Influence of nitrogen and phosphorus application on Opuntia engelmannii tissue N and P concentrations, biomass production and fruit yields

Journal of Arid Environments (1989) 16, 337-346

Influence of nitrogen and phosphorus application on Opuntia engelmannii tissue Nand P concentrations, biomass production and fruit yields Gideon N. Gathaara, * Peter Felkert & Margaret Landt Accepted 21 June 1988 Opuntia exhibits unique characteristics adapted for extreme drought tolerance and has found use as a human and livestock food since ancient times. Opuntia is lowin crude protein but high in carboydrates. Opuntiaengelmannii was planted in south Texas in 1984and fertilized with three levelsof nitrogen and phosphorus in factorial combination on a randorn,ized block design. The influence ofN and P on biomass productivity, fruiting, and crude protein was examined. A factorial design was used to test for interactions and possible future needs for factorial trials. The highest dry fruit production (734 kg ha -t y -1) was obtained from the o kg Nand 80 kg P ha -1 fertilizer application in 1986. Levels of Nand P soil applications were significantly correlated with fruiting (p < 0'01 and p < 0'00 1, respectively) in 1986 only. Fruit production was significantly related to cladode tissue Nand P concentrations using a non-linear model (p = 0'0015). Optimal N and P concentrations for fruiting appear to be 1'16% Nand 0'115% P. Total biomass productivity for 3-year growth was not correlated with Nand P soil applications (p > 0'05 and p > 0'05, respectively). The maximum total dry biomass obtained in 3 years was 25·7 Mglha corresponding to a mean annual growth of 8'6 Mg/ha. Canopy closure is far from complete and higher productivities would be expected with a greater stem area index. Introduction The genus Opuntia (prickly pear) holds considerable potential for providing fruit, vegetables and animal fodder on semi-arid lands (Russell & Felker, 1987a; SAG, 1976; Monjauze & Le Houerou, 1965). Opuntia exhibits Crassulacean Acid Metabolism (CAM) (Kluge & Ting, 1978) that provides this genus with drought tolerance and high water use efficiency. The plant is over 90% water (De Kock & Aucamp, 1970) as a result of high water storage capacity in its succulent modified stems (c1adodes). High water conservation in the pads of this genus and drought tolerance have significantly contributed towards the usefulness of Opuntia as an emergency livestock feed during times of extreme droughts (Griffiths, 1906; Shoop et al., 1977). In the early 1900s, an excellent series of articles described yields, chemical composition, and uses of various prickly pear cultivars (Griffiths, 1906; Griffiths & Hare, 1907; Griffiths, 1915). Contemporary research has examined use of prickly pear pulp in the manufacture of jam (Sawaya et al., 1983) and seasonal variations in nutritional quality as an animal feed (Retamal et al., 1987). Extension bulletins have described techniques and costs of prickly pear fruit production (SAG, 1976). Recent agronomic studies have • Ministry of Energy & Regional Development (Agroforestry), P.O. Box 30582, Nairobi, Kenya. t Center forSemi-Arid ForestResources, Caesar Kleberg Wildlife Research Institute,Texas A&I University, Kingsville, Texas 78363, U.S.A. t Department of Mathematics, Texas A&I University, Kingsville, Texas 78363, U.S.A. 0140-1963/89/030337+ 10 $03.0010

