Introduction

Orname Kenya ranks 78th in fertilizer use, globally; in sub-Saharan Africa (SSA), the country comes second after South Africa (Sikobe, 2009). The average national fertilizer rates (31.3 kg/ha) (The standard, 2013) are much higher than the average for SSA (9 kg/ha) (Amit, 2009). However, this is still below the internationally recommended standards of 50 kg/ha. Malawi comes second at 29kg/ha, followed by Zimbabwe (28 kg/ha) while Uganda comes fourth at 27 kg/ha (The standard, 2013). Globally, Netherlands uses 238 kg/ha while India comes second at 167 kg/ha. Kenya relies on world market for supply of fertilizer; major sources of fertilizer are USA, Europe, Middle East, Asia and South Africa (Mathenge, 2009). The most commonly used fertilizes for planting (basal application) are DAP, MAP, TSP, SSP, NPK 20:20:0 and NKP 23:23:0 while for top dressing, the most common ones are CAN, ASN, UREA, SA. These two groups account for 48.56% and 25.36% of the national fertilizer consumption, respectively (Mathenge, 2009).

 

In Kenya, less than 30% of the smallholder farmers in high potential areas use fertilizers while in low potential areas fertilizer use is less than 20% (Onyango, 2009). The low fertilizer use by the small-scale farmers can be attributed to lack of know-how and inability to afford the input. The Kenyan government has been promoting fertilizer use by small farmers through availing fertilizer at subsidized prices. For example, 50 kg of subsidized DAP is available at KSh. 2 500 (~25 USD) while private traders sell at between KSh. 3 500 (~35 USD) and KSh. 3 700 (~37 USD). The subsidized fertilizer can only be purchased from the National Cereals and Produce Board (NCPB) which is a local public institution. However, NCPB stores are far apart and farmers have to travel long distances; this discourages many small scale farmers from accessing this fertilizer and, left with no option, they buy expensive fertilizer from the local retail outlets. Due to the high cost of fertilizer from the private traders, farmers end up buying and applying less than the recommended fertilizer rates. Of all fertilizer used in Kenya, 51.7% goes to cereals, 17% goes to tea, 3.4 goes to coffee, 6.8% to special crops including flowers and 21.1% to all other crops (Sikobe, 2009). Among food crops, policy makers have put a lot of emphasis on maize (Sikobe, 2009; Ariga and Jayne, 2010) probably because it is the major staple food crop in Kenya. Interestingly, fertilizer use in potatoes is low despite the crop an important security food crop (FAO, 2013). There has been a general decline in potato production in Kenya (Gregory et al., 2013) because of a number of constraints among them low soil fertility (FAO, 2013). In addition, fertilizers are usually applied below the recommended rate (90 kg N ha_^-1+230 kg P₂O₅ ha_^-1) for potato production in Kenya (Kaguongo et al., 2008).

 

The most widely used fertilisers in Kenya since the 1960s contained nitrogen (N) and phosphorus (P) due to assumption that most Kenyan soils have adequate supplies of other essential nutrients (Kanyanjua and Agaya, 2006). The other reason for dependence on N and P fertilisers was that most of the fertilisers were donations and there were little or no opportunities to suggest the inclusion of other essential nutrients in the fertiliser programs (Kanyanjua and Agaya, 2006). Continuous sole application of N and P fertilisers may have created a deficiency of potassium and perhaps other essential nutrients. Although research results are scanty in this area, there is a general decline in soil organic matter and a drop in soil pH in most Kenyan soils, a trend that is common in degraded soils (Kamprath, 1984).

 

This review paper looks at the various fertilizers commonly used in potato production, their impact of soil pH and their likely impact on potato production in the Kenyan highlands.

