Weather, rooting depth and water relations of broad beans — a theoretical analysis

Agricultural Meteorology, 20(1979) 381--391 © Elsevier Scientific Publishing Company, Amsterdam - - Pr i n t ed in The Netherlands

WEATHER, ROOTING DEPTH AND WATER B E A N S -- A T H E O R E T I C A L A N A L Y S I S

RELATIONS

381

OF BROAD

H. R. ROWSE and A. BARNES

National Vegelable Research Station, Wellesbourne. Warwick. CV35 9EF (Greal Brilain) (Received June 24, 1978; accepted July 31, 1978)

ABSTRACT Rowse, H. R. and Barnes, A., 1979. Weather, rooting depth and water relations of broad beans -- a theoretical analysis. Agric. Meteorol., 20 : 381--391. Increases in yield due to deep cultivation vary from year to year, probably as a result of differences in the extent to which water extraction is improved by a more rapidly extending root system. A simulation model of crop water extraction has been used to identify the weather conditions under which such increases in root growth should be beneficial for a crop of broad beans growing in a sandy-loam soil. The simulated patterns of water extraction for a crop of broad beans grown in 1974 were similar to those measured. Additional simulations were carried out with weather data from other years and with roots which extended more rapidly than, at a similar rate to, or more slowly than those measured in the field. These showed that the water content of the plough layer was always higher in crops with rapidly extending roots. This should result in improved nutrient mobility and uptake as this zone usually contains the highest nutrient concentration. In all but low stress years, (years with low potential transpiration and high rainfall), increasing root extension rate relative to that measured in the field increased water extraction by the crop and it is argued that this would increase dry matter production, although the increases may be small in high stress years. A subsequent deep cultivation of a soil similar to that used for the test of the model, produced a large increase in yield of broad beans in a moderate streas year.

INTRODUCTION D e e p c u l t i v a t i o n h a s i n c r e a s e d c r o p y i e l d s in m a n y p a r t s o f t h e w o r l d ( E c k and Taylor, 1969; Schulte-Karring, 1975; El-Karouri and Gooderham, 1977), by up to 100% (Stone, 1978) or more (Bradford and Blanchar, 1977). The r e a s o n f o r t h e i m p r o v e m e n t is n o t a l w a y s c l e a r a n d m a y v a r y f r o m o n e site t o a n o t h e r . In h e a v y soils t h e p r i m a r y e f f e c t m a y be e n h a n c e d d r a i n a g e , b u t f r e q u e n t l y c r o p r e s p o n s e s a r e a s s o c i a t e d w i t h an i n c r e a s e in r o o t i n g d e p t h d u e to the disruption of a compact subsoil. Several workers have demonstrated that deep cultivation enables the crop to extract more water from the subsoil ( M e c h e t al., 1 9 6 7 ; E c k a n d T a y l o r , 1 9 6 9 ; S t i b b e a n d H a d a s , 1 9 7 7 ) a n d it s e e m s p r o b a b l e t h a t i n c r e a s e d y i e l d s a r e d u e at l e a s t in p a r t t o a d e c r e a s e in t h e

382 crop water stress. This hypothesis is supported by the finding that there is no response to deep cultivation when adequate irrigation is given (Mech et al., 1967; Eck and Taylor, 1969). More direct evidence for the involvement of water comes from the work of Reicosky et al. {1976) and Stone (1978) who showed an increase in leaf water potential and stomatal conductance on plots which had received deep cultivation. If the main effect of deep cultivation is via crop water stress, then variations in crop response to deep cultivation treatments from one year to another would be expected due to variation in rainfall and potential transpiration. The object of the present work was to use a simulation model to examine the likely effects of root extension occurring at various rates on the water extraction by a crop of broad beans, and to investigate how these effects might vary from year to year depending on the weather. THE MODEL The model calculates daily values of the distribution and flow of water in the soil, extraction of water by the roots, plant water potential and actual transpiration and evaporation from the crop and soil respectively. Input information consists of the hydraulic properties of the soil and roots and daily estimates of root distribution, crop cover, rainfall and potential transpiration. The model was written in FORTRAN but was essentially the same as an earlier version written in CSMP (Rowse and Stone, 1978; Rowse et al., 1978), and only the major features are described here. For the simulation the soil profile is divided into a number of horizontal layers and the period to be simulated is divided into hourly intervals. From the known water contents in each layer at the start of the simulation, new water contents are calculated every hour by estimating the rates of flow between adjacent layers and the rates of uptake by the roots. The calculations are based on the equation (Rowse et al., 1978): 5O_ 5 ~DSO 6t

