the influence of fertilizers, soil organic matter and soil compaction on maize yields on the surinam...

P l a n t a n d Soil 46, 445-458 (1977) Ms. 2996

THE I N F L U E N C E OF F E R T I L I Z E R S , SOIL ORGANIC MATTER A N D SOIL

COMPACTION ON MAIZE YIELDS ON THE SURINAM 'ZANDERIJ' SOILS*

by B. H. JANSSEN and R. VAN DER WEERT**

Centre for Agricultural Research in Surinam and Agricultural Experiment Station, Paramaribo, Surinam

SUMMARY

I n c o n n e c t i o n w i t h t h e growing i n t e r e s t in S u r i n a m to exp lo i t t h e soils in t h e h i n t e r l a n d , some t r ia l s were car r ied ou t on Zander i j soils to s t u d y pro- duc t iv i t y , fer t i l izer need a n d inf luence of some specia l soil f ac to rs on yield. T he t e s t c rop was maize.

T he yields were genera l ly low wh ich m i g h t p a r t l y be a t t r i b u t e d to t h e rela- t i ve d r y w e a t h e r cond i t ions d u r i n g t h e t r ia ls . On all soils i nves t iga t ed , y ie ld responses to p h o s p h a t e app l i ca t i on were high. On l o a m y soils t h e r e was Some- t i m e s a response to n i t r o g e n and on s a n d y soils to po t a s s ium. Yie lds on s a n d s were lower t h a n on l o a m y sands a n d s a n d y loams. The m o s t i m p o r t a n t yield d e t e r m i n i n g soil f ac to r p r o v e d to be organic m a t t e r . On c o m p a c t e d soils yields were cons ide rab ly lower w h i c h was due to inc reased m e c h a n i c a l im- p e d a n c e for roo t g r o w t h a n d in some cases to i n a d e q u a t e ae r a t i on .

INTRODUCTION AND SCOPE OF THE STUDY

Geomorphologically Surinam can be divided into four belts (Fig. 1). 1. Young Coastal Plain; heavy clay soils, in different stages of

ripening, occurring between sand ridges. 2. Old Coastal Plain; red and purple mottled clays and sand rid-

ges. 3. 'Zanderij' formation; bleached sands (40%) and unbleached

* Also published as Bulletin of the Agricultural Experiment Station, Paramaribo, Surinam.

** Present addresses: Agricultural University, Department of Soils and Fertilizers Wageningen, The Netherlands, and Ministry of Public Works, Kingston, Jamaica.

446 B. H. JANSSEN AND R. VAN DER WEERT

~ [ n a m

I I Young Coastat plain

Old Coastal Ptain

Zanderij formation

Uplands, residual soils

Fig. 1.

o, lo 20 3o, 4o, SOkm,

Schema t i c geomorpho log ica l m a p of S u r i n a m (af ter t3rinkman and Polls %

sediments (60%) with textures varying from sands to sandy loams and clays 7. Their stratigraphic name is 'Coesewiine series'.

4. Uplands; residual soils with strongly diverging textures formed on gneiss, schist, granite, diorite and dolorite.

Agriculture in Surinam is mainly confined to the young coastal plain. The intensive drainage which is required prohibits mechanisation on a large scale. Since labour is scarce in Surinam agriculture, it is very difficult to farm economically. Recently interest is shifting towards the almost uninhabited belt of Zanderij soils, which are more suited for mechanisation.

In the experiment area 'Coebiti', located between the rivers Sara- macca and Coesewijne (Fig. 1), a study is made of the possibilities of growing perennials like citrus, oil palm, coconut, banana and several grasses, as well as annuals like cassave, peanut and other pulses, sorghum and maize.

The present paper deals with an evaluation of tile prevailing soils,

MAIZE YIELDS ON THE SURINAS[ 'ZANDER¥' SOILS 447

with maize as test crop. The aim was to get information on product- ivity and fertilizer needs and to identify the soil properties determining maize yields. The results might be of interest not only for Surinam and neighbouring countries, but also for parts of West Africa and South East Asia where similar soils occur.

CLIMATE

According to K6ppen's classification, Surinam has a tropical rainforest climate. In the period 1931-1960, average monthly temperature at Paramari- bo Varied between 26.4 and 28.5°C, relative humidity between 75 and 85% and wind velocity between 1.1 and 1.7 m per sec. The distribution of rainfall over the year fluctuates strongly.