© 1989 Academic Press Limited

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examined the influence of weed control on growth (Felker & Russell, 1987), cold tolerance of useful cultivars (Russell & Felker, 1987b), and fertility influences on prickly pear biomass production (Nobel et al., 1987). Fertilizer application has improved Opuntia's biomass production. In Chile, SAG (1976) has recommended fertilizing individual Opuntias with 12 kg of goat manure to increase fruit productivity. Following this application, fruit yields of 1'3, 3'1, 6'3,15'6 and 7·5 t ha- I were achieved for 2,3,4-15,16-20 and 21-35 year- old plants respectively. This was with a 300-mm annual rainfall and two to four irrigations. Fruiting began during the second year of growth and climaxed at 20 years. Fertilizer trials in Tunisia found that manuring increased the growth over non-fertilized controls by 272%. In contrast an inorganic fertilizer application (20N kg, 20P kg and 20K kg ha- I) increased growth 246% over the controls (Monjauze & Le Houerou, 1965). These authors observed Opuntia to respond particularly well to phosphoric and nitrogenous fertilizer applications. Preplant applications of 20 to 30 t ha- I of manure, 20 kg Nand 100 kg P ha- I were recommended by Monjauze & Le Houerou (1965). These authors also recommended a 50 to 100 kg N ha -I v' and 50 kg P afPlication every 3 years after planting. Use of K was unnecessary. Yields of 60 t ha -I y- fresh weight were obtained from preplant fertilizer applications and yields of 200 t ha -I y-I fresh weight were obtained when fertilizer was applied before and after planting as specified above. In regions receiving over 400 mm annual rainfall, Monjauze & Le Houerou (1965) recommended a density of 4000 to 5000 plants ha-I with a spacing of 4 to 5 m between rows for non-mechanized farming systems. Where mechanization was to be used, they recommended a 6 to 7-m spacing. In regions with an annual rainfall of 150 to 400 mm, the plant density should be adjusted to 3000 plants ha -I with a 5 to 10 m spacing between rows. For industrial tuna farming, SAG (1976) recommended spacing Opuntia at 4 m x 4 m (625 plants ha-I) with four cladodes placed in each planting hole in a square pattern. To maintain high quality tunas, SAG (1976) recommended that young and old cladodes be thinned to leave four fruits and 8-10 fruits respectively. Opuntia is capable of achieving high biomass productivity. In an area with 290-mm rainfall, Acevedo et al., (1983) reported 13 t ha- I y-I dry matter for plants without fertilizer when three irrigations of 80 mm were provided. Xolocotzi (1970) reported 8 t ha -I y-I offresh fruit for plants that had been given a fertilizer application and grown at a density,of 2000 plants/ha. Since semi-arid soilsare low in nitrogen and fertility, a field trial was initiated to examine the influence ofN and P fertilizers on the biomass and fruiting productivity of Opuntia. An additional objective was to determine tissue concentrations of Nand P indicative of deficient and optimal concentrations for biomass and fruit productivity. It would have been desirable to have conducted these studies on a 'fruit cultivar' but sufficient planting stock was not available. Therefore the Texas native Opuntia engelmannii was used as a model system. In Texas the primary use ofO. engelmannii is as an emergency livestock feed during droughts after the spines are burned with propane torches. The young, tender regrowth of this species is cooked as a green vegetable with fish dishes during Lent. The native prickly pear fruit, although very abundant, are not sweet and are only consumed by livestock and wildlife. Materials and methods Medium aged cladodes (pads) of native Opuntia engelmannii were used as the planting stock. The soil was a Willacy fine sandy loam in the hyperthermic family of Udic Arguistolls. The top 15 em of soil on immediately adjacent plots had a total nitrogen content of 540 mg/kg, a sodium bicarbonate extractable phosphorus concentration of 1·3 mg/kg and a pH of 6·7. The soil was 75% sand, 17% silt and 10% clay (Wightman,

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manuscript in preparation). The soil was ploughed 50 em deep prior to planting and a preplant herbicide, oryzalin (Surflan), was incorporated into the soil 2 days before planting at 2·8 kg ha ~ '. The cladodes were planted on ridges 3 m apart, to facilitate drainage during heavy rains. This ridging has been subsequently shown to be undesirable because sporadic heavy rains erode the ridges, leaving exposed root systems that cannot adequately support the plants. The field was planted on 21 June 1984.

