Phosphorus Relationships in Potato Plants

Maximum potato (Solanum tuberosum L.) tuber yields occur when an active plant canopy is maintained until normal plant maturation. Plaid nutrient concentrations and uptake rates play a major role in maintaining an active plant top. The objectives of this study were to relate the plant P concentrations to the P and dry matter balance between tuber and total plant growth needs. Growth analysis data, plant and leaf total P concentrations and content, and the petiole soluble P concentrations were obtained on a 10-to 14-day sampling interval from P fertilization treatments in replicated field studies. The P concentration of the plant tops was significantly related to the petiole soluble P concentration and the P concentration of the active leaves. Total plant P uptake and dry matter production rates were not adequate for the tuber growth rate when the total P concentrations of the tops and active leaves were less than 2.2 g P kg-'. Soluble P concentrations in the fourth petiole down from the growing tip were less than 1000 and 700 mg kg1 when P uptake and dry matter production rates were not adequate for tuber growth, respectively. Final tuber yields increased from 30 to 70 Mg ha1 as the number of growing days past tuber set increased from 10 to 60 days for which the P concentration of the tops was above 2.2 g P kg-'. The petiole soluble P concentration decreased during the growing season following a semi-logarithmic relationship. This relationship enabled the prediction of the petiole soluble P concentration for the rest of the growing season and could be used to predict when to apply supplemental P fertilizer. Additional index words: P uptake, Soluble P concentration, Total P concentration, Seasonal monitoring, Tuber yields, Solarium tuberculin L. T HE evaluation of a plant's nutritional status is based upon a significant relationship between the nutrient in question and plant yields. This relationship is called a nutrient response curve and can identify nutrient concentrations that are deficient, adequate, and toxic. The transition zone between deficient and adequate is the critical nutrient concentration and is generally defined as that concentration where the growth or yield is 10% less than the maximum (17,23). This concentration is dependent upon plant growth stage, plant part and its physiological age, the form of the nutrient measured, and interactions with other nutrients (2, 17, 23). Efforts to remove some of these variables have been proposed by using a critical nutrient range (6, 13, 20) or the Diagnosis and Recommendation Integrated System—DRIS (16). The first technique requires identifying a critical nutrient range from the relationship between the nutrient concentration in a plant part at a particular growth stage and final yield. This gives a band of different critical nutrient concentrations during a crop's growth and development. The critical nutrient range generally decreases with plant age, possibly being explained by the declining absolute growth rate of plants as they become larger and older (21). The DRIS approach attempts to remove the growth stage variable by identifying significant nutrient ratios between two yield 'Contribution from the USDA-ARS. Snake River Conservation Research Center, Kimberly, 1D 83341, and Univ. of Idaho Research and Extension Center, Kimberly, /ID 83341. Received 7 June 1984. 2 Soil scientist and plant physiologist, respectively. Published in Agron. 77:490-494 (1985). levels which are then used to identify nutrient imbalances (16). Many growers and consultants are now using crop logging techniques on crops with a high cash value. The disease incidence, insect infestations, nutritional status of the soil and plant, and plant available soil water status are usually monitored during crop growth and development. This technique should allow a grower to detect or predict, and correct a potential problem before it affects yields. The relationships between critical nutrient ranges and final tuber yields are reported for potatoes (12, 13, 20), as well as the suitability of different plant parts and the form of plant nutrients for evaluating the P status (4, 7). The fourth petiole of the most recently matured leaf from the growing tip is usually the plant part used for nutrient analysis in potatoes (7, 13, 20). The soluble and total nutrient concentrations in the petiole are expressed on a dry weight basis. Final potato tuber yields are a function of tuber growth rates and the duration of tuber growth, particularly for indeterminate potato varieties (9, 10, 22). Full-season tuber growth requires nutrient uptake until the start of plant maturation since the tubers function as the major nutrient sink during their growth. Nutrient uptake rates less than those required for tuber growth will cause the loss of mobile nutrients from the other plant parts to the tubers, eventually causing a premature canopy senescence. Final tuber yields could be reduced if this senescence starts too early in the growing season when the environmental conditions are still favorable for growth. Nutrient uptake rates can slow or stop during normal maturation since most tuber growth during this growth stage is from the translocation of dry matter and nutrients from the other vegetative portions of the plant (9, 14). This report relates the potato plant P concentrations to the P and dry matter balance between tuber and total plant growth. METHODS AND MATERIALS Data presented here come from five replicated field experiments conducted on a Portneuf silt loam soil (coarsesilty, mixed, mesic Durixerollic . Calcioithid). This soil has a calcic layer starting near the 0.4 m soil depth that restricts root penetration but not water movement. These experiments were designed to evaluate different P fertilizer placement methods and to obtain soil test P correlation data for maximum potato tuber (Solanum tuberosum L.) yields. The effect of these treatments on the final tuber yield and total P uptake will be reported later, Standard cultural and fertilization practices were followed in all of the field experiments (18). The P treatments in this study were established by broadcast applications of monocalcium phosphate (0-45-0) followed by plowing or discing prior to planting. Sprinkler irrigations were scheduled according to tensiometers placed in the row at the seed-piece depth (0.2 m) when the plant available soil water dropped to 50 or 60% of the soil's field capacity. A randomized complete block design was used with four or five replications for each experiment. Russet Burbank potato seed [0.06 kg (seed