Here’s what to look for in fertilizer for North Texas soils

I was 26 years old when I came to the Metroplex as a county horticulturist with the Extension Service. I had two college degrees in horticulture, and I thought I was passingly smart. But the first time I saw a report sent back from the A&M Soil Testing Lab calling for only nitrogen to be applied to a tomato garden, I thought my Aggie buddies had flipped their crewcuts.

Soil science tells us we need nitrogen (N) to promote leaf and stem growth. It’s the first number in the 3-number analysis required to be posted on any product sold as a fertilizer in the state of Texas (and all other states).

The middle number, we all were taught at every ag school in the world, promotes roots, flowers, and fruit. That’s phosphorus (P), and that means that it encourages all growth that nitrogen does not.

Hold that note. We’ll come back to those two in just a moment, but let’s finish out our fertilizer analysis by discussing the third number. It’s potassium, also called potash. It goes by its chemical symbol of K, and it doesn’t especially promote any particular type of growth. What it does is give a plant more durability: winter and summer hardiness against rough growing conditions.

So, now let’s go back to the nitrogen and phosphorus thing. How can that renowned soil testing lab at A&M come up with a recommendation that a gardener add nothing but nitrogen to a vegetable patch where tomatoes were being grown? We want fruit! Nitrogen doesn’t promote fruit, or so we learned. I must admit it took me several years before I fully believed what they told me. You may already know the reason for this nitrogen thing, but in case you don’t, I’ll share the odd fact.

Clay soils like we have here in the Metroplex, and like a majority of Texans have for their gardens, have enormous amounts of surface area on which to capture and retain nutrients. Clay particles, you see, are microscopic in size. That’s why clay soils stay wet so much longer than sandy soils as another example.

Still awake? Add to that fact, phosphorus dissolves very slowly in the soil. Agronomists and horticulturists say that it is “very slowly available” for uptake by plants’ roots. It just hangs around, especially when clay particles make it so easy for it to remain attached.

The phosphorus accumulates to a point where it has harmful effects on availability of several trace elements, notably iron and zinc. That can lead to small leaves, off-color foliage, and stunted growth. Adding iron, zinc, and other trace elements may not help, because the phosphorus will still be there blocking their availability.

A dozen or so states, especially those in the Northeast where summer rainfalls are heavier, have even gone so far as to ban inclusion of phosphorus in agricultural and home lawn fertilizers. The restrictions vary from state to state. Some states apply them only to parts of their states or to parts of the year. They’re concerned about the excessive phosphorus being washed into waterways where it can cause algal blooms and oxygen depletion.

All of which is to ask the question, “Why should we be adding phosphorus to home lawns or gardens when soil tests from reputable labs show that we already have very high levels of phosphorus present?” That’s where we are in most parts of Texas.

Of course, the only way you’re going to know for sure is to have your soil tested every couple of years. Monitor levels and how they may shift from one year to the next. The Texas A&M lab is outstanding. You can find all the instructions and mailing address online. They’re busy this time of year as they work with both consumers and farmers and ranchers, but this is a long-term thing anyway, so you will still benefit from finding out.

Next comes the task of finding the best fertilizer. That’s the easy part. Use the 3-number analysis as your guide. Those are percentages of contents. Multiply the percentage of nitrogen times the total weight of the bag and you’ll be able to determine how many pounds of nitrogen you’ll be getting.

At the same time, let the vendor help you read the fine print on the back of the bag. Determine what percent of that nitrogen is in encapsulated or coated form. That would be the “slow-release” portion, and you want to have 30% to 40% of the total nitrogen to be available that way for sustained feeding. The remainder would be fast release for quick green-up. Do not choose for price alone. Ammonium sulfate (21-0-0) is the cheapest, but it’s all fast release, and it’s not good for your lawn.

Oh, and relative to potassium, the third number in the analysis, it just goes along for the ride. Most fertilizers will have some included and that’s fine. Our soils have ample supplies. It’s harmless, and it’s beneficial. Don’t be concerned about it.

Notice that I did not mention a brand name. There is no need to do so. There are many good brands. It all boils down to the cost per pound of actual nitrogen and the percent of slow-release N. Hopefully it will also have a “trace element package” included in it to supply iron, sulfur, zinc, and other minor elements. I would not buy anything else with the fertilizer (no insecticide, and certainly no weedkiller included within the bag).

What about root-stimulator and water-soluble foods?

You may be wondering about my recommendations to apply a high-phosphate “root-stimulator” fertilizer to all newly transplanted trees and shrubs that have been dug and relocated. Or about my recommendations of using a “complete-and-balanced,” water-soluble plant food for patio pots and hanging baskets.

In both cases, those are very precise and targeted applications of water-soluble phosphorus that will be utilized by those plants quickly and efficiently to (a) produce new roots following the transplanting and (b) produce flowers and fruit abundantly while being grown in containers.