Evaluation of Crop Rotation for High Value Cool Season Horticultural Crop Production in Organic and Sustainable Systems

Initial soil tests indicated that the ph was near neutral, soil test P levels were high and soil test K levels were low.  Consequently we used a Nature Safe 10-2-8 for the primary fertility source to avoid over applying P. Potassium levels were supplemented with KSO₄ as needed.  A nitrogen credit was given for cover crops based on values in the literature (Table 2).

There were no significant changes in the overall characteristics of soil in the study due to the different rotations. Soil pH ranged from 6.7-6.8 over the study. There were no differences in soil test phosphorus, which ranged from 138-181 mg/kg and remained in the very high range (>75 mg/kg) throughout the study.. Phosphorus was added at a rate of 10-35 lbs P₂O₅/ac each year.

There were no differences in potassium levels between the different rotations. Overal K levels decreased during the study from a medium level to a low level (<120mg/kg). Potassium was over applied on the strawberries, broccoli, lettuce, while under applied on the potatoes based on soil test recommendations(Table 2).

In both rotations A and B, there was a net loss of potentially mineralizable nitrogen of about 7-8 mg/kg (Figure 1). There was little change in total soil carbon over the three year period and no difference between rotations (Figure 2). This is not surprising because total C in the soil is known to change very slowly in response to management. There was an increase of 100-120 mg/kg in active carbon over the duration of the study; however, there was no difference between the two rotations.(Figure 3).  Active C is known to respond quickly to management changes.

See attachment below:  Figure 1. Average changes in mineralizable nitrogen over three growing seasons in two cool-season vegetable rotations at Watkinsville, Georgia. The error bars represent one standard deviation.

See attachment below:  Figure 2. Average changes in total carbon over three growing seasons within two cool-season vegetable rotations at Watkinsville, Georgia. The error bars represent one standard deviation.

See attachment below: Figure 3. Average changes in active soil carbon over three growing seasons between two cool-season vegetable rotations at Watkinsville, Georgia.  The error bars represent one standard deviation.

Generally there was low disease pressure in the study across all crops grown. There was a minor infection of Botrytis on onion foliage that did not affect yield. There are several species of Botrytis with B. squamosa causing Botrytis leaf blight in onions. Late in the season, approximately one month previous to harvest, purple blotch (Alternaria porri) occurred on the onions in both 2012 and 2013. This pathogen occurs during the warm wet weather of late spring. No significant differences were seen between rotations (figure 4).

In 2012, lettuce drop caused by Sclerotinia sclerotiorum resulted in a disease incidence of 5 to 7% (Table 3). This disease has a wide host range and form sclerotia in the soil that can persist for relatively long periods of time. In 2012, the mild wet spring weather favored development of the disease on lettuce. Crop rotation to non-hosts and maintaining soil moisture during periods of high soil temperature can reduce the sclerotia found in the soil thus reducing the potential for future infections. This pathogen was also found on the bean crop later in the 2012 season although losses were not significant.

In 2013, but not in the other two years, beans were found to be infected with southern blight (Sclerotium rolfsii). In 2013 there was abundant rainfall during the summer months, making for ideal disease conditions. This can be a difficult pathogen to control because of long lived sclerotia in the soil and the wide host range. Crop rotation to non-preferred hosts and cover crops, combined with adequate soil organic matter and moisture management to keep the crowns of plants dry, are the best methods for organic management. Interestingly, the affected beans were only in one of the subplots on the edge of the rotation study while the other subplots which were surrounded by other crops were not affected. This demonstrates the microclimate effects that can have an effect on disease suppression.

Root knot nematode (RKN) populations did increase from undetectable levels in one section of the field over the course of the study, although they did not consistently reach damaging levels in any one plot. Nematode sampling was conducted before crop harvest in January, June, and September. Detection of significant populations occurred in late summer 2012 after strawberries/bush bean crops and in late summer 2013 after the lettuce/sudax crops. Sunnhemp appeared to suppress these populations although a longer study would be needed to see if the increase in nematodes was a long term trend. RKN can be a major problem in organic vegetable production in the southeast. Adequate incorporation of high-quality soil organic matter combined with proper selection of rotations and cover crops can suppress RKN populations and other soil borne diseases, and this should be a consideration when scaling up organic production. Further investigations are needed to develop effective methods for suppressing RKN in organic vegetable production under southeastern conditions.

The strawberry leaves were infected with leaf spot (Mycosphaerella fragariae) during the fall and winter, but the disease was confined to older leaves and was not severe enough to cause any significant losses. There was an occasional low incidence of fruit rot from Botrytis after periods of rain but no evidence of root rot. The strawberries were remarkably free of disease.

Potatoes had some wireworm damage the first year, but no significant disease problems. Finally there were no significant diseases on the carrots.

See attachment below: Figure 4. Purple blotch disease on onion in 2012 (Rating: 1-5, 1-few lesions, 5-severe infection).

See attachment below: Table 3. Incidence of Sclerotinia Disease on Lettuce, 4/17/2012.

In both rotations, A and B, onions did as well or better than the average county yields for onions (Figures 5 & 6). Onions were grown on plastic with all the fertilizer preplant incorporated prior to bedding and then applying the plastic mulch. The plastic mulch offers several advantages including weed control, conservation of nutrients (less prone to leaching). Of course plastic mulch is a non-biodegradable product that is manufactured from non-renewable petroleum. There was also a significant reliance on the previous cover crop residue for nitrogen (Table 2). The sunnhemp cover crop appeared to supply about 90 lbs N/ac. Our modelling of N release in the 3^rd year indicated both the soil and the cover crop were significant sources of N.  Finally the relatively high onion yields may be partially due to the fact they were grown outside the traditional onion belt of southeast Georgia.

