Response of Hemp to Plant Population and Nitrogen Fertilisation

S. AMADUCCI 1, M. ERRANI 2, and G. VENTURI 2 †

1 Istituto di Agronomia generale e coltivazioni erbacee, Università Cattolica del Sacro Cuore, Piacenza, Italy2 Dipartimento di Agronomia, Università di Bologna, Italy

Corresponding Auhtor: S. Amaducci, Istituto di Agronomia Generale e Coltivazioni Erbacee, Università Cattolica del Sacro Cuore, via Emilia Parmense 84, 29100 Piacenza, Italy. Tel. +39 0523 599223; Fax: +39 0523599222; E-mail: samaducci@pc.unicatt.it

Received: 18 September 2001. Accepted: 15 July 2002.

Ital. J. Agron., 6, 2, 103-111

ABSTRACTBACKGROUND. Hemp (Cannabis sativa L.) used tobe a traditional industrial crop in many regions ofEurope and of the World. After many years of aban-don the crisis of agriculture and the need for natur-al raw materials cast a renewed interest over thiscrop. In the frame of the EU project “HEMP”, fieldexperiments were carried out to evaluate the effectof agronomic factors over yield and fibre quality.METHODS. The effects of plant density (from 30 to250 plants m-2) and available nitrogen (100, 160, and220 kg ha-1) on fibre hemp were studied at Cadriano(BO) over three years 1996 - 1998. These two factorswere studied on different genotypes, sowing and har-vest times. Significant results for major crop para-meters are reported.RESULTS. Results confirmed that optimal productionis obtained with a plant density of 90-100 plants m-2.The weight of a single plant decreased exponential-ly as plant density increased. The same trend was ob-served in different years and with different experi-mental treatments. Nitrogen fertilization and geno-type (monoecious and dioecious varieties) did not in-teract with plant density on hemp yield and qualityparameters. Delaying harvest time decreased plantdensity and increased average plant weight, due tothe higher competition. Plant density affected sever-al traits, which could influence fibre quality. Increas-ing plant population increased the ratio between cor-tical surface and volume of the stem, as well as thecortical surface per unit area. Nitrogen fertilizationincreased plant mortality but also average plantweight, and as a result stem production was in-creased. 100 kg ha-1 of nitrogen was the natural avail-ability in the soil, and each additional kg of nitrogensupplied via fertilisation increased stem dry matterproduction by 20 kg.CONCLUSIONS. The major effect of plant populationon plant biometrics, and yet the small effect on cropyield, suggest that plant population should be chosenaccording to the final destination of the crop.

Key-words: Cannabis sativa L., nitrogen fertilization,plant density, stem production.

INTRODUCTION

The response of hemp to environmental factorsand technical intervention has been the focusof much research. In general an optimum ca-pacity of adaptation to various cultivation andenvironmental situations was found in hempdue to various self-regulating mechanisms (Ven-turi and Amaducci, 1996; 1997). For example, inconditions of elevated plant density and theconsequential intraspecific competition, a partof the plants die, others stop growing, and onlythe remainder that grow normally contribute tothe final production. These harvested plantshowever have increased variability of dimen-sion, weight and volume.In contrast, at low density the plants grow re-markably either in height or diameter of thestem and they even branch out (in this situationthe destination of the crop should be changedto take advantage of its seed production). Atharvest variation in weight and dimensionamong the plants is relatively little. Conse-quently the quantity of seed to use depends onthe desired product (stem or seed), and also onthe destination of the stem. For production ofseeds it is suggested to use between 5 and 30plants m-2 (Martinov et al., 1996; Regazzi, 1955;Venturi and Amaducci, 1996; 1997).For stems destined for textile applications 100plants m-2 can be considered as optimal (Ven-turi, 1967; Amaducci, 1969; Dempsey, 1975; Mei-jer et al., 1995; Van der Werf et al., 1995b; DiCandilo et al., 1996; Venturi e Amaducci, 1997),though there can also be positive indications fora higher plant density (up to 250-300 plants m-2,Martinov et al., 1996). When stems are to be

