Groundnut yield potential is often greatest when soil pH ranges from 5.8 to 6.2. Many soils in Malawi have pH much lower than optimum, and yield and quality of groundnuts will not reach full potential regardless of other inputs unless soil pH is raised to the recommended level. When soil pH is too low, groundnuts won’t respond well to Bradyrhizobia-containing inoculant involved in BNF or to supplemental calcium in the form of gypsum (calcium sulfate).

Elements such as boron, manganese, molybdenum, and zinc can be affected by soil pH. The proper balance of all elements requires a soil pH around 6.0 for groundnuts. When soil pH is lower than 5.5, the bacteria in inoculant often does not survive.

Lime should be applied several months in advance of sowing groundnut to reduce acidity. The rate of lime needed to raise pH to 5.5 or higher depends on the current pH level and ability to incorporate lime into the upper 25 cm of soil. Although groundnut roots can grow deeper than 25 cm, neutralizing acidity of soils in the rooting zone is important. To increase soil pH by a factor of 0.5 units (for example, increasing soil pH from 5.0 to 5.5) requires approximately 1,000 kg/ha of dolomitic lime on a broadcast basis. To increase soil pH from 5.0 to the desired level of 6.0 requires approximately 2,000 kg/ha.

Soil samples taken in 2021 from more than 850 maize, groundnut, and soybean fields in five selected districts (Lilongwe, Salima, Ntchisi, Kasungu, and Mzimba) showed a range of pH levels, but the vast majority were between 4.0 and 6.0. Initial observations indicated that the average soil pH was approximately 5.2 which is lower than optimal. Inputs such as inoculants perform poorly under low soil pH and are most effective when the pH is close to 6.0.

Soil Fertility and Plant Nutrition

Groundnut can benefit from residual fertility and in some cases no additional fertilizer may be required if the crop is grown on well-managed land previously treated with a balanced fertilizer. However, if native fertility is low added fertilizer will increase yield. Most soils in rain fed agriculture contain very little phosphorus, so to ensure good plant establishment and high yield, farmers are encouraged to apply superphosphate before or at sowing. Use of single super phosphate, provides the crop with phosphate, sulphate and calcium which are all essential for crop growth and development.

Generally, groundnuts perform well following a well fertilized maize or tobacco crop. Phosphorus, calcium and sulfur-containing fertilizers such as 23:21:0+4S can increase groundnut yields even when applied to the previous crop. Superphosphate at 100 kg/ha applied at planting is recommended for groundnut in fields that have low native fertility levels. The fertilizer should be applied in a band on the ridge, or broadcast onto the soil and ploughed under before sowing. Studies in 2020 and 2021 indicate inconsistent, but often significant yield increases from the use of inorganic fertilizers. These inputs did not appear to be consistent across varieties and more research is needed.

Biological Nitrogen Fixation (BNF)

Many leguminous plants, including groundnut, can form a symbiotic relationship with specific bacteria (Bradyrhizobia spp.) that are commonly found in the soil. The bacteria infect the roots of the plant and take nitrogen from the air and make it available to help the plant grow (BNF) in exchange for nutrients from the plant. This exchange happens in small nodules found on the roots of the plants. Active BNF can be confirmed by slicing open a nodule to see if there is a red or rust color, indicating the active conversion of nitrogen. Specific strains of Bradyrhizobia have been identified to maximize this production for groundnut and are available as commercial inoculants that are usually coated to the seed prior to planting.

Unlike soy, which requires a specific strain of bacteria for BNF, groundnut may or may not benefit from inoculant. Inoculant can increase nodulation and nitrogen-fixing, leading to higher yield. However, in fields where groundnuts have been grown in the past, bacteria populations may already be adequate, making inoculant unnecessary. Or, if the soil pH is too low, inoculant may be ineffective until the pH is adjusted (a process described elsewhere in the guide). Recent trials conducted in Malawi indicate a benefit from the use of inoculants and increased biological nitrogen fixation and yield, but this was not true at every location or with every variety. Data trends do indicate value in using inoculant, particularly if soil pH is higher (above 5.5).

Since inoculants are living organisms, they can also be damaged by poor handling, such as storing in extreme heat. When using an inoculant, farmers should acquire their supply from a reputable dealer, follow the manufacturer’s directions carefully, apply the inoculant immediately before planting and purchase a new supply every year to ensure the maximum number of living bacteria. Pesticide seed treatments may harm inoculant performance, so farmers who use both should follow manufacturer’s directions closely. Direct contact with synthetic fertilizers in the soil also may harm inoculant health, so farmers should consider how fertilizers are applied at planting. Do not apply fertilizer in-furrow where it can contact the seed.

Apply Calcium at First Flowering

Calcium is the most limiting nutrient in sandy soils and where medium- to large-seeded varieties such as the Virginia-types are grown. The large-seeded Virginia types have larger kernels which often require more calcium to ensure kernel formation and seed development than small-seeded Spanish varieties do. Topdressing with gypsum when 30% of the plants have flowered will help to correct calcium deficiency and reduce the number of groundnut pods without kernels or malformed kernels. Gypsum should be applied at the rate of 200 kg/ha directly at the base of the plant where the pegs will enter the soil. Under low soil pH conditions, which is common in Malawi, the application of gypsum may reduce yield. For example, data from the USA showed that a positive response to gypsum occurs when soil pH is around 6.0. At pH around 5.5 a decrease in yield was noted when gypsum was applied, at pH 5.2 there was no response to gypsum, and at pH 4.5 gypsum increased yield. Other results indicated that increasing gypsum rates above those currently recommended also reduced yield when pH was 5.5. These results indicate that a clear description of gypsum rate and application pH.

Studies conducted in the 2020-2021 growing season on the impact of boron and calcium applications demonstrated a strong positive impact on yield across two locations, both Chitala and CG9 varieties, and when applied at different dates in fields with soil pH near 6.0.

pH and Soil Fertility