OrganicFarmer_AugSep2018_e
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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