This is the website of Luigi's Yard Services, the landscape component of Manrique International Agrotech (MIAT). MIAT is an international company specialized in tropical agriculture. MIAT publishes books on potato, cassava, sweet potato, taro, and other tropical crops.

Vegetables

Lettuce
Lettuce (Lactuca sativa L., family Compositae), is a crop native to Europe and Asia. Modern lettuce cultivars are generally divided in four types: (i) cabbage or head lettuce, which is further divided into crispheads (iceberg) and butterheads, (ii) cos or Romanian, (iii) "leaf" or curled, and (iv) fleshy stem type.
After tomato and cucumber, lettuce is by far the most important vegetable grown in the United States. Lettuce is a cool season crop normally grown in early spring or late summer. Commercial production is carried out during winter in California, Arizona, and other southern states. In the northern states, winter production is feasible in greenhouse systems.
Temperature affects plant growth and yield. The threshold night temperature for plant growth is about 4.5 oC providing that day temperatures are at least 10 oC higher. Optimum growth is attained at night and day temperatures of about 12 and 24 oC, respectively. Mean daily temperatures higher than 21 oC promote seed stalk elongation, puffy heads, bitterness, and an increasing tendency toward internal disorders. Temperatures below 13 oC, in turn, sharply reduce plant growth and N uptake.
Lettuce is somewhat sensitive to excess light (i.e., the crop gets saturated at 11 MJ/m2/d and levels of 19 MJ/m2/d or higher actually inhibit growth. If grown in high light regimes, the crop requires shading for optimum growth and yield.
Lettuce strongly responds to CO2 enrichment particularly at low light. At low light there is over 50% yield increase between enriched (1,000 mg CO2/kg) and non-enriched (400 mg CO2/kg).
For lettuce, optimum water supply is critical during vegetative and reproductive growth. Water stress, caused by high water demand and high air and soil temperatures, is the main cause for poor growth in the tropics. If exposed to water stress, the crop exhibits slow growth and tip-burn.
For lettuce the development of vigorous seedlings is a prerequisite for successful transplanting and subsequent growth. Yield at harvest normally depends upon the nutrient regime during the seedling period. A nutrient application program for optimum seedling production in the tropics consists of a daily foliar application of 200 to 600 mg/kg of 13N 11P 21K combined with pre-plant applications of up to 8 g of 8N 14P 7K per liter of growing media.
Temperature strongly affects plant growth and nutrient uptake. No differences in yield are reported among N sources but air temperatures below 13 oC sharply reduce plant growth and N uptake.
Soil acidity strongly affects growth and N uptake particularly if lettuce is grown in high rainfall areas. Overall, a pH ranging from 6.1 to 6.6 appears to be optimal for plant growth.
Lettuce meets its P needs from readily available soil P forms. In fact, about 0.3 mg/kg of P in the soil solution is required to produce 95% of maximum yields.
Tomato
The tomato is perhaps the most widely grown vegetable in the United States. The commercial tomato (Lycopersicon esculentum Miller) belongs to the genera Licopersicon (family Solanaceae). The species of the genera Licopersicon originated in the narrow coastal area of South America extending from Ecuador to Chile. There are two main types of tomato: (i) determinate or "bush" tomato, and (ii) indeterminate or "vine" tomato. Determinate cultivars are used mainly for processed food while indeterminate cultivars are used in greenhouse systems.
Minimum temperatures for seed germination range from 8 to 10 oC. The crop generally attains high yield at day and night temperatures of 18 and 14 oC, respectively but it can grow at suboptimal temperatures, (i.e., alternate night temperatures of 18 and 12 oC). High light is also a prerequisite for the tomato; at low light the crop grows vegetatively, does not set fruit, and requires supplemental lighting to yield. Supplemental light with an intensity of 100 umol/m2/s provided over 18 h days increases fresh tomato yield by 1.8 fold. The effect of supplemental lighting is greater if applied at key growth stages. Applying light (90 umol/m2/s for 18 h) from initial anthesis to early fruit set increases fruit number but the greatest increase in fruit weight occurs when the light was applied from initial fruit set to the mature green stage.
Carbon dioxide has been used on tomato since the early 1960s. Currently, CO2 enrichment is routinely used on a commercial basis. The customary method of enrichment is to inject CO2 at a constant rate of 56 kg/ha per h. Maximum benefits from the CO2 enrichment are attained at day temperatures of 20 to 25 oC.
The tomato is particularly sensitive to water stress during flower formation and fruit enlargement. A study applying water to meet 100, 75, 50, and 25% of the crop water demand show that the average number of flowers per truss decrease with decreasing water application. Severe stress, resulting from applying water to meet 25% of the water demand, reduces fruit set by 65%.
