At Landvik and Kvithamar 28 and 56%, respectively, of the total input of N over 5 years were from N- fixation (Table 2). At both sites the first year ley/seed production fields gave the highest yields of biologically fixed N (Tables 4 and 5). The decrease in fixation with time was related to a decrease in the proportion of clover in the leys. In three out of five years at Landvik there was 80% or more clover in the first year crop, whereas the clover content in the second year crop was as low as 5-15% (Table 4) (Aamlid, 1999). Low persistence of alsike clover was probably caused by the late harvest of clover- seeds, which did not allow any regrowth of clover in the autumn.
Table 2 Accumulated nutrient balances (kg ha-1) for the years 1994-1998 in two 6-year crop rotation
systems located at Landvik and Kvithamar. Means of the 6 fields constituting each of the two systems are given.
Landvik Kvithamar
Nutrient Balance (supplied - removed) Balance (supplied - removed)
Nitrogen 141 (459* - 317) 111 (523* - 412)
Phosphorus 33 (95 - 62) -25 (44 - 69)
Potassium - 367 (153 - 519) -300 (228 - 527)
Table 3 Nutrient balances (kg ha-1 yr-1) for the different crops in two organically farmed crop
rotation systems at The Norwegian Crop Research Institute, Landvik and Kvithamar for the years 1994-1998.
System and crop Means for N P K
Landvik
1. year seed crop 5 fields 61 - 8 - 106
2. year seed crop 4 fields - 18 - 7 - 62
Wheat (undersown) 5 fields - 19 0 - 23
Cabbage 3 fields 129 46 - 93
Carrots 5 fields 20 8 - 104
Potatoes 5 fields 27 11 - 82
Lettuce 2 fields 56 20 - 3
Kvithamar
Barley (undersown) 5 fields 32 2 36
1st year ley 7 fields 41 - 13 - 175
2nd year ley 5 fields 7 - 11 - 118
3rd year ley 3 fields 7 - 12 - 102
Swede 5 fields 46 8 9
Oat 5 fields - 17 - 5 19
Table 4 Nitrogen balance (kg N ha-1 yr-1) and clover content for the two crops with N-fixation at
Landvik, which is first and second year ley with timothy and clover seed production.
First year Second year Two years
Year % clover Balance % clover Balance Balance
1994+95 80 85 15 -29 55
1995+96 50 25 5 -21 4
1996+97 90 74 10 -10 64
1997+98 98 124 10 -9 115
Average 80 77 10 -18 59
Table 5 Estimated yearly nitrogen input (kg N ha-1 yr-1) from N-fixation in a 6-year crop rotation
system with barley undersown with grass/clover, 3 year clover ley (cut twice each year), swede and oats. Means (± SD) for the years 1994-1998 are given.
Crop Spring growth Regrowth Total
Barley undersown with grass and clover
10 ± 6 10
1st year ley 91 ± 53 48 ± 18 139
2nd year ley 50 ± 26 35 ± 18 85
Figure 1 Dry matter yields taken out of a 6-year crop rotation system located at The Norwegian Crop Research Institute, Landvik. Means for the years 1994 -1998 with standard deviations are given.
Figure 2 Dry matter yields taken out of a 6-year crop rotation system located at The Norwegian
Crop Research Institute, Kvithamar. Means for the years 1994-1998 with standard deviations are given. The straw was removed from fields with barley, but its weight is not included in the figure for barley yield.
Wheat 1st ley 2nd ley Potato Cabbage Lettuce Carrot
Dry m at te r y iel d (t ha -1 ) 0 2 4 6 8 10
Barley 1st ley 2nd ley 3rd ley Swede Oats
D ry ma tter y iel d (t ha -1 ) 0 2 4 6 8 10
The second year ley was dominated by timothy that yielded quite well (Figure 1), partly because of the significant N surplus accumulated by the clover the previous year (Table 4). Due to limitations to the use of non-organic compost implemented from 1996 onwards, the importance of effective nitrogen fixation in the Landvik system will increase.
Yields
Cereal yields were quite stable for wheat and within 65 to 85% of the average yields in conventional farming in the Landvik district (Figure 1; Eltun, 1996). At Kvithamar, the mean yields for barley and oats have been 1.5 and 3.5 t ha-1 (Figure2), which for oats are rather low and for barley very low
compared to yields obtained in conventional farming. As discussed by Haraldsen et al. (1999), the soil below the plough layer is very dense at Kvithamar, and root growth might have been severely restricted. The soil conditions are furthermore far from optimal regarding N-mineralisation and N-loss, especially under wet and cold weather conditions. Low yields from cereals might consequently have been caused by a shortage of plant available N, even if the nutrient account for N shows a positive balance for undersown barley (Table 3).