Experimental design A randomized complete block, factorial design with rates of 0,40, and 160 kg N ha -1 and 0, 20, and 80 kg P ha -1 was used. The nine fertilizer treatments were randomized in four blocks. Ammonium nitrate and triple super phosphate were banded into the ridges at a depth of 30 em. The cladodes of Opuntia engelmannii were planted 1 m within the rows and 3 m between the rows in all four blocks. Each replicate consisted of a five plant row-plot. The fruit and chemical determinations were taken on the inner three plants/row plot. The weeds were initially removed manually, but when this proved impracticable, a 2% solution of glyphosate (Roundup) was used. This caused no damage to mature cladodes but slightly damaged the immature cladodes. Unidentified scale insects damaged some Opuntias by burrowing 'tunnels' just below the cuticle layer. The symptoms appeared as a series of white lines on the green background. The systemic insecticide orthene (acephate) provided satisfactory control. The only freeze in the winter of 1986/1987 occurred very late in the year on 30 March 1987and destroyed the new regrowth and flowers on many kinds of plants. This late freeze may have influenced the 1987 fruit production.

Tissue nitrogen and phosphorus determinations A 3-cm diameter disk was taken from a fruited and from a non-fruited cladode from the three inner plants in each treatment. The total number of fruits was recorded from the inner three plants per plot for yield determinations. The number of fruits per cladode was recorded from the cladode from which the disks were taken for tissue Nand P determination. Fruits/cladode was then regressed against cladode % P and % N. To avoid the spines during sample collection, a 0'75 m-long stainless steel pipe of 3'2 em inner diameter was sharpened and driven through the pads held against a wooden frame essentially as described by Huffman & Iakoby, (1985). The cores were driven out with a wooden rod. The samples were collected in paper bags and immediately taken to an oven for drying at 50°C. The dried sample cores were ground in a stainless steel Wiley Mill with a 40 mesh screen.

Biomass determination Fruiting of all plants was recorded during the middle of the fruiting season in June of 1986 (2 years of age) and in May 1987. Three sets of 10 randomly selected ripe fruits were dried at 70°C in paper bags for moisture content determination. The average fruit fresh and dry weight were 33 g and 4 g respectively (dry weight = 12%). This is in good agreement with an average O. lindheimeri fruit weight of35 g (Griffiths & Hare, 1907)and a dry weight % of 12-3 (Monjauze & Le Houerou, 1965). All the plants were harvested in July 1987 to ground level. Thus the plants were just over 3 years old. Individual plants were placed on a 1·5 x 2·5 m frame with wire mesh interior that was suspended from a 91-kg spring scale from a forklift. The percent dry matter was determined from core samples of three replicates of four age classes in a companion study (Gregory, 1988). The mean ± S.E. for these 12 samples dried at 70°C was 10-35 ± 0-18.

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Nitrogen and phosphorus were measured colorimetrically after sulfuric acid and nitric acid digestions respectively, as previously described (Cline et al., 1986). Correlation between the fertilizer levels, fruit production and tissue concentrations of Nand P was determined utilizing an SAS procedure (SAS, 1985). Results In June 1985, at one year of age, the Opuntias had grown from a single cladode to over 1 m in height. The fruiting during the first season was negligible. The plants reached their first reproductive cycle in May and June of 1986 at 2 years of age. The peak fruiting season in 1987 was also in early June. However, the fruit size was smaller in 1987 than in 1986. Almost all the fruits arose from the areoles along the top edge of the pads but a small number in 1986 were observed to come out from the second set of areole rows from the edge of the pad. Thefruits in 1986were observed to start ripening towards the end of june with a deep red color. The Anova for fruit production as influenced by fertilizer application indicated highly significant treatment effects at 2 years of age but not at 3 years of age (Table 1). The effects of both Nand P were significant (p < 0'01 and p < 0'001, respectively) with effect of P being the most significant. The interaction of Nand P tended towards significance (p = 0'0691) in the second growing season. In 1986, the lowest fruit production was from the 160 kg N-20 P kg ha - I treatment. In 1986 the greatest fruit production of 734 kg dry matter ha -1 (Table 2) was obtained with no nitrogen and the greatest phosphorus. Surprisingly one year later this treatment had only 422 kg of dry fruitlha and was third in production after the 40 kg N-80 kg P treatment and the 0 kg N-D kg P treatment. A Tukey's HSD test indicated that in 1986, the 80 kg P ha -1 application gave significantly greater fruit yields than the 0 and 20 kg P ha -1 fertilizer applications. In contrast the Tukey's HSD test on N fertilizer levels indicated that the 40 and 160 N kg ha -1 fertilizer applications produced significantly less fruit than the 0 N kg ha -1 application. The nearly significant interaction observed in the Anova (Table 1) is consistent with a trend of decreasing fruit production with increasing tissue N content. Even though the Anova was not significant for the third season, in both years there was a trend .towards greater fruit production from the low N, high P fertilizer applications. However, the maximum production of 734 kg obtained in the 0 kg N-80 kg P treatment during the second year was not obtained by any treatment the third year. In the third growing season the mean fruit production of all treatments was 45% greater than in the second season. Many of the treatments with low production in 1986 greatly improved in 1987. Table 1. Anovafor Opuntiaengelmannii fruiting in number at 2 and 3 years of age as influenced by different levels of Nand P fertilizer applications to the plants at 1 m X 3 m spacing 2 years of age

