CULTIVATION PRACTICESFERTILIZATION
Maintaining a high-yielding potential in potatoes requires soil health and fertility management. Soil fertility depletion, however, remains the major biophysical cause of declining crop productivity on smallholder farms. In addition, soil degradation is increasing through loss of soil organic matter content, erosion, limited crop rotation, and use of inadequate fertilizers. These have led to depletion of specific nutrients and acidic soil conditions. Soil analysis amongst the farmers to ascertain the nutrient levels of their soils is also not a common practice.
This has resulted in farmers using the same fertilizer in all the crops, despite the knowledge of specific nutrient needs of different crops. Potatoes are most suitable for medium to light textured soils, with a pH level of about 5.5. They demand a high amount of nutrients, and so effective management of soil fertility is critical to potato production. Kenyan farmers mainly use Di-Ammonium Phosphate (DAP). Fertilizer application (average applications are at 300 kg/ha in Kenya) is below recommendations for Kenya.
In Kenya, DAP still is the most widely used fertilizer (sometimes cattle manure is added) among the farmers within the counties where the pilot project was conducted: more than 90% of the respondents used it on their potato plots. The Kenya Agricultural and Livestock Research Organization (KALRO) recommends a DAP application rate of 500 kg/ha for potatoes. However, the farmers’ application rates averaged at 300 kg/ha in Nyandarua and in Bungoma it was even lower at 137 kg/ha. It was also observed that farmers who apply the recommended amounts of fertilizer yield on average 3 t/ha more than the ones applying rates below recommendations.
The low rates of applications were attributed to lack of knowledge about the recommended rates and the high cost of fertilizer. For a 50-kg bag of fertilizer, the farmers pay KES 3,000–3,300 on average in Kenya depending on the area. Farmers with very small plots mostly purchase fertilizers in low quantities with a fixed price per kilogram. These farmers face the highest prices and also do not purchase the fertilizers in their original packaging. Therefore, they rely on the information provided by the wholesalers. As mentioned above soil analysis is not a common practice among farmers. However, it is a very crucial issue when it comes to the application of fertilizer. In Kenya, KALRO, together with CIP gathered soil samples and undertook soil analysis. Results showed that among the soil macro-nutrients, there was a range of phosphorus availability in the soil between the sites - most of the sites had medium amounts. Two sites in Kinangop and Naivasha reported relatively higher levels of phosphorus. Varying levels of potassium were observed in the demonstration sites.
Very low level of potassium was observed in Kinangop site, which experiences a lot of rainfall and serious water logging conditions. In three sites – OljoroOrok, University of Nairobi, and Naivasha – high levels of potassium were reported. Adequate levels of magnesium were observed in all the demonstration sites. Low levels of micro-nutrients were reported at all the sites, with some areas even missing some of the micro-nutrients like copper. Availability of these nutrients to the plants is greatly affected by low pH in the soil as observed in all the demonstration plots in Nyandarua County. Sites at University of Nairobi and Naivasha had relatively neutral soil pH.
For the field trials a potato-specific blend (i.e. N:P:K16:8:22 + 2MgO+2S) of fertilizer was applied which was developed by K+S Kali and MEA Ltd. in Kenya. This blend was applied on both local and new varieties after having conducted soil analysis in the pilot counties which confirmed a decreasing soil pH in the farms. Varieties responded differently to fertilizer types and application rates. The results are only for one season and thus cannot be conclusive. However, there are some useful indications that can be followed up further in the search for alternatives to the DAP that has been over-used by the farmers and led to the reduction of the soil pH.
The fertilizer blend NPK 16:8:22 + 2 MgO +2S provides a balanced nutrition to the potato crop. The trial can be followed up for several seasons to see the real benefit of using the balanced fertilizer as opposed to unbalanced fertilizers. Furthermore, knowledge of fertilizer application in respect to fertilizer types, rates of application, and timeliness of application is critical to modernizing potato production. Therefore, efforts to intensify farmer education (extension) by government agencies and private input dealers are recommended.