Tobacco farmers in Zimbabwe are preparing for the coming planting season. Part of this process is raising the seedlings that will be transplanted onto the fields later in the year.
To ensure the best possible outcome, the conditions in which the seedlings are planted need to be perfect. This includes having healthy soil that contains sufficient nutrients and has the correct pH level. Tobacco is sensitive to acidic soils, and the pH balance in the soil can make the difference between a good or bad harvest. It is therefore very important to know what the pH level and the chemical composition of the soil is. These principles of soil sampling also hold true for the production of other crops.
There are options when it comes to soil sampling. The most effortless, and also the most expensive one, is to have professionals come and take samples from your soil, and then test and analyse them in a laboratory.
The more cost-effective way is to take the samples yourself and send it to a laboratory for tests and analysis.
When taking your own soil samples, there are a number of factors to consider that can impact on the accuracy of the tests.
• When to take samples
• Where to take the samples
• Tools used
• Depth of sampling
• Correct handling of samples
The time of year that you take the soil samples is crucial to success. Samples should be taken just before you plan to use that particular piece of land. Keep in mind that you also cannot wait too long, because you need enough time to apply corrective measures if needed.
It is also best to collect samples from fields before any soil treatment is performed. Collecting samples after fertilisers or compost have been added may lead to inaccurate sampling. Also avoid sampling in areas where there is too much water. Soil should be moist, but not muddy, for the most accurate samples.
As with any other job, sampling is a lot easier with the right tools. It is recommended to use proper coring equipment because it allows the samples to clearly separate the different layers. This will enable you to get accurate results for each layer. Using an auger will result in mixing the layers and will cause less accurate results at the depth that you plan to plant.
To test the complete profile of the soil, you need to take samples fromthree different depths:
• First the topsoil that is found in the first 15 cm from the surface.
• The second layer is from 15 cm to 30 cm.
• The third layer that should be sampled is between 30 cm and 60 cm beneath the surface.
There are different methods of sampling that you can choose from. It is important to keep in mind that the sample should be as representative of the field as possible. The gradient of the soil surface and other variables can result in soil having different levels of nutrients in different areas. The best way to overcome this is by taking multiple samples from the same area.
The following guidelines will enable you to take samples that can produce the most accurate results:
• Between 15 and 20 samples should be taken to improve accuracy.
• Separate each core sample at the depths of 0 cm to 15 cm, 15 cm to 30 cm and 30 cm to 60 cm.
• After each of the cores have been separated into the different depth categories, the samples of similar depths can be mixed. Store these mixed samples in clearly marked containers. These containers should be plastic and not metal as the metal from the container could influence the samples.
• Take 500 g of each of the three mixed samples to send away for testing.
• When fields are sloped, take samples from halfway up the slope to get a good average soil quality.
Nutrients tend to accumulate at the bottom of a slope due to drainage. The top of the slopes will also have less than the average quantity of nutrients.
Ideally, you would test soil samples before every planting season. This is not always practical due to cost and the labour intensity of soil sampling. Some farmers tend to sample their soil every second year. This enables them to apply fertilisers as accurately as possible and using the previous year’s tests in alternating years.
When calculating the costs of these tests, keep in mind what the fertilisers will cost that might be applied ineffectively and the impact that it can have on the harvest. Carefully calculate the risks before deciding on a sampling strategy for your farm.