You fertilize the lawn, the neighbors' lawns green up, and yours stays patchy and pale. One explanation people rarely consider is soil pH. In Central and South Texas, where limestone bedrock runs close to the surface and caliche layers sit just below the topsoil, alkaline soil is extremely common. Soil pH above 7.5 changes the chemistry of the soil in ways that lock up certain nutrients and make them unavailable to grass roots even when you put them in the ground. Understanding the pH issue is what separates a lawn that responds to treatment from one that does not.
Quick answer
Most San Antonio lawns have alkaline soil (pH 7.5 to 8.5) due to the region's limestone bedrock and caliche layers. Alkaline soil locks up iron, manganese, and some phosphorus, making them unavailable to grass even when you fertilize. The result looks like nutrient deficiency: yellowing, slow growth, patchy green. Sulfur applications can lower pH over time, and iron sulfate supplements address iron chlorosis more directly. A soil test from Texas A&M is a fifteen-dollar investment that tells you exactly what you are dealing with.
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What Soil pH Actually Means
pH is a measure of how acidic or alkaline soil is, on a scale from 1 to 14, with 7 being neutral. Most turfgrasses prefer a slightly acidic to neutral range, roughly 6.0 to 7.0. In that range, nutrients dissolve properly into soil water where roots can absorb them. Outside that range, chemical reactions happen that convert nutrients into insoluble compounds that roots cannot take up.
San Antonio's soils tend to run between 7.5 and 8.5, which is moderately to strongly alkaline. The region sits on the Edwards Plateau limestone formation, and the calcium carbonate in that rock buffers soil strongly toward alkalinity. Even importing topsoil does not always help, because water from limestone-heavy aquifers keeps pushing the pH back up.
Iron Chlorosis: The Most Common Symptom
The most visible sign of alkalinity-related nutrient lockout is iron chlorosis: yellowing leaf tissue with veins that stay green, giving the blades a streaked or mottled look. Iron is essential for chlorophyll production, and it becomes essentially insoluble above pH 7.5. The grass is not iron-deficient in the sense that iron is missing from the soil. The iron is there. The alkalinity is just making it unavailable.
Iron sulfate applied to the lawn, either as a granular soil drench or as a foliar spray, can bypass the pH problem temporarily by delivering iron in a form the plant can use. The foliar spray gives faster visible results, often within a week. The soil application is slower but has longer-lasting effects. Neither is a permanent fix unless you also address the underlying pH.
Lowering pH With Sulfur
Elemental sulfur is the standard amendment for lowering soil pH. Soil bacteria convert it to sulfuric acid, which reacts with calcium carbonate in the soil and slowly brings pH down. The process takes months, not weeks, and in highly calcareous San Antonio soils the pH tends to rebound as new water introduces more calcium carbonate.
Sulfur applications work better in soils with lower caliche content and give slower results in heavily buffered soils. Texas A&M AgriLife recommends regular small applications over time rather than a single large dose, which can damage grass at high rates. Getting a soil test first tells you how much pH movement is actually possible and what rate is appropriate.
- Get a soil test to confirm pH and rule out actual nutrient deficiencies
- Apply iron sulfate for iron chlorosis as a holding measure
- Use elemental sulfur at recommended rates to lower pH over time
- Choose fertilizers with acidifying properties (ammonium sulfate) in high-pH soils
- Retest every one to two years to track movement
Getting a Soil Test
The Texas A&M AgriLife Extension Service offers soil testing through the Bexar County Extension office and online. The basic test covers pH, organic matter, and major nutrients for about fifteen dollars. The report comes with recommendations calibrated to Texas soil conditions and whatever grass type you specify.
The test tells you what is actually in the soil versus what the grass has access to, which are different questions. A lawn with adequate total nutrient levels but high pH may show deficiency symptoms that more fertilizer will not fix. The soil test is how you confirm which problem you actually have.
Fertilizer Choices That Work Better in Alkaline Soil
Some nitrogen fertilizers are more appropriate for high-pH soils. Ammonium sulfate has an acidifying effect and releases nitrogen in a form that is less likely to be lost to volatilization at high pH levels. Urea-based products can lose nitrogen as ammonia gas in warm, high-pH conditions if not watered in immediately after application.
For phosphorus and micronutrients, chelated forms are significantly more available at high pH than standard sulfate or oxide forms. Chelated iron, chelated zinc, and chelated manganese are worth specifying when buying micronutrient products for Central Texas soils. The chelation chemistry keeps them in solution at pH levels where unchelated forms precipitate out.