Managing the soil on your farm is just like stocking a grocery cupboard – so before you set off to buy fertilisers, you should know what your soil already contains. Otherwise, you could use the incorrect fertiliser which will lead to poor crops and could cost you a lot of money.

In an arid country such as South Africa, where grain is often produced under dry-land conditions, the soil’s ability to hold moisture for the crop is one of the most important factors when determining soil potential.

The other factors to consider when assessing the soil are:

• Effective soil depth.
• Soil tilth.
• Clay content.
• A chemical analysis.

The roots of the crop have to be able to penetrate the soil in order to access the moisture and the nutrients it will need to grow.

DIGGING PROFILE HOLES

Before buying land or deciding what to do with the land you already have, it is essential to determine the soil potential of the arable area of the farm. This is done by digging profile holes in selected spots.

The holes are usually 1 m x 1 m in size on the surface and are at least 1.2 m deep. On inspecting the holes, the profile or cross-section of layers will immediately indicate the main soil types on the farm.

ASSESSMENT

To assess the soil, get into the hole and, using a geologist’s hammer or a suitable hand tool (such as a penknife), chip away at the soil, from the top downwards, while taking note of any changes or differences that occur throughout.

The extent or zone of a particular form on an arable area can be determined more exactly by using soil augurs to measure effective root depth and to identify any soil type layers that might limit agricultural potential. These could be plough pan or compacted silt layers near the surface of the soil at cultivatable depth or impenetrable rock or other layers deeper in the profile that would limit root development.

SOIL POTENTIAL

Usually each production area contains soils that are peculiar to a farming area. High-potential soils in each area could look very different in profile, but combined with climatic and fertility factors could be high for a particular area.

This might mean that a high-potential Avalon soil could produce 3,5 tons per hectare, or more, of maize in a good year but cannot be compared to a Hutton soil producing more than 6 tons per hectare of maize in a good year.

The determination of soil potential will take into account:

• Effective root depth.
• Impervious layers.
• Annual rainfall.
• Moisture-holding capacity.
• Fertility.

SOIL NUTRIENTS

As you are well aware, crops need nutrients in order to grow and produce a yield such as grain. The plant takes these nutrients from the soil.
It could be said that the soil is the equivalent of the grocery cupboard. Before you go shopping for food, you need to assess what food there is in the cupboard – otherwise you could be buying more of the foods you already have and forgetting to buy those that you have run out of.

Taking soil samples is similar to taking stock in the grocery cupboard. It is crucial that you take soil samples early, so that you can have the results back in good time to enable you to plan your next crop properly. If you have the results of the soil samples, you will know if liming is required on your land and also which fertiliser to apply, at what rate per hectare.

SOIL SAMPLING

It is essential to take a soil sample before planting a crop on any particular piece of land, in order to determine which fertilisers have to be used if you are to get a good crop.

Here’s how to take a soil sample:
1. Take a number of samples (5 to 10, depending on the size of the field) from the topsoil (the top 15 cm of the soil profile).
2. Mix all the topsoil samples taken from that field, then take a 1 kg sample from the mixture.
3. Take a number of samples (5 to 10, depending on the size of the field) from the subsoil (15 cm to 30 cm below the surface of the soil).
4. Mix all the samples of subsoil from that field, then take a 1 kg sample from the mixture.
5. Label, date and number each sample clearly.
The following information should be included on the label:

• Name of the farmer.
• Name of the farm.
• Postal address of the farmer – including the town.
• Number of the field (make sure you number all your fields and keep a list).

6. Submit the soil samples to a laboratory for analysis.

Remember: if the soil in the field varies considerably, it will be necessary to number it differently – divide the field into parts (according to number of soil types) and give each part a separate number.

Also read: Get the most from your fertiliser

TIP

It would be a good idea to refer to the book: Soil Classification – A Binomial System for South Africa, ISBN 0-621-10784-0.

• Information for this article was obtained from the Grain SA Advanced Production Course.