The Sunday Mail
Word from the Market
with
Kudakwashe Mashanda
MANY farmers still base their crop nutrition programmes on the broad fertiliser guides that are available for the major crops grown in Zimbabwe. While many basic fertilisation guides exist and are helpful tools, they fall short on giving farmers full information on what their specific soils need.
These guides can be used to provide the general direction for crop production, but for maximum impact, farmers need to take advantage of the relationship between nutrients and their soil.
A full analysis is a treasure trove of information if it is interpreted the right way. Following a tailor-made fertiliser programme will ensure farmers invest in exactly what their crop requires, minimising waste and maximising potential output.
Most farmers who do any form of soil testing are focused purely on their pH levels, which does not give them a full picture. Full soil analysis costs around US$40.
At that price, it is a no-brainer that there is need for any serious farmer to invest in a full soil analysis test at the start of the cropping season. In this article, we shall explore some of the information that a soil test will bring out besides the pH levels.
To raise any plant successfully, farmers need to create the right environment, with the correct nutrients, for plants to thrive.
Cation exchange capacity
Cation exchange capacity (CEC) can be simplified as a figure representing the type of clay, clay content and organic matter that soil contains. CEC gives a good indication of how much water, nutrients, chemicals and fertiliser the soil can hold.
Soils with low CEC hold less water and nutrients, compared to those with high CEC.
CEC also helps in determining the amount of pre-emergence herbicide to be used before planting. In general, sandy soils have a low CEC and will require lower rates of pre-emergence herbicide, compared to clay soils, which have a high CEC.
For practical application, the CEC of soil can be used to make decisions on how to manage nutrients such as nitrogen and potassium.
Soil with low CEC does not hold nutrients well and the likes of nitrogen and potassium may need split application for effective uptake by crops.
To understand how CEC will help in guiding crop nutrition, let us see the example below: To estimate the amount of nitrogen soil can hold, one must multiply their CEC by a factor of 11,2.
Let us say the CEC is 5, it means the soil cannot hold more than 56kg of nitrogen at any particular time.
If the farmer applies more than 56kg of nitrogen, the extra nitrogen would likely be leached before it is used by plants.
If the soil already contains some nitrogen, then this should be subtracted before applying additional nitrogen through fertiliser.
Below are the CEC ranges for general soil types:
Sandy soils 0-10
Loam soils 10-15
Clay loams 15-30
Clay soils 30 and above
Relationships between nutrients
Soil analysis reports give an indication of the relationships between different nutrients. Too much of one nutrient can inhibit the uptake of another.
Too much calcium in alkaline soils can inhibit the uptake of potassium and magnesium, and too much potassium can have the same effect on the uptake of magnesium.
The CEC ratios below are ideal for optimal production:
Potassium > 4-8 percent
Calcium > 65-80 percent
Magnesium > 8-12 percent
Salinity
Salinity refers to the accumulation of water-soluble salts like sodium, calcium and magnesium as chlorides, sulphates or carbonates. Soils with high salinity inhibit the uptake of water by creating high osmotic pressure.
To understand osmotic pressure, think of trying to suck thick liquid through a straw and the energy this takes.
The same is true for a plant — the roots have to work hard to pull water from the soil, and the plant wastes valuable energy trying to get water, instead of promoting growth.
This knowledge of the soil’s sodium and salt levels is imperative when selecting the right fertilisers to use.
Kudakwashe Mashanda is an agronomist with Superfert Fertiliser. Word from the Market is a column produced by the Agricultural Marketing Authority (AMA) to promote market-driven production. Feedback: [email protected] or WhatsApp/Call +263781706212.
