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Rural Diary
1 April 2020 Jay Howes

Soil as a reservoir

When soil moisture is discussed, certain terms are used to describe its status, but what exactly do they mean? 

Water moves through soil and is held in pores and on soil surfaces. The relative size and proportion of the pores in the soil determine the vital soil physical characteristics, such as drainage and aeration. It is by understanding these differences in pore size that we can explain water movement and storage in soils. 

During heavy rain, many soils become saturated and the pores are filled with water. After the rain, water drains from the largest pores, called macropores, unless there is an impediment to drainage. This might take a couple of days and is an important process as once these big pores have emptied, air can get into the soil. At this stage, the soil might be thought of as a ‘reservoir’ for the plants, although not all the water left in the soil is available to plants. 

The actual size of the ‘reservoir’ and how much of it is available to plants varies markedly between soil types. Coarse-textured soils, known as ‘light’ sandy soils, have mainly large pores which drain quickly. Fine-textured soils, known as ‘heavy’ clay soils, have mainly small pores called micropores, which drain slowly, if at all. Neither of these pore sizes is ideal in terms of water storage for plants. The macropores drain quickly and don’t hold water for a long enough period for plant roots to access it. Conversely, micropores hold onto water with so much suction that this water is extremely hard to access. 

In between these two pore sizes are the ‘goldilocks’ zone of intermediate sized pores. These pores ‘hold onto’ the water and don’t drain, but the water in these pores is also easily extracted by plant roots. These pores tend to be most numerous in intermediate-textured soils. However, all soils have a combination of macro, micro and intermediate pores, and the proportion of sand, silt and clay in a soil play the major role in determining the drainage and water holding capacity of the soil.

As we go from spring into summer, we can often describe changes in soil moisture in the following sequence. If all pores are filled with water, the soil is said to be ’saturated’, for instance water may appear at the surface. If this saturated soil can drain, then the macropores start to empty. When most of this drainage is completed, the soil is said to be at ‘Field Capacity’ (FC). At FC, the water and air contents of the soil are ideal for crop growth. Gradually the water stored in the soil is taken up by the plant roots for transpiration, or evaporated from the bare soil surface. 

If no additional water is supplied to the soil, it gradually dries out. The dryer the soil becomes, the more tightly the water is retained and the more difficult it is for the plant roots to extract it. The uptake of water is insufficient to meet plant needs. The plant attempts to cope with this stress by wilting. If the soil continues to dry, the leaves may change colour from green to yellow and in extreme cases die. When the plant has taken all the water that it can get, the soil is said to be at ‘Permanent Wilting Point’ (PWP). The difference between FC and PWP is the ‘Profile Available Water’ (PAW). The PAW of a given soil is, therefore, the size of the ‘reservoir’ that is available to the plants. This usually tends to be larger in intermediate-textured soils and smaller in coarse sandy soils.

For more information on managing various levels of soil moisture, contact your local PGG Wrightson Technical Field Representative.

Jay Howes

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Cropping programme and partnership delivers

01 March 2020

Wairarapa Arable Farmer, Richard Kershaw has worked alongside PGG Wrightson for the last 20 years, growing and selling multiple crops for seed and feed. 

The Kershaw family owns Moiki Farm on the East Coast, in the flat country between Martinborough and Greytown. The homeblock is 260 ha, with another 200 ha lease block. Different members of the family manage various aspects of the farm. 

The bulk of the business is cropping for seed production. They grow approximately 20 ha of ryegrass for seed, 20 ha of red clover for seed, 35 ha of hybrid maize seed and 12 ha of Milton oats. They also produce around 50 ha of barley and 110 ha of  maize for feed grain. In addition to this, the Kershaw’s fatten lambs and graze beef cattle.

Richard notes that their challenges are much the same as everyone else operating in the dry East Coast conditions. The farm is fully irrigated, and production has risen accordingly. He adds that there’s occasional but devastating biosecurity issues to contend with, such as the introduction of pea weevil into the Wairarapa about four years ago. Not growing the crop has been the only way to eradicate the pest.  

Working alongside Richard for the past two decades has been PGG Wrightson Technical Field Representative, Geoff Horrobin. In fact, Geoff has been with PGG Wrightson for approximately 40 years, servicing Masterton south to Martinborough for all that time, bar three years in the South Island.

“Geoff is our agronomist and we have relied on his expertise for many years” confirms Richard. “His knowledge of the area is second to none, plus he knows our business so well. 

“We count him very much part of our team and our cropping success. Annually, we sit down together to review our cropping programme. Geoff then takes responsibility for organising all our seed, fertiliser and ag-chem requirements as we need them.

“Crop monitoring is Geoff’s area of expertise, I’d say he is probably on farm two or three times a week in the height of the season, checking on each of our crops and reviewing our weed spray and fungicide programmes."

