Agronomists warns excess fertiliser application may halve crop yield

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.

Half har­vests

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

In 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.

Crop Nu­tri­tion Lab can be reached on +254720639933 or support@​cropnuts.​com