Soil testing is a valuable tool for determining the fertilizer and limestone needs of field crops, lawns and gardens. Collecting a representative soil sample is the first step in getting accurate, reliable results. Soil pH is a measure of the acidity of the soil. The University of Missouri Soil Testing Laboratory measures salt pH. Some labs measure the soil pH in water. As a general rule, water pH is 0.5 units higher than salt pH.

When the soil test report indicates the pH is too low for optimum nutrient uptake by growing plants, limestone will need to be applied. Autumn is a good time to make limestone applications since it takes three to six months for limestone to breakdown and have the full effect on soil pH. Most plants require a near neutral pH between 6.0 and 6.5 for optimum growth and production. There are exceptions, for example blueberries need a more acidic soil, requiring a pH near 5.0. When soil pH is too high or too low, nutrients are tied up in the soil and are not available for plant growth.

Research at the University of Missouri reported that soil pH can have a dramatic effect on nutrient availability. When soil pH was at the optimum level of 6.5, 100 percent of the nitrogen, phosphorus and potassium applied was available for plant growth. But, as pH decreased, so did nutrient availability. When soil pH was 4.0 it took three to four times as much fertilizer to obtain the same level of nutrient availability. Only 30% of the nitrogen, 23% of the phosphorus, and 33% of the potassium applied were available for plant uptake in this study when soil pH dropped to 4.0.

For more information, contact Valerie Tate, University of Missouri Extension Agronomist at or 660-895-5123. MU Extension programs are open to all.