The editor of this magazine asked that I put together a back-to-basics-type series of articles to start the fall off right. Here’s a refresher on the basics of soil composition and some real-world examples of why it matters (see sidebars). This is the first of three articles on the topic. Next month we’ll talk about soil zones, organic matter and microorganisms.
Just what is soil, anyway? In any give pound of topsoil about five per cent (give or take, depending on the soil zone) will be organic matter and the other 95 per cent is mineral material. Soil texture is nothing more or less than the amount of sand, silt and clay in the mineral portion of the soil.
Sand has a particle size of two to 0.05 mm (millimetre) in diameter; silt has a particle size from 0.05 to 0.002 mm in diameter; and clay has particle sizes less than 0.002 mm in diameter.
We all know that a sand pile does not make a good agricultural soil; clay is king when it comes to rating soil because the percentage of clay particles determines the moisture and nutrient-retention capacity of a soil (see Figure 1 below). sucked all it can. Air-dry soil only occurs at the soil surface in the first few inches.
2) Wilting point (WP): This is the water left after plants have sucked up all they can.
3) Field capacity (FC): This is the water present after a rain and after enough time for the large pores to empty. A sandy soil will come to FC in a day or two but a heavy clay soil will require several days to come to FC. A good commonsense test is this: when the field is able to be driven on after a heavy rain, the soil has come to FC. In 2010, there were many fields that did not come to FC when spraying HAD to be done.
4) Available water: This is the amount a soil holds between FC and WP.
In sandy soils, this is about one inch of available water for each foot of soil that is at field capacity. In loams and clay loams, there is about 1.5 inches of available water for each foot of soil that is at field capacity, and clay soils hold about two inches of available water for each foot of soil that is at field capacity.
The amount of available water that is present at seeding is the key piece of information that determines the yield potential of crops over much of the Palliser Triangle (the Brown and Dark Brown soil zones) of Saskatchewan and Alberta. In 2010 that was not true, but most years, it is.
5) Saturation: This is the water content when all pores are filled with water (i. e, at the height of water table). In 2010, we got to see a lot of the water table; when water is laying in a field that is the water table. We often say that the sloughs “dry up.” For the most part, sloughs do not dry up — they simply drain. If the water sits there for a long time it means that the water table is high. The water in the slough will not disappear until plants use up water round abouts and the overall water table drops. I observed that in spades this year on Blackstrap Farm
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Look past the top four feet of soil
Farmers and soil scientists alike usually think very shallow. The soil is four feet deep and anything below that will not affect us, right? Nonsense. This year provides a good example of how soil moisture is affected by depths greater than the root zone.
In the Nipawin area of Saskatchewan the Nipawin Loam soils are a special kind of Grey Black soil with special drainage qualities. The soil is a few feet of loam soil on top of approximately 40 feet of sand. The sand sits on clay that slopes to the Saskatchewan River, and the water table that forms in the sand above the clay drains to the river as springs in the valleys that drain to the river. So, when excess water comes it simply increases that spring flow and soil stays “farmable” and fieldwork can resume. In 2010 internal drainage characteristics of the Nipawin soils separated them from nearby Weirdale or Carrot River soils where no such internal drainage system exists.
Mother Nature is great, but we must understand how she functions if we want to play this game called farming.
Clay content is used to define texture groups as follows: <15% Clay = Sand 15%-25%Clay = Loam 25%-40% Clay = Clay Loam 40%-60 % Clay = Clay 60% Clay = Heavy Clay
The myth of shallow rooted crops
How many times have you heard that a wet spring means shallow-rooted crops, and that these shallow roots will fry when the rain stops? It’s simply not true. I’ve been following my 2010 wheat crop’s root development with the soil probe every week or so. The roots were down to a foot or so before tillering was done and have advanced steadily, even though steady rains have meant no moisture stress. At the time of writing at the end of July, the crop is done flowering and roots are down four feet deep with soil at field capacity (except for the top few inches).