10 BULLETIN 71, HAWAII EXPERIMENT STATION 



to prevent surface washing. Cover crops that are planted dur- 

 ing the dry season, unless irrigation is practiced, will hardly 

 reach usable size before the heavy rains begin. 



The crop should be started in the early fall so that the 

 ground will be well covered by the first of December. There 

 should be sufficient moisture for both the cover crop and the 

 orchard trees, and the cover crop may be plowed under as soon 

 as its herbage is ample. The entire plant should be plowed 

 under before the seeds mature sufficiently to germinate. A cover 

 crop should be turned under from 2 to 6 weeks before another 

 crop is planted. Piper and Pieters (20, p. 21) found that "green 

 manures do not increase soil acidity, or, if so, that such a con- 

 dition is transitory." Deep-rooted cover crops should be planted 

 on hard subsoils that form a plow-sole. The cheese weed (Malva 

 rotundifolia; M. parviflora) usually yields an abundance of herb- 

 age, and the large, penetrating roots, when severed by the plow, 

 decay and leave deep drainage canals. Among the green-manure 

 crops used as cover crops at the station are mungo beans (Phas- 

 ealus mungo), sunn hemp (Crotalaria juncea), cowpeas (Vigna 

 catjang), and velvet beans (Stizolobium aterrimum; S. deering- 

 ianum) . 



The mungo bean attained a desirable size in 3!/2 months and 

 apparently made a fair green manure. The small seeds were 

 planted with the drill in rows 2 feet apart at the rate of 20 

 pounds per acre. 



Sunn hemp and several other Crotalarias commonly grown 

 in Hawaii as green manures stand among the highest in the pro- 

 duction of nitrogen, as is shown by Johnson, Thompson, and 

 Sahr (19, p. 5, 8) . The seeds were planted with the drill in rows 

 3 feet apart at the rate of 15 to 20 pounds per acre. 



The cowpea produced ample herbage in less than 5 months. 

 For green manure the seeds were planted 2 to the hill, forming 

 rows 2 feet apart each way. This arrangement requires about 

 8 pounds of seed per acre. When drilled in rows 2 feet apart, 

 10 to 12 pounds of cowpea seed are required per acre. Hume 

 (18, p. 290) states that cowpeas cannot be recommended for all 

 soils in Florida because of the presence of "a nematode worm." 

 He also states that so far as known this nematode does not 

 attack orange roots. 



For several years the Black Mauritius variety of velvet bean 

 has been used for green manure in the station citrus orchard. 

 This variety makes strong vine growth. The seed may be drilled 

 in rows 3 or 4 feet apart, or it may be planted at the rate of 

 3 or 4 to each hill. 



Fertilizing 



The addition of a complete fertilizer to the soil in which citrus 

 is growing has been proved beneficial at the station. The nec- 

 essary elements of plant food are nitrogen, phosphorus, potash, 

 and calcium. Standard complete commercial fertilizers for citrus 

 trees may be purchased on the market and contain about 5 per- 

 cent of nitrogen, 6 to 8 percent of phosphorus, and 2 to 3 percent 

 potash. There is some calcium in the compounds carrying the 