† All the Authors equally contributed to the paper.

used for paper pulp a lower plant density is ac-ceptable (Martinov et al., 1996).It used to be that the density of plants at har-vest was not considered but instead the quanti-ty of seeds sown. In environmentally lessfavourable situations of Central and NorthernEurope comes the recommendation to sow 400-600 viable seeds m-2, yet 200-350 m-2 in South-ern and Eastern Europe (Paskovic, 1966; Starce-vic, 1966). In both cases autoregulation of thehemp relative to the density, and especially thegrowth of the plant, was relied upon. The effectsof density however vary in time. In the initialphase the relation between density and mass ofdry matter is linear. Successively, the competi-tion between plants commences, first only at el-evated plant densities, then with progressivegrowth, and subsequently at lower densities. Thelinear relation between plant density and drymatter is substituted by a curvelinear relation inwhich the production level increases consider-ably until a maximum value that becomes a con-stant for all the higher densities. Production canthen be defined by the relationship: averageweight × density = K, where the average weightof the plant becomes inversely proportional tothe density. The effect of plant population thenvaries in time, with the obvious consequence onthe influence of harvest time on the quantityand quality of production.In fact, if the effects of density can be minimalwith regard to biomass production they can in-stead profoundly alter the composition and thusinfluence the qualitative and technological char-acteristics of the fibre through modification ofdiverse components of production.The above mentioned relationships have beenknown for some time but the intensity of theirresult varies as a function of the environmentalconditions and the harvesting system used (Ven-turi, 1963; Venturi, 1967; Amaducci, 1969; Bassoet al., 1976; Marras e Spanu, 1979; Hoppner eMenge-Hartmann, 1994; Meijer et al., 1995; Vander Werf et al., 1995a; 1995b; Di Candilo et al.,1996; Venturi e Amaducci, 1997).Another factor of note that influences the quan-tity and quality of the production of hemp is ni-trogen fertilisation. The effect of increasing itsdosage from 120 kg ha-1 (Van Geel and Van derWerf,1994; Van der Werf and Van Gell, 1994; Vander Werf et al., 1995a) to 225 kg ha-1 (Di Can-dilo et al., 1996) varies as a function of envi-

ronment, cultivar and agro-technique. On aver-age, nitrogen increases biomass because of anincrease of leaf canopy and humidity, but itworsens qualitative characteristics because ofthe minor content of fibre (Venturi and Ama-ducci, 1997).With the intention of quantifying the above re-lations within a model of wide applicability, aresearch project was designed that would run si-multaneously in England, The Netherlands andItaly. For the model used the effects of plantpopulation and increased nitrogen fertilisationon quantity and quality of production were ver-ified for environmentally diverse situations(North and South Europe, from 53° to 45° par-allel North) in a wide range of latitudes. Pho-toperiod and intensity of radiation were partic-ularly different between the chosen locations.Moreover, by harvesting at time intervals, we in-tended to determine if the effects of plant pop-ulation and nitrogen fertilisation differ betweenmonoecious and dioecious types.This paper presents some of the results obtainedfrom a series of trials carried out in the frame-work of the European Union project “Hemp forEurope”. Only data on the effect of the harvestdone close to the agronomic maturity of thecrop will be reported.

MATERIALS AND METHODS

During three years 1996-1998 at Cadriano,Bologna, lat. 43° 30’ N; long. 11° 21’ E; alt. 32a.s.l. different trials were carried out to studythe effects of plant density on genotypes withdifferent lengths of cycle, dioecious and mo-noecious, and grown with different combina-tions of agrotechniques, in particular for harvesttime and nitrogen availability. In the first twoyears the effects of nitrogen on monoecious anddioecious cultivars (Futura 77 and Kompolti, re-spectively) was also studied by comparing threedoses in different factorial combinations withcultivar and plant population. Nitrogen was de-termined as the mineral element present in thesoil at the beginning of the season (analysedwith CaCl2 method). To obtain three levels ofinitial availability equal to 100, 160 and 200 kgha-1 nitrogen was distributed as urea beforeseeding.In all trials inter-row distance was 18 cm, which