Fig. 2.

ram/month

300- Eo 250-

200-

150-

100-

50-

I ~ I i ] ; i i i i i i

0 J F N A N J J A S 0 N D

Average monthly precipitation (P) and free water evaporation (Eo) at Republiek, for the years 1906-1972 except 1912 and 1957.

Fig. 2 presents rainfall and free water evaporation data gathered at Repu- bliek, the nearest place to Coebiti (ca 35 km) where rainfall was measured over a long period. Free water evaporation was calculated by the method of L e n s e l i n k and V a n de r W e e r t s Four seasons can be distinguished: major rainy season, April-July; major dry season, August-November; minor rainy season, December-January and minor dry season, February-March. The transitions from one season to another are not sharp and particularly the minor rainy and minor dry seasons set in irregularly.

448 ]3. H. JANSSEN AND R. VAN DER WEERT

SOILS General

In the Zanderij formation Enti-, Oxi-, and Spodosols are found. The bleached white sands of the Zanderij formation consist for over 99% of

quartz, the unbleached sands have yellow and brown colours due to the presence of oxidized iron components. The clay fraction contains 85-90% kaolinite, about 5% goethite and low amounts of gibbsite, quartz and other minerals 2 7. Organic matter content varies between 0.5 and 4.0%, and pH- KC1 between 3.8 and 4.5. P-Bray I is mostly lower than 5 ppm P. Cation exchange capacity ranges from 1-6 meq, while the ranges for exchangeable Ca, Mg, K and N a are 0.3-2.0, 0.1-I. 0, 0.01-0.15 and 0.05-0.10 meq per 100 g soil, respectively. The cation exchange capacity depends largely on soil organic matter, the kaolinitic clay being of minor importance. The C/N quotient is 30-60 under savanna and forest 2 and 10-25 after some years of agricultural use. The amounts of potentially available soil moisture (between pF 2.0 and pF 4.2) are low, mean values for one meter depth being 46, 88 and 145 mm for white sands, brown sands and sandy loams, respectively.

The textures of the Coebiti soils, which mainly belong to the Oxisols, range from sands to sandy loams.

ln/luence o/mechanical [orest clearing on soil conditions

V a n de r W e e r t and L e n s e l i n k 10 11 and V a n de r W e e r t 9 have shown that soils can be severely compacted by mechanical clearing of forest like was practiced at Coebiti. This compaction causes a decrease in root penetrability, in aeration, infiltration capacity and water permeability and, hence, increases the hazards of erosion and water logging.

V a n de r W e e r t and L e n s e l i n k divided pore space into macropores ( > 180 microns), mesopores (30-180 microns) and mieropores ( < 30 microns). Based on the study of W i e r s u m 14 the volume of macropores was considered to be an index of mechanical impedance to roots. The volume of meso- and macropores was seen as an index of aeration under moist conditions.

S A N D LOAMY SAND SANDY LOAM control after clearing control after clearing control vo[.o/oaftcrcla~'}ng

30 30 30

20 20

10 10

[--0~o--]macropores, > 180 p

rncsopores,30-180 p

micropores, < 30 p

Fig. 3. Pore space and pore size distribution at 10-15 cm depth, outside (control) and inside (after clearing) the Coebiti experiment area.

MAIZE YIELDS ON THE SURINAM 'ZANDERY' SOILS 449

Compaction went mainly at the expense of macropores (Fig. 3). The degree of compaction varied widely over short distances, both in horizontal and ver- tical direction. Usually the 10-30 cm layer was most strongly compacted (Fig. 4).

Organic matter content, depth of topsoil and microrelief also varied widely over short distances, because during clearing locally topsoils had been removed and elsewhere piled up, treestumps had been buried and elsewhere holes of removed stumps had been filled with adjacent soil.

Fig. 4.

----_2 10

20.

x: 30

4 0

50-

60-

Bulk density (g/cm 3 ) 1.3 1.4 1,5 1.6 1,7 1.8

~ l i i p i l " , . \ \ . \ ',, o

2 ~ . . - . . . . . . . . .

- o . . J. i "4-. ~,

/ ~ / j )t.. Z / 1 / J ..... --'i

/ / / I'1 / ,," / i /1 J." l i o ' : ~A" f ',, I I . ~ / I ,,, ! i z '

/ ",,7, ; /

/ / )i ~ -

Bulk density profiles, determined at six points within an area of 10 × 10 m in the Coebiti experiment area.

