Fertilising beans

With dry beans, general fertility is better than direct fertilisation.

A land of kidney beans coming into flower.
Photo: Bill Kerr

Dry beans must be planted in soil that has been previously well fertilised. General fertility is more advantageous than direct fertilisation, because beans are sensitive to high concentrations of minerals. Have your soil analysed and consult an expert about your fertilisation requirements. The following is an overview of the nutrients required.

Macronutrients
About 36kg nitrogen (N), 8kg phosphorus (P) and 18kg potassium (K) are withdrawn from the soil per 1t dry bean seed produced.

Nitrogen
Inoculation of dry bean seed is ineffective. Dry beans cannot meet all their nitrogen requirements through N fixation. It is therefore a good idea to apply all the nitrogen at planting, particularly where undecomposed material has been ploughed in before planting. Nitrogen deficiency is indicated by the lower leaves turning light green, then becoming yellow and eventually dying. Young leaves may be a lighter green than normal. Apply 15kg N/ha where yield potential is 1,5t/ ha; 30kg/ha for 2t/ha; and 45kg/ha for 2,5t/ha.

Phosphorus
Phosphorus fertilisation does not usually produce a dramatic response in yield in dry beans under commercial production. In smaller operations, however, where smaller quantities of fertiliser are applied, yield can be affected. Phosphorus fertiliser must be bandplaced when planting. In low pH soils, utilise P efficiently by bandplacing 3,5cm to the side and 5cm below the seed.
Deficiency symptoms to watch out for are young leaves that are small and dark green, older leaves that age prematurely, plants with short internodes, and reduced branching. Soil analysis is important when it comes to phosphorus. See Table 1 for DAFF’s application guidelines.

Potassium
When dry beans are grown in soils with a high clay content, potassium is not normally a limiting factor. Deficiencies are most likely to occur in sandy soils. The main sign of deficiency is a bright yellow colour that appears initially at the leaf margins and then extends rapidly inwards. See Table 2 for application guidelines.

Micronutrients

Molybdenum
If the soil has a pH (H2O) of less than six, a seed treatment of 100g sodium molybdate per 50kg seed and/or a foliar spray of sodium molybdate at 100g/ha should be given. Deficiency symptoms are similar to those of nitrogen because molybdenum is important for N metabolism.

Zinc
The critical level of zinc in bean tissue is 15ppm to 20ppm (parts per million). Levels higher than 120ppm can decrease yield. The availability of zinc is highest in slightly acidic soils (pH 6,0 to 6,8) and lowest when the pH is higher than 7,4. Deficiency symptoms include pale, yellow leaves, especially between veins and near the tips, and pods that form slowly. The plants generally mature slowly and may become deformed and die.

Manganese
Deficiencies only occur in soils with a high pH value. Symptoms: small leaves with a mosaic yellowing, and veins that remain green. Deficiencies can be corrected by applying manganese sulphate at 15kg/ha to 20kg/ha.

Boron
With boron, toxicity is a more frequent problem than deficiency. Symptoms include chlorosis and dwarfing. Over time, the chlorosis increases and resembles burn, with the leaf margins curling in. Beans should not follow a sunflower crop that has received boron fertiliser.

Iron
Deficiency occurs in saline soils (where pH is higher than 7,4), causing bright yellow leaves. Apply a 1% iron sulphate solution or chelate as a foliar spray.

Source: Dry Bean Production, edited by Dr AJ Liebenberg (Directorate Agricultural Information Services, department of agriculture, in co-operation with the ARC-Grain Crops Institute).