104 Amaducci et al.

is considered optimal for the growth of hempfibre in the environmental conditions used(Crescini, 1940; Di Candilo et al., 1996; Venturi1967; 1969; Venturi and Amaducci, 1999). Depthof sowing was 3-4 cm and the seeds were alwaystreated with antrachinone (1 g kg-1) to guardagainst birds, and with captano (1 g kg-1) to pro-tect against fungi. Sowing was done, dependingon the specific trial and the year, betweenMarch 29th and April 27th. Monoecious varietiesused were Felina 34 and Futura 77. Dioeciousvarieties were Carmagnola and Kompolti. Theexperimental plant densities were achieved bymanual thinning after complete emergence.With scaled harvests, different in both totalnumber and time according to the experiment,both biometric and productive measurementswere carried out. For calculating cortical surfaceand volume the stems were assumed to be con-ical. The average effects of plant population forthe cultivar Futura 77 were estimated by meansof regression analysis, using the data obtainedin all three years for the different combinations(48 in total) of environmental situations andcropping techniques.For the cultivar Futura it was possible in 1997and 1998 also to do a cumulative elaboration(homogeneous variance in the Bartlett test) ofdata obtained in three harvests comparable forthe vegetative phase.The characteristics of the soil in which the ex-periments were done are presented in Table 1and the meteorological report is in Figure 1.

Meteorological reportThe three years proved to be diverse either for

Plant Population and N Fertilisation of Hemp 105

Table 1. Soil physical and chemical characteristics.(†) Method Braj I (‡) Method Ammonium Acetate

1996 1997 1997 1998

Texture (Boyoucous):sand (0.02 < Ø < 2 mm) (%) 2237.3 2233.5 2231 2247.4silt (0.002 < Ø < 0.02 mm) (%) 2233.3 2235.5 2237.52 2230.4clay (Ø < 0.002 mm) (%) 2230.3 2232.5 2232.52 2223.4

pH 2227.3 2227.0 2227.42 2226.3C.E.C. meq 100g-1 2248.3 2253.1 2244.02 2238.5CaCO3 total (%) 2221.6 2220.7 2221.42 2220.5CaCO3 active (Met. Droineau-Gehu) (%) 2220.9 2220.5 2221.15 2220.5Organic matter (Walkley Black) (%) 2222.0 2221.7 2222.02 2221.4Total Nitrogen (Kjeldahl) (‰) 2220.9 2220.8 2221.12 2220.8P available (Olsen) (ppm) (†) 2238.3 2230.5 2225.52 2262.4K exchangeable (Ba Cl2 + TEA) (ppm) (‡) 2184.3 2202.5 2207.52 2298.4Ca (Ba Cl2 + TEA) (ppm) (‡) 4461.3 4369.5 4792.52 2457.

4

Figure 1. Average temperature and rainfall measured at 10days intervals during the trial period.

temperature or for the total amount and distri-bution of the rain (Figure 1). This had a strik-ing influence on the duration of the croppingcycle in 1998, a particularly hot and dry year.The cycle was especially short with respect topreceding years. From March/April until themiddle of August the precipitation levels were325, 274 and 228 mm respectively in the threeyears of experiment. In the same period thethermal sum was 2826, 2891 and 2973 °C.

RESULTS AND DISCUSSION

General AspectsOver an average of the three years, consideringonly the final harvest, the production of biomassfluctuated from 30 to 50 t ha-1, with humidity nor-mally around 65-70%, from which 10-15% camefrom the fresh leaves, 80-85% from the stems andthe remaining part from inflorescence and seeds.The production of dry stems varied from 10 to 18t ha-1, with the highest frequency at 12-13.On average the stems had reached a height of180-240 cm and an average weight (dry matter)of 10-18 g per plant.