 

1 Primary Soil Mineral Nutrients Required by Potatoes

Optimum potato growth and profitable production depend on many management factors, one of which is ensuring a sufficient supply of nutrients. When the supply of nutrients from the soil is not adequate to meet the demands for growth, fertilizer application becomes necessary. Therefore, a comprehensive nutrient management program is essential for maintaining a healthy potato crop, optimizing tuber yield and quality, and minimizing undesirable impacts on the environment. Potatoes have a sparse and shallow root system (Onder et al., 2005). Consequently, potatoes are often unable to exploit nutrients and soil moisture fully within a soil profile and this result in their relatively high demand for many nutrients. Nutrients uptake is at its greatest during tuber bulking stage; it is important to supply the required plant nutrients during the tuber bulking stage in right N-P-K ratio and in ample quantities (Haifa, 2014). The daily nutrient requirements of potatoes during the critical bulking stage are 4.5 kg/ha N, 0.3 kg/ha P and 6.0 kg/ha K (Haifa, 2014). Tuber yield and quality are significantly affected by plant nutrition. Phosphorus deficiency and/or excessive nitrogen may lead to lower tuber specific gravity. To keep specific gravity high, maintain adequate soil P concentrations. Phosphorus and nitrogen also influence net development (skin set) on tubers; P deficiency reduces netting when nitrogen levels are adequate or excessive (Tindall et al., 1993).

 

1.1 Nitrogen (N)

Of all the nutrients essential for plant growth, N is often the most limiting for potato production. Application of fertilizer N is usually necessary to ensure profitable potato production because soil N is largely tied up in organic matter and not readily available for uptake. Potatoes are highly responsive to N fertilizer (Rourke, 1985). Both N rate and timing can have important impacts on potato yield and quality. Factors to consider when deciding on the rate of N to apply include: potato variety, yield potential or goal, growing season, soil organic matter content, and previous crop. If manure is used, then an estimate of N availability from the manure should be incorporated into the overall N applied (Bernie et al., 2007). In general, early maturing varieties and those grown for early markets require less N than late maturing varieties. Too much N will delay tuber initiation and maturity leading to excessive vegetative growth at the expense of tuber growth. Excessive foliage growth can lead to an increase in vine rot diseases. An adequate early season N supply is important to support vegetative growth but excessive soil N later in the season will suppress tuber initiation, reduce yields, and decrease tuber specific gravity (News and Views, 2006; Bernie et al., 2007). Excess soil N late in the season can delay maturity of the tubers and result in poor skin set, which harms the tuber quality and storage properties. Delayed maturity can result in tubers with low specific gravity. On the other hand, lack of N can increase the early blight infestations (PEI, 2014).

 

In general, split applications of N are recommended for potatoes from both a production and an environmental standpoint. A portion of the N should be applied preplant or at planting and the remainder at emergence and hilling (News and Views, 2006). Nitrogen uptake by the potato plant is highest during the tuber bulking stage. Split applications will generally improve N use efficiency by reducing leaching losses due to excessive rainfall and providing available N when it is needed for tuber growth. Applications of N after hilling should be based on petiole nitrate analysis (Haifa, 2014).

 

1.2 Phosphorus (P)

Phosphorus is a vital component of ATP, ATP provides energy for plant processes such as ion uptake and transport (CropNutrition, 2016a). Phosphorus play an essential role in plant health and root development, which directly impacts yield and quality (Marschner, 1995). During the early growth stages, P stimulates the development of a vigorous root system and healthy tops. Phosphorus requirement of potatoes is frequently higher than for many field crops due to the high nutrient demand of potatoes and their relatively shallow root system. Potato plants require an adequate supply of P throughout the growing season to achieve optimum quality and yield (Yara International, 2016; Tindall et al., 1993). Maximum potato yield occurs when sufficient P is available during early vegetative development and the entire period of tuber growth. Potato plant P uptake increases rapidly during tuber initiation, levels off to a constant rate during tuber bulking, and ceases with plant maturation (Tindall et al., 1993). Potato demand for P peaks at tuber set and early bulking, and then slows during later bulking when much of the nutrient demand of the developing tubers is met by translocating P from the tops of the plants and the roots to the tubers (News and Views, 2006). In potatoes, P is important for early root and shoot development, early tuber initiation and it enhances tuber maturation. At tuber initiation, an adequate supply of phosphorus ensures formation of optimum number of tubers Yara International, 2016). Since P generally moves very little in soil, it is important to place the P within the root zone to stimulate the early-season growth required for high yields (Tindall et al., 1993). While potatoes are very responsive to fresh soil phosphate, the economic optimum rate is often very difficult to define. Rates will depend on soil type and soil test results (Yara International, 2016). Phosphorus deficiency results in reduced tuber yield, reduced tuber size and reduced tuber specific gravity (Yara International, 2016; Tindall et al., 1993). The concentration of soluble phosphate in the soil solution is low and phosphorus is relatively immobile in the soil (PSU, 2002). Plant roots absorb phosphate ions only when they are dissolved in the soil water. Phosphorus deficiencies can occur even in soils with abundant available P if drought, low temperatures, or disease interfere with P diffusion to the root through the soil solution (News and Views, 2006; Tindall et al., 1993). Where sufficient soil phosphorus is not available for growth, foliar phosphate ensures rapid availability. Applied just before tuber initiation, foliar phosphate increases total tuber number.

