MISCELLANEOUS PUBLICATIONS 1251, U.S. DEPT. OF AGRICULTURE 



The quality or spectral composition of light 

 affects the growth and development of plants (-4). 

 An improper balance of light quality can result in 

 abnormal growth. 



Different plant species vary in their photoperiod 

 requirements for best growth. For example, barley 

 requires a photoperiod greater than 12 hours for 

 good flowering, whereas soybeans can mature 

 under a 12-hour day. The photoperiod can be 

 regulated to hasten or delay flowering (|). In the 

 growth chambers the plants can be given what- 

 ever photoperiod is required for optimum plant 

 growth. Some suggested photoperiods are given in 

 table 2. 



In the greenhouse many different species of 

 plants are grown. Some of these may require long 

 days for flowering and others short days. It is 

 impossible to provide the optimum light require- 

 ments for each species, but many plants can be 

 grown satisfactorily. During the winter a 12-hour 

 photoperiod is generally used as a compromise day 

 length under which reasonable vegetative growth 

 can be maintained. The short natural photoperiod 

 in the winter is supplemented with fluorescent 

 light. 



Temperature 



During the summer it is often difficult to main- 

 tain cool enough temperatures in the greenhouses 

 for certain species of plants. For example, head 

 lettuce and peas will often grow poorly in mid- 

 summer. It is not recommended to grow these 

 species until conditions are more favorable. 



Relative Humidity 



When moisture supply to the roots is adequate, 

 the degree of water vapor saturation in the air 

 exerts a major effect on the rate of transpiration of 

 many plants. An average of 50-percent relative 

 humidity is satisfactory for many species. 



A relative humidity of 100 percent is often 

 needed to germinate very small seeds such as those 

 of tobacco or bluegrass or to root sugarcane stem 

 sections. The required humidity can be obtained 

 by covering the plant containers with plastic bags. 

 If mildew infection is a problem under high 

 humidity, a light dusting with sulfur may be 

 sufficient to control it. 



Growth Media 



At the Fargo laboratory the plants are not 

 grown in soil but in either vermiculite or deionized 

 water. Vermiculite of a medium fine texture, some- 

 times called horticultural grade, is readily obtained 

 from local dealers. Local tapwater may be too 

 saline or alkaline for use in nutrient culture. It is 

 usually deionized by passage through a commercial 

 mixed-bed deionizer. The resins remove cations 

 and anions such as potassium and chloride ions 

 from the tapwater. 



Nutrition 



Environmental conditions affect the amount and 

 rate of nutrient uptake by individual plants. If 

 many different kinds of plants are to be grown, 

 considerable variation in nutrient requirements 

 can be expected. By careful observation, early 

 deficiency or toxicity symptoms can be diagnosed 

 and readily cured (2, 13). If fairly exact nutrient 

 requirements are not known for a given species, it 

 is best to use a standard solution, such as that 

 given on pages 13 and 14, taking care not to over- 

 fertilize, for it is easier to correct a nutrient defi- 

 ciency than a toxicity. Many plants seem to 

 tolerate wide variations in nutrient supply. It is 

 naturally desirable to supply nutrients at the 

 optimum level. 



Nutritional guidelines for several species are 

 listed in table 3. These have proved very satis- 

 factory at the Fargo laboratory. One should not 

 expect that these methods are optimum for other 

 growth conditions, and some alterations may be re- 

 quired. In this respect nothing replaces common 

 sense, careful observation, and a little "tender, 

 loving care." 



In the Fargo laboratory we use a modification 

 of one basic nutrient solution (9), in which a 

 chelated form of iron is substituted for the iron 

 salt originally reported. This is satisfactory for 

 most of the plants. The full-strength solution 

 is made up and the concentration varied simply 

 by diluting to three-fourths, one-half, or one- 

 third strength with deionized water. Tapwater 

 may be substituted for deionized water in some 

 localities. We find that generally the full-strength 

 solution is too concentrated for many species, 

 particularly in early stages of growth. 