The seed production fields gave excellent yields, sometimes higher than in conventional farming (data not shown). Alsike clover was the major component in the first ley year, and the seed-yields were stable at about the same level as in conventional farming. The second year crop was dominated by timothy. Seed yields of timothy after a clover-rich first year ley were high, and sometimes higher than for conventional farming. This is partly due to the fact that lodging never occurred in the organic seed- crops.
The yields of vegetables were variable at both Landvik and Kvithamar (Figures 1 and 2) and highly affected by weather-conditions and pests. In some years difficult weather conditions in the spring and early attack of potato blight reduced the yields in potatoes. In the best years the potato yields were almost as high as in conventional farming. The yields of cabbage in the three first years were high. This is, however, a nitrogen-demanding crop and limitations to the use of external compost from non- organic sources caused change from cabbage to lettuce from 1997 and onwards.
The carrots were also vulnerable to climatic conditions and pests. Two years of excellent yields in the beginning of the study were not repeated. Problems during seed bed preparation with resulting weed- problems gave poor stands of carrots and reduced yields significantly.
Although of some importance, pests were not the main explanation for the rather low and variable yields of swede at Kvithamar (Figure 2). The demand for N might not have been met, and the harrowing aimed at weed control and soil aeration were unfortunately left out or badly timed in some years.
Nutrient balances
As regards nitrogen, the input from compost, farmyard manure and N-fixation was higher than the removal in the yields in both systems (Table 2). Haraldsen et al. (1999) still suggest that the actual balance is close to zero when estimated leaching is taken into account. Although the input of N from biological fixation was considerably higher at Kvithamar than at Landvik both in absolute and relative terms, it was of importance for the total balance at both sites (Tables 2 and 3). The remarkably high surplus that occured on the cabbage and lettuce fields at Landvik (Table 3) was due to a high supply of
compost in some years before the limitations to the input was set in 1996. The total surplus of phosphorus at Landvik is also partly the result of the high supply of compost to cabbage and lettuce (Tables 2 and 3). Nearly twice as much was supplied of this nutrient at Landvik as at Kvithamar, whereas almost the same amount was taken out by the yields at the two sites (Table 2).
The K-account shows big deficits in both cropping systems (Table 2). More than 70% of the potassium taken out at Landvik and more than 50% of the potassium taken out at Kvithamar were from soil reserves (Table 2). According to plant analyses, the total supply from fertilisers and soils seem to have been sufficient for all crops (data not shown). Soil analyses to be undertaken in autumn 1999 will reveal whether the pool of K in the soil has been significantly and seriously depleted. At Kvithamar most of the K taken out has been in yields from ley, whereas yields from cabbage, carrots, potatoes and seed leys have all contributed to the negative balance at Landvik (Table 3).
Conclusions
With the exception of undersown barley at Kvithamar, all crops have returned what we regard as acceptable yields in an organically farmed system, for at least one of the five years from 1994 to 1998. It will be an important challenge to minimise the between-year variation in yield that has occurred so far, both for leys, cereals and vegetables. A more frequent or properly timed weed control might improve the yields from cereals and vegetables. Both groups of crops would probably have benefitted from a higher supply of plant available N in the spring because the soil and weather conditions have frequently been far from optimal for N-mineralisation.
As regards nitrogen and phosphorus, the nutrient-accounts show an adequate balance after 5 years. It is, however, likely that the net removal of more than 300 kg ha-1 of potassium from the systems will
have depleted the soil reserves of this element, at least at Landvik. The initial reserves of K in the soil were smaller at this site than at Kvithamar.
By the autumn 1999, the 6-year rotation will be complete, and several soil analyses will be carried out. On the basis of the current soil status and previous recordings and experiences, plans for the continued studies of both systems will be outlined. Important issues will be how to interpret and deal with the potassium deficit and how to improve soil structure in order to optimise the turnover of organic matter and nitrogen. Changes in the crop rotation will also be considered. Some of the presently grown vegetable crops are probably too nitrogen-demanding for a system without its own organic fertiliser- source. Due to the maximum limits to the use of non-organic compost implemented from 1996 onwards, the importance of effective nitrogen fixation in this system will increase.
References
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