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Agronomist: Excess fertiliser application may cut crop yields by half

Jeremy Musila French beans
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Farmer applying fertiliser to his French beans. Excess application of fertiliser to crops can cut production by half leading to losses. Photo courtesy.

Adding more commercial and organic fertiliser may cut by half the season harvest due to accumulation of ‘unnecessary’ nutrients in the soil that lead to poor performance of crops.

According to Crop Nutrition Lab Agronomist Karanja Kiiru, excess nutrients lead to blockage of absorption of other essential elements besides encouraging pest attack.

“It is false to think that the more the fertiliser applied, the more the yields. Excess application of farm yard manure for instance leads to accumulation of nitrogen, which suppresses other crucial elements from being accessed by crops for example molybdenum,” he said.

The agronomist said farmers must regularly test soils to know the composition of nutrients. This will dictate the elements one will require to add or the crop to grow the following season to “flash out the imbalance”.

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Excess elements like nitrogen from farm manure encourage pest attacks to crops.

“Farmers stand to save about 40 per cent of the total production cost by only testing the soil. They will not buy fertilisers supplying elements, which are already in excess. Besides, the cost of dealing with pests and related diseases will not be incurred,” Kiiru said.

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Half harvests

Patrick Njenga, a Kiambu County farmer harvested 22 110kg bags of Irish potatoes from a three quarter acre after the first season.

The second season, the yield dropped to 21 110kg even after adding three tonnes of animal manure as well as 150kg commercial fertiliser. He repeated the same in the third season and the harvest dwindled further instead of going up. The quality of the tubers was also dismal.

Kiiru carried out soil testing for Njenga and in consultation with a crop pathologist, they found that fusarium wilt was setting in because of the repeated use of the same planting materials.

Nitrogen was also in excess.

“He asked me to use clean planting materials as well as add gypsum (acid neutraliser) to the soil. I used one tonne of manure and 50 kg of commercial fertiliser in my third planting. The harvest rose from the 21, and 22 to 41 bags,” Njenga said.

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Cost of analysis

Crop Nutrition Lab offers basic soil testing at a Sh2,500. Complete analysis costs Sh5,000.

Basic testing entails analysis of soil PH and the major elements such as potassium, sulphur, phosphorus, calcium, nitrogen as well as organic matter.

Complete test involves detailed analysis of the elements such as iron, manganese, boron, copper, zinc and their presence ratio. An example is the calcium-nitrogen ratio.

Based on the outcome, the farmer is advised on the steps to take to realise the desired results.

Kiiru can be reached on +254737576066

Crop Nutrition Lab can be reached on +254720639933 or [email protected]

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