The process of soil restoration at Project Wadi Attir draws on a variety of fascinating techniques and tools. To answer this question about what is possible, let us consider the following sub-questions…

Are dryland soils notoriously infertile and useless for agriculture?

Not so! Many dryland soils can be highly productive when properly managed, and are resilient to disturbances and degradation. Some areas of the ‘Fertile Crescent’ have been under continuous agricultural exploitation for 10000 years. Many sandy and loess soils in dry areas are highly suitable for agriculture. In combination with fertilization and adequate soil organic matter content excellent farming soils can be created and maintained due to high water and nutrient binding capabilities. Nevertheless, ongoing mismanagement and overexploitation without the necessary care leads to degradation of such soils and massive erosion, both in the Middle East and China.

Can the application of synthetic fertilizer restore soil fertility and productivity?

Synthetic fertilizers are only of limited use in restoring fertility to degraded dryland soils. Due to lacking soil microbial life and SOM, nutrients are not biologically fixed and remain susceptible to erosion or washing out. Phosphate binds strongly to top-soil particles and is easily lost by wind or water erosion, while nitrate does bind poorly to soil particles and is washed down to deep soil, not accessible to the roots of annual vegetation.

 Example of Industrial Agriculture

Example of Industrial Agriculture

Only enrichment of SOM (Soil Organic Matter) together with nutrients leads to biological fixation and mobilization of those essential nutrients by microorganisms for the subsequent use by plants. Soil restoration for intensive irrigation agriculture is therefore best done by tilling of manure or compost into the topsoil whereby fertile humus with functional soil microbiology can be restored. This however does not work for rain-fed dryland soils because nutrients are deposited too deep to be reached by plant roots, and tilling will cause further damage to soil structure and induce erosion.

Can overlaying dryland soil with compost or manure restore soil fertility?

Degraded dryland soils differ fundamentally from other soil categories and therefore require different treatment to overcome specific problems. Dryland soils are often crusted, either as the result of degradation or because of microbial soil crusts. In both cases, water infiltration is low, and water runoff is high, and the soil surface exposed directly to the intensive sun dries out rapidly. Any kind of organic soil cover, leaf litter, compost or manure over-laid on dryland soil will enhance water retention, allowing it to infiltrate instead of running off. Such litter layers also protect the soil from heating up due to direct radiation thus reducing evaporation. Furthermore limiting amounts of precipitation often penetrate only a few cm into the top soil so that deep lying nutrients are not accessible to plant roots. Plant litter or manure layers on top of the soil will release their nutrients in accordance with available moisture, and provide those nutrients exactly where soil moisture is available and where plant roots are developing.

Fig. 1: Express soil and productivity restoration at Project Wadi Attir: the green, highly productive area in the middle was created by over-laying of excess compost in 2013. The good 2014/15 winter rains transformed this area into a highly productive mixed grass-land, while the control areas in front and in the back remain essentially unproductive.

Because organic matter decomposes slowly under dry conditions, organic litter cover has a pronounced long term effect on soil fertility. A site over-laid with manure at Project Wadi Attir displays ten-fold productivity compared to nearby untreated degraded soil since at least eight years. Similarly Acacia woodland nearby with a closed leaf litter layer displays ten-fold productivity compared to nearby degraded shrubland. A natural status of a closed organic layer can be slowly restored by recovering dryland vegetation if grazing, wind and water erosion are properly controlled. The plot shown in Fig. 2 in the same climate zone has been recovering without disturbance for at least 15 years and has now achieved a state of maximal productivity.

Fig. 2: spontaneously restored highly fertile dryland top-soil thanks to accumulation of a layer of decomposing grass litter. Similar to the examples above this soil has 5 – 10 times higher productivity that surrounding degraded plots.

Can farmed dryland soils be restored while in continuous use?