 

1.3 Potassium (K)

Potassium has an important role in the control of the plant water status and internal ionic concentration of the plant tissues, especially the stomatal functioning. Potatoes require large amounts of soil K because this nutrient is crucial for metabolic functions such as the movement of sugars from the leaves to the tubers and the transformation of sugar into potato starch (Haifa, 2014). Potassium enhances root growth. In addition, it enhances plant’s tolerance to external stress such as frost, drought, heat, and high light intensity. It also reduces stress from disease andinsect damage (News and Views, 2006). Potassium plays important roles in tuber yield and quality. Potassium reduces discolouration of raw tuber and after-cooking blackening. It also improves resistance to harvesting/handling damage and improves storability by allowing tubers to mature fully. In addition, it minimises the content of reducing sugars thereby ensuring that tubers are better suited for processing into chips and crisps. Potassium regulates osmotic turgor of the cells and water balance; adequate K therefore assists plants to survive drought better. Of all the nutrients, potassium is absorbed in the greatest quantities by the potato crop (Potato grower, 2013); potato is considered a luxury consumer of K (Haifa, 2014). The highest requirement for potassium is during the bulking up stage; beginning of this morphological stage is indicated by plant flowering (Haifa, 2014). Potassium deficiencies reduce the yield, size, and quality of the potato crop. A lack of adequate soil K is also associated with low specific gravity in potatoes. (News and Views, 2006; Haifa, 2014). Potassium deficiency leads to increased incidence of internal black spot bruising and hollow heart disorder. The deficiency also decreases photosynthesis, thereby reducing starch formation for tuber yield. It also results in inefficient use of other nutrients such as N and P. Excess K application decreases tuber specific gravity; potassium chloride has a greater effect than potassium sulphate (0-0-50) at equivalent K rates. In general, potatoes do not respond to K fertilizer application if soil test K levels are 200 ppm or more (Potato grower, 2013).

 

2 Effects of Soil pH on Nutrient Availability

Soil pH is a characteristic that describes the relative acidity or alkalinity of the soil (CropNutrition, 2016b). The “ideal” soil pH is close to neutral; neutral soils range from a slightly acidic pH of 6.5 to slightly alkaline pH of 7.5 (IPNI, 2010). It has been determined that most plant nutrients are optimally available to plants within this 6.5 to 7.5 pH range (CropNutrition, 2016 b; IPNI, 2010). In addition, this pH range is generally good for plant root growth. Soils are considered acidic below a pH of 5, and very acidic below a pH of 4 (IPNI, 2010). Conversely, soils are considered alkaline above a pH of 7.5 and very alkaline above a pH of 8 (Table 1).

 

 

The pH levels control many chemical processes that take place in the soil specifically plant nutrient availability (CropNutrition, 2016b). Nitrogen, K, and Sulphur (S) are major plant nutrients that appear to be less affected directly by soil pH, but still are affected to some extent. Phosphorus, however, is directly affected. The pH range of greatest phosphorus availability is 6.0 to 7.0 (PSU, 2002). At pH values greater than pH 7.5, HPO_4^2- phosphate ions tend to react quickly with Calcium (Ca) and Magnesium (Mg) to form less soluble compounds. At acidic pH values, H₂PO_4^-phosphate ions react with aluminum (Al) and iron (Fe) to again form less soluble compounds (IPNI, 2010). Most of the other nutrients (especially micronutrients) tend to be less available when soil pH is above 7.5, and in fact are optimally available at a slightly acidic pH, e.g. 6.5 to 6.8. Exception is Molybdenum (Mo), which appears to be less available under acidic pH and more available at moderately alkaline pH values (Figure 1).

