Most soils in Uganda are less productive than they were years ago, according to Prof. Julius Kitungulu Zaake, a soil scientist.
Zaake says the soil fertility in terms of the proportions is low with high to medium fertility at 26%.
He says medium to low fertility stands at 47%, while low to negligible fertility stands at 27% across the country.
According to Zaake, fertile volcanic soils in mountainous areas are not as productive as they should be due to the rapid population growth in these areas and the resultant pressure on land for both cultivation and settlement.
He adds that that kind of soil has been exacerbated by weather effects, such as floods, which run off with top soils, depriving it of vital nutrients.
Agriculture is the mainstay of Uganda’s economy, employing 65.6% of the population, according to a 2010 report from the Uganda National Bureau of Statistics on the labour force.
Agriculture also contributes 21% to the GDP. Currently, agricultural production is dominated by smallholder farmers. From these activities, the country is one of the leading producer of coffee and bananas and a major producer of tea, cotton, tobacco, cereals and grains.
However, productivity of most of these crops has been reducing over time.
The reasons include high cost of inputs, poor production techniques, limited extension services, over dependency on rain- fed agriculture, limited markets, land tenure challenges and reducing soil fertility.
Reducing soil fertility is considered by many scholars as the biggest factor affecting agriculture production.
How soil loses fertility
Since soil is made up of different nutrients, such as organic matter, which is washed away through soil erosion and bush burning, which leads to loss of nutrients such as nitrogen and sulphur, as these escape into thin air.
This leaves the soil bare, leading to loss of soil fertility.
“Such soils cannot allow root penetration, but also the larger spaces that take in air will be blocked and nutrients, including oxygen, will not find their way into the soil to get to the roots,” Zaake says.
According to Zaake, continuous tillage affects the larger spaces for air, by exposing the soil to direct heat and rains.
Loss of soil productiveness
Most farmers do not replace lost nutrients after harvesting their crops.
“When we harvest crops, we have to replenish the lost nutrients through application of fertilisers, which has not been embraced by most Ugandans,” he says.
“If farmers do not replace the lost nutrients after harvest, production goes down. It is just like having a shop from which you take out products, but you are reluctant to restock,” Zaake warns.
Conducting soil analysis
According to Julius Mabuya from PAAT Soil Clinic, soil sampling and analysis starts with having experts visit the garden, where samples are taken.
While this is happening, such areas should not have any trace of organic manure, such as cow dung, chicken manure or any other fertiliser as these could compromise the final results.
He adds that during the sampling process, experts look out for the PH, which is the acidity of the soil, organic matter, NPK, sodium, calcium and the proportion of sand clay and silt, in addition to physical properties.
“The whole process shows that such work shouldn’t be hijacked by farmers, but should be left to experts so as to benefit. Many times farmers are tempted to bring samples to testing centres, which may not give a true representation of the nutrients in the soil,” he adds.
To maintain soil fertility, Zaake says farmers should apply both organic and inorganic fertilisers.
Organic fertilisers can be got from cow dung, chicken and goat droppings
What determines soil fertility
According to Zaake, a soil scientist, the soil is made up of over 15 different nutrients, including both the large and small spaces.
Zaake says the small spaces take in water, while the large spaces take in oxygen.
For the plants to grow as expected, the soils must have all the needed nutrients.
In addition, the soil must have physical properties, such as small and large air and chemical nutrients such as Nitrogen, Phosphorus and Potassium (NPK).
Other nutrients are magnesium, calcium and sulphur, required in small quantities, while iron, copper, boron, manganese, zinc, chlorine and sodium are required in minute amounts.
Zaake, however, says there is a misconception among farmers that NPK is a single fertiliser or nutrient for the soil, which is not true.
He says NPK is a combination of three nutrients needed by plants at different stages of growth.