Optimum N supply is critical for the tomato but excess N may lead to excessive vegetative growth and delayed maturity. Plant tissue analysis for NO3 N and total N are customarily used to determine excess or deficiency. Usually the NO3 N content in petioles is more indicative of the N status than total N content. One study reports a critical NO3 N level of 500 mg/kg for the second leaf from the leaf tip, while another study reports a critical level of 4,000 mg/kg for the 5th leaf and severe deficiency at 1,000 mg/kg. In terms of soil N, a soil NO3-N up to 48mg/kg was found following a N application of 90 kg/ha which resulted in petiole concentration of 14,500 mg NO3 N/kg.
The tomato requires a high P level (i.e., a level of 0.2 mg/kg is often associated with 95% of maximum yields) in the soil solution to produce maximum yields. Yields are depressed when the P level in the soil solution is greater than 0.4 mg/kg.
Cucumber
The cucumber normally grown in greenhouses is the Cucumis sativus L. (family Cucurbitaceae). Its cultivation goes back to the onset of the Christian era as it was found growing in North Africa, Italy, Greece, and other places. The crop was introduced to England in the early 1300s but was not cultivated until 1550.
The cucumber is a coarse, prostrate, annual vine plant with stiff hairs or spines on leaves and stems. Un-branched lateral tendrils develop at the leaf axil, and vine begins after two or three leaves are formed. Branching also begins at this time. As soon as branching begins, flower clusters appear at the leaf axils. Cucumber cultivars are generally classified into four main groups: (i) field cucumbers with prominent black or white spines, (ii) greenhouse or forcing cultivars, also referred as English cucumbers, (iii) Sikkim cultivars with reddish orange fruits, and (iv) small- fruited cultivars often used for pickling. The cucumber normally requires high temperatures for optimum plant growth, i.e., temperatures from 11 to 15 oC for germination and night and day temperatures of 19 and 21 oC from planting to the first two months. Afterwards, day and night temperatures of 19 and 17 oC are usually required.
Temperature affects not only plant growth but flower formation and abscission as well. Although flower abscission takes place at all temperatures, flower numbers are high at low than at high temperatures primarily because competition between buds increases with increasing temperature.
Constant water supply is essential for optimum growth particularly during flowering and fruiting. Water stress has shown to reduce vine elongation and node number of fruiting plants after one week of water stress and completely inhibit growth after 2 weeks. Under water stress, fruit growth rate is severely reduced with large fruits being less affected than small ones.
Under field conditions, a water application of 380 mm or more per season is customarily used for maximum production. Other recommendations include water applications of 25 to 38 mm per week, depending upon soil type, or when the available water content is depleted between 48 to 64% in the upper 90 cm of the soil profile.
Specific fertilizer recommendations for the cucumber are not normally available. Instead general recommendations are used for a wide variety of crops including the cucumber. As a general rule, pre-plant N application from 34 to 134 kg/ha with two additional side-dress applications of 28 to 45 kg N/ha are recommended.
The cucumber is generally sensitive to N deficiency and grows slowly and produces fewer and smaller fruits in low N regimes. The crop rapidly responds to N application but the response is not well defined, i.e., in one study a positive yield response is reported to N application up to 90 kg/ha while in another study pre-plant rates of 67 to 134 kg N/ha result in greater yields than rates of 201 or 268 kg N/ha. A study applying lime (0 to 4.5 Mg/ha), K (0, 32, 64, and 96 kg/ha), N (56, 112, and 168 kg/ha), and Mg (0, 32, and 64 kg/ha) show an increase in yield when liming increased the soil pH from 4.6 to 5.0, a reduction in yield at N rates greater than 56 kg/ha, and no yield response when K and Mg were applied.
Nitrate concentration in plant tissues is a good indicator of the N requirements of cucumbers. A critical NO3 N concentration in petioles of about 2,000 mg/kg is often reported. In another study NO3-N concentrations in petioles smaller than 0.8% or greater than 1.5% at harvest are associated with reduced yields.
Spinach
Spinach (Spinacia oleracea L., family Chenopodiaceae), is native to Iran and was cultivated by the Persians 2,000 years ago. The crop was introduced to Europe from Southwest Asia around the 1400s and later reached North America in the early 1800s. The modern spinach is a leafy vegetable, which in its vegetative stage is characterized by a compact rosette of ovate to triangular leaves that may be crinkled or smooth.
Spinach requires low temperatures for germination with some studies reporting high germination at temperatures from 0.6 to 3 oC and low germination at temperatures above 12 oC. Overall, spinach is a cool season short day crop generally grown in the fall, winter, and early spring under often freezing temperatures and low light levels.
In greenhouse systems, peat moss and perlite are common components of growing media for spinach. One problem of these components is their relatively high content of soluble F which is reported to be toxic to plants. Foliar symptoms of F toxicity are often overlooked partly because they resemble symptoms associated with other soil stresses, i.e., salinity, water stress, nutrient deficiency, and so on. If F toxicity is detected, liming and to a lesser degree charcoal application, are effective ways to ameliorate the adverse effects of F.