5z ~ - ~ - z +

K]~

-

S ( z , t)

(1)

where z and t are depth (cm) and time (days) and 0, D and K are the soil volumetric water content, diffusivity (cm~/day) and hydraulic conductivity (cm/day), respectively: The extraction term S (day "l ) for a soil layer of unit thickness is given by: S = L(hs -- hp)/(Rs

+ Rp)

(2)

where L is the length of root per unit volume of soil (cm-2), Rs a n d R p are the soil and plant resistances associated with unit length of root (day cm -l) and hs is the bulk soil pressure head (sum of matrix and gravitational potentials in cm). The plant water potential hp is considered to be uniform througho u t the xylem of the root system. It is fixed initially by finding a value which

383

results in the total uptake from all soil layers (ES) being equal to the atmospheric demand which is calculated from the Penman potential transpiration and the proportion of crop cover. If this value is below (more negative than) the plant water potential at which stomatal closure occurs (hp*), then the plant water potential is assumed to be hp*. In these circumstances the total uptake ( E S ) is less than the atmospheric demand and the crop loses water at less than the potential rate. The model also calculates the redistribution of water following rainfall and evaporation from the soil surface. The flow at the b o t t o m of the lowest soil layer {lower boundary condition) was assumed to be such that the lowest layer {90--100 cm) remained at a nominal field capacity equivalent to a matrix potential of --30 cm. Field measurements showed this to be a realistic assumption. The predominant flow required to maintain this condition was usually drainage (downward flow), the m a x i m u m upward flow occurred in dry years with deep rooted crops and only a m o u n t e d to a total of about 6 mm throughout the season.

0

Root concentration (cm/cm 3} 0.4 0.8 0 0.4 0.8 0 0.4 0.8

0

20 u J~2

40

Q_ £3

60

28 Nay R=8 3

4 June R =12'0

31 Jury R = 15.z,

80

Fig. 1. R o o t d i s t r i b u t i o n s o n v a r i o u s o c c a s i o n s in 1 9 7 4 . M e a s u r e d ( p o i n t s ) a n d c a l c u l a t e d f r o m eq. 3 (lines) w i t h m e a s u r e d R values a n d b = 5.0.

The plant parameters used were based on those of a broad bean crop grown in 1974. The root distributions measured on three occasions beneath this crop are shown in Fig.1. The lines in Fig.l)*. Eq. 3was used i n t h e simulaempirical equation similar to that used by Rowse (1974): l

1

C = (R/b2~ - exp(--Z/bl~)

(3)

where C is the mean concentration of roots in cm of root per cm a of soil at depth Z (cm), and b is a constant (5.0 in Fig.l).* Eq. 3 was used in the simulation studies because for a given total root length the depth of the root system is controlled by the single term b. For example the soil depth containing 90% of the roots is given by 2.303bR~. The effect on root distribution of varying b is shown in Fig.2. Fig.3 shows the measured variation of R with time (points) and that used in the model (line). Finite quantities of root are calculated by *R is t o t a l r o o t l e n g t h b e n e a t h 1 c m 2 o f soil s u r f a c e .

3 S4 Root concentration {cm/cm'-~! 05 1,0

y

20

15

sE

15 E

b=50

.

-= 10 #

oJ

~:~0

-

I I

o

~

6O

b= 10'0

,/,~"

Si~ul°ti°r'

,~-- ,

' 100 120 140 160 180 200 220 April I Hay I June J July I Time (days from 1 Jan)

o

@@

5

[3:

!