106 Amaducci et al.

In situations of average plant densities the basediameter varied from 7 to 10 mm, average vol-ume of the stem was 30 to 40 cm3 and the cor-tical surface was around 500 cm2. With plantpopulations around 90-100 plants the corticalsurface had reached 4-5 m2 per m2 of soil.

Effects of Plant PopulationTaking an average from diverse situations ofmeteorological condition, time of harvest, geno-type, and nitrogen availability, the effects ofplant population on the total production weremodest and insignificant, though the effectswere evident in the biometric and qualitativecharacteristics.Considering a large range of plant densities(from 80% less than and 120% more than theoptimal of 90-100 plants m-2), the productionvalues (total biomass, and fresh and dry weightof stem and leaves) showed only a slight ten-dency to increase with increasing plant density.Such a tendency was relatively more evident inthe fresh weights than in the dry.Examining situations disaggregated for factorsof production (time of harvest, year, cultivar)very similar development was found.

Figure 2. Effects of plant density on whole plant and stem weight of fibre hemp.

Plant Population and N Fertilisation of Hemp 107

Figure 3. Effects of plant density on some biometric traits of fibre hemp.

This supports the plasticity of the hemp, in thatthe plant is capable to fit well to various situa-tions of intraspecific competition autoregulatingthe actual increase.The average fresh and dry weight of the wholeplant, and in a greater way the one of the stemalone, decreased sharply with increasing plantpopulation until 100 plants m-2, from there it de-creased more steadily with higher plant popu-lations. For example, with populations of 40-50plants m-2 the average weight of the stem was

around 80 g in the fresh state, and 30 g whendry. Weight reduction was from 30 g to 10 g with100 plants m-2, and from 20 g down to 8 g with150 plant m-2 (Figure 2). This obviously altersthe architecture of the crop and the biometriccharacteristics. Limiting the examinations to thebiometric characteristics there was a slight ten-dency of the height to decrease in conditions ofhigher population (Figure 3a). This, like the de-crease in diameter (Figure 3b), was very no-ticeable passing from the lower density to the

intermediate, and subsequently to the higherdensities.This behaviour reduced the cortical surface (byaround 5 cm2 for each additional plant between50 and 100 plants m-2) and the average volumeof the stems, but improved the relation betweenthese two factors (Figure 3c, d, e). With 50 plantsm-2 the relation exceeds 10, and was around 20for 150 plants m-2. This might represent a qual-itative index and give an indication on the bark-core relation. If not the single plant, but the en-tire crop is considered the effect of plant pop-ulation becomes evident; in fact the cortical sur-face per metre of surface cultivated grows byaround 20 cm2 for each additional 10 plants(Figure 3f).The above mentioned developments occurred inall situations, but the intensity of the relation-ship between plant density and each character-istic varied depending on the phase of growth,and the relative thermal sum. For example, therelationship between base diameter and popu-lation appeared to be stronger (higher values ofR2) halfway through the crop cycle with a ther-mal sum inclusive between 1250 and 1800 °C,with respect to the successive phases. Analogousconsiderations can be formulated also for corti-cal surface, stem volume, and their relationship.Relative to the cortical surface per unit area ofsurface cultivated, the strongest relationshipswere with thermal sums from 2700-3300 °C, thusin the latest phase of the crop cycle.The effects of plant density don’t appear to bedifferent as a function of genotype, nor as afunction of nitrogen availability; they changedinstead as a function of harvest time, thoughlimitedly for some characteristics. This result isevident considering the significant effects ob-

