The cost of fertiliser is a substantial proportion of the total production cost of wheat and the optimalisation of fertilising practices is therefore of the utmost importance.
The development of specifically adapted cultivars over the past few years has necessitated the planning of a fertilisation programme by the producer on an annual basis. As with cultivar choice, a fertilisation programme is planned on the basis of a specific yield potential or target yield. The following guidelines can be used as a reference to plan such a programme for a given situation.
Reliable soil analysis data is essential for planning an effective fertilization programme. The regular sampling of lands to timeously identify problems, such as soil acidification, is absolutely essential.
Soil sampling for analysis
Soil is analysed to determine its ability to supply the necessary plant nutrients to the crop concerned. Soil analyses are related to potential nutrient uptake, supplementation of plant nutrients through fertilisation and the target yield. From plant nutrient research programmes that take these factors into account, guidelines that will be valid in a given situation are laid down.
Therefore, to make the best possible use of these guidelines, it is essential that the soil samples that are interpreted are representative of the particular land. To achieve this, the following standard procedures are required when handling soil samples:
• Homogeneous units that are also practical for crop production purposes must be sampled. (Homogeneity is determined by previous crop performance, topography and the soil depth, colour and texture).
• A soil sample must represent a homogeneous unit of not more than 50 ha.
• Homogeneous units must be numbered clearly and separately.
• Problem/poor patches must be indicated and sampled separately.
• When taking the sample, all foreign matter (grass, twigs, loose stones) must be removed at the sampling point. In the case of very rocky soils an estimate must be made of the rock percentage per volume.
• Twenty to 40 samples must be taken at random over the entire area of each homogeneous unit of the land. Conspicuously poor patches, headlands, places were animals gather, et cetera, must be avoided.
• The recommended depth for sampling the topsoil is about 200 mm, in other words the 0-200 mm portion of the topsoil is sampled.
• Subsoil samples must be taken from the 300-600 mm layer of the profile for dryland cultivation, and at 300-600 and from 600-1 200 mm for irrigation.
• If the land has been ploughed, random samples must be taken from the entire area. If the rows of the previous crop are still visible, the samples must be taken randomly between and in the rows.
• To compare results, sampling should be done at more or less the same time of the year every year, or during the same phase of the cultivation programme, but at least once every 3 years.
• The 20-40 samples from which the final sample is to be compiled must be collected in a clean bag. (Farmers are warned against using salt bags, fertiliser bags or other contaminated containers). Clods must be crushed, foreign matter removed, and the soil must be mixed thoroughly. After spreading the soil in a thin layer, small scoops are taken evenly over the whole depth and area and placed in a clean plastic bag or carton. This final sample, representative of a homogeneous unit, must have a mass of 0,5-1,0 kg.
Additional information about the properties of the soil, climate, as well as the production and fertilisation history should also be furnished, since recommendations cannot be based on soil analysis alone. The Soil Laboratory at ARC-Small Grain Institute offers free sample boxes and fertiliser recommendations for wheat. The laboratory can be contacted for further information at 058-307 3501.