It is possible to do so, however traditional extensive wheat cultivation in drylands is highly destructive. This has likely contributed to massive expansion of the Sahara thousands of years ago. The same mechanism, mismanagement of dryland soils has caused the Dust Bowl phenomenon in the American Great Plains during the 1930ties. Most of the Negev’s wheat growing areas are profoundly degraded due to poor management and repeated unnecessary tilling. Tilling now as then has a large number of negative impacts such as:

  • Compaction of soil below the depth of tillage (formation of a tillage pan);
  • Crusting of soil when soil pulverization is followed by rain, stimulating weed seed germination and inhibiting crop emergence;
  • Increased susceptibility to water and wind erosion associated with residue removal and soil loosening;
  • Accelerated decomposition of organic matter, which is undesirable from a long-term perspective;
  • Destruction of soil microbial life and soil nutrient cycling
  • Cost of equipment purchase and operation;
  • Energy cost of tillage operations;
  • Labor and temporal obligations; and
  • Alteration of the soil food-web, shifting populations away from larger, longer-lived organisms to smaller, shorter-lived organisms.

No till agriculture, is capable to provide the same impact on farmlands, as litter cover does in rangelands. Both approaches restore proper soil structure, and enhance PUE, higher soil nutrient cycling and SOM. The guiding principle here is to leave sufficient waste biomass (normally 1/3 to ½ of the straw) on the field to establish an organic matter layer for continuous soil improvement. No-till agriculture in the Northern Negev is reported to enhance productivity up to two-fold and was especially effective during drought years.

30 Years of No-Till Conservation Tillage at Utah State University

If the fields are not cultivated, sowing a (best nitrogen fixing) cover crop such as clover will further assist in restoring such farmland to an enhanced state of maximal rain-fed yield with high SOM content and nutrient inventories. Worldwide application of such dry farmland restoration technologies could contribute significantly to food security, and sequester huge amounts of the greenhouse gas carbon dioxide into restored soil.

Fig. 3: Structure of tilled versus no till soil

How can tree planting be used to restore soil fertility in degraded drylands?

Adequate dryland tree species are not only helpful in restoring soil fertility, they are actually essential in long-term maintenance of soil fertility and ecosystem stability in drylands. Among the many services provided by trees, the most crucial ones in this respect are the following:

  • Recovery of nutrients from deep soil that are subsequently contributed as leaf litter to enrich the top soil is essential to return essential minerals, especially nitrate to the top soil, that can be washed into the deep soil layers by occasional strong rains.
  • Trees strongly reduce wind intensity and raindrop intensity to reduce both wind and water erosion of both soil and organic material.
  • Leaf litter shed by trees contributes to building a litter and compost layer, adding fertility to top soil, enhancing water infiltration and reducing evaporation and water runoff.

However, tree species planted, and planting technologies must be carefully selected to avoid grave mistakes made in the past. Eucalyptus trees planted do very little for soil improvement since their leaves suppress germination of annual vegetation. Furthermore Eucalyptus and other fast growing dryland trees deplete top soil humidity and thus out-compete other vegetation. The most suitable trees for soil improvement approaches are rather slow growing nitrogen fixing trees with easily decomposing leaves low in allelochemicals, such as Acacia, Carob or Albizia.

 Acacia victoria stabilizing an erosion gully

Acacia victoria stabilizing an erosion gully

Planting technologies applied also must be carefully weighed. The widely used contour trenching technique can be specifically harmful, as large scale soil movement destroys all existing soil structures, reduces soil organic matter and nutrients and finally creates large areas of crusted and degraded soil enhancing water runoff and soil erosion.

Adequate soil conserving tree planting technologies are thus the most critical considerations when considering soil restoration by trees in drylands. The use of drip irrigation is the preferred technology for rapid establishment of tree populations, wherever irrigation water is available. A number of dryland trees, especially Acacia species, can be planted into small ditches by application of a few buckets of water, or during the rainy season, and normally manage to survive the critical first dry season.

Try this quiz and test your knowledge of the question:

How can degraded dryland soils be restored?

Utilizing the information presented in this section, do you best to answer the following questions to test your knowledge of this topic question.