3 years of age

Source

dJ.

Sum of squares

F

p

Sum of squares

F

p

Block

3 2 2 4

1196 6876 10374 4798

0'75 6·45 9·74 2-25

0·5260 0'0023 0·0001 0·0691

4794 120 678 1059

3'91 0·15 0'83 0·65

0'02 0·86 0'45 0·63

N P NxP

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Table 2. Fruit and biomass productivity of Opuntia engelmanni as influenced by nitrogen and phosphorus fertilizer application kg dryfruits ha- I * 1987

Total dI( weight t ha- after 3 years' growth

P Mean + SE

Mean + SE

Mean + SE

o 165 ± 20 258 ± 80 734 ± o 156 ± 20 81 ± 80 317 ±

448 ± 216 257 ± 86 422 ± 146 149 ± 62 336 ± 139 463 ± 268 291 ± 89 356 ± 106 397 ± 164

18'0 ± 2'4 23·0 ± 1'9 23'1 ± 2'9 19·9 ± 3'0 18·5 ± 1'4 21-6 ± 3'7 25'7 ± 2·8 18'7 ± 2'0 24'6 ± 2'6

346

21·4 ± 2·8

Fertilizer kg ha- 1 N

o o o

40 40 40 160 160 160

1986

85 124 257 94 14 42 o 131 ± 38 20 80 ± 32 80 215 ± 60

Mean

237

* The mean fruit fresh weight was 33'3 g and the fruit dry matter percentwas 12·0%. The dry weight was 10·35% oftotal above ground biomass fresh weight. Due to significant influence of phosphorus fertilizer application on yields of prickly pear fruit, the relationships betwen fruiting and cladode tissue concentrations ofN and P were examined. Spearman correlation coefficients indicated cladode tissue P to be significantly correlated with fruits per cladode (r = 0'379, P = 0'022) and mean fruits per treatment (r = 0'359,p = 0'031). Tissue N was negatively but non-significantly correlated (p > 0,20) with these two variables. The raw data for the values for tissue % N and tissue % P for both non-fruited cladodes and fruited cladodes (Table 3) were not normally distributed (p < 0'01 by the Wilks test), thus invalidating the use of a Student's r-test. Use of a non-parametric sign test found that the tissue % N in the fruited cladodes was significantly greater than the non-fruited cladodes (p < 0'02). Using this non-parametric test, the level of significance was even greater for the difference in tissue % P between the fruited and non-fruited cladodes (p < 0'0001). Linear and quadratic regressions were examined between the Nand P concentration of the plants and the fruit production per cladode. Backward elimination regressions were examined on data sets containing from 27 to 36 points after residual analyses (SAS, 1985) and apparent outliers were deleted. The quadratic term for the cladode phosphorus concentration was consistently the most significant term in these models. The cladode nitrogen concentration was often significant. The interaction term of cladode nitrogen versus cladode phosphorus varied in significance (p < 0'10) depending on which outliers were deleted. It is not surprising that deletion of outliers identified with the SAS procedure substantially influenced the model, since these are the points that most significantly influence the regression. A representative model for 32 data points is provided below in which both the interaction term and the phosphorus concentration were not significant (p> 0'24). Fruitlcladode = -34'3 + 56·2 (N) - 22·5 (N)2 + 480'4 (pi This equation had an R 2 of 0'417 (p = 0'0015). The (N), (Ni, and (p)2 were significant at

p

= 0'021,p = 0'017, andp

=

0·0003 respectively.

