Nitrogen is one of the most critical nutrients affecting sweetpotato yield. This study was conducted to 1) evaluate the growth and yield response of sweetpotato NSIC SP 33 to different levels of N; and 2) determine the relationship between N level and root yield. A screenhouse experiment laid out in Completely Randomized Design (CRD) was carried out using five (5) N levels: 0,40, 80, 120 and 160kg ha-1, each replicated thrice.Thetotal amount of N (as urea) was applied twice, one-half at planting and the other half at six weeks after planting (6 WAP) along with a blanket application of solophos and muriate of potash at 40 and 60kg ha-1 P2O5and K2O. An alluvial Umingan sandy loam soil (lnceptisol) was used as medium. Levels of N significantly increased the leaf area, fresh herbage, total root yield, biomass, dry matter yield, shoot-root ratio, weight and number of marketable roots. Harvest index (HI) of plants applied with 40 and 80kg N ha-1 were comparable to those applied with 0 N. Increasing levels of N from 0 to 1 60kg ha-1 resulted in higher total crude protein (CP) and total N in the leaves. N uptake was higher in N-applied plants. Application of 40 and 80kg ha-1 N produced the highest root yield, above which root yield declined. Relationship between N level and root yield developed through regression analysis showed a quadratic function of Y=111.1794+1.2098x-0.00897x2 and R-square of 0.9904 which suggest that inherent soil N affected root development.

Until now, no studies have been conducted in the Philippines on the use of the Mitscherlich-Bray equation to formulate NPK fertilizer recommendation for sweetpotato. This study used the Mitscherlich-Bray equation to formulate NPK requirements for sweetpotato. Independent experimental set-ups of N (7 application levels), P (6 levels), and K (9 levels) arranged in RCBD with three replications were simultaneously conducted. Theoretical maximum yield, NPK constants c, and c, NPK fertilizer recommendations for sweetpotato at different soil fertility levels, and optimum fertilizer rates were calculated. Fertilizer recommendations for a common range of soil test values were developed but needed further field verification trials. Theoretical maximum yields determined by the Mitscherlich-Bray equation were 19.05, 12.66, and 14.88t ha-1 for NPK, respectively. The study showed that inherent soil fertility is vital in the development of fertilizer recommendation for sweetpotato not only to increase root yield but likewise to increase overall productivity. It showed that 30, 50, and 60 percent of the maximum possible yield was attributed to the inherent soil N, P, and K, respectively. N, P2O5, and K2O recommendations for sweetpotato were computed based on a common range of soil test values ranging from 50 to 300, 5to 40, and 200 to 700kg ha1 NPK, respectively.

There is an urgent need for soil erosion data from marginal uplands in the country. This study evaluated the occurrences of soil erosion in the marginal uplands of lnopacan, Leyte. Field soil erosion indicators were assessed in different portions of the study site and erosion plots were established in the corn and sweetpotato fertilizer experiments to measure erosion rates. Field indicators of soil erosion such as rills, cracks across slopes, exposed rocks, thin surface soil and eroded sediments in waterways were common in various parts of the marginal upland studied. Soil erosion rates measured on erosion plots were higher from the corn field than from the sweetpotato field. Application of chicken manure and vermicast resulted in lower soil erosion rates due to improved soil structure. Plots without crop cover gave the highest erosion rates. The degree of soil erosion in the marginal upland can be considered as moderate to severe.

Several insect pests attack the foliage of sweetpotato at different stages of crop growth which contribute to yield reduction. Weeds belonging to the same genera as the main host can act as alternate hosts of insect pests. The study evaluated the biology of tortoiseshell beetle in the laboratory at an ambient temperature of 27-30oC and 85-90% RH on sweetpotato (Ipomoea batatas) as main host and four other Ipomoea species: three-lobe morning glory (I. triloba), beach morning glory (Ipes-caprae), swamp morning glory (I. aquatica), and five-finger morning glory (I. pentaphylla Syn. I. cairica) as alternate hosts. The beetle underwent four developmental stages – egg, larva, pupa, and adult. It completed its development within 4-6 weeks from egg-laying to adult emergence of 26-40 days, an average of 27.55 to 39.01 days. The total development period of male and female A. miliaris on I. batatas, I. triloba, I. pes-caprae, I. aquatica, and I. pentaphylla were 27.55 and 30.72, 27.52 and 30.49, 29.94 and 35.06, 33.25 and 38.11, and 33.70 and 39.01 days, respectively. Adult longevity of males and females was longer on I. batatas and I. triloba, followed by I. aquatica), I. pentaphylla, and lastly, I. pes-caprae. The highest number of eggs laid was recorded in I. batatas (202.7), followed by I. triloba (173.2), I. pes-caprae (76.0), I. aquatica (71.7), and I. pentaphylla (59.7). The highest egg viability of more than 90% was recorded in I. batatas and I. triloba. Mortality occurred towards the later part of larval development, with the lowest mortality of 8.33% in I. batatas. The host plants ‘ high protein, N, P, and K contents influenced the shorter life cycle, higher reproductive rate, high percentage egg hatchability, lower mortality, and longer life span. Increased mortality and abnormal wet frass in I. pes-caprae could be accounted for by exceptionally high sugar content in the leaves and secondary metabolites present, especially in other host plants that may have insecticidal activity. Since A. miliaris was also able to complete its life cycle and reproduce on the four weed species, the insect continues to survive without sweetpotato. Any management strategy for this insect by destroying the Ipomoea weed hosts eliminates other sources of infestation in the field.