Due to the nature of their business, the Kershaws work with a group of people from PGG Wrightson, including the Rural Supplies store team, as well as PGG Wrightson Seed and Corson Maize. 
“We find PGG Wrightson exceptionally supportive of our business” concludes Richard. “Not only do we rely on them for quality inputs and advice, they bring us new contracts regularly.

“I’d say a fair chunk of our income has been a result of working with PGG Wrightson.”

The benefits of measuring soil variability

01 April 2020

When soil sampling, we usually collect the sample by block or by paddock. This is purely driven by ease of sampling logistics, such as crop area or fencing around a paddock, rather than yield variability and soil type, which in fact have the biggest influence on available plant nutrient.

Zonal soil sampling uses knowledge of historical management and spatial factors to direct where to take samples and determine if these areas have different fertiliser needs. Tools such as yield maps, crop sensor maps, Electro Magnetic (EM) soil maps and aerial imagery provide more information about variability in the field and where soil sampling can help interpret variability.

Soil depth and texture (sand, silt, and clay) change constantly across a paddock, impacting on the soil’s ability to hold on to moisture and nutrients, for instance a sandy soil has a lower water and nutrient holding capacity than a clay soil. By measuring the variability in sand, silt, and clay across a paddock and plotting it using GPS, we can create a map showing zones by soil texture and then sample those areas separately. This can be done either by a soil scientist taking soil cores and creating a soil classification report, or by using a scanning device such as an Electro Magnetic (EM) scanner or an Electrical Conductivity (EC) scanner. 

The scanner is towed across the paddock or area to be scanned usually at 12 metres swaths and sends an electrical pulse into the soil about every second as it drives forwards. The electrical pulse then bounces back to a receiver on the scanner having arced through moisture in the soils pores. Then, with the aid of an algorithm, a geo located reading of the soil porosity is done linking to the soil particle size and giving a soil texture indication.

A map is then created showing areas of soil texture change which can be sampled separately. A fertiliser plan can then be created by zone rather than by paddock. The spreader used must have the ability to variably apply fertiliser or lime. The map can also be used to variably apply seed, slug bait and even cultivations and help place soil moisture probes if you have irrigation.

The benefits of measuring soil variability are that you can apply lime and plant nutrients where they are required instead of carrying out a blanket application across the area to be fertilised. There are potential savings and environmental benefits by avoiding over fertilising areas, and crop output benefits from not under-fertilising areas, however the jury is still out with proven benefits of variable applied nitrogen (N).

For more information on zonal soil sampling, please speak to your local PGG Wrightson Technical Field Representative.

Technical advice core to business relationship

01 April 2020

The business relationship between Nelson contractors Kevin and Andrew Fry (K & A Fry Contracting) and PGG Wrightson has grown so strong over the last five years, that both parties now promote and refer one another to new farmers in the district.

Established in 2002, K & A Fry Contracting are the biggest agri-contractors in the local area, offering hay silage, groundwork, feed conservation and spraying services. What started as a one-man business with a tractor and a seed drill, has grown to a sizeable partnership between Andrew and his father, Kevin. They now have a fleet of five tractors and a spray truck, operating with a team of six full-time staff. 

Andrew notes that spray contracting is the biggest part of their business and being able to rely on PGG Wrightson Technical Field Representative, Andrew Young, helps with inputs, advice and daily support in both their agri-contracting business and cropping ventures. 

“Andrew (Young) is outstanding for both his service and knowledge. We’re in constant communication with him for advice when we’re out on a job, and we talk to him daily about spraying.

“In many instances, we’re working for PGG Wrightson customers, but sometimes not. That doesn’t matter to Andrew, he’s always happy to help.

“He knows what he’s talking about and gives us sound advice. He’s quick to respond, and if there’s something he’s not sure about, he’s upfront in telling us that he needs to do some research first.”
The Fry’s also have 75 ha of crops under irrigation, growing maize for PGG Wrightson Seeds and Lucerne for baleage. They’ve recently added 11 ha hops, too. With the contracting business, Kevin and Andrew are not on the farm a lot of the time. Here, they rely heavily on Andrew Young to monitor the crops and regularly revise their crop management plans. 

 “Andrew is probably here at least three times a week checking on the crops and keeping things on track,” says Andrew Fry. “That gives us real peace of mind.”

He adds that they order everything through PGG Wrightson, “we find PGG Wrightson to be consistently reliable. We rarely have to wait for a product. Andrew normally delivers it too, unless it’s a big volume.

“We’re loyal to PGG Wrightson, but they’ve earned that loyalty. We consistently get good deals and Andrew’s support is invaluable in all areas of our business.”

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