G. N. GATHAARA, P. FELKER & M. LAND

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Table 3. Fruiting and tissue % P and % N for cladodes of Opuntia engelmannii at different levels of fertilizer applications planted at l·Om x 3·Om

Fertilizer level ON ON ON ON ON ON ON ON ON ON ON ON 40N 40N 40N 40N 40N 40N 40N 40N 40N 40N 40N 40N 160N 160N 160N 160N 160N 160N 160N 160N 160N 160N 160N 160N

Block OP OP OP OP 20P* 20P* 20P 20P 80P 80P 80P 80P OP OP OP OP 20P 20P 20P 20P 80P 80P 80P 80P OP OP OP OP 20P* 20P 20P 20P 80P 80P 80P 80P

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

fruits Cladode % N per frtd cladode nonfr 1 3 0 1 4 1 4 5 6 8 3 7 2 0 1 1 4 1 4 1 6 6 8 8 2 2 2 2 3 1 3 4 2 3 1 4

0'868 1-416 1'410 0'833 0'905 1'178 0·873 0·982 0·758 1'293 0·949 10345 0'829 1·064 1'133 1'077 0·693 0·923 1·170 1'091 1·054 1·088 1'053 0'695 0·851 0'750 1'436 1·249 10302 0'778 1'054 1'163 1·037 0·997 1'312 1'673

1'197 1·163 0·884 1'277 0·159 1·626 1'162 1'212 1'425 1'099 1'006 1'161 1-097 1'145 1·115 1·186 1-028 1'192 1·141 1-616 HOI 1·164 1·179 1'160 1'286 1-334 1'409 1'147 2-837 1·284 1'192 1·295 1'199 1·276 1-657 1·549

Cladode % P nonfr

frtd

0'002 0'128 0'049 0'005 0·034 0'177 0'023 0'026 0·054 0'058 0·051 1'013 0'025 0·031 0·009 0'008 0'018 0'033 0·060 0'008 0·037 0·056 0'014 0'040 0·025 0'025 0·013 0'009 0·017 0'007 0'027 0·010 0'022 0·044 0'067 0·066

0'053 0·132 0'081 0·046 0·068 0'149 0'072 0'071 0·107 0·090 0·105 0'071 0'066 0·084 0'070 0'046 0'052 0·080 0·094 0·058 0·095 0·090 0'104 0'115 0·076 0·066 0·077 0'071 0'063 0'059 0'078 0'041 0'067 0'083 0'105 0'098

* indicates these values were determined to be outliers and were excluded from the data set for the regression equations. nonfr, nonfruited dadodes; frtd, fruited cladodes. This equation is presented graphically in Fig. lover the ranges of 0·884 to 1'657 % N and 0·041 to 0'115 % P. The estimated maximum value over the observed range of 32 points was seven fruits/cladode with corresponding tissue concentrations of 1'16% Nand 0'115% P. This response surface indicated that increasing P tissue concentrations nearly always increased fruit production. It does not appear that the plants in this study had reached their maximum fruit production with respect to phosphorus with the maximal concentra-

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7-64

1-72

r---"::::::"'lL 1.60

~----~~------,L::::::-=::~::::::::t 0-093

-1-24 0·120

0-067

Pad P (%)

Figure 1. Opuntia fruit production as function of cladode % Nand % P.

Table 4. ANOVA for Opuntia engelmannii totalaboveground biomass at 3 years of ageas

influenced by N and P fertilizer applications

Source

d.f.

