H HARDENING a standardized implement (penetrometer) as it is pushed into the soil divided by the cross-sectional area of its tip. Hardening or induration of a soil takes place by the loss of Introduction void space by compaction or filling with fine materials. Hardpans, hard layers, or compacted horizons, either surface or subsurface, are widespread problems that limit Bibliography crop production. Hard layers can be caused by traffic or soil Chesworth, W. (ed.). 2008. Encyclopedia of Soil Science, Springer, genetic properties that result in horizons with high density p. 303. or cemented soil particles (Hamza and Anderson, 2005); these horizons have elevated penetration resistances that limit root growth and reduce water and airflow. Limited root HARDPAN growth leads to limited crop water and nutrient uptake. Reduced water flow prevents rainfall or irrigation water from filtering into the soil profile where it can be stored A compacted, impermeable layer of soil at or near the for plant growth. Reduced airflow limits oxygen and carbon surface. dioxide exchange with the atmosphere; exchange is needed for plant and microorganism respiration. These limitations reduce crop productivity. HARDPAN SOILS: MANAGEMENT* Improving the hard layer consists of reducing its hardness or penetration resistance. When we reduce the ’ Warren J. Busscher layer s hardness, we assume that it and/or the layers below Agricultural Research Service, US Department of it have properties conducive to plant growth. As the hard Agriculture, Coastal Plain Soil, Water and Plant Research layer softens, water and air are able to move into and/or Center, Florence, SC, USA through it and into the layers below, improving conditions for root growth and with its productivity. There are several ways to improve hard layers; the most common is tillage; Synonyms but other solutions exist in the forms of water/crop manage- Hard-layer soils; Management of hard-layered soils ment and soil amendments. Definition Tillage Hardpan soil. A soil that has a layer whose physical char- Tillage has been and is the common method used to reme- acteristics limit root penetration and restrict water diate hard-layer problems; it physically breaks up hard movement. layers. Tillage by hand involves digging with a spade, Penetration resistance. The penetration resistance (or soil broad fork, or U-fork. In large-scale mechanical agricul- strength) is usually measured as the force exerted on ture, tillage involves using a tractor to pull any of a number of tines or shanks through the soil. In the *All rights reserved mechanical method, shallow hard layers (<5 cm) can be Jan Gliński, Józef Horabik & Jerzy Lipiec (eds.), Encyclopedia of Agrophysics, DOI 10.1007/978-90-481-3585-1, # Springer Science+Business Media B.V. 2011 358 HARDPAN SOILS: MANAGEMENT broken up with tines or cultivators that disrupt the surface performed repeatedly at prescribed frequencies, often sea- soil. Deeper hard layers (>15 cm) can be broken up with sonally or annually. Frequent tillage can be expensive shanks. Shanks are sized or adjusted so they are pulled because it often requires large tractors (14–20 kg weight through the soil at the depth of the hard layer shattering per shank), 20–40 min haÀ1 of labor, and 20–25 L haÀ1 it and decreasing its resistance to root growth. Different of fuel. Eventually, the producer has to make the decision shank designs that are manufactured by various tillage whether or not to till based on the value of increased yield companies produce different results or work with different by tillage vs. the cost of tillage (Bolliger et al., 2006). efficiencies depending on the type of hard layer and the In an effort to save time, fuel, and production costs, type of soil. Consider the example seen in Figure 1 where deep tillage studies have included soil disruption on the hard layer was located in loamy sand between 20-cm a multiple-year rotation. In many cases, not tilling every and 40-cm depths. Tillage in this example was performed year reduces yield to levels that may (or may not) be with an older 5-cm thick shank that produced wider zones acceptable given the increase in fuel costs. Additionally, of disruption and used more energy than narrower shanks. annual deep tillage may not be needed for some crops, Also seen in Figure 1, the process that reduced soil pene- such as cotton, to maintain yields. Deep tilling every tration resistance under the row increased it under the traf- 2–3 years may be just as effective as deep tilling annually ficked mid row because of the tractor weight. (Busscher et al., 2010). This will depend on the crop and To meet conservation goals, deep tillage such as that variety grown, amount of re-compaction, and other crop shown in Figure 1 can be performed in such a way that it management techniques such as row width and traffic/ does not invert soil; equipment companies have developed compaction patterns. shanks that break up soil with minimal surface