Understanding the effects of soil pH on crop productivity

Soil pH

• The effects of soil pH on crop productivity

Key Points:

• Soil pH is a measure of the concentration of hydrogen ions in the soil solution. The lower the pH of soil, the greater the acidity.
• A well maintained soil pH will maintain the value of the soil resource, maximize crop and pasture choice and avoid production losses due to low pH.

Soil pH

• Soil acidity is measured in pH units. Soil pH is a measure of the concentration of hydrogen ions in the soil solution. Soils become acidic when basic elements such as calcium, magnesium, sodium and potassium held by soil colloids are replaced by hydrogen ions. The lower the pH of soil, the greater the acidity. pH is measured on a logarithmic scale from 1 to 14, with 7 being neutral. A soil with a pH of 4 has 10 times more acid than a soil with a pH of 5 and 100 times more acid than a soil with a pH of 6.

Desirable soil pH for optimum crop production pH range

• The desirable pH range for optimum plant growth varies among crops. While some crops grow best in the 6.0 to 7.0 range, others grow well under slightly acidic conditions. Soil properties that influence the need for and response to lime vary by region. A knowledge of the soil and the crop is important in managing soil pH for the best crop performance.

Effects of soil acidity:

Nutrient availability

• Plant growth and most soil processes are favored by a soil pH range of 5.5 – 6.5. Acid soils particularly in the sub-surface will also restrict root access to water and nutrients. In very acid soils, all the major plant nutrients – nitrogen, phosphorus, potassium, sulphur, calcium, manganese and also the trace element molybdenum may be unavailable, or only available in insufficient quantities. Plants can show deficiency symptoms despite adequate fertilizer application.

Aluminium toxicity

• When soil pH drops, aluminium becomes soluble. A small drop in pH can result in a large increase in soluble aluminium. In this form aluminium retards root growth, restricting access to water and nutrients. Poor crop and pasture growth, yield reduction and smaller grain size occur as a result of inadequate water and nutrition. The effects of aluminium toxicity on crops are usually most noticeable in seasons with a dry finish as plants have restricted access to stored subsoil water for grain filling.

Herbicide persistence

• Soil pH affects how long some herbicides persist for and how available they are for plant uptake and soil binding.

Microbial activity

• Low pH in top soils may affect microbial activity, most notably decreasing legume nodulation. The resulting nitrogen deficiency may be indicated by reddening of stems and petioles on pasture legumes, or yellowing and death of oldest leaves on grain legumes. Rhizobia bacteria are greatly reduced in acid soils. Some pasture legumes may fail to persist due to the inability of reduced rhizobia populations to successfully nodulate roots and form a functioning symbiosis.

In the next article we will highlight on the relevance of soil testing and how to take soil samples for analysis to maximize crop productivity.

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