Soil and Crop Management July 2002 SCM-4 Nutrient Deficiencies and Excesses in Taro Susan C. Miyasaka1, Randall T. Hamasaki2, and Ramon S. de la Pena3 Departments of 1Tropical Plant and Soil Sciences, 2Plant and Environmental Protection Sciences, and 3Natural Resources and Environmental Management dentifying and correcting plant nutrient deficiencies toms appear first on older leaves, because excess mineral Iand toxicities are essential for good crop management elements tend to accumulate in mature leaves. The dia- and contribute to higher economic returns. Failure to gram on page 2 provides a systematic key for diagnosing correct soil problems or to apply sufficient amounts of visual mineral deficiency and toxicity symptoms in taro. fertilizers can result in poor yields and wasted effort. Applying too much or the wrong kind of fertilizer can Methods to determine deficiencies have many negative consequences, including and toxicities in taro • nutrient toxicities or imbalances that reduce plant Three different methods were used to determine symp- growth and yield toms of nutrient deficiencies and toxicities of taro. First, • excessive foliage growth that invites damage by plant taro plants were grown in hydroponic culture, and the diseases and insect pests element of interest was reduced or deleted from the nu- • environmental contamination from runoff into surface trient solution. Second, taro plants were grown in pots water bodies and leaching into the groundwater, and with a soil that had been identified previously to result • economic loss due to wasted fertilizer. in a particular nutrient deficiency or toxicity. Third, plants in farmers’ fields were diagnosed as having a This publication explains the role of essential plant nu- particular nutrient deficiency or toxicity, based on symp- trients in taro (Colocasia esculenta) and shows the vi- toms and analyses of the soil and the plant tissues; then, sual symptoms that occur when there is a nutrient inad- correction of the particular nutrient problem was done equacy (deficiency) or excess (toxicity) in the taro plant. in the field to confirm the diagnosis. Diagnosis of nutrient imbalances Essential plant nutrients using visual symptoms Thirteen essential mineral nutrients are Diagnosis of nutritional disorders must be required by all plants. Six of these min- done in a systematic manner. Deficiency erals are required in large amounts and symptoms first appear on either the are called macronutrients: nitrogen (N), younger or the older leaves of the plant, phosphorus (P), potassium (K), calcium depending on the way the particular nu- (Ca), magnesium (Mg), and sulfur (S). trient is mobilized by the plant’s metabo- The seven minerals required by plants lism. Deficiency symptoms first appear in only small amounts are called micro- on younger leaves and growing points if nutrients: iron (Fe), manganese (Mn), the plant is not able to break down stored boron (B), zinc (Zn), copper (Cu), mo- organic compounds containing the nutri- lybdenum (Mo), and chlorine (Cl). The ent in mature leaves and then transport it metallic element aluminum (Al) is not to young, growing tissues. If the plant is essential as a nutrient, but it is consid- able to do this, deficiency symptoms ap- Normal taro leaf blades, without ered here because it is toxic to plants pear first on older leaves. Toxicity symp- signs of nutrient deficiency. when taken up in excessive amounts. Published by the College of Tropical Agriculture and Human Resources (CTAHR) and issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Andrew G. Hashimoto, Director/Dean, Cooperative Extension Service/CTAHR, University of Hawai‘i at Mänoa, Honolulu, Hawai‘i 96822. An equal opportunity/affirmative action institution providing programs and services to the people of Hawai‘i without regard to race, sex, age, religion, color, national origin, ancestry, disability, marital status, arrest and court record, sexual orientation, or status as a covered veteran. CTAHR publications can be found on the Web site <http://www.ctahr.hawaii.edu/freepubs>. Nutrient Deficiencies and Excesses in Taro Key to taro nutrient disorders (deficiencies and toxicities) Plant part Visual symptoms Nutrient disorder Manganese deficiency Uniform Sulfur deficiency Yellowing Between veins Iron deficiency Young leaf blades, Browning, Manganese deficiency growing points death of tissues Calcium deficiency Deformation Nitrogen deficiency Uniform Yellowing Magnesium deficiency Between veins Manganese toxicity Boron toxicity Potassium deficiency Older and mature Between veins Zinc, boron toxicity leaf blades Browning, death of tissues Manganese toxicity Brown spots Sodium chloride toxicity Boron toxicity Leaf margins