Chinese Cabbage
Chinese cabbage (Brassica pekinensis, family Cruciferae), probably originated in Asia where it was cultivated for centuries before it was introduced into the United States in the late nineteenth century. There are two types of Chinese cabbage cultivated in the United States: (i) the heading type or pet sai, with cylindrical to round heads and light green leaves, and (ii) the non-heading type or pak choy, which has shiny dark green leaves with petioles.
Chinese cabbage is the most popular oriental vegetable grown in the United States. Chinese cabbage is a cool season crop requiring temperatures of 15 to 16 oC for head formation and good quality production. Premature bolting (a rapid development of the flower stalk at low temperatures) is a major problem during spring production in northern climates. Bolting is induced by low temperatures, i.e., as low as 2 oC.
Research on bolting indicates an interactive effect of high temperatures and short days on bolting during initial growth. Temperatures above 18 oC during the seedling period appear to effectively delay bolting. Below 20 oC, short days delay bolting compared with long days. Apparently it seems important to grow Chinese cabbage either at high temperatures or short days from emergence until a sufficient number of leaves for head formation are formed.
In greenhouse systems, pot size, transplant age, and plant spacing appear to influence plant growth and yield of Chinese cabbage. In Hawaii, 76 to 79% greater head weights are obtained at plant spacing of 43 cm than at 29 cm. Transplant age appear not to affect either maturity or yield while plants grown in 7.5 cm diameter by 6.4 cm height pots apparently mature 7 days earlier, produce 10% heavier heads, and yield 21 to 31% more than those grown in 2.5 cm by 6.4 cm pots.
Because of the high likelihood of premature bolting in spring, Chinese cabbage production is mainly limited to summer and early fall when the crop is exposed to high temperatures which often promote infestation of soil borne diseases such as bacterial soft rot (Erwinia carotovora). Several agronomic practices such as raising beds and plant densities are used to ameliorate the effects of bacterial soft rot. Rising beds in some instances are beneficial in reducing disease incidence and progression but no distinctive effects of plant density are observed except that plants grown at 30 cm within row spacing produce fewer marketable heads that plants at 46, 61, or 76 cm.
Chinese and head cabbage (Brassica olearacea L.) appear to grow well in soils with low P concentration in the soil solution. One study reports Chinese and head cabbage attaining 75% of maximum yields at low P levels (0.05 and 0.012 mg P/kg, respectively). This indicative of the intrinsic ability of these crops to grow and produce yields at suboptimal P. Ninety five percent of maximum yields are reported at P levels of 0.2 and 0.04 mg/kg, respectively.
Bell Peppers
Bell peppers (Capsicum annuum L.) originated in Central and South America. Numerous species were used by the natives for centuries before Columbus discovered America. Afterwards the popularity of peppers spread quickly throughout Europe and Asia.
Several cultivars are grown in the United States but the sweet bell pepper is the most popular. Although bell peppers are perennials, they grow as annuals in temperate regions primarily because of their sensitivity to low temperatures. Pepper requires higher temperatures than tomato. Fruit set does not occur below 16 oC or above 32 oC and maximum fruit set takes place between 16 to 21 oC.
Bud and flower abscission is a major problem during summer. The main factor causing flower abscission is high temperatures. Shading is used to reduce air temperature and to protect plants from excessive light. For example, shading to decrease light by 12 to 26% apparently produces the highest yield of high quality fruits. Sun scald damage in the fruits is also reduced from 36% in full sunlight to about 3% under 26 and 47% shading but 47% shading causes a significant reduction in fruit set.
Low root zone temperatures at high latitudes strongly affect growth and yield of bell peppers. Soil warming appears to be a suitable alternative to offset the detrimental effects of low night temperatures. A study of the effects of five root zone temperatures (12 , 18 , 24 , 30 , and 36 oC) on ten week old plants grown for 8 weeks show maximum shoot dry weight and leaf area at 24 oC and 30 oC. Fruit weight was the highest but earliness was delayed at 30 oC.
Optimum N supply is essential for peppers, i.e., high yields of chili peppers are reported at N rates of 100 to 150 kg ha 1. Nitrogen use efficiency is particularly low for bell peppers partly because the crop grows slowly during the first 38% of the growing season and absorbs only 7% of the total N. During this slow growth period the applied N is subjected to leaching losses. Thus proper fertilizer management (i.e., split application, placement, and slow release N sources) is critical to minimize losses.
Nutrient concentrations in plant tissues generally correlate well with pepper yields. The optimum N concentration for 42 day transplants is reported to be 3.7% while the optimum N concentration in mature vegetative tissues is reported to be 1.6%. Optimum pepper leaf P concentrations are reported to range from 0.3 to 0.4% with evidences showing increases in leaf P concentrations from 0.2 to 0.5% with increasing P fertilization. Other studies report plant removals at 98 days after transplanting of 118 kg/ha of N which correspond to a N concentration of 2.5%.
References
Manrique, L.A. 1993. Greenhouse crops: A review. J. Plant Nutrition 16:2411-2477.
Manrique, L.A. 1994. Technology for greenhouse systems. Manrique International Agrotech, Honolulu, HI. 263p.

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