 

 

Low soil pH can lead to nutrient imbalances; once the pH drops below 4.9, nutrient deficiencies and toxicities become more common. In particular, Manganese (Mn) and Aluminium (Al) toxicity as well as P, K, Ca, and Mg deficiencies may occur in these low pH soils. On the other hand, high soil pH can limit availability of Fe, Zinc (Zn) and Manganese (Ochapa, 1983).

 

Soil pH also affects efficiency of fertilizer utilization by plants. Plants can take up N in the ammonium (NH_4⁺) or nitrate (NO_3^-) form. At pH values near neutral (pH 7), the microbial conversion of NH_4⁺ to nitrate (nitrification) is rapid, and crops generally take up nitrate. In acid soils (pH < 6), nitrification is slow, and plants with the ability to take up NH_4⁺ may have an advantage (USDA, 2007). Likewise, soil pH also plays an important role in volatization losses. An ammonium ion (NH_4⁺) in the soil solution exists in equilibrium with ammonia (NH₃) in the soil solution. The equilibrium is strongly pH dependent. There is a tendency for the equilibrium to favour conversion of ammonium to ammonia because solution NH₃ is subject to gaseous losses to the atmosphere (USDA, 2007). The difference between NH₃ and NH_4⁺ is a H_⁺. For example, if NH_4⁺ were applied to a soil at pH 7, the equilibrium condition would be 99% NH_4⁺ and 1% NH₃. At pH 8, approximately 10% would exist as NH₃. The equilibrium is dynamic; as soon as one molecule of NH₃ escapes from the soil, one molecule of NH_4⁺ converts to NH₃ to maintain the equilibrium. Under conditions of low soil moisture or poor fertilizer incorporation, volatilization loss can be considerable even at pH values as low as 5.5. This means that a fertilizer like urea (46-0-0) is generally subject to higher volatilization losses at higher soil Ph especially if surface-applied and not incorporated (Nielsen, 2006). Volatilization risk is also high on lighter textured soils with low buffer capacity (Nielsen, 2006). Soil pH also affects nitrogen fixation in legumes. The survival and activity of Rhizobium, the bacteria responsible for N fixation in association with legumes, declines as soil acidity increases (IPNI, 2010).

 

The form and availability of soil P is also highly pH dependent; plants take up soluble P from the soil solution, but this pool tends to be extremely low. The limited solubility of P relates to its tendency to form a wide range of stable minerals in soil. Under alkaline soil conditions, P fertilizers such as mono-ammonium phosphate (MAP) (11-55-0) generally form more stable (less soluble) minerals through reactions with Calcium and Magnesium (Mooso et al., 2013; FTRC, 2013). However, the P in these Ca-P minerals will still contribute to crop P requirements; as plants remove P from the soil solution, the more soluble of the Ca-P minerals dissolve, and solution P levels are replenished. In addition, over 90 per cent of the fertilizer P tied up this year in Ca-P minerals will still be available to crops in subsequent years. The fate of added P in acidic soils is somewhat different as reactions occur with aluminium (Al) and iron (Fe) leading to formation of insoluble compounds (Mooso et al., 2013). The lock-up of P in Al-P and Fe-P minerals under acidic conditions tends to be more permanent than in Ca-P minerals (Ochapa, 1983).

 

The fixation of potassium (K) and entrapment at specific sites between clay layers tends to be lower under acid conditions. This situation is thought to be due to the presence of soluble aluminium that occupies the binding sites. Liming increases K availability, most likely due to displacement of exchangeable K by Ca in the binding sites (Jensen, 2010).