Fig. 2. E f f e c t of varying b o n r o o t d i s t r i b u t i o n s calculated f r o m eq. 3. R = 15.4. Fig.3. M e a s u r e d t o t a l r o o t l e n g t h b e n e a t h 1 c m ~ of soil surface R ( p o i n t s ) a n d t h a t used in the m o d e l (line). S = sowing d a t e ; E = time of 50% e m e r g e n c e .

eq. 3 at all finite d e p t h s , b u t unless o t h e r w i s e s t a t e d a r o o t limit was set at I 0 c m a n d no r o o t s w e r e a l l o w e d to g r o w b e y o n d this d e p t h . RESULTS

Fig.4 shows a comparison of the soil water extraction patterns measured in 1974 under a crop of broad beans with those simulated by the FORTRAN model using root distribution data obtained by interpolation of the measureDeficit relative to 5May (%volume) 0

40

0

•

4

80

4

4

8

12

I

2O

30

,

Ij i

.c 40

~

--

i

i/ so

i SIll

50

i i]

70 8O

June

17 June

I/. July

22 July

Fig.4. Water e x t r a c t i o n profiles in 1974. M e a n m e a s u r e d values + I S,D. s h o w n b y h o r i z o n t a l lines, s i m u l a t e d values using m e a s u ~ d r o o t d i s t r i b u t i o n s (M) a n d s i m u l a t e d values using r o o t d i s t r i b u t i o n s calculated f r o m e q u a t i o n 3 (C).

385

ments shown in Fig.1. Details of the e x p e r i m e n t have been published elsewhere (Rowse et al., 1978). There is reasonable agreement between measured and simulated results and the probable reasons for some of the discrepancies h a v e b e e n discussed by Rowse et al. (1978). Also shown in Fig.4 are the e x t r a c t i o n profiles calculated for the same period using r o o t distribution data calculated f r o m eq. 3 with b = 5.0 and R as shown in Fig.3. The two simulated profiles are similar e x c e p t that on 17 June the results calculated using eq. 3 show mo r e extraction between 30 and 50 cm indicating t hat at, or shortly before this time, eq. 3 overestimated the q u a n t i t y of root s in this zone. However, the conclusions f r om the present study do not rely on an exact simulation o f the p attern of r o o t growth which in any case varies from year to year and site to site, and the a p p r o x i m a t i o n given by eq. 3 is therefore considered adequate. Simulations were repeated for 1974 with either deep (b = 10.0) or shallow (b = 2.5) r o o t systems, all ot her parameters remaining unchanged. T he total q u a n t i t y o f water extracted per unit volume of soil for various depths is shown in Fig.5. As might be expect ed the deeper r o o t system was predicted Total

0

0.1

water extraction(cm3,Cm3)

0.2

0.3

04.

0-5

0-6

0-7

10 2O

.c /.0

50

=

i

=

•

7O 80 Fig.5. Simulated

total water extraction

in 1 9 7 4

for various calculated

root distributions.

to ex tr act more water f r o m the deeper parts of the soil profile and less from the surface soil. Between a b o u t 10 and 30 cm, however, the total ext ract i on t h r o u g h o u t the season was very similar for all r o o t systems, despite considerable differences in r o o t distribution (Fig.2). Fig.6a shows that the rate at which water was e xt r act ed from a dept h of 10 cm differed between the various theoretical r o o t distributions. Initially extraction by the shallow r o o t system was faster but this caused a more rapid drying of the soil and a c o n s e q u e n t earlier decline in uptake. These differences were reflected in the simulated soil water c o n t e n t at 10 cm which was always higher for t he deeper

386

r o o t e d crop (Fig.6b). The altered water distribution near the soil surface had o nly a small effect on calculated total evaporation from the soil, which was 3.2 cm for the deeper r o o t e d plants and 2.9 cm for the shallow root ed plants. ~10 [(0) b=2.5 ~

f'"'i ,

I 2 I

~

:V'~',: ,

22 rib)

-1~1-

I

,

.......

~' ;,...... ,

,~.

IRA/',',,

~ 1 8 I"

:

,

"4

/,?/10"0

. '

b=lO,O

.'~ ",

/,,

b=2.~

",

'

:,

/,

~

% ",...

:., ,,

,'.~v,-~

,,,,:v ~:

.k\4.

,,1 .

~. ' \ ....

~,

,

',.

140 150 160 170 180 190 200 Moy I June I July Time (d(:]ys from 1 J o n }

210

Fig.6. S i m u l a t e d results a t 10 c m d e p t h in 1 9 7 4 w i t h e i t h e r shallow (b = 2 . 5 ) o r deep (b = 1 0 . 0 ) roots. (a) M e a n u p t a k e rate averaged over 3-day intervals (U). (b) Soil v o l u m e t r i c w a t e r c o n t e n t (0).