tained on average in the two years 1997-1998 inthe three harvests carried out in the interval be-tween 25 and 100% of flowering (Table 2, 3, 4).As a function of plant population, the total drybiomass tended to differentiate more in the in-termediate harvest; the difference between thelow and high plant population exceeded 15%,most probably as a consequence of the leafcanopy. For biomass, the fraction represented byleaves gave its highest contribution to the in-crease of density only in the latest harvest; thedifference between the crops cultivated and thelow and high density was almost 65% (Table 2).The effects of plant population appeared dif-ferently with time, especially with regard to theaverage dry weight of the whole plant and of thestem (Table 3). Generally, comparing sparse cropswith intermediate and denser conditions the av-erage weight obtained was 53-58% and 25-30%less, respectively. Average weights of the wholeplant and the stem increased differently: at low-er density it increased by 28-30% in the intervalbetween the first and second harvest, and by 76-80% between the first and third harvest. At in-termediate density the increments were 21 and17% from the first to second harvest, and 64 and58% from first to the third. For these same in-tervals at high density the increases were by 50and 60% and 117 and 100%, respectively.It seemed then that final weight stabilised soon-er in the density considered optimal, with a cer-tain delay in the sparse crops, and with moredelay in the dense crops.The surface to volume relationship changedthrough time in a different way according withthe plant density (Table 4). Increasing the den-sity in average decreased height, diameter, cor-tical surface and stem volume and increased

108 Amaducci et al.

Table 2. Effect of planned plant density on plant population and yield components at three harvest times. For each para-meter, values of main effects or of interaction marked by the same letter indicate differences not significant at P ≤ 0.05(Duncan’s Multiple Range Test).(†) Harvest I = 25% flowering(‡) Harvest II = between 50 and 75% flowering(¥) Harvest III = 100% flowering

Programmed plant Plant density at harvest Total dry biomass Leaves dry Fresh leaves biomassdensity (m-2) (m-2) (t ha-1) mass (t ha-1) (% on total)

I (†) II (‡) III (¥) I II III I II III

45 246 e 244 E 243 e 11.5 fe 13.7 ce 18.4 ae 1.4 ae 17.4 a 14.9 b 4.5 de90 283 d 286 D 278 d 11.8 ef 14.1 cd 17.6 ab 1.6 ab 18.5 a 13.8 b 6.1 cd180 196 a 179 B 143 c 12.1 df 15.9 bc 17.4 ab 1.7 ae 17.6 a 14.1 b 7.4 cemean 108 a 103 A 288 b 11.8 ce 14.6 be 17.8 ae 1.6 ee 17.8 a 14.2 b 6.0 ce

cortical surface per unit of ground surface andsurface - volume relationship. With respect tothe low plant population this last parameter was33% higher than the intermediate density, and117% higher than the most dense, at the earli-est harvest; it was 36 and 55% higher, respec-tively at intermediate harvest and only 27 and55% higher in the latest harvest. In fact the re-lationship tended to decrease in time muchmore markedly that the increases in plant pop-ulation.Plant population increased the homogeneity ofstem diameter (standard deviation decreased),and stem height, but not significantly. The re-duction of disformity among the plants with re-gard to diameter and height was apparentlymore noticeable in the latest harvest. Thisseemed due to the increase in time of the effectof competition, with consequently more deathsof small and thin plants.

Effects of Nitrogen FertilisationThe effects of nitrogen fertilisation, in the two

years 1996-1997, are explained only on a limit-ed number of characteristics, and in no case didthe result significantly interact with genotype. Itseemed that cultivar of different cycle (mo-noecious and dioecious), responded in a similarway to the availability of the element. On av-erage an increase in the dose increased plantdeath, production of fresh and dry stems, aver-age weight of the whole plant and the stems,while diameter and volume of the stems tend-ed to diminish in one year and increase in theanother (Table 5).For fresh and dry production of stems the ef-fect of nitrogen dosage on weight was similar,with increments of 10% and 15% for each ad-ditional 60 kg of nitrogen respectively. On theaverage weight of the whole plant and of thestem the effect of the lower dose of fertiliserwas more effective then the higher one: 60 kgha-1 of additional nitrogen increased whole plantand stem dry matter by 25% and 29% respec-tively compared to the not fertilised crop, whilethe higher dose of fertiliser produced an addi-

Plant Population and N Fertilisation of Hemp 109

Table 3. Effects of plant density on average plant weight as a function of harvest time. For each parameter, values of maineffects or of interaction marked by the same letter indicate differences not significant at P ≤ 0.05 (Duncan’s Multiple RangeTest).(†) Harvest I = 25% flowering(‡) Harvest II = between 50 and 75% flowering(¥) Harvest III = 100% flowering(¶) Significant differences by comparison to interaction.