 

3 Effects of Fertilizer Types on Soil pH

Of all the major fertilizer nutrients, nitrogen is the main nutrient affecting soil pH because it is added in the largest quantities, and soils can become more acidic or more alkaline depending on the type of nitrogen fertilizer applied. Nitrate-based products are the least acidifying of the nitrogen fertilizers, while ammonium-based products have the greatest potential to acidify soil (CropNutrition, 2016c). Soils tend to acidify over time, particularly when large applications of NH_4⁺ based fertilizers are used or there is a high proportion of legumes in the rotation (IPNI, 2010). When considering nitrogenous fertilizer to apply, a balanced ammonium / nitrate ratio is very important at planting time; too much ammonium nitrogen lowers root-zone pH thereby promoting Rhizoctonia disease. Nitrate-nitrogen enhances the uptake of cations such as calcium, potassium and magnesium; these cations are required for high specific gravity of the tubers (Haifa, 2014).

 

The form of N and the fate of N in the soil-plant system is probably the major driver of changes in soil pH in agricultural systems. The predominant forms of fertilizer N used are urea [CO(NH₂)₂], monoammonium phosphate (MAP)(NH₄H₂PO₄), diammonium phosphate (DAP) [(NH₄)₂HPO₄], ammonium nitrate (AN)(NH₄NO₃), calcium ammonium nitrate (CAN)(CaCO₃+NH₄NO₃), ammonium sulphate (AS)[(NH4)₂SO₄], urea ammonium nitrate (UAN) (a mixture of urea and ammonium nitrate) and ammonium polyphosphate (APP)[(NH₄PO₃)n] (FTRC. 2013).

 

The key molecules of N in terms of changes in soil pH are the uncharged urea molecule ([CO (NH₂)₂]^₀), the cation ammonium (NH_4⁺) and the anion nitrate (NO_3^-). The conversion of N from one form to the other involves the generation or consumption of acidity, and the uptake of urea, ammonium or nitrate by plants will also affect acidity of the soil. Ammonium-based fertilizers will acidify soil as they generate two H⁺ ions for each ammonium molecule nitrified to nitrate. The extent of acidification depends on whether the nitrate produced from ammonium is leached or is taken up by plants. If nitrate is taken up by plants the net acidification per molecule of ammonium is halved compared to the scenario when nitrate is leached. This is due to the consumption of one H⁺ ion (or excretion of OH^-) for each molecule of nitrate taken up by the plant; this is often observed as pH increases in the rhizosphere (Smiley and Cook, 1973). Anhydrous ammonia (NH₃) and urea have a lower acidification potential compared to ammonium-based products as one H^₊ ion is consumed in the conversion to ammonium. Anhydrous ammonia (82% N), Urea (46% N), Ammonium nitrate (34% N) and Urea ammonium nitrate (UAN) solutions (32% and 28% N) are less acidifying than DAP, MAP, or ammonium sulphate (NSW Agriculture, 1999). Unlike DAP and MAP, anhydrous ammonia and urea do not leave any phosphoric acid residue after they dissolve in soil solution (FTRC, 2013). Ammonium sulphate leaves sulphuric acid residue as it dissolves. With ammonium nitrate and UAN solutions, only part of the total N is in the ammonium form, so these products result in less nitrification than fertilizers in which all the N is in the ammonia or ammonium form. Ammonium sulphate (21% N, 24% S) and MAP (11% N, 52% P₂O₅) are more acidifying than DAP (18% N, 46% P₂O₅). Ammonium sulphate not only results in acidification through the process of nitrification, but one of the dissolution byproducts in sulphuric acid (Smiley and Cook, 1973).

 

Ammonium phosphates, such as MAP and DAP fertilizers, are extremely soluble in soil solution, and dissolve easily (KSU, 2013; IPNI, 2010). Monoammonium phosphate (MAP)(11% N, 52% P₂O₅)is slightly more acidifying than DAP (18% N, 46% P₂O₅) when applied at the same N rate.

 

Placed in a band, DAP initially increases pH, in contrast to MAP which lowers it. However, these pH changes are temporary and localized to the band; within a few weeks, soil pH is lower with DAP than MAP (Sanderson et al., 2002).

