Soil pH: The source of plant growth problems

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Recently, I have talked with a number of people that have observed plant growth problems in corn. Fortunately, some of these had taken both soil and plant tissue samples from the problem area and the good area. In each of these cases, the soil in the poor growth area was quite acidic, pH in the 5.0 to 5.4 range. Available soil nutrient levels were generally 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 5.5, 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 5.5, 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, crop residues and manures.

It is not unusual to see acid spots in fields that have overall soil pHs in the 6.5 to 7.0 range. These may range in size from 50 feet in diameter to half an acre or more. Theses spots are most likely related to differences in the underlying subsoil which results in these areas becoming acidic more quickly than the field as a whole. When diagnosing plant growth problems, be sure to check soil pH first and look at the plant roots.

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