To examine effects caused by variations in weather conditions the model was run with several r o o t distributions (b = 2.5, 3.3, 5.0, 6.7 and 10.0), but always with the same total r o o t length, R, for any day as shown in Fig.3, and with weather data f r om various years. Weather data was obtained f r o m the NVRS records for selected years between 1959 and 1976 to include years with ex tr emes of rainfall and potential transpiration as well as m ore average years. Th e total potential transpiration and rainfall during the simulation period (14 May--29 July) for the years are shown in Table I. TABLE I W e a t h e r d a t a in various years for the s i m u l a t i o n p e r i o d Year

1959 1960 1964 1967 1968 1971 1974 1975 1976

Type*

H M M L L M M H H

Rainfall (cm)

6.3 13.5 13.9 13.3 21.7 12.3 9.3 6.5 7.3

Potential transpiration

Bright sunshine

(cm)

(%)

15.3 13.6 12.4 9.1 9.2 11.1 13.3 16.2 18.0

44.3 39.5 30.7 12.8 23.8 28.4 40.7 48.5 46.4

* Years designated as high (H), moderate (M) or low (L) stress according to results shown in Figs.7, 8 and 9.

387

The a m o u n t o f water transpired by the crop was pl ot t ed as a f u n c t i o n of b. T h e results for years w i t h o u t e x t r e m e s of rainfall or potential transpiration (hereafter called m o d e r a t e stress years) are shown in Fig. 7. The total

14

¥

1960 f

1974

,

1

u

t /

1964.

=1

_~

1971

s"

_.--"

],o Moderate

Stress

Years

, //--~ 6 8 10 b ( i n c r e a s i n g root depth---,-)

oo

Fig.7. E f f e c t o f b o n t o t a l c r o p w a t e r use in m o d e r a t e s t r e s s years.

transpiration at a value of b m ar ke d oo represents the a m o u n t of water t hat would have been transpired had there been no stomatal closure (i.e., the a c c u mu lated potential transpiration over the growth period corrected for crop cover). Fig.8 shows the same pl ot for high stress years including the results of a simulation when t he roots were allowed to grow to 110 cm, and Fig.9 shows the results for two low stress years. DISCUSSION

In the simulations eq. 3 was used to alter r o o t dept h w i t h o u t altering the total q u a n t i t y o f roots. In reality factors which p r o m o t e deeper rooting would p r o b a b l y also increase total r o o t length, and deep r o o t e d plants may t h e r ef o r e e x t r a c t m or e water than predicted by the model. It has also been assumed th at crop cover was the same as that measured in the 1974 e x p e r i m e n t and was a f u n ction only of time. However, it is known that leaf expansion rates are reduced by water stress (Hsiao et al., 1976) and in reality crop cover would increase m o r e or less rapidly than t h a t measured in the 1974 e x p e r i m e n t according to the degree of water stress. This again would result in an underestimate o f the water e x t r a c t i o n of the deeper r o o t e d crops since these are • likely to have lower levels o f water stress and t h e r e f o r e faster expanding leaves providing a greater evaporative surface. Our analysis examines the p e r f o r m a n c e of different root' s y s t e m s u n d e r various weather conditions and it t h e r e f o r e ignores the fact t hat adaptive r o o t

388 18

rr

t 1

/ 16

ii t1

/,'/

1976 14

//

u

II i

1975

,,,,jl ~

o. 10 o

o

/

High

,

,

4 b

6 (increasing

Stress

Yeors

//.-,

,

root

8 depths)

10

oo

Fig.8. E f f e c t o f b o n total crop water use in high stress years. R L = 1 1 0 c m s h o w s e f f e c t o f a l l o w i n g r o o t s to grow t o 1 1 0 cm.

f

1968

f

(J

1967

Low i

i

Stress Years i

6 8 b (increasing root d e p t h s )

i

10

Fig.9. E f f e c t o f b o n total crop water use in l o w stress years.

growth ctm occur and the weather conditions themselves may alter root distribution (Rowse, 1974). The topic is beyond the scope o f this paper but it seems likely that when soil conditions restrict rooting depth then the degree to which the plant root system can adapt is probably limited. For the reasons outlined above we regard our results as qualitative rather than quantitative, and the true extent of dif~rences caused by different root depths could be much larger than we predict.

