A farmer in Kiambu has increased his tomato harvest by one-quarter by improving the calcium of his soil with eggshells and bones.
Ethan Njeru, who grows greenhouse tomatoes at his family farm in Kiambu, started using these unconventional calcium sources in 2021 after fertiliser prices hit a historic Sh7,000 high.
Up to 80 per cent of Kenya’s soils are classified as tropical soils. They lose a lot of essential nutrients like calcium because of high temperatures and intense rainfall. A lack of calcium discolours and deforms leaves, curling them inwards and stunting plants.
Calcium deficiency also causes diseases such as blossom end rot, a common disease in tomatoes and peppers. Bitter pit causes a bitter taste in fruits and tipburn in leafy vegetables like sukuma and cabbages, where leaf edges brown and wither.
Bone meal is prepared by boiling animal bones from chicken and beef to remove any meat, fat, or cartilage.
“I toss the bones in water and boil them for five hours in a charcoal fire to soften them and make sure they have no grease.
Chicken bones usually become soft in under three hours, while cow bones take up to seven hours to be ready.
I also add a tablespoon of vinegar to help soften the bones quicker,” Njeru informed.
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The boiled bones are spread on a clean surface where they are hit by direct sunlight for four days to a week, depending on the weather, until they become soft and can be easily broken.
The bones are then hammered with a stone or hammer and ground using a mortar and pestle into small shards, grain mill or blender.
The finer the powder, the faster the calcium is released into the soil.
The bonemeal can be stored in a dry, airtight container.
“For the eggshells, I just collect them in a bucket and later empty them in a gunia by trampling on it,” he said.
Ground eggshells are made almost entirely of calcium (Ca), while bones contain both Ca and phosphorus nutrients. Ca is essential in providing balanced nutrition for crops and is especially heavily used by tomatoes and peppers.
The calcium from eggshells and bone meal powder also prevents blossom end rot. This is a common disease, especially in tomatoes, caused by calcium deficiency.
It also adjusts soil pH by neutralising acidity in soils. This makes other soil nutrients more available for plants improving their growth and increases the amount of small animals in the soil.
There is also some evidence that the sharp scraps of eggshells deter slugs or snails from getting into shambas.
The calcium in eggshells and bones also increases water and air movement in soils, especially clays.
Bone meal also contains phosphorus, which helps improve root development, flowering, and fruiting in crops and trees. Phosphorus is vital for bulbs, roots, and flowering plants such as onions, sweet potatoes and fruit trees.
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Bones also contain trace minerals such as nitrogen and zinc.
“There is no strict limit to the amount of bone meal or crushed eggs added to compost, but I usually put in about a cup of each for about 20 kilograms of compost material. This ensures that the nutrients do not overwhelm the microbial balance of the soil.
The crushed eggshells and bone meal need to be thoroughly mixed into the compost to ensure even distribution.
Finely ground eggshells decompose faster, releasing calcium faster.