Although onions did well in comparison to average farm gate yields, the onion sizes were generally smaller than what is traditionally produced in conventional production. Most of the onions were in the medium size class (2-3 inches), while in conventional onions most are in the jumbo size (≥3 inches) (Figure 6). Conventional onion production will have approximately 80% of the onions in the jumbo size class. It should be noted that there is a consumer preference for the medium size onion, apparently because this is the right size for cooking without leftover onion pieces.

Strawberries on the other hand did not do very well with yields going down over the course of the three years. In all years yields were significantly lower than the farm gate estimates (Figure 5). The relatively high yields the first year may have been due to previous soil management.

Bush bean yields were modest at best with the first year’s yields a complete loss. In the first year, the variety used was ‘Yellow Wax Bean’, which did poorly and was eventually abandoned. In subsequent years, the grower recommended green bean variety ‘Provider’ was used.

The potato variety used in this study was ‘Yukon Gold’ a yellow fleshed variety. Yields improved from one year to the next, but never reached the level of reported farm gate yields.

Southernpeas were included in rotation A because they offered the benefits of a summer legume cover crop and the potential as a fresh market vegetable crop. The variety chosen was ‘Pink Eye Purple Hull’, a popular and widely available southernpea variety. Unfortunately, there were no yields to report in years 1 or 3 and the year 2 yields were modest. Part of the difficulty with yields in this crop was the short time period between when the onions were harvested and the followed strawberry crop needed to be planted.

In year 1, broccoli was grown on plastic mulch to minimize weed problems. In years 2 and 3 the crop was grown on bare ground since the winter weed pressure was minimal. Yields improved over the course of the study with the third year yields equivalent to the reported average farm gate yields.

Lettuce production overall was successful in all three years of the study with yields better than double the average farm gate yields. There is not much conventional lettuce production in Georgia; however, it is a popular crop among organic growers. In the first year, the lettuce was grown on plastic mulch behind the broccoli. This practice was abandoned in subsequent years because the plastic ended up with too many holes (punched for the broccoli and then for the lettuce). The lettuce variety that was used was ‘Winter Density’, which was pelleted with a clay coating. The pelleting made the seed easier to handle for transplant production.

Carrots yields were disappointing overall. They are a difficult crop to grow organically because they have small seed, are direct seeded, and are slow growing. Weed control is particularly difficult because of the close spacing and slow growth. Weeds must be controlled very close to the plants particularly during early growth, which is very time consuming and expensive. Yields were less than half the average farm gate values for Georgia.  Carrots yields may have been adversely affected by the previous Egyptian wheat/cowpea cover crop.  Eyptian wheat is in the sorghum family and is a locally produced seed.  The cover crop was chosen because it has good weed suppression properties and both cowpeas and sorghums may also help reduce nematode pressure, which would be important before a root crop like carrots. However, the Egyptian wheat consistently dominated the cover crop mixture and decomposed slowly. These could have created two negative effects on the carrots. First, the sorghum family can have allelopathic properties, which may have affected seed germination.  Second, our modelling of N release in the 3^rd year indicated there was N immobilization following the Egyptian wheat/cowpea incorporation.  Consequently, we may have had too little N for good carrot production.

See attachment below: Figure 5. Revenue generating crop yields from rotation A at Watksinsville , Georgia, including annual yields (2011-13) and average yields from the 2012 Georgia Farm Gate Value Report.

See attachment below: Figure 6. Revenue generating crop yields from rotation B at Watksinsville, Georgia, including annual yields (2011-13) and average yields from the 2012 Georgia Farm Gate Value Report.

See attachment below: Figure 7. Distribution of onion yield at Watkinsville, Georgia by size class (smalls 4 inches).

In conclusion, yield results were mixed in this study. Onions definitely did well in all years consistently producing good yields. This was also true for lettuce. Broccoli and potatoes did reasonably well and would be good choices for organic production. Strawberries were inconsistent with good yields in the first year, but subsequent years were disappointing. Beans, southernpeas, and carrots, based on this study, did not perform well in these rotations.

• Table 2. Soil test based fertilizer recommendations and actual nutrient application rates for vegetable crops and cover crops
• Figure 6. Revenue generating crop yields from rotation B at Watksinsville, Georgia, including annual yields (2011-13) and average yields from the 2012 Georgia Farm Gate Value Report
• Figure 7. Distribution of onion yield at Watkinsville, Georgia by size class (smalls <2 inches, mediums =2 inches & <3 inches, jumbos =3 inches, and Colossal >4 inches)
• Figure 3. Average changes in active soil carbon over three growing seasons between two cool-season vegetable rotations at Watkinsville, Georgia
• Table 3. Incidence of Sclerotinia Disease on Lettuce, 4/17/2012
• Figure 5. Revenue generating crop yields from rotation A at Watksinsville , Georgia, including annual yields (2011-13) and average yields from the 2012 Georgia Farm Gate Value Report
• Figure 1. Average changes in mineralizable nitrogen over three growing seasons in two cool-season vegetable rotations at Watkinsville, Georgia
• Figure 2. Average changes in total carbon over three growing seasons within two cool-season vegetable rotations at Watkinsville, Georgia
• Figure 4. Purple blotch disease on onion in 2012 (Rating: 1-5, 1-few lesions, 5-severe infection)