Soils with more than 20% gravels will be gravelly soils to a considerable extent , with leaching  and  percolation  of applied nutrients and water as maximum. but , in such soils , fruit crops come up very nicely , with little bit of care on soil surface remaining covered with cover crops , sufficient organic manuring , deep rooted and shallow rooted crops alternate , lots of options to optimise the crop  productivity 

Similar problems, we face in some shallow depth soils of central India where fruits crops come so well , simply because of two reasons, better soils to be used for food grains and such soils of marginal nature should be brought under active agriculture.We add lot of mulches alongside cover crops , select perennial fruits requiring comparatively lesser water, drip irrigation etc..but conceptually so well suited for organic farming or even conservation agriculture , depending upon other resources available or not..

in general the increase in gravel% in the soil means decrease in weight of clay per a certain weight of soil or per certain volume or area(hectare  ,square meter ...etc) since the clay and fine silts are the source of essential nutrients for plants  ,it means  increase in gravel causes decrease in nutrients per area  or per certain volume of the soil which causes decrease in plant growth and development. On the other hand some times  (under drought condition or water stress condition) gravels on soil surface causes  decrease in evaporation which had positive effect on plant growth. 

The gravels are not considered as soil separates. Thus, its direct effect on plant nutrition and growth is expected to be minimal. However, it can indirectly affect crop or soil productivity. For example, its presence on the soil surface can reduce water erosion caused beating action of the rain droplets, or can increase surface roughness to prevent wind erosion. Nevertheless, root zone containing significant amounts of gravels can interfere with the root penetration, especially in shallow soils.

Soils with more than 20% gravels will be gravelly soils to a considerable extent , with leaching  and  percolation  of applied nutrients and water as maximum. but , in such soils , fruit crops come up very nicely , with little bit of care on soil surface remaining covered with cover crops , sufficient organic manuring , deep rooted and shallow rooted crops alternate , lots of options to optimise the crop  productivity 

The surface gravel can protect the soil but in depth the generic development of the roots.

Trees and vines in particular are suitable for this type of soil

We also need soil and water conservation practices to operate effectively in such soils to do well , regardless of crops...

The size of the gravel may also interfere with this.

Small gravels can improve the porosity and aeration of heavy soils.

Not all gravels are the same.  Quartz gravel will have (close to) zero available water holding capacity, while ironstone gravel can have considerable available water holding capacity.  See our paper (Brouwer & Anderson) on ResearchGate.  Gravel originating from weathered sedimentary rocks may also be porous.  You may want to check the water holding characteristics of the gravel in your soils.

Good point  Brouwer. Like in smectite dominated soils developed on basalt , most of the Entisols have weathered basalt ( more than 20-30%gravels), but citrus come exceedingly well and qualitywise on par... 

Presence of too much gravel, interfere with land preparation for annual crops. In such areas, the most economical agricultural activity is fruits orchards and agroforestry. in arid regions, where water scarcity is common, a gravel mulch would significantly reduce soil water evaporation and conserve water in the soil. If the soil is saline, and the environment is arid, xerophytes and halophyte shrubs and trees with economic use (agroforestry, fruit, oil seeds, etc.) is an option, even in gravelly soils.

Good luck.

In arboriculture and in the case of heavy soils the presence of gravels can improve the infiltration of the water and facilitate the drainage

Dear Professor Sidi Mohamed Ousmane

is the gravels present in heavy clay soil or correlate well with sand light soil

there are two distinct conditions , gravels distributed more or less uniformly within entire soil profile or alternatively heavy textured top soil is  underlained by gravelly layers..

Thank you all colleagues for informative comments.

if the gravelly land is hilly and the slope is too high for economic land leveling, fruit orchard and agroforestry, is more applicable than annual crops.

Dear colleagues,

the work of Marion Tetegan available below will  be very usefull to answer.

Best regards.

JJ

Article The contribution of rock fragments to the available water co...

Conference Paper The role of rock fragments in soils hydric properties

Article The effect of soil stoniness on the estimation of water rete...

Article Modeling of water retention properties of stony soils : appl...

It is also possible to envisage intercropping between the lines of the trees

The following project document /report probably answers some of the questions raised.I find the publication interesting.

The impact of gravel on crop management-a desktop study By Bowden,B. and Ambasador,WMG  www.coggo.net.au/..../9.3.5 The impact of gravel on crop management final report May14.pdf

Please find enclosed some interesting work...

Soil profile gravel concentration and its effect on rainfed crop yields (Plant and Soil 81(1):75-83 · February 1984,DOI: 10.1007/BF02206890

Abstract : Soil characteristics in the crop root zone are critical to soil water and nutrient availability to rainfed crops and determine crop production in coarse textured soils. A four-year field study was conducted in the foot-hills of North Himalayas near Chandigarh (India) on a coarse textured soil (Gravelly udic ustocrepts) to evaluate the effect of varying soil profile gravel concentration on the yield of rainfed crops of Taramira (Eruca sativa Mill.) in winter followed by maize (Zea mays L.), sorghum (Sorghum vulgare Pers.), cowpea (Vigna unguiculata L.) and sesamum (Sesamum indicum L.) in summer. Taramira gave a mean grain yield of 683, 410 and 275 kg ha–1 at gravel concentration (GC) of 18, 28 and 40 percent by volume in the surface one metre soil depth. The grain and forage yield of summer crops decreased with the increasing GC. The gross monetary returns decreased in the order: Sorghum fodder, cowpea, sesamum and maize. The dilution of soil mass with increasing GC and corresponding decrease in nutrient and water holding capacity of the soil appears to have depressed the crop yields. The results indicated that the legume which can also conserve rainwater with dense canopy like cowpea or crops having vigorous fibrous root system and are relatively drought tolerant like sorghum may provide better economic returns in light textured soil containing gravel upto 40 percent

Yes Dr. Anoop, soil characteristics in the crop root zone are critical to soil water and nutrient availability in the graveled soils which may affect plant growth and development. We need to have cost effective practical solutions.

New term nowdays we frequently used , called rhizosphere , the properties of which  are so important for a plant to perform optimally ..instead of worrying about bulk soils..