Jan. 31, 2001
Soil's pH is one of its most important properties. Extremes in pH can seriously affect plant growth, herbicide interactions and resistance to disease.
The term pH defines the relative acidity or alkalinity of a substance. The pH scale ranges from 0 (most acid) to 14 (most alkaline); a pH value of 7.0 is neutral.
Each unit of change represents a tenfold change in the degree of acidity or alkalinity. In other words, a pH of 8.0 is 10 times more alkaline than a pH of 7.0 and a pH of 5.0 is 10 times more acid than a pH of 6.0.
Factors that influence soil pH include:
- The parent rock from which the soil was formed (alkaline or acid)
- Rain and water leaching through the soil; basic elements (especially calcium and magnesium) are replaced by acidic elements (hydrogen, manganese and aluminum)
- Vegetation (forest soils are more acidic than soils under grasslands)
- Fertilization (as nitrogen rates increase, soil acidity increases)
- Crop removal of alkaline elements (a 50 bushel soybean crop removes 10 pounds per acre of calcium and 15 pounds per acre of magnesium) Crops can grow in a wide soil pH range. However, yields are higher and stresses lower at or near the ideal pH for the given crop.
Nutrient Availability Influenced by Soil pH
Nutrient availability changes with pH - the higher the availability, the greater the uptake by the plant. A fine balance is needed to avoid stress due to too little or too much of a given nutrient. In the range from pH 6.0 to 7.0, most of the essential nutrients are available.
Managing pH in Acid Soils
The application of lime corrects low-pH soils and provides measurable benefits to crop production.
- Liming:
- Improves soil's physical condition
- Stimulates microbial activity in the soil
- Makes minerals more available to plants
- Supplies calcium and magnesium for plants
- Improves symbiotic N fixation by legumes.
The most important factor that determines the effectiveness of lime is placement. Distributing lime in the soil so that moisture is contacted is essential for lime-soil reaction to occur. Lime placed on the soil surface reacts more slowly than - and not as completely as - lime mixed with the soil. Pelletized lime has a high neutralizing value and excellent fineness (usually 100 percent passing through 50 mesh screens).
How to Manage High-pH Soils
High-pH soils can be managed for improved productivity by:
- Banding of fertilizer
- Using acid-forming fertilizer
- Applying gypsum (CaSO4) annually
In 1998 and 1999, Agri-Growth researchers tested how adding pelletized gypsum (SuperCal SO4) affects the yields of corn and soybeans. The soil pH ranged from 7.5 to 7.9. Yields were increased an average of 10.5 bushels per acre in corn and 7.1 bushels per acre in soybeans over the two-year study.
Managing soil pH can mean the difference between healthy growing crops and crop failure. Applying lime to acid soils and gypsum to alkaline soils will make a more favorable soil environment.
Dr. James Ladlie is the founder of Agri-Growth Inc., Hollandale, Minn. Dr. Ladlie conducts workshops in North America for ag professionals on crop production systems. Article edited by Dan Hopper.