 

The acidic reaction of MAP in the soil can be an advantage in neutral and high pH soils. However, any theoretical benefits of MAP or DAP based on different pH of dissolution are rarely transferred to consistent performance in the field. The exception is in highly calcareous soils where it is now widely recognized that acidic MAP generally out performs DAP; consequently, MAP is used in preference to DAP on alkaline soils (News and Views, 2002). Differences between MAP and DAP are smallest and mostly non-existent when the fertilizers are broadcast and incorporated into the soil or when they are drilled in preplanting into neutral or acid soils (Bernie et al., 2007).  However, another concern regarding MAP or DAP selection, is potential ammonia toxicity to germinating seeds in dry soils. Because DAP contains about twice as much ammonium-N as MAP, and because its pH of dissolution is more alkaline than MAP, DAP has greater potential for nitrogen loss through ammonia volatilization when broadcast onto neutral to alkaline soils (News and Views, 2002). The free ammonia could cause seed germination problems, seedling injury and potentially interfere with root development (News and Views, 2002; PSU, 2002). Phosphorus is taken up from soil solutions by roots as orthophosphate ions H₂PO_4^- or HPO_4^2-. The H₂PO_4^- is the principal ion taken by plant while HPO_4^2- is taken at a far lesser extent (FAO, 2008). The acidic soil solution in MAP favours the formation of H₂PO_4^-, thus more potential for P uptake by the plants.

 

Nitrate-based fertilizers have no acidification potential and actually can increase soil pH as one H_⁺ ion is absorbed by the plant (or OH^- excreted) in the uptake of nitrate (News and Views, 2002).

 

Generally most soluble P fertilizers i.e. MAP, DAP and triple superphosphate (TSP) have similar P use efficiency in most soils provided there are no other limitations to crop growth (FTRC, 2013). Soil acidification due to use of phosphorus fertilizers is small compared to that attributed to nitrogen because of the lower amounts of this nutrient used and the lower acidification per kg of phosphorus. The form of P fertilizer added to soil can affect soil acidity, principally through the release or gain of H⁺ ions by the phosphate molecule depending on soil pH. Phosphoric acid (PA) is the most acidifying phosphorus fertilizer. If phosphoric acid is added to soil, the molecule will always acidify soil as H⁺ ions will be released, one H⁺ ion if the soil pH is less than 6.2 and two H⁺ ions if the soil pH is above 8.2. Monoammonium phosphate (MAP), single superphosphate (SSP) and triple superphosphate (TSP) all add P to the soil in form of H₂PO₄^-, which can acidify soil with a pH greater than 7.2 but has no effect on soil pH in acidic soils. The form of P in di-ammonium phosphate (DAP) is HPO_4^2- which can make acidic soils (pH<7.2) more alkaline but has no effect on soil with a pH >7.2. The hydrolysis of ammonium polyphosphate (APP), where the P present as the P₂O_7^4- molecule converts to HPO_4^2- is pH neutral and hence any acidification due to adding P can be regarded as similar to DAP. Crop uptake of P has little effect on soil acidity due to the small amounts of fertilizer P taken up in any one year. Phosphorus fertilizers containing ammonium can increase P availability by reducing soil pH (FTRC, 2013) especially in alkaline soils.

 

The acidification potential of various N and P fertilizers can be expressed in terms of kg lime equivalent to neutralize the acidity generated by one kg of nutrient added in different forms. Nitrogen fertilizers are more acidifying than P fertilizers per kg of nutrient applied (Table 2).

 

 

Potassium fertilizers have little or no effect on soil pH. The form in which K is added to soil either as muriate of potash (KCl) or sulphate of potash (K₂SO₄) has no effect on soil acidification (FTRC, 2013).

 

Soil acidification is a widespread natural phenomenon in regions with medium to high rainfall. Agricultural production systems can accelerate soil acidification through perturbation of the natural cycles of nitrogen (N), phosphorus (P) and sulphur (S) in the soil, through removal of agricultural produce from the land, and through addition of fertilizers and soil amendments that can either acidify soil or make it more alkaline (Kennedy, 1986). The four main causes of soil acidity are removal of products from the farm or paddock, leaching of nitrogen below the plant root zone, inappropriate use of nitrogenous fertilisers and build up in organic matter (NSW Agriculture, 1999).