RESPONSE TO FERTILIZERS

Experimental After clearing of the forest in 1969, the Coebiti experiment area was sown

with kudzu (Pueraria phaseoloides), which covered the ground in 1971, except on places where the soil had been compacted severely.

During the period Ju ly-November 1972 the response to urea, triplesuper- phosphate, muriate of potash and ground chalk was studied in nine 2 4 factorial experiments; 2 a factorial experiments with nitrogen, potassium and lime as factors were conducted between December 1972 and April 1973. In the latter period phosphate was applied as a basal dressing. Rates were 120 kg N, 90 kg P205, 120 kg K20 and 3000 kg CaCO3 per ha. After the first har- vest root development was assessed in the NPKCa-plots by counting the number of roots visible along the walls of profile pits.

450 B. H. J A N S S E N AND R. VAN D E R W E E R T

B e s i d e s , t h e p a t t e r n o f v a r i a b i l i t y in y i e ld s w a s i n v e s t i g a t e d b e t w e e n J a n -

u a r y a n d M a y 1973. T w o s t r i p s , e a c h a b o u t 3 0 0 - 4 0 0 m l o n g a n d 30 m wide ,

w e r e s u b d i v i d e d i n t o f o u r s u b s t r i p s w h i c h r e c e i v e d no fe r t i l i zer , P , N P K a n d

N P K C a , r e s p e c t i v e l y . R a t e s w e r e as m e n t i o n e d a b o v e . T h e s t r i p s w e r e h a r -

v e s t e d in p l o t s of 36 m 2 n e t a r ea .

Results

The variation in yields was very high. As far as could be seen in the field, low yields were found where soils had been compacted and deprived from topsoils and high yields where topsoils and organic remnants had been accumulated. The compacted areas partly coin- cided with former pathways of clearing machines. In general, however, the pattern of patches of poor and good growth was very irregular.

In nearly all experiments there was a significant response to phosphate (Tables 1 and 2). Especially compacted soils needed phosphate, mean yields being 50 kg without and 1000 kg grain per ha with phosphate application, respectively. Soil compaction reduces phosphate availability by impeding root growth.

Liming did not increase yields. In this, maize contrasts with groundnut, soyabean, eowpea and green gram, for which liming proved necessary 13.

Potassium increased yields more often on sands than on loamy soils, whereas nitrogen dressing proved more successful on loamy soils than on sands.

T A B L E 1

Mean responses (kg per ha) to fertilizers and mean highest yields obtained in the factorial experiments*

Sands Loamy sands and sandy loams

Mean response to N 49 315 P 380 1149 K 525 78 lime -- 10 341

Mean highest yield 2126 2794 Number of experiments 10 8

* Since s tandard deviations varied widely with fields, also values for least significant differences diverged widely, i.e. from 300-1200 kg per ha for individual fields (P > 10%),.

MAIZE YIELDS ON THE .SURINAM 'ZANDERY' SOILS

TABLE 2

Mean yields of maize in the strips (kg per ha)*

451

Sands Loamy sands Sandy loams

No fe r t i l i ze r s 692 e 405 ef 365 f

P 1835 c d 1806 c d 1502 d

N P K 2140 b c 2658 a 2609 a

N P K C a 2011 c d 2662 a 2487 a

N u m b e r of p lo t s 15 18"~ 13

* Values followed by the same letter do not differ significantly (P < 10~o).

When fertilized with NPK or NPKCa loamy sands and sandy loams yielded 400-600 kg more than sands.

The variation in both root intensity and rooting depth was large (Fig. 5). In some cases also in horizontal direction the root pattern was very irregular. Reduction in rootgrowth was severe at bulk densities of 1.58-1.64 g cm -3 or when the volume of macropores was less than 6-70/0 .

0 I0 20 , , 310 0 1

10 . . . . . . I

2oi-~ ~ r ....

c~ 30 ,rJ~ ~

40'

. . . . . . . . . _d I

m a x i m u m

Fig. 5.

number of roots per dm 2

I

Minimum, m a x i m u m and m e a n root d i s t r ibu t ion of nine fields, measured at 5 c m - d e p t h intervals .

The amount of potentially available soil moisture over the rooting depth ranged from 18-46 ram, means being 24 mm for sands and 36 mm for sandy loams.

YIELD-DETERMINING SOIL FACTORS

Experimental To inves t iga te to which soil factors t he large var ia t ions in yie!d were re-

lated, in t he N P K C a subs t r ips 55 sites were selected which were r ep re sen ta t - ive of good, mode ra t e and poor maize growth . These sites were two maize rows wide and 3 m long, t hus compris ing an area of 5.4 m e. I t was no t possible to f ind larger sites w i th un i form growth .