Programmed plant Fresh weight (g) Dry weight (g)density(m-2) Whole plant Stem Whole plant Stem

I (†) II (‡) III (¥) mean (¶) I II III mean

45 93 a 74 a 25 cc 32 b 44 a 34 a 20 cc 26 bc 36 ac 28 a90 49 b 39 b 14 de 17 d 23 c 18 b 12 de 14 dc 19 cc 15 b180 25 c 20 c 26 fc 29 f 13 e 29 c 25 gc 28 fg 10 ef 28 cmean 56 c 44 c 15 cc 19 b 26 a 20 c 12 cc 16 bc 22 ac 17 c

Table 4. Effects of plant density on some biometrical characteristics as a function of harvest time. For each parameter, val-ues of main effects or of interaction marked by the same letter indicate differences not significant at P ≤ 0.05 (Duncan’sMultiple Range Test).(†) Harvest I = 25% flowering(‡) Harvest II = between 50 and 75% flowering(¥) Harvest III = 100% flowering(¶) Significant differences by comparison to interaction.

Programmed plant Height Diameter Cortical Volume Surface/volume ratio Corticaldensity (m-2) (cm) (mm) surface (cm2) (cm3) I (†) II (‡) III (¥) mean (¶) (m2 m-2 soil)

45 234 a 10.4 a 765 a 68 a 12 cd 11 dd 11 dd 12 c 3.4 c90 224 b 28.1 b 581 b 42 b 16 bd 15 bc 14 bd 15 b 4.8 b180 205 c 26.1 c 404 c 24 c 26 ad 17 bd 17 bd 22 a 6.5 amean 221 c 28.2 c 584 c 44 c 16 ad 14 bd 14 bd 16 d 4.9 d

tional increase of 10% and 14% on the sameparameters. Similar results were found for freshmatter production.With the low dose of nitrogen there was an in-crease of dry matter of 17 kg per kg of the el-ement provided, and a 21 kg increase with thehigher dose.Diameter and volume of the stems in the firstyear reached a particularly high value andseemed to have a negative effect due to nitro-gen fertilisation, where as in the second yearthey increased from the nitrogen.

CONCLUSIONS

The research carried out in the three years 1996-1998, with the objective of getting data for cre-ating a model with a range of applications, hasconfirmed the results obtained in previous ex-periments conducted in the same environment:− effects of plant population on total produc-

tion seemed modest in a range of differentsituations, and confirmed the optimal densityas 90-100 plants per m2;

− above the optimal density the considerablylimited competition of the plants increasessuch that the product density x averageweight became constant;

− at the optimal density, density and averageplant weight was inversely related. At lowerdensities there was a linear relation. Obvi-ously this was related to the potential maxi-mum growth of a single plant in conditionsof very low density;

− the above mentioned results seem to havegeneral value since they were obtained in awide range of conditions different for bothenvironmental factors and cropping tech-nique;

− effects of plant population did not appear tointeract either with genotype (monoecious,

dioecious, and different cycle) or with the ni-trogen fertilisation. There was an interactionthough with the time of harvest as a functionof the increased competitive effects in time;

− generally, increased density had a tendency toincrease the surface/volume relationship ofthe stems, and the cortical surface per unit ofsurface cultivated;

− nitrogen caused a high mortality, probablydue to competitive effects in the initial phaseof the cycle and, favouring a more than pro-portional increment of average plant weight,lead to a high production of dry stems;

− for each kg of nitrogen provided there wasan increase of around 20 kg in dry stems.