 

4 Implication of Soil pH on Potato Production

Potatoes are more tolerant to low pH than most other crops. The crop prefers a slightly acidic soil (pH of around 5.5-6.0) (KARI, 2006; Haifa, 2014) but will grow in a range from 5.0 to 6.5. Soil pH below 4.8 generally results in impaired growth (Haifa, 2014). However, incidence of common scabs of potatoes (Strepotmyces scabies) are low where soil pH is lower than 5.4; the problem becomes widespread when soil pH is above 5.5. Although potatoes tolerate acid soil, there are benefits from raising the pH up to 6.0-6.5; optimal soil pH for nutrient availability is between 6 and 7.

 

5 Soil Fertility Situation in Potato Growing Kenyan Highlands

In Kenya, potato is an important food crop, second after maize in volumes produced (MoA, 1998; FAO, 2013, 2014). Its ability to grow in high altitude areas where maize does not do well and itshigh nutritional value makes it an important food crop.

 

Despite the importance of potato in Kenya, its production is faced by a number of constraints, mainly low soil fertility (Kaguongo et al., 2008). Low soil fertility is mainly caused by continuous cultivation without adequate replenishment of mined nutrients (Kiiya et al., 2006). Continuous cultivation, without a fallow period, is worsened by small farm sizes which make proper crop rotation difficult (Kaguongo et al., 2008). In addition, inorganic fertilizers are usually applied below the recommended rate for potato production (90 kg Nha_^-1+230 kg P₂O₅ha_^-1) (Kaguongo et al., 2008). Even application of organic matter to the fields is limited because crop residues are used as fodder; where cattle manure is used, the quantities applied are below the recommended rates and the quality is questionable (Lekasi et al., 1998). Nitrogen and phosphorus are the major nutrients limiting potato production in Kenya (Recke et al., 1997). Soil phosphorus in major potato growing parts of Kenya is as low as 2.9 ppm (modified Olsen) while total nitrogen may be lower than 0.15% (Recke et al., 1997).

 

In addition to low soil fertility, high soil acidity limits potato production in Kenya. Soils in most potato growing areas in Kenya are acidic. This is mainly due to the fact that the soils in the highlands are derived from acidic volcanic rocks and have been highly leached by high rainfall (Recke et al., 1997; Kanyanjua and Agaya, 2006; Jaetzold et al., 2006). The situation has been worsened by the continuous use of (DAP) (18:46:0) for production of most food crops (Kiiya et al., 2006); this fertilizer has been shown to increase soil acidity. Consequently, most of potato growing areas in Kenya have a soil pH of less than 5.5 while values of 4 to 5 are most common (Recke et al.,1997; Kiiya et al., 2006). A pH of less than 5.5 severely limits availability of potassium, nitrogen, phosphorus, sulphur, calcium and magnesium, and excessive levels ofaluminium, manganese, boron, iron, copper and zinc (Recke et al., 1997; Kanyanjua and Agaya, 2006). It is quite possible that the problem of low soil pH has led to nutrient imbalances that lead to even further decline of potato yields (Janssen et al., 2013).

 

In Kenya, application of di-ammonium phosphate (DAP) (N 18%: P₂O₅ 46%) at 500 kgha^_-1 is the recommended for potato production (KARI, 2008). A recent study conducted on eight sites across Nyandarua County, which is the main potato growing county in Kenya, showed that soil pH ranged from 4.0 to 5.72 with a mean of about 5.0 (Martha et al., 2014). The same study showed that P was generally low ranging from 10 to 30 ppm with a mean of 20 ppm (Bray II method). Di-ammonium phosphate (DAP) (N 18%: P₂O₅ 46%) fertilizer has been in use at KARLO Tigoni Centre for a long time despite increasing acidity at the Centre and declining potato yields. Soil samples collected from KALRO Tigoni Centre in 2015 showed pH has been on decline (Table 3).

 

 

Soils at KALRO Tigoni are generally acidic (Table 3); the low pH could be responsible for the declining tuber yields. There is need to lime the soils so as to raise the pH. In addition, there is need to shift from using DAP (18:46:0) as the basal planting fertilizer to SSP or TSP accompanied with topdressing using CAN later. The situation in Nyandarua and at KALRO Tigoni just indicates just how serious soil fertility problems are in the potato growing Kenyan highlands.

 

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