Potassium, magnesium deficiency Blotching Phosphorus deficiency Deformation Manganese toxicity Sodium chloride toxicity Effect of the taro cropping system lion. Under anaerobic conditions soil pH increases, and on plant nutrients thus soil acidity and aluminum toxicity are not prob- Taro is grown under both “upland” (nonflooded) and lems in wetland taro. “wetland” (flooded) soil conditions. In a wetland sys- tem, where the soil is anaerobic (oxygen-depleted), ni- Soil testing trogen can be lost through denitrification. Also, in an A basic soil test measures the soil pH and the plant-avail- anaerobic environment certain elements can be chemi- able levels of the nutrient elements phosphorus, potas- cally “reduced,” increasing their availability to plants; sium, calcium, and magnesium. This information is used these elements include phosphorus, manganese, and iron. to determine which type of soil amendment (such as Taro’s adaptation to flooded conditions apparently in- lime) might be needed and to estimate the amounts of volves a tolerance of high levels of manganese. In ex- fertilizers required to supplement nutrients in the soil to periments, Mn toxicity was not observed until its con- produce a good crop. Other specialized soil analyses can centration in the leaf blade exceeded 2000 parts per mil- provide information on the levels of soil salinity, organic 2 UH–CTAHR carbon, aluminum, nitrogen, and micronutrients. For The index tissue for taro is leaf blade 2 information on soil analysis, see Testing Your Soil, Why and How to Take a Soil-Test Sample, CTAHR publica- tion AS-4. Plant tissue analysis Plant tissue analysis is done to monitor the nutrient lev- els in plant tissues. Nutrient content data are most use- ful in combination with soil analysis data and records of 1 past fertilizer applications and crop performance. Plant 2 tissue analysis measures the elements in an “index tis- sue,” a particular plant part determined by experimen- 0 tation to be the most reliable indicator of the plant’s nutrient status. For taro, the index tissue is the “leaf number 2,” the second leaf blade below the first, youngest expanded leaf blade with a new leaf beginning to emerge from its 3 petiole (see figure at right). The newly emerging leaf blade is counted as “leaf zero,” the first fully expanded (mature) leaf blade is counted as “leaf number 1,” and the next older mature leaf blade is counted as “leaf num- ber 2.” To sample taro for tissue analysis, collect index tissue from at least five relatively healthy main (“mother”) plants randomly located throughout one dis- tinct planting area (such as a field or lo’i). Remove leaf Table 1 (p. 4) gives the ranges of nutrient concen- number 2 with a sharp knife, discarding the petiole and trations in taro associated with deficiency, sufficiency, placing the leaf blade into a clean plastic bag. Avoid and toxicity. A gap between sufficient and toxic levels collecting diseased or damaged leaves. Protect the of a nutrient could occur because the plant is able to sample from overheating during transport by placing it absorb amounts of a nutrient in excess of what is re- into an ice chest. quired. Earlier recommendations called for the third leaf In Hawaii, research is ongoing to calibrate soil fer- blade (“leaf number 3”), but Phytophthora leaf blight tility levels with taro plant tissue levels and crop yields. often resulted in diseased tissue, making this older leaf Best management practices and interim fertilizer rec- unsuitable for sampling. ommendations are used to assist taro growers to help The tissue nutrient analysis results are compared increase yields, avoid excessive fertilizer applications with sufficiency ranges (standards) established for the and costs, limit disease severity resulting from over-fer- particular crop. Within the sufficiency range for a nutri- tilizing with nitrogen, and reduce environmental pollu- ent, adequate growth can be expected, as far as that par- tion. Best management practices are given in Taro, ticular nutrient is concerned. In the deficiency range, Mauka to Makai: A Taro Production and Business Guide visible nutrient deficiency symptoms are evident and for Hawaii Growers. Fertilizer recommendations for crop yield is reduced. When levels of a nutrient are in wetland taro are given in Interim Fertilizer Recommen- excess (above the sufficiency range), nutrient imbalances dations for Wet (Flooded) Taro, CTAHR publication PM- can occur, and the plants may become prone to diseases 1a. Additional information on soil management, plant or physiological disorders. For example, it was found nutrition and diagnosis of nutrient deficiencies is avail- that excessive nitrogen can promote taro leaf blight able in CTAHR’s