On each site compos i te samples were t a k e n of t he 0-20 cm layer. The soil


was analysed for organic carbon, cation exchange capacity and clay content. Organic carbon was determined according to the Walkley-Black method; organic matter content was assumed to be twice the value of organic carbon 6 The cation exchange capacity was determined by the ammonium-acetate method at pH 7. The clay content was estimated by comparing the samples with a series of soil samples of known textural composition.

On each site two additional core samples of approximately 400 ml each were taken at I0-15 cm depth, which were used to determine bulk density and micro-, meso-, and macropore volume.

Potentially available soil moisture (% by volume) was calculated as the difference in soil moisture contents at pF 2.0 and pF 4.2. The latter was estimated from the clay content 12.

Results

Some da t a ob ta ined at the sites are s u m m a r i z e d in Table 3. Many

soil pa r ame te r s were m u t u a l l y corre la ted (Table 4). Some relat ions are causal, e.g. bo th cat ion exchange capac i ty and avai lable soil

mois ture are l inked with organic m a t t e r and c lay content . CEC prov-

ed 8-9 meq per I00 g clay and 1.1-1.3 meq per g organic ma t t e r . The relat ionships be tween avai lable soil mois ture and organic m a t t e r

and c lay conten t have been discussed b y V a n d e r W e e r t and L e n s e l i n k 12

We decided to leave out of considerat ion soil p a r a m e t e r s like

CEC and avai lable mois ture which are depending on a m o u n t and qua l i ty of p r i m a r y soil const i tuents , and to confine the s t u d y to the

basic character is t ics : organic ma t t e r , t ex tu re and s t ructure . Table 4

shows tha t organic m a t t e r was highly s ignif icant ly and t h a t vo lume

TABLE 3

Values of yields and some soil parameters. Means per textural class and ranges for all sites

Mean values Range

Sand Loamy Sandy sand loam

Yield (kg per ha, 15% moisture) 2480 3094 3069 231-4800 Clay {%) 5.1 9.8 14.8 2-20

Organic matter {%) 2.2 2.6 2.7 0.95-3.75 CEC (meq per 100 g soil) 3.3 4.5 4.8 1.9 -5.6 Bulk density (g per cm a) 1.55 1.57 1.56 1.29-1.82 Maeropore volume (%) 5.0 5.7 5.4 0.2 -13.2 Meso- + macropore volume (%) 24.2 16.2 13.4 5.4 -35.6 Available moisture volume (%) 14.1 18.5 19.2 7.6 -25.1

MAIZE Y I E L D S ON THE SURINAM ' Z A N D E R Y ' SOILS 4 5 3

of macropores was significantly correlated with yield, while the correlation between clay and yield was almost significant.

Apart from mutual correlations, the investigation towards yield- determining soil factors was complicated by interactions and by non- linearity of some relationships. Especially soil texture proved to influence the relations between other parameters. Therefore soils were divided into three classes: sands, loamy sands and sandy loams (Table 3).

In Fig. 6 the relations between organic matter, volume of macropores and yields of maize are shown for the textural classes sand and sandy loam. For sands a close relation was found between yield and organic mat ter (left side, first quadrant), except for some points, indicated with dark dots. The same soils have also a relatively low macropore-volume (fourth quadrant), whereas their relation between yield and macropore-volume does not differ systematically from that found for soils of other sites (second quadrant). This might be inter- preted as follows. The beneficial influence of organic matter on yield is exerted at least partly through an increase in macropore-volume and hence in root penetrability. Therefore the relation between yield and organic matter cannot be maintained on the same level, when the relation between organic matter and macropore-volume has been disturbed by compaction.

