Monday, June 29, 2009

Landscape and Turf - Low pH and Poor Plant Performance

The following is information on low pH and poor plant performance in the landscape.

Over the years, I have observed plant growth problems in the landscape associated with low pH. In most of these cases, the soil in the poor growth area was quite acidic, pH in the 5.2 or lower. Available soil nutrient levels were often adequate. However, the plant tissue analysis in these samples showed low phosphorus and magnesium levels and in some cases elevated levels of iron and aluminum. Without soil pH information, one may have concluded that the plant growth problem was due to inadequate available phosphorus or magnesium in the soil.

The remedy is applied needed lime to those individual spots, not applying phosphorus or magnesium. When the soil pH is below 5.2, availability of aluminum, iron and manganese increases significantly. This results in increased uptake of these three elements. Aluminum and manganese begin to accumulate to the point of becoming slightly toxic and reducing root growth. Reduced root growth results in greatly reduced uptake of phosphorus, which is relatively immobile in the soil. Uptake of other nutrients will also be reduced, resulting in poor growth. Magnesium availability decreases as the soil pH decreases, especially below 5.2, resulting in low levels in the plant. Also, under acidic soil conditions microbial activity is reduced, resulting in reduced mineralization of nitrogen and sulfur from soil organic matter and organic soil ammendment.

It is not unusual to see acid spots in landscapes even with overall soil pHs in the 5.8 to 6.5 range. These spots can vary greatly in size. Theses spots are usually found in newly developed properties where subsoil was exposed or inadequate topsoil replaced, in the sandiest areas, or in turf were excessive N fertilizer is being used. Subsoil is naturally acid in DE and exposed subsoil or inadequate replacement of topsoil will result in acid spots. Excessive applications of N fertilizers results in areas becoming acidic more quickly. In very sandy spots, excess leaching of basic cations results pH dropping rapidly. When diagnosing plant growth problems, be sure to check soil pH first and look at the plant roots.

Adapted from "Soil pH, the source of plant growth problems" from Darryl Warncke, Crop and Soil Sciences, Michigan State University on the MSU Field Crop Advisory Alert web site.

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