disruption. Another effort to save fuel and production costs involves Non-inversion tillage leaves most crop residue on the soil varying the tillage depth. Deep tillage is often performed surface protecting it from erosion, surface crusting/compac- with implements set to a fixed depth. But depth to the tion, and excessive evaporation (Raper, 2007). Though compacted layer varies throughout a field. What depth early studies with non-inversion and reduced tillage demon- should the implement have? On the one hand, if tillage depth strated little or no yield advantage, improvements in is based on the deeper zones of the compacted layer, the planters, residue management, and soil/crop management implement disrupts too much soil where the compacted layer practices increased the success of conservation systems by is shallow; this wastes fuel. On the other hand, if tillage optimizing factors that affected seed germination and vigor. depth is based on the shallower zones, the implement will The problem with tillage is that the reduction of pene- not disrupt the whole compacted layer, leaving hard zones tration resistance is temporary. For some soils, temporary that limit root growth. Technologies are now available that means a few to several years. For others, it can mean only allow tillage to vary with the depth of the compacted layer; a few months (Raper et al., 2005a). Most often it is this can be accomplished by mapping the hard layer of effective for only months. In either case, over time, soil a field or placing sensors on the shanks. Shanks are then reconsolidates leading to reduced water/airflow, reduced raised and lowered as needed. This action can save energy root growth, and lower crop yields (Håkansson and Lipiec, without sacrificing crop yields. Research has shown that this 2000). Even if the reconsolidated soil’s penetration resis- “site-specific tillage” produced yields equivalent to those of tance is not as high as it was originally, it can be high uniform deep tillage while reducing tractor draft forces, enough to limit growth. As a result, tillage has to be drawbar power, and fuel used (Raper et al., 2005b). nonwheel track Other solutions in-row Wheel track mid row mid row Soil Organic Matter: For the past few decades, soil scien- 0 tists and producers have been trying to increase organic 3 matter levels in soils (Carter, 2002). This improves fertil- ity, decreases strength in hard layers (especially those 2 close to the surface), and increases yield (Soane, 1990). 1 But with the increase in fuel prices comes the need for 30 organic matter/residue in the form of cellulose. The same 0 organic matter that scientists and producers were trying Soil depth (cm) Penetration to increase in soils may be removed to produce ethanol. resistance Both increased organic matter and removed cellulose (MPa) might be attainable; but only after some research. 60 Research on organic matter removal had started during the 1970s fuel crisis; but because the crisis did not con- Hardpan Soils: Management, Figure 1 Soil penetration tinue, the research priority decreased as funding ceased. resistance for a loamy sand that has a hard layer at 20- to 40-cm Results from the 1970s showed that some residue could depths. The soil was tilled to a depth of about 45 cm with be removed provided that nutrients were replaced with shank that did not invert the soil. fertilizers. The problem with this finding is that fertilizer HARDPAN SOILS: MANAGEMENT 359 production requires large amounts of energy. The previ- in the profile, the amount of PAM and its mixing into the ously unfinished research has resumed asking questions soil will cost several hundred euros per hectare. Given about the sustainability, economic efficiency, energy effi- the high cost of fuel, this cost might be feasible if the ciency of residue removal, and the effect of the removal PAM could last multiple years. Current estimates have on soil properties such as penetration resistance. the PAM breaking down at a rate of 10% per year. To ameliorate hard layers, additions of organic matter Another amendment that has attracted attention in need not come from crop residues. Another way to add the past few years is biochar. Biochar captured the it, especially to subsurface hard layers, is through root attention of the agricultural community as a result of growth (Yunusa and Newton, 2003). In this method, cover archeological/agricultural findings of charcoal-amended crops are grown between growing seasons aimed at pene- soils in the Amazon and other historically old areas. trating the hard layers with their roots. Cover crop roots Charcoal- or biochar-amended soils were found to have are able to penetrate soil where production crops cannot supported larger populations 500–1,000 years ago than either because conditions between growing seasons are previously estimated and today they are still more produc- different, for example, cold and wet, or because the cover tive than expected.