TABLE 4

Coefficients of correlation between some parameters

No. Parameters 1 2 3 4 5

1 Yield

2 Clay 0.25 3 Organic mat te r 0.67 0.47 4 Bulk densi ty --0.24 0.05 --0.25 5 Maeropores 0.35 -- 0.07 0.30 6 Meso + macropores --0.07 --0.66 --0.28

--0.81 --0.63 0.59

P (R > 0.34) = 0.01 P (R > 0.26) ~ 0.05

For sandy loams the situation was different. There is no rg:!ation between organic-matter content and macropore-volume (second quadrant) ; a distinction can be made between compacted (dark dots) and not-compacted sites (open circles). They do not differ in the relation between organic-matter content and yield (first quadrant),


S A N O S

yie ld (tons per ha)

o o

° ~ •

i | i i i 8 4

macropor~-vo[um~: (%) >

o • •

° ° •

0'8 ' 116 ' 2'.4 ' J.~ ' ;2 • org.matter(°lo)

o ° o .

o o

o o

o o

o

o o

o

' ; ;.

rnacropore-votume (%)

SANDY LOANS

yield (tons per ha)

:'Jo • ./ * * * , o / ,

• / e

r ~ i p , i i , i i 0.8 1.6 2.4 ?.2

o r g . ~ a t t e r (%1

• •oa

>

o ° o 8

o o o

12

Fig. 6. Relations between organic-matter content, volume of macropores and yield of maize for sands and sandy loams.

and there is no relation between macropore-volume and yield (second quadrant). In these loamy soils the roots are apparently less dependent for soil penetration on the existence of macropores than they are in sands. In loams soil particles can be pushed aside more easily than in the more rigid matrix of a sandy soil. The higher organic matter content of the loams was a second cause of their higher deformability and hence better penetrability.

The relationship between certain parameters was not a linear one, which might be due to the existance of an optimum value or a threshold value of the relevant parameter. For sandy loams the meso + macropore volume seemed to be such a parameter, with a threshold value between 8 and 12% and an optimum range between 12 and 17%. Below this threshold, apparently aeration becomes the limiting factor ( c /Boeke l 1).

The sands have larger meso- + macropore volumes. If there was


an optimum range, which was not very obvious, it lies between 23 and 33% volume of meso- + macropores. At lower values not aeration, but the volume of macropores is becoming the limiting factor. At large volumes of meso- + macropores, likely the amount of available moisture becomes limiting.

Mathematical treatment

The above discussed phenomena of mutual correlation, interaction and non-linearity were responsible for the fact that neither multiple linear regression analysis, nor component analysis could set forth many new facts. Regression analysis showed once again the overrid- ing influence of organic matter, especially on sands. Component analysis indicated that the variance of yield was explained for about 65% by two components. The first component was linked with the variance of organic matter, clay, available moisture and CEC, the second with the variance of bulk density and the volumes of macropores and of meso- + macropores.

DISCUSSION AND CONCLUSIONS

The yields obtained in these experiments were characterized by a large variability and were in general low. A part of the variation could be accounted for by organic matter and related factors and by pore size distribution. Still much of the yield variation remained unexplained. Other factors than those included in the present study might have been of influence, like rooting depth and structure at other depths than the sampled 10-15 cm layer.

Phosphate proved to be the nutrient limiting yield most. A response to nitrogen might be obtained on loamy soils and to potassium on sandy soils. Liming had no effect on maize yield. Since no deficiency symptoms were visible (except of iron deficiency on bleached sands), it is not likely that other nutrients than nitrogen, phospho- rus and potassium were limiting, at least not at the present yield levels.

It is assumed that maize yields should surpass the level of 3 tons per ha to be economical in Surinam. To allow for such yields, Zan- derij soils should contain at least 2.5-3% organic matter which brings about a CEC of 4-5 meq per 100 g of soil. These values should not be considered as a guarantee, but as a minimum requirement. They


are in fair agreement with the 5-7 meq CEC (depending on growing season) which proved necessary to obtain 3 tons of maize yields on terrace soils along the Surinam river 5.

This requirement implies that nearly all sands and a large part of the loamy sands of the Zanderij formation are not suited for maize growing. After some years of cultivation the area suited for maize growing might be still more restricted, as the content of organic matter steadily decreases. This content was followed in six top-soils (0-20 cm) from August 1972 to April 1973. The average content of organic matter was 2.77, 2.30 and 2.22~o in August, November and April, respectively. This emphasizes the need of measures to conserve organic matter.

The low level of the yields might have been caused by lack of moisture. According to Da le and S h a w 4 maize yields depend largely on the number of non-moisture stress days in the period from six weeks before to three weeks after silking, especially if this number of days falls below 40. Analogous to the procedure described by D a l e and S h a w the number of non-moisture stress days was estimated for a series of increasing amounts of potentially available moisture (Table 5). In the present experiments the number of non-

TABLE 5

Number of non-stress days in the period from 6 weeks before to 3 weeks after silking of the maize, calculated for fixed amounts of plant available moisture

Plant available moisture (ram) 20 40 60 80 100 Non-stress days 1st period 23 29 31 32 33

2rid period 15 23 25 26 28

stress days was below 40, even for a potential available moisture content of 100 mm. As mentioned above the amounts of available soil moisture within the rooting zone varied between 18 and 46 mm. The corresponding number of non-stress days is low, so that moisture stress was at least partly the cause of the low yields and the yield differences between sands and loams. Higher yields might be obtained during a growing season from April-May till August-September, coinciding with the major rainy season. In that case the critical organic matter content might be less than 2.5-3%.

