This paper analyses the effects of foliar application of the seaweed extracts Bio-algeen S90 (Ascophyllum nodosum) and Kelpak SL (Ecklonia maxima), as well as the humic and fulvic acids ini HumiPlant (leonardite extract), on the assimilation area and chlorophyll content of very early potato cultivars (‘Denar’, ‘Lord’, Miłek’). The field experiment was carried out in central-eastern Poland over three growing seasons, using Luvisol. The biostimulants were applied according to the manufacturers’ recommendations. The use of biostimulants resulted in enlargement of the assimilation area, but had no effect on the specific leaf area (SLA) or chlorophyll content (Soil Plant Analysis Development (SPAD) value). The assimilation area was larger, on average, by 0.0505 m2 and leaf area index (LAI) was higher by 0.30 compared with the plants from the control group without a biostimulant. The SLA and SPAD depend on the cultivar and weather conditions, or nitrogen and magnesium content in soil, to a greater extent. The biostimulants enhanced abiotic stress tolerance and increased marketable tuber yield (diameter above 30 mm) 75 days after planting (the end of June), on average by 2.15 t·ha−1. Bio-algeen S90 and Keplak SL produced better results in a warm and very wet growing season, whereas HumiPlant produced better results in a year with lower air temperature and with drought periods during potato growth. No correlations were found between the tuber yield and assimilation area or between the tuber yield and SPAD value, although a significant negative correlation was found between the tuber yield and SLA.

The effect of perforated polyethylene foil and polypropylene fibre cover on the development of the early potato cultivars Aksamitka and Cykada was investigated. The use of the covers resulted in an increase in the soil temperature at the depth of 100 mm on average by 2.2°C at 8 a.m. and by 4.0°C at 2 p.m. in the year with warm spring and by 1.5°C and 3.2°C in the year with cold spring, respectively. A higher increase in the soil temperature, on average by 1-2°C, was caused by the use of the perforated foil than by the use of the polypropylene fibre. The increase in the soil temperature as a result of covering forced the plant emergence on average by 5 days and shortened the period from planting to tuber setting by 5 days in comparison with the cultivation with no plant covering. The higher favourable effect of covering was obtained in the year with the lowest air and soil temperatures in May. The use of the covers at that time forced the plant emergence and shortened the period from planting to tuber setting on average by 6 days. A higher soil temperature increase was observed under the perforated foil than under the polypropylene fibre, which resulted in an earlier occurrence of the successive plant development phases, on average by 1-2 days.

A condition for achieving a high potato tuber yield on an early harvest is to provide plants with good growth conditions. In modern horticulture, plant growth stimulants have been gaining increasing importance. This study examined the effect of dose (0.2 dm 3 ha-1 or 0.4 dm 3 ha-1) and date (leaf development stage-BBCH 14-16, tuber formation stage-BBCH 41-43, leaf development stage and tuber formation stage) of Tytanit ® application (8.5 g Ti in 1 dm 3) on the growth of very early-maturing potato cultivars ('Lord', 'Miłek'). Following the Tytanit ® application, the plants were higher and produced a greater aboveground biomass and tuber weight, however, the leaf weight ratio (LWR) and leaf area ratio (LAR) were lower than in the cultivation without the growth stimulant. Tytanit ® had a greater effect on the leaf weight ratio (LAR) and tuber weight for the 'Lord' cultivar. Tytanit ® dose had no effect on the plant growth. A double Tytanit ® application resulted in a reduction in the length and weight of stems and tuber weight. The date of Tytanit ® application had no effect on the weight of leaves. The leaf area ratio (LAR) was the highest when Tytanit ® was only applied in the tuber formation stage, and the leaf weight ratio (LWR) was the highest when Tytanit ® was applied twice. A positive correlation was found between the tuber weight and the LWR and LAR.

SummaryThe effect of cover material (perforated polyethylene film, polypropylene nonwoven) and date of its removal (2 and 3 weeks after plant emergence) on the marketable value of the yield of early potato cultivar (Aksamitka, Cykada) was investigated. Potatoes were harvested 60 and 75 days after planting, in the second and third decade of June. Plant vegetation forcing by covering resulted in an increase in the marketable tuber yield (diameter >30 mm) and improved marketable value due to a reduced share of small tubers, with the diameter of 31-40 mm, especially at a very early date of potato harvest. In potato cultivation for very early crop, the cover material had a greater effect on the tuber yield and its marketable value than the length of plant covering period. Perforated film was a better kind of cover than polypropylene nonwoven, especially in the year with cold spring. At the delayed date of potato harvest the marketable tuber yield depended, to a higher degree on the date of cover removal. Higher yield was obtained when the cover was removed 2 weeks after plant emergence.

In this six-year research study the effect of nonwoven polypropylene covering on the quantity and quality of early potato yield was estimated. The use of nonwoven polypropylene covers resulted in an increase in the tuber yield and smaller yield variability in the various years, when compared with the traditional cultivation, especially at a very early date of potato harvest. In the cultivation with nonwoven polypropylene covering, the marketable tuber yield 60 days a�er planting was higher by 23.34% on average and 75 days a�er planting by 10.92% in the six-year period of the study, compared with the cultivation with no plant covering. The higher profitable effect of covering was obtained in the years with cold spring. The cultivation method of the potato did not affect the chemical composition of the tubers harvested a�er 60 days from planting. A�er 75 days from planting the tubers of potato cultivated under nonwoven polypropylene covers contained, on average, more dry ma�er and starch by 0.81% and 0.85%, respectively.Keywords: early potato; nonwoven polypropylene cover; yield; tuber quality PLANT SOIL ENVIRON., 51, 2005 (5): 226-231 227 60 kg P 2 O 5 and 90 kg K 2 O for potatoes harvested 60 days after planting and 90 kg N, 90 kg P 2 O 5 and 135 kg K 2 O per 1 ha in the case of potatoes harvested 75 days after planting. Immediately after planting and before covering the herbicide was applied (linuron -trade name Afalon 450 SC). The material used in the experiment was nonwoven polypropylene of 17 g/m 2 weight (trade name Pegas Agro 17). The cover was removed 2 weeks after plant emergence. The total and marketable yield of tubers (diameter above 30 mm) was estimated. For laboratory studies, 50 tubers of different size, according to the proportional participation in the yield of each plot were sampled. The dry matter, starch (using the polarimetric method), total protein (using the Kjeldahl method) and vitamin C (using the Pijanowski method) contents in potato tubers were determined (Rutkowska 1981). Chemical analyses were made on fresh material just after the harvest.The results of the experiment were analysed statistically by means of analysis of variance. The significance of differences was veryfied using the Tukey's test at P = 0.05. The yield and the content of examined components in potato tubers were characterized by the mean, standard deviation and variability coefficient (%).In the six-year period of the study, only in 1998 and 2002 were the climatic conditions favourable for potato cultivation for the early crop (Table 1). In 1997, a very cold April and a long-lasting snow cover made it impossible to plant the potatoes early. RESULTS AND DISCUSSIONDiverse climatic conditions during the potato vegetation period significantly affected the tuber yield. The yield variability after 60 days from planting was higher than that obtained after 75 days from planting. The variability of marketable tuber yield was higher than that of total tuber yield, particularly at the earlier date of potato harvest (Tabl...