ACKNOWLEDGEMENTS

The research has been carried out in the frame-work of the E.U. project “Hemp for Europe –manufacture and production systems”.

REFERENCES

Amaducci M.T., 1969. Ricerche sulla tecnica colturaledelle canape monoiche utilizzate per fabbricazione dicarte pregiate. Sementi Elette, 3, 166-179.

Basso F., Ruggiero C., 1976. Influenza della conci-mazione azotata e dell’epoca di raccolta sulla pro-duzione di fibra e cellulosa di cultivar e ibridi di cana-pa. Nota I. Cellulosa e Carta, XXVI, 3, 17-26.

Dempsey J.M., 1975. Fiber crops. Univ. Press of Florida.

Di Candilo M., Ranalli P., Marino A., 1996. Influenzadell’investimento e della concimazione azotata sulla pro-duzione di canapa da cellulosa (Cannabis sativa L.). Riv.Agron., 30, 3, 258-263.

Hoppner F., Menge-Hartmann U., 1994. Field trials withfibre hemp on nitrogen fertilizer application and standdensity. Landb.-Volkenrode, 44, 4, 314-324.

Marras G.F., Spanu A., 1979. Aspetti di tecnica colturalein canapa da cellulosa. Densità di semina e concimazioneazotata. Ann. Fac. Agric. Univ. Sassari, XXVII.

110 Amaducci et al.

Table 5. Effects of nitrogen fertilisation on some yield components. For each parameter, values of main effects or of in-teraction marked by the same letter indicate differences not significant at P ≤ 0,05 (Duncan’s Multiple Range Test).

N doses Plants (m-2) Stem (t ha-1) Fresh weight (g) Dry weight (g) Stem Diameter (mm) Volume (cm3)(kg ha-1) Fresh Dry Whole plant Stem Whole plant Stem 1996 1997 1996 1997

100 88 a 25.7 bb 9.5 bb 42.0 b 29.6 b 14.7 b 10.9 b 10.0 ab 6.6 c2 80 a2 24 d2160 75 b 28.4 ab 10.5 ab 52.5 a 38.3 a 18.5 a 14.1 a 29.2 ab 7.4 c2 61 ac 34 cd220 76 b 32.9 ab 12.0 ab 57.6 a 43.5 a 19.9 a 15.7 a 29.4 ab 8.3 bc 65 ac 42 bdmean 80 b 29.0 bb 10.7 bb 50.7 b 37.1 b 17.7 b 13.6 b 29.7 b 7.4 22 69 22 33 22

* * * ** * *** ** * *

Martinov M., Markovic D., Tesic M., Grozdanic N., 1996.Hemp harvesting mechanization. Agric. Engin., 2, 1-2,23-38.

Meijer W.J.M., Van Der Werf H.M.G., MathijssenE.W.J.M., Van Der Brink P.W.M., 1995. Constraints todry matter production in fiber hemp (Cannabis sativaL.). Eur. J. Agron., 4, 109-117.

Paskovic F., 1966. Predivo bilje, konoplja, lan i pamuk.Nakladni Zavod Znanje, Zagreb.

Regazzi G., 1955. La coltivazione della canapa nell’Italiasettentrionale, 1-42, 53-94. In: Canapicoltura moderna.Calderini, Bologna.

Starcevic LJ., 1996. Production technology of fiber hemp.Agric. Engin., 2, 1-2, 12-22.

Van Der Werf H.M.G., Van Geel W.C.A., 1994. Fibrehemp as a raw material for paper. Proefstation voor deAkkerbouwn en de Groenteteelt in de Vollegrond, no.,177, 1-68.

Van Der Werf H.M.G., Van Geel W.C.A., Van Gils, L.J.C.,Haverkort A.J., 1995a. Nitrogen fertilization and rowwidth affect self-thinning and productivity of fibre hemp(Cannabis sativa L.). Field Crops Res., 42, 27-37.

