can be adapted to your needs.  Usually you will be given a bag that contains instructions on how to prepare your sample.  They ask that you prepare 10-15 dig sites per 10 acres.  I personally did 5 on a little more than 1/2 an acre.

Basically, a soil test is this.  Dig a 12″ deep hole.  Scrape a side an inch deep from each site and place that in the bucket.  This is to get the soil layers.  Also dig a 1″ square from the bottom of each site.  This is for nutrient penetration.

Mix the samples and “strain” by hand any large pieces of soil clods breaking them up or removing them.  Rocks, wires, and any foreign materials should be removed as well as any big pieces of compost.  I had broken pieces of cow manure which would contaminate the sample. Seal it up and send it off.

After you send off the sample, it takes anywhere from 3 weeks to a month to get a result.  Here is mine:


pH:  8.5 (Moderately Alkaline)

I already knew that my soil was alkaline.  How much was to be determined.  Optimum growing conditions for vegetables are somewhere around a Ph of 6.5-7.0 So I know now that I need to assess my pH and the availability of nutrients to my plants.  The main thing I need to keep in mind is a possible zinc deficiency and the availability of the phosphorous, potassium, magnesium, iron,  and copper, in the soil.  It isn’t enough to have them in the soil.  They need to be available for the plants to uptake into their systems.

Conductivity: 384 umho/cm

So I had no idea what this meant.  After a quick bit of digging I found a scientific answer:

Apparent soil electrical conductivity is influenced by
a combination of physico-chemical properties including soluble salts, clay content and mineralogy,
soil water content, bulk density, organic matter, and soil temperature; consequently, measurements
of ECa have been used at field scales to map the spatial variation of several edaphic properties: soil
salinity, clay content or depth to clay-rich layers, soil water content, the depth of flood deposited
sands, and organic matter.

I have the answer but this definition was confusing.  I went to the Agrilife extension website and found that the precise definition from above basically means how much soluble salt is in your soil. This determines how much your plants will be affected by the salt in the soil.  If it is high, your plants could be stunted, not produce well, or even die.  I found a chart to elaborate.

So mine is 384 umho or units of millimhos per centimeter.  I fall in the very slightly saline which means some of the most sensitive plants could be affected.

The crazy thing is I had already noticed a little bit of this in my soil.  I have planted strawberries in native soil and in a bed of miracle-gro organic soil.  The ones in the organic soil have flourished and have begun fruiting whilst the native soil plantings are struggling.  There is no difference in sun, water, or spacing. (The miracle gro bed actually is closer spaced than the native soil.)  It confirms what nature has already hinted at.  The valley soil is not the most hospitable towards strawberries.

Nutrients:  (N,P,K,Ca,Mg,S,Na,Fe,Zn,Mn,Cu, & B)

These are both macro and micro nutrients determined to be beneficial for a plant’s complete life cycle.  The only nutrient that seems low is, of course, nitrogen.  The great thing about having your soil professionally tested is that the lab will give you a recommended fertilizer rate.  My rate is around .3 lbs of nitrogen per 1000 sq/ft.  There is an additional recommendation of 1lb nitrogen/1000 sq ft. every 4-6 weeks to maintain vegetative growth.

Final Thoughts

The most important aspect of growing anything, be it grass, vegetables, or fruit, is the soil.  Maintaining a healthy soil will ensure a successful crop for many years to come.

Have a soil test kit on hand to take quick readings.  I recommend the LaMotte’s Gardener’s kit.  It is refillable and give more detailed readings than the 3 dollar kit from home depot.  It is pricey but will last a while. These quick readings can help you if something doesn’t look right with your plants so you can make adjustments.

Get your soil tested at least 2x a year from a professional agency. Usually, the land grant university has a home or market garden program where you can test your soil for around twenty dollars.  Forty a year is not bad with all the information you are given.

Follow the recommendations, but compare results  The experts have given you a recommendation, but they are not on your land as much as you are.  They can give you a general idea, but the specifics are left up to you.  If you modify their feedback and get better results. Bravo.





About ideadonnafarm

I am the resident farmer for IDEA Public Schools in Donna, Texas. I, along with the students, tend an acre right on the school grounds. The things I grow go straight to the students plate, fresh and healthy. This blog is about the projects, ideas (pun), and all around happenings on the IDEA Donna farm. If you live in the area and want to help out, feel free to email me at
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