389 Several workers have observed t hat the water c o n t e n t of the surface soil tends to be lower under shallow r o o t e d crops (Mech et al., 1967; Stone, 1978). The effect is n o t always found, however, and Fig.6b suggests that the differences may n o t be apparent if measurements are made shortly after heavy rainfall or after prolonged drought. Fig.6b also suggests that one consequence of a deep r o o t system is to improve n u tr ien t uptake from the surface soil because nutrient mobility is enhanced in moist soil (Dunham and Nye, 1976). The effect would be important if, as is usually the case, the surface soil contains the highest concent rat i on of nutrients. The relationship between transpiration and dry matter p r o d u c t i o n (De Wit, 1958) can be used to interpret the simulated water use data shown in Figs.7, 8 and 9. Cowan and T r o u g h t o n (1971) have shown theoretically t hat there is no unique relation between transpiration and assimilation but that its form depends on the relative magnitudes of the resistances to carbon dioxide and water vapour. However, De Wit (1958) quotes numerous experiments which illustrate a linear relationship for m a n y species grown under t em perat e conditions. F o r our purposes it is sufficient that under temperate conditions an increase in transpiration is accompanied by an increase in dry m at t er production. The relationship need not be linear nor need it be the same every year. For crops grown in low stress years (Fig.9) there is no increase in water use (and hence it is assumed associated dry m a t ter p r o d u c t i o n ) by increasing b to above 5.0, the value f o u n d in the field. Indeed even a very shallow r o o t system with b = 3.3 would be sufficient to extract water at the rate dem anded by the atmosphere. The results for m o d e r a t e stress years (Fig.7) show t hat some benefit might be e x p e c t e d by increasing b above 5.0, which suggest t hat the r o o t system measured in 1974 Ca m ode r at e stress year) was less than adequate. This was confirmed by the large increase in yield of broad beans obtained in 1977, (also a m o d e r a t e stress year), following the deep cultivation of a soil similar to that used in the 1974 e x p e r i m e n t (Stone, 1978). The lack of response to increasing b b e y o n d a value of a bout 6.0 is due to the rather severe constraint in the simulation that roots c a n n o t grow b e y o n d 70 cm. It can be seen from Fig.8 th at when the roots are allowed to grow to 110 cm then there is a response to increasing b b e y o n d 6.0. Two i m p o r t a n t conclusions can be drawn from this result. Firstly, it appears that a very small r o o t c o n c e n t r a t i o n deep in the soil profile (see Fig.2) can have a di s pr opor t i onat e effect on crop water ext ract i on and even small departures f r om the r o o t distribution described by eq. 3 could have a p r o f o u n d ef f ect on crop water extraction. In the absence of furt her experimental data there is little point in speculating what might have happened had other empirical m e t h o d s been used for estimating r o o t distribution, but it seems clear that in experimental studies particular at t ent i on must be given to measuring these small r o o t concent r a t i ons deep in the profile. Secondly, the

390 possibility of increasing root growth between 70 and 110 cm is probably limited, the typical depth of subsoiling in Great Britain being 35--40 cm. However, the disruption of a distinct pan above 40 cm may well advance the arrival of the roots at 70 cm and so produce a deeper root system. According to De Wit (1958) there is a linear relation between transpiration and dry matter production in temperate regions defined as those with about 35% direct sunlight. In more arid regions with about 70% direct sunlight he concludes that the ratio of transpiration to dry matter production is not constant but increases with open water evaporation Eo, (i.e., an increase in transpiration is not reflected in dry matter production). The percentages of bright sunlight for high stress years (Table I) are above 35%. It is n o t certain therefore that the linear relation between dry matter production and transpiration is applicable, and the extra water extracted by the deep rooted crop (Fig.8) may not be very effective in increasing dry matter production. What is more certain is that in moderate stress years a deep root system will allow the crop to extract water at or near the rate demanded by the atmosphere and so prevent serious water stress. In high stress years, even though an increase in root depth enables more water to be extracted, the deep root system still cannot extract water at the rate demanded by the atmosphere and crop yields will inevitably be reduced by water stress. However, the magnitude of the yield reduction is difficult to assess. An examination of the simulated plant water potentials showed that the mid-day potentials were invariably higher (less negative) in plants with a deep root system. The differences between the plant potential of the various simulated root patterns were small when the soil water deficits were very small or very large and were most marked at intermediate deficits. When soil water deficits were large, however, the duration of low water potentials during the day was longer in plants with shallow root systems. There was no evidence of higher potentials in shallow rooted crops later in the season due to their having conserved water deep in the soil profile (Passioura, 1972), but this p h e n o m e n o n would n o t be expected in our simulations if it occured as a result of adaptive root growth, as discussed previously.