Block N P NxP

3 2 2

4

Sum of squares

F

p

55 142 146 459

0·14 0·55 0'57 0·89

0·9329 0·5819 0'5737 0·4824

tion of 0·12% P. While fruit production increased with increasing tissue phosphorus concentration, high concentrations of nitrogen, i.e. 1·6% appeared to depress fruit production. The highest total dry biomass (25'7 t ha -I for 3 years' growth) was obtained from 160 kg N by 0 kg P ha -I soil fertilizer application whereas the lowest was from the 0 kg N by 0 kg P ha- I soil fertilizer application (18'0 t ha- I for 3 years' growth). These dry weight calculations are sensitive to determination of the % dry matter, since a 1% difference in dry biomass is equivalent to 2000 kg/ha dry weight. None of the soil fertilizer applications were significantly correlated with biomass productivity (Table 4) for the 3-year growth period. Discussion Maximum fruiting was observed at 1·16% Nand 0·115% P for an optimal N to P ratio of 10:1. In contrast, studies with legumes such as Prosopis have observed maximum growth at 3'0% Nand 0'16% P for an N to P ratio of 18:8. Most reports of Opwuia indicate the nitrogen content is in the range of I- 3% N to 1'9% N. This low N content could prevent

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Opuntia from being N limited, by its high efficiency in converting water to dry matter. It appears as if fruiting in Opuntia can easily be reduced by excessive N applications. Excessive N applications have been reported to reduce the harvest index in rice by stimulating excessive vegetative growth (Donald & Hamblin, 1976). These authors also cited field studies in which phosphorus and nitrogen factorial applications were made to sorghum in India. Here phosphorus increased the harvest index over the maximum obtainable with nitrogen applications. Several reports (SAG, 1976; Monjauze & Le Houerou, 1965)have suggested high rates of manure are necessary for stimulation of fruit production. The data here suggests that if the manure had a high NIP concentration, high application rates might increase vegetative growth at the expense of fruit production. Perhaps manure should be supplemented with mineral phosphorus fertilizers? The high phosphorus applications significantly stimulated fruit production in the second growing season but not during the third. Possibly the phosphorus may have been depleted, or rendered unavailable in these slightly alkaline soils, by the third growing season. This explanation is consistent with the observation that the fruit production in the high P-Iow N treatment (most fruit in the second season) declined in the third growing season while the production from all other treatments increased. The nitrogen and phosphorus concentrations measured on these plots one year earlier by Nobel et al. (1987)were significantly greater than the values we observed. For instance in the 0 kg Nlha-80 kg Plha these authors obtained Nand P tissue concentrations of 2'0% and O· 23% respectively whereas the mean values for these nutrients observed one year later in our study was 1·17% Nand 0·093% P. This lends credence to the hypothesis that lack of a treatment effect the third season after application, may be due to depletion of the fertilizers. The optimum values of 1'16% Nand 0'115% P for fruit production in Opuntia engZemannii may not produce optimal fruit production in economically important species such as Opuntia ficus-indica. Some literature values for the cladode Nand P contents for O. ficus-indica are considerably higher than the values we have observed. For example Nobel, (1983) reported concentrations of 2'45% Nand O'3% P for Opuntia ficus-indica being grown commercially for fruit production in California. However, tissue chlorenchyma concentrations for 5- and 12-yearold commercial Opuntia ficus-indica plantations in Chile (Nobel, 1983)were much lower, i.e. 1·53% Nand 0·12% Pfor 5-year-old plantations and l- 25% Nand 0·99% P for 12-year-old plantations. The tissue Nand P concentrations for the entire stem in the California plantations were only about 10% less than the chlorenchyma values. Gregory (1988) has observed large differences in the mean % Nand % P of eight commercially important Opunti fodder and vegetable cultivars in south Texas field trials. Thus it may be necessary to develop tissue % Nand % P diagnostics for each of the major Opuntia clones. Greenhouse sand culture experiments using a one quarter strength Hoaglands solution produced much higher concentrations of tissue % P, i.e, about 0'3% (Berry & Nobel, 1985). Retamal et al. (1987) examined the seasonal variation in chemical composition of commercial Opuntiaficus-indica plantations in Spain. Depending on the season of the year, these workers found that the young cladodes without fruits varied from 1'7% N to 2'4% and that the old cladodes without fruit varied from 0'7% to 1'8% N. The young cladodes with fruit were 1'2% N in July, and the mature cladodes with fruit were 1'1% in March, 1·3% in September and 1'6% N in October. These latter nitrogen concentrations are similar to the values we observed for optimum fruit production. Earlier work by Nobel et al. (1987) demonstrated a significant increase in biomass in response to the fertilizer treatments. The final harvest data reported here did not observe such a treatment effect. Perhaps the fertilizer stimulated total biomass production at an early age, similar to its effect on fruit, but that this effect was diluted by the time the plants became older.