Related to moisture stress is the reduced rooting depth, caused by compaction. On the sandy soils, the negative influence of corn-


paction is brought about by a decrease of macropore-volume (increase in mechanical impedance). On the loamy soils also aeration might be limiting due to a decreased volume of meso- + macropores. On sands this volume was always more than 13%, so that aeration problems were not likely to occur there. It should be kept in mind, however, that the weather was relatively dry during the present trials. Under more wet conditions during the major rainy season, the meso- q- macropore volume that is critical for aeration, might be higher than 8-10°/o .

A C K N O W L E D G E M E N T

T h e a u t h o r s a r e m u c h i n d e b t e d t o M r s R . M. T j o n g - E n g - S o e - M o n s a n t o

( C e n t r e fo r A g r i c u l t u r a l R e s e a r c h in S u r i n a m ) , Mr . H . M a h e s h (Agr i cu l -

t u r a l E x p e r i m e n t a l S t a t i o n ) a n d c o w o r k e r s for t h e i r t e c h n i c a l a s s i s t a n c e .

Received 17 October 1975

R E F E R E N C E S

1 B o e k e l , P., Soil s t ructure and plant growth. Neth. J. Agric. Sci. 11, 120-127 (1963). 2 B r i n k m a n , R., A rapid reconnaissance of the main clay mineral provinces ill the

Guyanas . Geol. MJjnbouw 46, 446-452 (1967). 3 B r i n k m a n , R. and Ports , L. J., A pedo-geomorphological classification and map of

the Holocene sediments in the Coastal Plain of the three Guianas. Soil Survey Papers No. 4. Soil Survey Inst . , Wageningen, The Netherlands (1968).

4 Da le , F. and S h a w , R. H., Effect on corn yields of moisture stress and s tand at two fertility levels. Agron. J. 57, 475-481 (1965).

5 J a n s s e n , B. H., Onderzoek naar de vruchtbaarheid van enkele terrasgronden langs de Surinamerivier. CELOS rapporten 91, Paramaribo, also: Interne Meded. Lab. Landbouwscheik. 13, Wageningen (1973).

6 J a n s s e n , B. H., Short investigations on methods of soil analysis 1. Organic mat ter . CELOS bull. 18, 47 (1973).

7 K r o o k , L. and M u l d e r s , M, A., Geological and related pedological aspects of the Upper Coesewijne formation. Geol. Mijnbouwk. Dienst Suriname Meded. 21, 183-208 (1971).

8 L e n s e l i n k , K. J. and V a n de r W e e r t , R., Es t imat ing Iree water evaporation in Surinam. Sur inaamse Landbouw 21, 70-79 (1973).

9 W e e r t , R. v a n der , Influence of mechanical forest clearing on soil conditions and the result ing effects on root growth. Trop. Agric. (Trinidad) 51, 325-331 (1974).

10 W e e r t , R. v a n d e r and L e n s e l i n k , K. J., The influence of mechanical clearing of forest on some physical and chemical soil properties, Sur inaamse Landbouw 20, 2-14 (1972).

11 W e e r t , R. v a n d e r and L e n s e l i n k , K. J., The influence of mechanical clearing of forest oil p lant growth, Sur inaamse Landbouw 21, 100-111 (1973).

4 5 8 MAIZE YIELDS ON THE SURINAM 'ZANDERY' SOILS

12 Wee r t , R. vail de r and L e n s e l i n k , K. J., Moisture characteristics of a number o5 interior soils in Surinam. Surinaamse Landbouw 22, 13-22 (1974).

13 Wienk , J. F., Observation plots of soyabeans, cowpea and green gram. Groundnut trials. CELOS bull. 28, 17-21 (1975).

14 W i e r s u m , L. K., The relationship of the size and structural rigidity of pores to their penetration by roots. Plant and Soil 9, 75-85 (1957).

the influence of fertilizers, soil organic matter and soil compaction on maize yields on the surinam...

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