CONCLUSIONS Analysis of the 1974 broad bean experiment suggests t h a t the measured root system was less than adequate to supply the water requirements of the crop in a moderate stress year. Large increases in yield obtained in a deep cultivation experiment on a similar soil in 1977 support this conclusioa. Our inability to represent adequately the adaption of root and shoot growth to water stress means t h a t the more general interpretation of our results must be very tentative. With this proviso our analysis suggests t h a t crop responses to factors which increase root depth will be greatest in years w i t h o u t extremes o f rainfall or potential transpiration, and that because a deep root system

391 c a n d e l a y t h e d r y i n g o f t h e s u r f a c e soil h o r i z o n s , a d e e p r o o t s y s t e m m a y e n a b l e b e t t e r n u t r i e n t u p t a k e f r o m t h e soil s u r f a c e . ACKNOWLEDGEMENTS We t h a n k p a r t i c u l a r l y Dr. D. J. G r e e n w o o d a n d also Dr. M. A. S c a i f e , Mr. D. A. S t o n e a n d Dr. A. W a l k e r f o r t h e i r c o m m e n t s o n t h e m a n u s c r i p t . We also t h a n k R o s e m a r y S t o n e f o r p r e p a r i n g t h e d i a g r a m s .

REFERENCES Bradford, J. M. and Blanchar, R. W., 1977. Profile modification of a fragiudalf to increase crop production. Soil Sci. Soc. Am. J., 41: 127--131. Cowan, I. R. and Troughton, J. H., 1971. The relative role of stomata in transpiration and assimilation. Planta (Berl.), 97: 325--336. De Wit, C. T., 1958. Transpiration and crop yields. Versl. Landbouwk Onderz., No. 64.6. Dunham, R. J. and Nye, P. H., 1976. The influence of soil water content on the uptake of ions by roots. J. Appl. Ecol., 13: 967--984. Eck, H. V. and Taylor, H. M., 1969. Profile modification of a slowly permeable soil. Soil Sci. Soc. Am. Proc., 33: 779--783. EI-Karouri, M. O. H. and Gooderham, P. T., 1977. The effect of soil phsyical conditions and nitrogen fertilizer on the yield of Italian ryegrass. J. Agric. Sci., Camb., 88: 743--751. Hsiao, T. C., Alcevedo, E., Fereres, E. and Henderson, D. W., 1976. Water stress, growth, and osmotic adjustment. Philos. Trans. R. Soc. Lond., B 273, pp. 479--500. Mech, S. J., Horner, G. M., Cox, L. M. and Cary, E. E., 1967. Soil profile modification by backhoe mixing and deep ploughing. Am. Soc. Agric. Eng., Trans., 10: 775--779. Passioura, J. B., 1972. The effect of root geometry on the yield of wheat growing on stored water. Aust. J. Agric. Res., 23: 745--752. Reicosky, D. C., Campbell, R. B. and Doty, C. W., 1976. Corn plant water stress as influenced by chiseling irrigation and water table depth. Agron. J., 68: 499--503. Rowse, H. R., 1974. The effect of irrigation on the length, weight and diameter of lettuce roots. Plant Soil, 40: 381--391. Rowse, H. R. and Stone, D. A., 1978. Simulation of the water distribution in soil, I. Measurement of soil hydraulic properties and the model for an uncropped soil. Plant Soil. In press. Rowse, H. R., Stone, D. A. and Gerwitz, A., 1978. Simulation of the water distribution in soil, II. The model for a cropped soil and its comparison with experiment. Plant Soil. In press. Schulte-Karring, H., 1975. A new soil amelioration technique. Bull. No. 4, National Committee of the German Federal Republic on Irrigation and Drainage (ICID). Stibbe, E. and Hadas, A., 1977. Responses of dryland cotton plant growth, soil-water uptake and lint yield to two extreme types of tillage. Agron. J., 69: 701--704. Stone, D. A., 1978. Subsoiling and deep incorporation of nutrients. Rep. Natn. Veg. Res. Sta., 1977. In press.