Herself's Houston Garden

Conservation through cultivation

Soil testing is the best bargain out there for gardeners




I bet you all know that you can ship off a pint of soil to be tested and the government will tell you all about it. I’ve known about it for years. I’ve even thought about doing it several times. But I never seemed to get there. This year I will be doing it.

If you go to Texas A&M University Soil, Water and Forage Testing Lab’s webpage you can get detailed instructions, a form to fill out and everything you need to do a soil sample. You can also go to your local extension office and pick up the form and a soil sample bag.

So why is this the best bargain for gardeners? It’s cheap, dirt cheap actually. For $10 they tell you your soil’s pH, Nitrogen, Phosphorus, Potassium, Calcium, Magnesium, Sodium, and Conductivity. For $15 they add in Zinc, Iron, Copper and Manganese, for $20 they check the organic matter also and for $25 they add in the Micro nutrients.

Soil testing will tell you exactly what you need to add to keep your plants thriving. So your plants thrive and you are not wasting money on chemicals you don’t need. Fertilizer is expensive. And probably you are using way more than you need. Unless it is a lawn, vegetable or other crop you take away there is likely plenty of most of what you need already in the soil.

The most important part for you is to get a good sample. You want to grab about 10 soil samples. Fetch some from each bed. Samples should be about 1″ across and 6″ deep. Just put your shovel in and grab a 6″ clod of soil. Then just grab the middle column.

Put all your samples in a clean plastic bucket. Mix well. Then fill the soil bag with the mixture and ship it off with your form and a check.

I mailed the test out Jan. 19th and received the results Feb. 2. Amazing, it is a wonder any thing is alive and no wonder I wasn’t getting any tomatoes. Here’s the results:

Analysis Results Critical Level ExtLow, VLow, Low, Mod, High, VHigh, Excess Recommendation
pH 8.0 6.5 Mod. Alkaline
Conductivity 181 None
Nitrate 3 ExtLow 1.3 lbs N/1,000 sqft
Phosphorus 34 50 Moderate Need 1.2 lbs P2O5/1,000 sqft
Potassium 80 175 Low 2.1 lbs K2O/1,000 sqft
Calcium 3349 180 High
Magnesium 131 50 High
Sulfur 24 13 High
Sodium 298 Low
Iron 27.06 4.25 High
Zinc 3.54 .27 High
Manganese 7.9 1.00 High
Copper .96 .16 High

4.23 % Organic matter in soil


So I need to find fertilizer with a similar ratio to 1.3:1.2:2.1 ( 1:1:2 or 1:1:1 ).Then I want to match the nitrogen needed. 1.3 lbs per 1,000 sqft.I have ~5,000 sqft of lawn and garden so 1.3 * 5 = 6.5 lbs N are needed.Now fertilizer is sold in volume per weight so a bag of 10:10:10 is 10% N. A 20 lb bag would give me 2 lbs of N, I’d need a little more than 3 bags spread evenly over the lawn and gardens. The Extension recommends matching the ratio as close as you can, then buying the correct amount of N.

Since Nitrates are the most important of the plant nutrients and mine were below Extremely Low on the chart I’m not surprised some plants are showing nutritional problems. The recommendation is to put out 1.3 lbs Nitrogen, 1.2 lbs Phosphorus, 2.1 lbs Potassium/1,000 now, and 1.3 lbs Nitrogen/1,000 monthly.

I received a separate recommendation for garden, roses and lawn. But I’m removing the lawn and the rose and garden recommendations were the same. So I’ll just follow those.

Soil conductivity runs from 0-5 for sandy soils, 5-25 for topsoils, and 10-1000 for clay soils. Soil conductivity measures the amount of soluble salts in a soil. Some plants are sensitive to high salts in the soil. Too much may also inhibit seed germination. Usually high salts are caused from too much fertilizer being added to the soil.

To convert umho/cm to ppm multiply your umho/cm * 640 then divide by 1000. So we have about 116 ppm salts in the soil, which is fine. Until it goes over 1400 ppm you shouldn’t see a problem. But remember some plants are very sensitive to salt so it depends on your plant.

Fresh water (1000)< Brackish ( 10,000 )< Saline ( 100,000)< Brine ( 100,000)

Organic matter breaks down quickly in Houston’s climate. Organic matter usually ranges between 2% to 8%. The more there is the better. If it is less than 2% you’ll want to add some organic matter ( compost, mulch, leaves etc ). There are several methods for testing organic matter. So if you are trying to compare from year to year, use the same lab to do the testing.