Van Der Werf H.M.G., Wijlhuizen M., De SchutterJ.A.A., 1995b. Plant density and self-thinning affect yieldand quality of fibre hemp (Cannabis sativa L.). FieldCrops Res., 40, 3, 153-164.

Van Gell W. C. A., Van Der Werf H.M.G., 1994. Nitro-gen fertilization of fibre hemp. Proefstation voor deAkkerbouw en de Groenteteelt in de Volleground, 73 A,123-128.

Venturi G., 1963. Ricerche per migliorare la fibra di unanuova varietà di canapa. Prog. Agr., XI, 12, 1-20.

Venturi G., 1967. Risultati di un sessennio di sperimen-tazione sulla canapa. Riv. Agron., 3, 137-150.

Venturi G., Amaducci M.T., 1996. Carateristiche bio-logiche di alcune cultivar di canapa. Sementi Elette,XLII, 6, 23-32.

Venturi G., Amaducci M.T., 1997. Effetti di dosi di azo-to e densità di semina su produzione e caratteristichetecnologiche di Cannabis sativa L. Riv. Agron., XXXI,3, 616-623.

Venturi G., Amaducci M.T., 1999. Colture da fibra, 33-55. Edagricole, Bologna.

Plant Population and N Fertilisation of Hemp 111

RISPOSTA DELLA CANAPA ALLA DENSITÀ DI INVESTIMENTO E ALLA CONCIMA-ZIONE AZOTATA

SCOPO. Nel passato la canapa (Cannabis sativa L.) è stata una coltura di notevole importanza industriale in nume-rose regioni europee ed anche a livello mondiale. In conseguenza della crisi agricola attuale ed alla crescente do-manda di materie prime di origine naturale, la canapa è oggetto di un rinnovato interesse. Nel contesto del pro-getto Europeo “HEMP”, sono stati realizzati esperimenti di campo per valutare l’effetto di tecniche agronomichesulla resa e sulla qualità della fibra di canapa.METODO. Gli effetti della densità d’investimento (30-250 piante per m2), e quelli della disponibilità di azoto all’im-pianto (100, 160 e 220 kg ha-1) sono stati studiati in canapa da fibra, nel triennio 1996-1998. I due fattori sono sta-ti valutati in situazioni diversificate per combinazioni fra genotipi, epoca di semina e di raccolta. Per i principali pa-rametri vengono sintetizzati i risultati di effetti significativi.RISULTATI. La densità d’investimento ottimale, per biomassa totale, peso fresco e secco di steli e foglie si è confer-mata attorno a 90-100 piante m-2 sia nell’insieme delle situazioni considerate sia in situazioni disaggregate per fat-tori della produzione. La relazione inversa tra peso medio di una pianta e numero di piante per m2 è stata di tipoesponenziale, e si è ripetuta in una gamma di situazioni variabili per fattori ambientali e di tecnica colturale. Ladensità d’investimento non ha mostrato interazioni con genotipo (diverso ciclo, tipi dioici e monoici), e concima-zione azotata; ha interagito con l’epoca di raccolta per l’aumento nel tempo degli effetti competitivi. Al cresceredella fittezza tende ad aumentare il rapporto superficie volume degli steli, superficie corticale per unità di superfi-cie coltivata (2 cm2 per ogni pianta aggiunta). L’apporto di azoto ha aumentato la mortalità delle piante e, in mag-gior misura, il loro peso medio; e quindi la produzione di steli. A partire da disponibilità iniziale di 100 kg ha-1, ognikg di azoto apportato (fino a 120 kg ha-1) ha incrementato di circa 20 kg la produzione di steli secchi.CONCLUSIONE. La densità di semina ha dimostrato uno scarso effetto sulla resa in biomassa mentre ha largamenteinfluenzato le caratteristiche biometriche della pianta. Essendo queste legate alla qualità della fibra si ritiene chela densità di semina sia da scegliere in funzione della destinazione finale della produzione.

Parole chiave: Cannabis sativa L., concimazione azotata, densità di investimento, resa in fusti.