Know Your Soil pH
Soil pH is a measure of the acidity or alkalinity found in soils, and it is a key indicator of basic soil chemistry. We define pH as the negative logarithm (base 10) of hydrogen ion (H+) activity in solution. Soils with a pH less than 7.0 are acidic, those with a pH greater than 7.0 are alkaline, while soils with a pH equal to 7.0 are considered neutral. Soil pH is a parameter which can be easily monitored over time with consistent sampling and laboratory techniques.
Soil pH can have direct impacts on nutrient availability and ion toxicity. It is generally agreed that essential plant nutrients are optimally available when the soil pH is between 6.5 and 7.5. Micronutrients
such as copper, iron, manganese and zinc tend to be more available when soils are slightly acidic. Molybdenum becomes more available when soils are slightly alkaline. Aluminum (not an essential plant nutrient) and manganese become much more active in the soil solution when the soil pH falls below 5.2. Their activity can be extremely detrimental to root development and plant growth, and subsequent accumulation can result in toxicity symptoms and effects in the plant. Often, application of lime to the soil is highly beneficial in such situations.
Microbial activity, both beneficial and pathogenic, may be influenced by soil pH. This has important implications in crop residue degradation, nutrient cycling, soil-borne disease development and beneficial associations such as mycorrhizae for efficient nutrient uptake and Rhizobium for nitrogen fixation. It is important to understand the crop you are growing and how soil pH may impact its growth.
Free calcium carbonate, excessive salinity, high levels of organic matter and an abundance of clay minerals all have a tendency to buffer soil pH. Buffering simply means the soil is resistant to a change in pH. For example, many western soils with excess calcium carbonate will often have soil pH measuring somewhere between 7.2 to 8.5. It is difficult to significantly alter the pH of a calcareous soil as long as free calcium carbonate remains. Typically, large quantities of acid-forming amendments are required to remove this free calcium carbonate. It may be more appropriate to manage around this problem. Consider nutrient sources that perform better and resist tie-up at the higher soil pH or placement methods that protect the nutrient and offer extended uptake opportunity. In some situations, smaller amounts of nutrient applied more frequently may improve uptake efficiency. Foliar application of nutrients can bypass the difficulties associated with soil chemistry and is often an efficient and effective means for providing supplemental nutrient needs at key growth stages. Soil pH exceeding 9.0 usually indicates excessive salinity present in the form of sodium. Such soils will benefit from proper amendment application and subsequent leaching to remove the excess salinity.
HyGround® soil management services are an excellent method for monitoring soil pH over extended periods of time. Using guided (GPS) soil sampling, where soil cores are pulled from approximately the same locations year after year, reduces some of the sampling inconsistencies associated with composite samples of random cores within a field. One or two soil cores with free lime present in a composite sample of random cores are sufficient to introduce a high soil pH bias for the entire sample, even though portions of the field may have had significantly lower soil pH readings. Further refinement and definition of spatial soil pH variability can be improved by management zone delineation and smart sampling methods associated with HyGround’s EC mapping functions. Where variable rate application equipment is available, consider using HyGround soil pH surface maps to create a variable rate
recommendation of lime, gypsum, elemental sulfur or other appropriate amendments to manage soil pH and improve productivity.
- Michael Larkin, Manager, Precision Information