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The mean dry matter production at the end of 3 years growth was 21'4 Mg/ha corresponding to a mean annual growth of 7'1 Mg/ha. The current annual growth of perennials, such as trees, generally increases until canopy closure occurs. At the time of harvest the Opuntia's were overlapping in the rows (1 m in-row spacing) but the canopies were at least 2 m from touching between the rows. Thus we would expect higher biomass estimates after canopy closure was complete. Using regression equations on the new cladode regrowth, and assuming the Opunua's fully occupied the site with an optimal stem area index of 4 to 5, Nobel et al. (1987) estimated the yields at the end of the first growing season to be 15 Mg/ha. However, the actual dry weight measured by Nobel et al. (1987) at the existing plant density was 4970 Mg/ha in the high N-high P treatment. If this is subtracted from the total dry weight at the end of three seasons' growth (25'7 Mg/ha), the mean annual growth for the second and third season would be 10'4 Mg/ha. A productivity lower than 10'4 Mg/ha in the second season and greater than 10·4 Mg/ha in the third season would be expected due to greater stem area. As pointed out by Nobel et al. (1987) these productivities compare favorably to many current agronomic crops. As the planting stock for this experiment was random selection of native germplasm, considerably greater productivity probably could have been obtained with well adapted high biomass producing Opuntia ficus-indica clones. Our maximum fruit yields at 734 kg ha- I y-I dry weight (equivalent to 5 t ha- I fresh weight at 87'94% moisture content) compare favorably to those reported by Xolocotzi (1970) and Acevedo et al. (1983) at 8 t ha -I y - fresh weight and 3 t ha -I y -I dry weight, respectively. The yields obtained here are almost four times greater than for equivalent aged (2-year-old) plants fertilized with guano in Chile (SAG, 1976).

Conclusions Total dry biomass productivity of Opuntia engelmannii was not significantly influenced by fertilizer applications after three growing seasons. Fruit yields were significantly influenced by fertilizer applications and the greatest fruit yields were obtained with a combination of 0 kg Nand 80 kg P ha -I. The nearly significant interaction in fruit yields as a function of Nand P applications suggests that high N applications may be detrimental to fruit production. The fruit yields were responsive to Nand P fertilizer in the second, but not the third season. The drop in fruit yields by the highest yielding treatment in the third season combined with overall improvement in yields, suggests that soil phosphorus became unavailable by the third season. Tissue P and N concentrations may serve as a useful guide in predicting when fertilizer applications are needed for fruit production. We thank the U.S. National Science Foundation grant PCM 8315760 for initial support for this research. G.N.G. thanks the Governments of the Netherlands, Kenya and the U.S. Agency for International Development for financial support. Statistical consultation with R.L. Bingham is gratefully acknowledged. References Acevedo, E., Badila, I. & Nobel, P. S. (1983). Water relations, diurnal acidity changes, and productivity of a cultivated cactus, Opuntia ficus-indica. Plant Physiology, 72: 775-780. Berry,W. L. & Nobel,P. S. (1985). Influence of soiland mineral stresses on cacti.Journal of Plant Nutrition, 8: 679-697.

Cline, G., Dwight,R. & Felker,P. (1986). Micronutrient, phosphorus and pH influences on growth and leaf tissue nutrient levels of Prosopis alba and Prosopis glandulosa. Forest Ecology and Management, 16: 81-93.

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