Table 8 Nutrient flows for the feed component for the average specialized dairy farm in the middle of the eighties, the prognoses at the start
of ‘De Marke’ and the values realized in the years 19931994 and 19941995 kgha ‘De Marke’ prognoses
19931994 19941995
Characteristic ‘Average’ 19831986
P N
N P
N P
N P
Input 48.0
276 37.3
275 398
35.2 Harvestable crop
270 34.5
32.0 41
5.9 82
15.0 Purchased feed
84 182
11.5 Output
74.0 278
Feed intake 40.1
496 336
47.1 330
43.8 8
1.2 Feed sold
− 35
− 2.3
Mutation feed stock 17
0.8 6.0
39.0 3.1
48 4.2
Input–output
a
7 84
1.4
a
Grazing, harvesting, conservation and feeding losses.
short grazing season — careful silage making, and utilization of maize straw and beet leaves,
that at the current farm are left in the field. Inputs of N and P with feed from outside the farm and
from the feed stocks were substantially higher than predicted because of a larger herd size Table
8. Moreover, more N and P was imported with feed than necessary according to the feeding stan-
dards. Fodder with ‘adapted’ nutrient contents is rather expensive and it is cheaper to limit the
input of N and P on the farm through restricted fertilizer purchases. Differences between years in
the balance of inputs and outputs are partly due to inaccuracies in determining the feed stocks.
6. The fate of the nutrient surplus
6
.
1
. Nitrogen The nitrogen surplus of the farm Table 3 is
the sum of ammonia volatilization, denitrification, surface run-off, leaching and modifications in the
soil store. Surface run-off does not play a role, as there is hardly any surface water at ‘De Marke’.
Only at extremely high rainfall will water be removed through ditches. About 22 kg of N per
ha was lost as ammonia from manure and an estimated 2 kg from crop residues. Total loss of
ammonia-N thus equals 24 kg plus an unknown quantity from ensiled products.
Through denitrification a maximum of some tens of kg Nha were lost annually from the top
soil or the grass sward Corre´, 1996. Denitrifica- tion can be important in parts of the plots where
water stagnates lack of oxygen and there is ample supply of easily decomposable organic mat-
ter. Denitrification from deeper layers is consid- ered unimportant at ‘De Marke’ because of the
deep groundwater level, the low contents of or- ganic matter in the subsoil and the limited
amount of nitrate that leaches from the top soil. It has been assumed that during both years deni-
trification was 25 kg Nha.
In Fig. 1 the time course of nitrate content in the upper groundwater is given, measured by
RIVM National Institute of Public Health and Enviromental Protection, averaged for the farm.
Fig. 1. The concentration of nitrate in the upper groundwater of ‘De Marke’.
The nitrate contents strongly decreased over time and were, on the last reported date, on average
slightly below 50 mgl, the requirement for drink- ing water. Nitrate contents under current plots
close to ‘De Marke’ measured by the Drinking Water Company Oostelijk Gelderland, in the au-
tumn of 1993, were almost four times as high. Apparently, the adapted management results in
realisation of the objective ‘clean groundwater’ within a few years, where a much longer period
was
expected Goossensen
and Meeuwissen,
1990. No clear differences were found among crops, nor an effect of irrigation. On the basis of
precipitation surplus and the measured nitrate contents in the groundwater annual leaching is
estimated at 50 kg Nha.
The sum
of N-losses
through ammonia
volatilization, denitrification and leaching of ni- trate equals 99 kgha. The average surplus at farm
level is 169 kg Nha. The difference between these two values 70 kg Nha must be attributed to
measuring errors, losses from ensiled feed and changes in the store of soil organic-N. On aver-
age, the store of soil organic-N appeared to have increased annually by 17 kgha between 1990 and
1995, but obviously that estimate is not very accurate.
At ‘De Marke’ net mineralization showed large variations among plots and among years. Gener-
ally, the results suggest that the more humid the conditions during the growing season the higher
net mineralization Aarts, 1996a,b. This could imply that — if the soil store is in equilibrium in
the long run — it decreases in relatively wet years and increases during dry years.
6
.
2
. Phosphorus The P-content in the upper groundwater was
only 0.13 mgl when sampled in October 1994, i.e. much lower than the target value for sandy soil
0.4 mgl; Willems and Fraters 1995. As hardly any P leached it is most likely that the small
P-surplus accumulated in the soil. This is in agree- ment with the observation that the total amount
of P in the soil did not change in the period 19891990 – 19951996 Habekotte´ et al., 1998.
However, despite a slightly positive P-surplus, the P-status of the soil in terms of plant-available P
P-Al and P
w
; Van der Paauw 1956 and Van der Paauw 1971 decreased by some 2 – 4 annually.
This might be the expected response to the instan- taneous reduction in the input of external P
slurry and chemical fertilizer. Because of the high inputs of P prior to implementation of ‘De
Marke’, the labile pool of P in the upper layer of the soil is relatively high compared to the stable
pool, which results in net transfer from the former to the latter Janssen et al., 1987; Wolf et al.,
1987. This process continues until the reverse flow is in equilibrium again. As P
w
and P-Al are most closely related to the size of the labile pool,
these soil fertility indicators may be expected to further decrease in the near future.
The decrease in P
w
and P-Al, — in combina- tion with the low P-fertilizer level — caused
problems in a number of plots during the early growth of maize Aarts, 1995. Soil analyses indi-
cate that the solubility of P has decreased, espe- cially
where the
soil contained
high concentrations of iron and aluminum Schou-
mans, 1995. Until now, the significantly lower P-status of the soil and the associated slower early
growth of maize have not resulted in lower yields Habekotte´ et al., 1998.
7. Discussion and conclusions
