Soil Water Holding Capacity

What is water holding capacity?

A soil is a sponge with pores of every size. After a heavy rain all of them fill first, then water drains from the coarse ones back down. What stays in the medium and fine pores is the store plants live on in the weeks that follow. Water holding capacity describes the size of this store: how much water a soil holds against gravity, and how much of it roots can draw back out.

In a climate with a dry summer it is one of the most important properties of a site. The water balance shows when rain and evaporation cancel out and when the account turns negative. The soil is the account in between. The more it carries over from the rainy season, the longer plants get through the summer without watering.

Field capacity and wilting point

Two limits define the store. Field capacity is the amount of water a well-drained soil holds against gravity, that is, what remains once downward drainage after a soaking rain has markedly slowed. The wilting point is the water content at which plants wilt permanently. Below it there is still water in the soil, but it clings so tightly to the soil particles that roots can no longer extract it.

What lies between the two limits is what FAO, in its guidelines on crop evapotranspiration, calls the total available water. It is also known as available water capacity. It is expressed as a depth of water, in millimetres per metre of soil. One millimetre is one litre on every square metre.

The store in numbers

FAO calculates available water from the difference between the water content at field capacity and at the wilting point, multiplied by the rooting depth. In its worked examples a loamy sand holds about 15 percent of its volume as water at field capacity and about 6 percent at the wilting point. The difference gives around 90 millimetres of available water per metre of soil. A silt holds about 32 and 15 percent and so reaches around 170 millimetres, a silty clay with about 35 and 23 percent around 120.

Plants cannot use all of this store. The drier the soil becomes, the more tightly it holds the remaining water, and long before the wilting point plants begin to close their stomata and grow less. FAO calls the part a plant can use without stress the readily available water. Its share lies roughly between 0.3 and 0.7 depending on the plant and the weather; for many crops 0.5 is used. Of 90 millimetres in a loamy sand, around 45 millimetres remain before growth suffers.

The second quantity is rooting depth. In one FAO example an onion roots 0.4 metres deep, maize 1.2 metres. In the same soil the maize therefore draws on three times the store. For a tree that can root deeply, the whole rootable depth counts; for a seedling, only the hand's breadth it has reached so far.

What sets the store

Texture. It sets the order of magnitude. Sand has coarse pores from which water drains quickly. Clay has very fine pores and holds a lot of water, but a large part of it below the wilting point. Silt-rich soils hold the most plant-available water. In the analysis of the US soil database by Libohova and colleagues, silt content was the property most strongly related to available water. The texture-by-feel test determines texture by hand.

Structure. Crumbs create pores between the mineral particles, and a loose soil structure lets water and roots go deeper. In the same analysis available water rose as bulk density fell, but the relationship was weak and varied widely. The effect of soil compaction on depth is clearer: a dense layer stops roots, and whatever lies beneath it no longer belongs to the usable store.

Stones. Rock fragments hold almost no water. The US Natural Resources Conservation Service assumes that they reduce available water in proportion to their volume, unless the rock itself is porous. A soil that is one third stones holds, by calculation, only two thirds of what its fine earth alone would hold.

Depth. A shallow soil over rock has a small store even with the best texture. Texture, stone content and rootable depth together decide how many litres per square metre are available in the end.

Humus and water: what the studies show

Few sentences about soil are repeated as often as this one: one percent more humus stores so many litres of water per hectare. A Cornell University page gives 20,000 US gallons of plant-available water per acre for every percent more organic matter, which converts to about 187,000 litres per hectare, or almost 19 litres per square metre. No soil depth is given. The measured data paint a much smaller picture.

In a 2018 meta-analysis, Budiman Minasny and Alex McBratney evaluated 60 published studies and large databases with more than 50,000 measurements. One percent more organic carbon, that is 10 grams per kilogram of soil, raised available water by 1.16 millimetres per 10 centimetres of soil on average. Three quarters of the studies lay between 0.7 and 2 millimetres. Over the top 30 centimetres that comes to about 3.5 litres per square metre. The effect was largest in sandy soils, smaller in loams and smallest in clays. The authors consider the claim that building up soil carbon can substantially enlarge the soil's water store questionable.

A group led by Zamir Libohova of the US Department of Agriculture, working with the database of the National Cooperative Soil Survey, reached the same conclusion. At contents of 0 to 8 percent organic matter, the range of most farmland, available water was only weakly related to it. By calculation, one percent more organic matter gave about 1.5 to 1.7 percent by volume more available water. The authors put this at around 10,800 litres per acre in a layer 15 centimetres thick, which converts to almost 2.7 litres per square metre. Here too the effect was stronger in sandy soils than in silty ones.

The two studies measure slightly different things, organic carbon in one case and organic matter in the other, and cannot be compared figure by figure. Their direction is the same: humus enlarges the water store, but by a few litres per square metre, not by dozens. On top of this, one percent more humus in the topsoil builds up over years and decades, and in a warm climate rather more slowly.

None of this makes humus less valuable. Its greatest effect on water lies elsewhere. It stabilises crumbs, keeps the surface open and helps rain soak in instead of running off; Libohova and colleagues name better infiltration and less erosion explicitly. A store that is never filled is of no use. In sandy soils, where the store is small, the direct contribution of humus also counts most.

Reading the store in the field

Texture. Texture by feel tells whether a soil is rather sandy, silty, loamy or clayey. With that, the order of magnitude of the store is known before any laboratory figure arrives.

Depth and obstacles. The spade test shows how deep roots reach, where a compacted layer turns them aside and how many stones are in the soil. A soil auger or a deeper hole in a few places adds to it.

Moisture. A few days after a soaking rain, once the excess water has drained away, the soil is close to field capacity. The feel and appearance method of the Natural Resources Conservation Service judges a handful of soil by how it looks and whether it can be squeezed into a ball. Soil that leaves a moist imprint and keeps its shape still holds plenty of water; soil that crumbles and runs through the fingers has given up most of it. Repeating this at the same depth over a few weeks shows how fast the store empties.

What protects the store

The size of the store changes only slowly. How full it gets and how long it lasts can be influenced at once.

Filling. Water that runs off the surface never reaches the store. Covered soil with open pores lets rain soak in; bare, sealed soil sends it down the slope.

Covering. A layer of mulch shades the surface and slows evaporation, so that the water stays for the roots. Mulch in place at the end of the wet season carries soil moisture further into the summer.

Avoiding compaction. Wet soil is at its most vulnerable. Driving on it then, or walking the same line again and again, presses pores together and takes depth away from roots.

Keeping shade. For cutting, this wiki follows a rule that comes from the water balance: we prune in the rainy season, from the first heavy autumn rains into early spring; in summer every bit of shade stays. The cuttings from chop and drop thus become mulch before the summer arrives.

How Terra Humana understands water holding capacity

For Terra Humana, water holding capacity is the quantity that decides how long a hillside carries itself through the summer. Texture, stones and depth are given, and we read them with texture by feel and the spade test before we plant. What we can influence is how much rain gets into the soil and how much of it stays into the summer: through cover, mulch, open pores and shade. We build humus because it makes the soil alive and permeable. As a water store, we promise of it only what the measurements show.