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Continued from Page 20<br />

control.<br />

Sources of Nitrogen<br />

For example, in the vegetable-strawberry<br />

rotation, common in the Watsonville-Salinas<br />

area, the crop residue left<br />

in the field by lettuce, broccoli or cover<br />

crop preceding strawberry can provide<br />

substantial quantities of nitrogen (ranging<br />

from 100 to 200 lbs of N per acre).<br />

Strawberry nitrogen uptake is modest at<br />

Organic fertilizer being flushed at<br />

the tail end of the drip tapes.<br />

the beginning of the season (Bottoms,<br />

2013), and is not synchronized with the<br />

release of nitrogen (N) by crop residue.<br />

Crop residue with N content of >2.5<br />

percent mineralizes about half of the N<br />

in the first four to six weeks and then<br />

releases the remainder of the N in a<br />

slow and steady fashion, similar to organic<br />

matter (Hartz, in press). The rate<br />

of release depends on soil temperature,<br />

moisture and aeration (Muramoto et<br />

al., 2012). Nitrate mineralized from the<br />

previous crop is susceptible to leaching,<br />

by excessive irrigation and rain. Under<br />

these conditions, controlling this<br />

phenomenon poses a serious challenge<br />

to growers since residual nitrogen<br />

becomes available in the period around<br />

planting, when plants are small and<br />

crop uptake is slow. Another source of<br />

nitrogen for the crop is nitrate in the<br />

irrigation water. This source of nitrogen<br />

has been shown to be equivalent<br />

to fertilizer as a source of nitrogen to<br />

supply crop needs (Cahn et al. 2016). It<br />

is not uncommon for agricultural wells<br />

to have high levels of nitrate. More than<br />

20 inches of water are applied seasonally<br />

to a strawberry crop so even when<br />

the well water nitrate concentration is<br />

relatively low, irrigation can provide a<br />

substantial source of nitrogen in organic<br />

production. To understand how much<br />

N is being provided by the irrigation<br />

water, multiply ppm (parts per million)<br />

nitrate-N by 0.23 to calculate pounds of<br />

N per acre inch of water. For example,<br />

with a well NO3-N concentration of 10<br />

ppm, when applying 20 inches of irrigation<br />

water, 46 pounds of nitrogen would<br />

be applied to the crop. Water with higher<br />

concentrations of nitrate can supply a<br />

significant proportion of the N needs of<br />

the crop.<br />

Challenges of Organic Nitrogen<br />

Fertilizers<br />

Another complication of nitrogen<br />

management in organic systems,<br />

compared to conventional, is that the<br />

fertilizers available in organic production<br />

are somewhat limited in number<br />

and their concentration of nitrogen is<br />

relatively low (for example, a very popular<br />

fish emulsion liquid fertilizer used by<br />

organic strawberry growers contains 2.5<br />

percent N). The nitrogen fertilizers used<br />

in strawberry production can be divided<br />

into two broad categories: pre-plant<br />

fertilizers, which are applied to the soil<br />

in the fall before listing the beds, and<br />

liquid fertilizers that are injected in the<br />

drip line during production season. Preplant<br />

fertilizers such as feather or blood<br />

meal contain up to 13 percent N. Liquid<br />

fertilizers (mostly made from fish or<br />

grain) are commonly injected in the<br />

irrigation system and contain 4 percent<br />

N maximum. They are rapidly available<br />

(Hartz et al., 2010) but are prone<br />

to causing plugging of drip systems<br />

because they are not fully soluble. Compost<br />

is applied to build up levels of soil<br />

organic matter. Depending on the type<br />

of compost, it generally provides a longterm<br />

source of N. Most composts have<br />

lower concentration of N (2 percent on<br />

average) and are applied pre-plant in organic<br />

strawberry production at a rate up<br />

Figure 1. Nitrogen balance of the organic ranches in the study.<br />

20<br />

Organic Famer August/September 2018

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